North American Terrestrial Vegetation Second Edition Edited by Michael G. Barbour William Dwight Billings =corred deciduous io'ess; 5aands; .^'=.Aopzlachian _re<u; P= coaszl plain íorQ=:rup:ca! róresr$. Bou.rd- g ro W. D. Bi!:ngs; zn CAMBRIDGE UNIVERSITY PRESS Contri Prefac Prefac PLSLKHHED BY THE PRE55 SYVD!CATE OF THE C\:VER:T: iF CAVBR:D-E Tne Fitt Building, Trumpington Street, Car.:bridge, Lnited Kir,dom CAYMSRI DGE C4.ERS,TY PRESS The Edinbur,h Building, Cambrid _ e CB? _RU, LUnited Kingdom r ,1+'ti..c.:p.cam.acuk 40 \Yest ? O:h`Srreet, Ne w York, NY 1 001;=21 i, CSA ,+'x-vr. cap.org 10 Stamford Road , Oakieigh, Melbou :-te 3; ^5. Australia Ruiz de Alarcon 13, 25014 ,-Madrid, Spain CHAPTER 1 Arctic Latrrer CHAPTER 2 The Ti Debor. CHAPTER 3 Forest Robe, CHAPTER 4 Pacifi, Jerry F CHAPTER 5 Califo Micha CHAPTER 6 Chapa Jon E. CHAPTER 7 Interr N'eii E CHAPTER 8 Wa rn James CHAPTER 9 Grass Phi!1ip CHAPTER 10 Easte; Hzzc, CHAPTER 11 Veget N'or,m, CHAPTER 12 Fresh Curar= CHAPTER 13 Salta. In iro Cs Cambnd ,e Cniversity Press 2958, 2300 This book is in copyright. Subject to sta.`cton' escephon and lo the pro\isions o£ relevant collectice '. icensn, agreements, no reproduc tlon of am- part mav take place t. ithout the ., r.tten permission of Cambridge Universri' Press. First publ!shed 1955 Second edi on 2000 Pnnted in the Cnited States of Amenca Tv?cfr:e, 9/11 Palatino pt. DeskTopPro Ln IRFI A ca:aing rcccrd for ilds book is n,:.riloble 5em f e Eritislr Librare. Lrr,-v of Corgress Ca:aI ^i,rq-in-A,b'ics;ian Gas North American terrestrial vegetation / edited by \S:hael G. Barbour, William D,.'ight Billings. - 2nd ed. p. cm. 'nclndes bibliographiczl referentes (p. ) en-' índex. ISBN 0-521-55027- 0 (hardhcund) 1. P!ant commnni:ies - North America. 2. Plant ecology - North .America. Phvtereo,raphv - North A-, eric a. 1. Barbour , '1ichael G. II. Biilings, W. D. Q1'illiam Ació t), 1910-;997. QK11 O.N854 1999 551.722097 - de i ISBN 0 531 55027 0 hardback ISBN 0 521 55956 3 paperback 9729C'cl CIP CHAPTER 14 Alpir. CHAPTER 15 Meza, A!e;., CHAPTER 16 The C Are! CHAPTER 17 Tro,n Contents Contribulors Preface to the First Edition Preface to the Second Edition HAPTER 1 Arctic Tundra and Polar Desert Biome page vi¡ ix xi 1 Lawrence C. 3liss 41 CHAPTER 2 The Taiga and Boreal Forest Deborah L. Ellon-Fisk CHAPTER 3 Forests and Meadows oí the Rocky Mountains Roben K. Pee: CHAPTER 4 Pacific Northwest Forests Ierry F. Frankii❑ and Charles B. Ha!pera CHAPTER 5 Californian Upland Forests and Woodlands Michael C. Barbourand Richard A. Minnich 161 CHAPTER 6 Chaparral Ion E. Keeiey 203 CHAPTER 7 Intermountain Valleys and Lower Mountain Slopes %eii E. L1'est and la .mes A. Young 255 CHAPTER 8 Warm Deserts James A. Mac.^;ahon 285 CHAPTER 9 Grasslands Phil!ip L_ Sims and Paul C. Risser 323 CHAPTER 10 Eastern Deciduous Forests Haze! R. Delcoun and Paul A. Delcou: r 357 CHAPTER 11 Vegetation oí the Southeastern Coastal Plain 397 23 Norman L. Christensen 449 CHAPTER 12 Freshwater Wetlands CHAPTER 13 Saltrnarshes and.\langroves Irvin, A. Mendelssohn and Karen L. McKee 501 CHAPTER 14 Alpine Vegetation William Dwight Billings 53% CHAPTER 15 Mexican Temperate Vegetation Alejandro Velázquez, Victor Manuel Toledo, and !solda Luna 573 Cunisl. Richardson CHAPTER 16 The Caribbean 593 Ariel E. Lugo, Julio Figueroa Colón, and Frederick N. Scarena CHAPTER 17 Tropical and Subtropical Vegetation of Mesoamerica Gary S. Hanshorn 623 W. D. Bi/!ings ,.d 'oil develop\!'_:dro^. C- ac er, G. Molenazr. L, :crs in z:Cic and unes, R. W. Hoharn, fed>-) The ecologv Carnbridge. ;.,D. 1\'a!ker, and .,. studies of .enze +_.-S ^,'5. Czrboh drate ^Le plants..amer. of paterned bu'.l. Geo_c<ta Rica und itere -, rrit den Ho.n 1?athematischiz^.: gan,e XR 3. und der Literz;ar. :I lir.ts of vascular Chapter 15 Mexicali Temperate Vegetation ALEJANDRO VELÁZQUEZ VICTOR MANTUEL TOLEDO ISOLDA LUNA S74 A. Velázquez, DIVERSITY AND NEARCTIC AFFINITIES Mexico has a high diversitv of ecosvstems iRamamoorthv, Bye, Lot, and Fa 1993). Mexico includes six oí the ten major terrestrial biomes oí the world -extra-drv vegetation, mediterranean, temperate forest, temperate grassland, montane, and tropical rain forest (Cox and Moore 1993) - and it is one of the ten most megadiverse countries oí the w'orld (Mlttermeier 1988), harborirg 10-12% oí the world's vascular species (Toledo and Ordoñez 1993)Mexico's vide elevation range (0-3000 rn), its location astride the Tropic oí Cancer, and the influence oí two oceans across its reiatively narrow' continental mass probable are determining factors for the most significant features oí Mexico's climatic diversity. The Tropic oí Cancer is a significant therrnal demarcation and also deli.-nits the transition between arid and semiarid climates arid anticyclone high pressures toward the north versus humid and semihumid trade winds and cyclones in the south. The complex ph\siography, together with the differences determined by latihude and altitude, result in a climatic mosaic with a great number oí yariations (García 1951). Maximum average temperatures (288-30° C) are recorded in the low-hing regions oí the Balsas Depression, w-hereas adjacent zones at -he top oí Pico de Orizaba in Veracruz hace the iorest average temperatures (-6' C). Some mountains Nave glaciers and permanent snow. Apart from these tw•o extremes, the range oí temperatures most frequently recorded caries from 10 lo 2S° C. Precipitation also presents notable contrasts: from <50 mm annuallv and no wet season (as in parts oí Baja California) lo >5500 mm annually and almost no drv season (as in parts oí Tabasco and Chiapas). As a consequence oí this climatic diversity, Mexico has a largo variety oí vegetation types, comparable only lo India or Pena. Altnough detailed studies haye distinguished up to 70 different units of vegetation, based on phy'siognomc and floristic composition, it is possible to differentiate fewer principal tapes of vegetation in Mexico at the biome category (e.g., West 1971; Rzedow ski 1978, 1993; Flores 1993). At such a scale, it is apparent 7har.ks lo Martha Cual and R. M. Fonseca for support en floristics. Comments by Jorge Llorecte, Javier Madrigal, and Richerd Minnici on an ear!y iersion, and bv >tichaei Barbour on a late' version, are acknow!edged. J. Rzedowski fully encouraged the preparation oí this chapter, and it vas further supported by a grant-in-a.'d from DGAPA-GN.AM grant IN-209094, and FGO's Universiw of Amsterdam. V. M. Toledo and 1. Luna Temperater 1 egetation that Mexico mar be divided relatively easilv. Fin ure 15.1 shows the distribution oí the main vegetation tepes oí Mexico as a function oí two clima tic attributes: precipitation and temperature (the latter is represented by eievation in the figure). Although mane Cosmopolitan and Paleoarctie taxa are present L-i Mexico, its geegraphic location has favored the establishment cf hiotic elements characteristic oí %vo main regions, Nearctic and Neotropical. Mexico is si;uated un a transitional ,; adient from Neotropical to Nearctic environmer,ts. A large part oí Mexico is dominated by ecosv°terns oí northem af fiiiation (Beard 1944; Rzedowski 1978). Two main historical events may explain the present dominarce of arctic biota: (1) most oí Mexico's northem territorv has been linked permznen;k lo the rest oí North America, and (2) the last glaciation (ca- IS,000 Nr ago) promoted the moyement south oí .mana northem taza within Mexico s present political horders (Fe:rusquia-VÜlafranca 1993; Velázquez 1993:. Temperate vegetation tvres on long north-south mountain chains tcpifv Mexico's arctic affinities. The Madrean Region from no7inem Mexico south through the Xeovolcanic Transversal Belt lo Las Cañadas de Chiapas is dominated by oak, alder, p:.ne, and fir species. The absolute dominance oí species oí Nearctic origLn che tree iayer and a large number oí species oí Neotropicai origin in the shrub and herb layers is commonly obsen'ed (Gadow 1930). T,tic compiex ceeetztion patiem becoma more dominated by the Neotropical to the south and bv +-he Nearctic lo tE:e north. T'ne Neocolcanic Transversal Belt tenis the heart oí the gradient, and it contains a lame number oí endemic taxa (Fa 1959; Rzedow.wski 1993). Data from a lar ge nuniber oí botanical and zoological expeditioti throughout tñe present centun provide strong evidente oí `e great affinity of montane regions of Mexico wi:h the rest oí North America (Beard 1911; Goldman and Moore 1945; Srnith 1940; Troll 1952; Wagner 1964). Structural classficatiors oí Mexican vegetation tupes equated temperate ecosvstems •.with mountanious regions (Sanders 1921; Shelford 1926). Temperate montan e vegetation tepes pave ra-ieusl;- been called (Table 15.1) montane raLn. forest, high m .ountain forest, and páramo (Beard 1944; Braman 1962); pine-oak forest (Leopold 1950); low e.erreen forest, conifer-oak forest, oak forest, and :-,amo (Miranda and Hernández-X. 1963); and mest recent!v cloud montane forest, cornierous forest, uak iurest, and grassiand (Rzedowsski 1978). Research that defines the relationship oí these vegetation tapes lo ecological facters such as humidity, ssoil suitabilit', 5000 -____-___ 2000 -__ i :000 --TRi e I 3000 Hvperhumid Hu: Figure 75.1. Maior.M,exka a!ong gradients oí tempera: rainfali in mi!!imeters). Abb (TRF), subtr-oical caduciró tbbus iorest T(1F), shrublan Table 15.1. Equivaleni r tarious authcrs Beard (i 955) Herr Montare rain iorest Deci( .Montare iorest Pine Páramo Alpin Source: Rzedcvrski. (1978). fire, number oí days w'iti and mean anual prec: scanty or restricted lo sr initions oí temperate -\le eeer, Nave been ¡nade and Ordoñez 1993). M. matic ecoregions are the humid temperate, and 15.2). 3 and I. Luna Temperare l'ege:ai;on (Mierico 575 v easily. Figmain vegen,o climatjc cre (the latter _.e). Cold :d Paleoarctjc spic location 5c elements _ .carctic and a trans_itional rctic environ. Semicold Températe Températe ted by ecosvs1; Rzedosyski ay explain the :m,ost of Mexinked perraaand (2) the last ted the moveaitin Mexico's uja-Villafranca ng north-south rctic affinites. Mexico south sal Belt to Las by oak, alder, dom ,Lnance of ce layer and a pical origin in ronly observed ic n pa ttern be'cupical to the crth. The Neoe heart of the ,.umber of en+3). nical and zooresent centnrvv eat aff ruty of Test of Nonh 3 Moore 1945; 64). Structural hiles equated :nious region erate montane n calied (Tabie ,untain forest, 962); p,e-oak n forest, coniamo (Miranda recently cloud ,ak forest, and ;h that defines types to ecooil suitability', Sem v arm \Yarm \'erv \1'arrn Semiarid Ard Hv^perarid Sub.hum d Hvperhumid F. =d Transition Trarsidon Figure 15.7. Major Mercan vegetation ;upes ordir,ated alon;S g^adien.s or ;erPper arare and precipCation (anm:al rainiall in milümeter<i A±brevi tions tropical ruin tares; (TRF,t subtropical caduc;rolius [ores; (Sri, tropical caduci- (SAG), t.homshwb (T5), oak lores; !OF', cloud forest (CF), rir [ores; !FR. pine lorest (PF), and alpine grassland (A C). The nical zone is IC. (Modified rrom Toledo d RzedoHSki, 1995.) [ollas [ores; (TCFh shrub avd íA, subafpine erassiaad Tab'.e 15.1. Equ[va!er„ carnes ci ASexican :enoerate s ege'a?ion rypes ,iver, by varicurs authors Toledo & Miranda & Beard (1955; . ernandez.X . ; 1963) ntontane ram lorest D eciducus lorest .Montare forest Páramo PI^-e-ilr lorest - .or ne bun..cir grass!acd Rzedo,ski ; 19781 C:'ou ores; Comer lorest 'taca;on.al" Ordoñez;1993) Hurnid temperare Subhunid temperare Cool temperare Source: Rzc Jowsk;, t19-2:. fire, number of daos -.,:th temperatures below teto, and mean anual precipation is, on the schole, scantv or restricted to specific places. General definitiors of temperate Mexican environments, hos+ever, hace been made (Rzedowski 1978; Toledo and Ordoñez 1993). Major temperate biogeoclimatic ecoregions are the humid temperate, the subhumid temperate, and the cool (or alpine) (Fig. 15.2). Tre humid temperase repon is characterized by cloud forests dominated by oaks. Tse forests' tloristic composition includes beth boreal and tropical elementü. This region nccupies yery restricted sites of 600-3200 m elevation, mainly on slopes facing the Gulf of Mexico from Tamaulipas tu Chiapas. Distrbuted in 21 states, it covers an atea of approximateiy 10,000 km'. The subhumid temperate region covers the 576 Temperate Vegetation .4. Velazquez, V. . M. Toledo and 1. Cuna 0 2). Figure 15.2. Climatograrx representad; e of thr ee nain ele'a:ion befts or nountain environmernr temperare humid (Tlanchinoli, temperare subhumid iEl Guarda), and tempera:e cool :Hueuedaco). Months Jancan-DecemSeri are arranaed along the horizontal axis, mean monrh„S remperature along the lert rerlical axis, and monrhly p-ecipiration greatest parí of the mountains of Mexico at elevatiotu of 2500-3000 ni. The characteristic vegetation is forest of fir, pine, oak, or mixtures. It s distributed through 20 states (principally Chihuahua, Michoacán, Durango, and Oaxaca) and covers an area of approximately 3_3,000 km'. The cool (or alpine) region is located aboye timberline (>4000 m) on the 12 his,hest mountairs of Mexico. It is dominated by alpine bunchgrasses, or zaca tonales. GEOGRAPHICAL DISTRIBUTION OF TEMPERATE VEGETATION Mexico is a ver, mountainous countn,, with over half of its territon- >1000 m in elevation. Arid and semiarid vegetation dorninates the high plateau of central and northern Mexico, whereas temperate vegetation (as defined in this chapter) covers the steep and more humid areas ahoye 1000.n. Temperate vegetation thus covers 2200 of the Mexican territon•. Mexico is also a countn- with evidente of past volcanic activity. The most spectacular volcanic area is the great Neo;olcanic Transversal Belt, which crosses Mexico from west to east at the latitude of Mexico City (19-204). its landscape is characterized bv thousands of old cinder tones and dczens of tal) volcanic peaks (Fig. 15.3). Volcanism continues todas', with many active or temporariiv dormant volcanoes. Earthquake activity is common, mostly along the Pacific Coast and the Gulf of California. Earthquakes are also frequent in the Neovolcanic Trans- along tne right verical axil. F'e Bical shading represe,-'s soi1.rnoisture recharee ; solid srac,nc reoresenr$ precipi:ation bevond soil smraee caoacn, and doned shadi, represe%-5 soil mo .ivu: e derlc;r. Pa, ;Sienta) ir,rorma:/on is e, e'a:ion, mean annual tempera:cre. acd an.- jzl precip;:arion. Cii.ma:ic data a,e a' ere; es or :930-1990. verse) Belt, often ca ❑sing considerable damage in this heavily populated region. Mexico can be divided finto five general rea':ras: ext-atropical dn'lands, tropical highlands, tropical lowlands, extratropical l gF.lands, and subhumid lowlands (STest 1971). Diese realms match most Mexican phvsiographic regions - Lar exampie, the Baja California and Buried Rarees of northwest Mexico, the western and eastem Sierra Macires, the Neovoicanic Transversal Be!',, and the Highlands of southem Mexico. Four repon q are considered temperate: the western Sierra Madre, the eastem Sierra Madre, the Necvolcanic Transversal Belt, and th,e southem Sierra Madre. ic westem and ezstern Sierra Madres forra dissected borders of the;vestern and eastem, edees of :he central piateau. The croad crest of the western Sierra Madre vises up to 3000 m. The upeer portion of the range is covered with thick lavers ef lava. The western slope of -,he range forras rueged can\ons and narrow ridges dropping down to the Pacific coastal plain. The eastem Sierra Madre rises to a sharper crest on the eastem rim of the central piateau, with elevations up to 4000 m. In the North, the Sierra is comprised of severa; irregular ridges separated bv baria descending graduaily to the Golf coastal p lain. Tne Neoyoicanic Transversal Beft forms a maior geological break ;, ith the central plateau. The belt is hnrdered en the north bv a series of high bas:nS on the south the )and drops sharply mito the drrr Balsas Depression. included in this volcanic area are Mexico's highest and best known peaks: Pico Figure 15.3. o gital eles me Vallev of México ' e: de Orizaba (5700 m), cíhuati (52S5 m), and The Highlands o: logically complex re tions b'c the Isthmus Sierra Madre in the ;ande in the east. ??-. highly dissected mu vailevs, a discontinu a few highland basin are dcminated bs- tl-, teau rising up to an These regions oc Coahuila, Nuevo Lec Luis Potosí, jalisco, c Distrito Federal, Pu. choacan, Oaxaca, ar tened regions in the c species: high elevatic Baja California Sur, C erra Fria in Aquasca Sierra Lacandona aa MAJOR TEMPERAT VEGETATION TYPI } The four major ter^p ico are comp^xd (s,,:su Rzedow 'ki Beard 1941 and Brau can classifications ar, laven and at ihe gene phasis masks consic leve). Fir forest (. 577 Temperate L'egeiation of ,\lexico Digital £1"ation Nade] Ajusto ( 3990 s) Fi;ure 75.3. Dita! eleuaon model or southern portion o7 crea oi 805 kr are >J00 vo,'caric eones. From Velázrhe Vallev o México ;vericel exagaeration 3x:. )vithin Ihis quez 7993.) 7990. .' dam age in _._ral reair^s: ropical ..;, subhumid _ match rnost epa-:ple, the cf northwest 7a Madres, the e Hi^iilands of _r_.dered tem,e_stem Sierra 1 .,_.., and re .ladres forro cartero ed-_es st cf tire westm. The upper .,ick lavers oí _ orms rugged g do••Vn to the ora Madre r,ses .. of the central :33 m. In C,e .coral irregular ng gradua„\ to t forros a major '.ateau. The belt of high basins v finto the deep s co!canic area n peaks: Pico ce Orizaba (5700 m), Popocatépetl ( 5.1=2 m), lxtaccíhuatl (5255 rn), and Nevado de Toluca (4392 m). The Highlands of southem Mexico are a geologically complex region separated oto hvo sections bv the isthmus of Tehuantepec : 'he southem Sierra Madre in the west and tire Chiapas Highlands in tire east. The southem Sierra Madre is a highly dissected mow,tain svstern w'ith narrow vallevs, a discontinuas Pacific coastal plain, and a few high!and basins. Tire southeastem highlands are domi:nated by tire altiplano of Chiapas, a pieteau rising u,,- te en ele\ation of 2500 m. Tpese regio.r.s occur in Chihuahua, Durango, Coahuila, Nuevo Leon, Tamaulipas, Zacatecs, San Luis Potosí, jalisco, Guanajuato, Hidalgo, Mexico, Distrito Federal, Puebla, Veracruz, Morelos, Michoacan, Oaxaca, and Chiapas. Sorne other scattered regions in tire country also harbor temperate species: high elevations of Sierra de San Lazaro in Baja California Sur, Cerro Potosí in Tamaulipas, Sierra Fria in Aquascalientes, and high e.evatior<of Sierra Lacandona and las Canarias in Chiapas. 1tA)OR TEti1PERATE VEGETATION TYPES Tire four major temperate vegetation upes of Mexico are comprsed of many plant comrtunities (ser;e;r Rzedowski 1978) or associations (sensu Beard 1914 and Braun-Elanquet 1951). Most Mexican ciassifications are purely based on tire canopv layer and at tire generic level (Tabie 15.2). Tnis ernphasis rnasks considerable variation at tire species level. Fir forest (A.cies), for instante, occurs Tahle 15.2. Chzracreri;tic eecera or the Tour majo temperare vegetar:on rape; o7.tfexico. ;hese genera mav van' iro.m slooe :o sfone and moro sierra to siena, so thar no single genus can be round :hroughout a single temperare +egetation rype. Cloud Gene a a^^< t'.ein.vu.n,nia forest Pirre ic'est Y X x Liquidaabar x Acer x iüa x Aluhtenéerla ti Ribes x x x x He:e-ia x Cier^NJm x Roícana 3odd!eia T udwm Sh !ro pia S2iz Cirr:a Srmphoricarpos Alplne grassland x cr.=e.tha«1:ia Celos Bacchariss Fir forest X X x x x x x Ca!amagros:is x A^roslis x Ttiselum L'.mbilica ria Bry oepihrophyllum Siereocaulon Cladonia x X X X X 5 78 throughout Mexico, but co-dominant shrub species and fir species change from north to south (Velázquez and Cleef 1993; Islebe, Cleef, and Velázquez 1995). Only a few phvtosociolodca] studies have been conducted in central Mexico, sufficient to define associations and alliances based en fine-scale differences in species..Another limitation to our ability to summarize the vegetation is that a large part oí the territorv where temperate vegetation types are distributed has not been sun eved in detall. This lack oí homogeneous informa tion does not permit us to provide a thorough description oí al¡ communities. Thus a detailed description oí temperate vegetation is beyond the scope this chapter. Our objective is to provide an ovenview oí major vegetation tepes and to emphasize those tha; have been best studied. We also intend te outline the consenation possibilities for temperate vegetation in Mexico. Additional details on the vegetation oí Mexican forests, grasslznds, deserts, alpine, and wetlands are found in Chapters 3, 8, 9, 13 and 14 oí this volume. Cloud Forest ( Humid Temperate Forests) Large biological heterogeneity t-pifies a cioud forest, which is a mix oí northem, southem, and endemic taxa , and oí ]ow- and upper-elevation laxa. Because oí its diversit-, various .-.ames have been given to Lhis vegetation tvpe; bosque mesófilo de montaña (Miranda 1947), caducife'ious forest (Miranda and Hernández -X. 1963), temperate deciduous forest (Rzedowski 1963). Five environmental requirements seem to govem the presence oí cloud forest Li Mexico: high relative humiditv, montarte en;ironments. irregular topography, Jeep litter laye:, aad temperate el¡mate. Cloud foress covers at most 1% oí the total Mexican surface, but it includes about 3000 vascular species (Rzedowski 1993), which is 12% oí the countrv's vascular flora (Toledo and Ordoñez 1993). Currently, there are only a few large preserves oí cloud foress but thev are scatered throughout the range oí the tepe. Aiong an elevation gradient, the structural complexity oí cloud forest decreases totvard high e'.evations and varíes from slope te slope. Elevation ranges from 600-3200 m, though trae vegetation a best developed at 1000-1750 m. Precipitation :s IS00-5500 mm }-r', and doudiness is commo;t throughout the year. Freezing temperatures are rare. Major temperature changes are seasonal, in contrast to large daily changes un alpine environments at higher elevatiora. A. Ve!azquez, V. M. Toledo and 1. Luna Phvsiognomically, cloud forest is dense, 15-40 m high, and multilayered (Fig. 15.4). Some oí the tree genera that reach more than 40 m are Engeihardia ard Pi.^.tanus. Tne upper free la}•er is dominated bv caducifolious (deciduous) taxa, the lower tree laver bv perenrüo!ious (evergreen) ones. The most diagnostic species is Liquidmnhar rnncnephu:'.: (see Fig- and the most common associated boreal elements are Ca-vinos caroliniar.a, Cornos disnjlora, Ti! mexicana, .4.'nos f, rn:yfolia, and Qnercus car;dicros (Miranda 1947; Puig 1970). Some species shared rvith the eastem deciduous forest oí North America are Acer r:egundo, Carpinus carolinana, Carca orcta, Cornos onda, Fagus mexicana, lllici:un ^9oriu'.aruat. Liquica.n:ber nz:rophy!la, Nyssa sylt'afica. Oshva -i•cir, irisa, Pra:us serotina, Ti!ia foridana, and Taxurs globosa. Species shared with western (oíten riparian) foress are Arsu:us xa!cpersis. Cei:is pailid C. ret:cu!uta, and 5„mbucus mexicana. Mesoamerican Laxa f:eeuently, foud in cloud forest are Clet!:ra spp., l"+eir:n:a; ✓ :: spp., Arctataphvlos argufn, Ilex discokr, Litsen gia c scens, Magnolia schiedca nt, Pinos :r;ontezun::.e, P. ps:udestrobos, Prunus brachyhotr✓a, arad Uimus mexisma. Among the endemic taza are Canco eruta var. mexicana, Ilcx pring!ei, Juglans mol!:s, and P!atanus mexicana (Rzedowski 1978). Commonly, there are wo shrub layers, both with Neo:ropical al les- T,.e families Compost tae, Gesneriaceae, Ciusiaceae, Labiatae, Legurninosae, Malvaceae, Me astomataceae, MyrsLnaceae, Piperaceae, and Rubiaceae varousiv dominate, dependLng en elevation, !atitude, and humidity. The herb laver Lncreases cover and diversitc when the oversto:v is disturbed. Arboreal fems (e.g., Cya:%rea) are comnion, as well as herbaceous species. Mosses are aba-ndant. Flowering piants are in the fzmilies Asclepiadeacea, Begoniaceae, Bromeliaceae (especialls' the genus Tillandsia), Cvperaceae, Compositae, Convolvulaceae, Cucurbitaceae, Dioscoriaceae, Equisetaceae, Gramineae, Liliaceae, Lvcopodiaceae, Orchidaceae, Piperaceae, Solanaceae, Urticaceae aad 1'erbenaceae. This ]ayer has a Nectropical afí n-:, :.ith only a few rnosses and mushrooms be", ci boreal afiinity (Crum 1951; Guzmán 1973; Delgadillo 1979). Pine Forest (Subhumid Temperate Forests) Mexico contains about naif oí the 1, orld's pire species (Critchfield aid Little 1966; Shles 1993). in mu,i arras pino forests are co-dominants with other broadleaf trees (AL;us and Quercus) and other conifer species (Abies and Juuipen:s). Collectiveh they, cover 1596 oí Mexico: on sandy soils of coastal Temperate t < lag -- plains (Pinos michoacmrus, ,' (P. hartwegñ), 15, C- 10)a not seem te ment oí pine sociated with Hernández 19 Rzedowski disturbance i^ of conifers, b, shown that succession tm study is n =edc grazL 0 en pir fleco and 1. Luna Temperare Vegetation oi.Mexico 579 is dense, 15-40 Some oí the m are Engel_e laver is domi) tasa, the lower ,,creen) ones. The ,,tbar mr<crophypa nmon associated 1 tia ra, Cor,;us dfsand Quercas Some species s forest of North 6:us carolfniana. -ruL'xicmm, illiciuru NyEsa sul:'atica, Tilia floridana, +ith westem (of,.. apersis, Ce;ris _ a:exicano. Meso7, cloud forest are . os tapl:ylus arguta, ?cro!ia sczicdenna, ?ranus brachuong the endemic I:ex prir:giei, jugara (Rzedowski :n b lavers, hoth amilies Composiabiatae, Legurnieae, Myrsinaceae, :s!v domínate, dend humidity. per and dicersity J Arboreal fems ell as herbaceous wering plants are 3egoniaceae, Bro'd:anisia), Cvper:eae, Cucurbitare, Gramineae, cene, Piperaceae, acere. This laver nlv a few mosses i affi dty (Crum ,79). ate Forests) sorld's pine speStvles 1993). In -domL,ants with uercus) and other -ia). Collecñvely, '.v soils of coastal FI, ure 15.=. Asoect oí cloud forest at encelo anea (2600 m). ICounesy oí 1. L. Convexas) plairs (.Pü¢us car:b..ca, P. oocarpa), en lava flo•.+s (P. michoacanus, P. *..-..,e.urrac), and at high elevarions (P. kartaegif), (Rzedos+ski 197S). Temperature (ca. 15- C = 10) and precipitation (S00 = 150 mm) do not seem to be .actors for the establishment oí pine speces. Acid soils, how+wever, are associated with pine íorests (Aguilera, Do,,', and Hernández 1962). Rzedo,+'ski and v?cVaugh (1966) state that fire disturbance in pire forest favors the establishment of corifers, bu t Sa ..chez and Huguet (1959) have shows that fire, loó ing, and grazing induce succession toward pine-alder-bunchgrass. More studv is needed to document t-he e.`fect of fire and grazing on pine forest (Velázquez 1994). Major tupes of pine forest inciude alder-pire, ponderosa pire, pine-oak, Hartxeg pine, and mixed pine. Each is described in the following sections. Alder-pne forest. Alder-pire forest is made up of four structural lay.ers: (1) coniferous tree layer (50% cc.ver, masimum height 22 m) of Pinus: (2) sh,-ub layer (4 m height) of Alnus `nnifolia, Senecio cinerrricdes, and S (3) dense burchgrass ]ayer dorninated mai.nly by ,,luhlenbcrqia nmcrcur a and Frstaca tolucensis; and (4) ground layer componed mairly of Alcheudlia pronm:bens and Arenaría lycarndic:d s. The bunchgrass )ayer is characterized by compact bunchgrasses -,+ith 580 A. Velazgoez, V. Al. Toledo and 1. Luna Temperate Vei Litsea glnucesc laver. 'Ylidstai mentioned, bt are more dom sir and 5,un:pic Cure stage is laur;na and Q single species dant subcanr (Gonzáiez-Ese phase is one a. dominated by understory (th depends stron Intensive r comrnon in t. bun agriculh: ited, mainly a Pinus hartwegh scribed this poor. 11, is mai: pedregal lava rather flat ut:; 330373 me (1 m) are loam Figure 73.5. Aspect oí pine íorest at Tláloc volcano (3300 m). broad, long, tough grass leaves that seem to be ,veil adapted to fire and browsing (Fig. 15.5). The íorest is restricted to ver] dissected, rol'.ino toeep slopes, and lava flo,vs at 2700-3500 m elevation. Soils are shallow i+ith grave'}- sandy loam texture. Constant diagnostic species include Alims frmi,`clfa, Arhutas glanduloso, Buddleia pare?ora, Ery»gium carlmae, Pere cn;on ge»tiarcides. P. campar:+Iatus, Pivus monte:Inliae, Qt4'rcus r;urina, Sicyos 1,ar,'ij7erus, Stellaria cuspidata, and Striu nm»arditblis. Cervantes (1950) and González (1952) described this ttpe of mixed forest, and Rzedowski (1951, 1975) referred to it as a mosaic of Air::rs finni olía forest and M;dacuhcrgin gandridentata , assiand and suggested that repeated buming and grating are the main causes of the bunchgrass. Disturbed conifer forest in some parts of central Mexico is replaced by alder forest or by suba!pine coarse bunehgrassland of .\hdiler. bcrgia and Festuca. Velázquez and Cleef (1993) desccbed four associations ivithin alder-pine forest in central Mexico: Trisctunt alti];i um-Ahlus Frmifoi:r. Pinus-A6:us fr; folia, Ervngiunt cr. rlinae-A.bu+c, ün :fol(o, and Pi. 77m7ife:unce-Al::us Jtrmi^ólia. Pinus ponderosa forest. Ponderosa pine is chieflv found in the western Sierra Madre ori granitic or volcanic steep slopes or plains at 1021500 m elevation. Fires are frequent. Ponderosa pine dom: nates large stands, are citen mixed ,vid: Abies cor;cnlm. Studies of :...- forest's distribution and d,-namics are scanty in con trast to many publications about it in ti-e Urited States (Styles 1993). Pirre-oak Forest ;Pinus oocarpa-Quercus aurina). .\lost landscapes at high elevations (23002i00 m) of Chiapas are covered by pine-oak forest in sorne stage of recocerv from disturbance (1\'agner 1964; González-Espinosa et al. 1991). Onh' a fe" patches of oíd-ero,v th forest remain. There are no freezinng davs dur'ng the %ear. Soils are moderately deep (10 cm) calcarecus or clavey loaras (Breedlove 1951). Tnree seral stages - early. middle, and matare forest - are apparer.t. At ec c stages, Pimis oocur;'a. P. oaxacar:o. Quere2j9 Q. crass;fnlia, and Q. rugosa prevail. Beio,,' the overstor' canopy is a ser ind, io,,cr roe !:ccr i,,niinatei by Ra,anea ¡re,. genserií and P: u :us serctira. Scla»urn 7igrica7:s and The physio layers: (1) an c (2) a herb-bun hergia. Festuca, (3) a ground la enana lucepec:: tbis forest are qu.uiridentata; :: Muhlenhergia tuca tohrce»sis, ter hvo bunchs alpine grasslar quez, and Lur: The forest layers, unlike and Pelado yo' ,va,•s hace a de is considered t Em (1973), hoy ,vas a successic religiosa forest, alpine slopes a gro,vs in a mal i e_:rnu:e, and Q .Haca, Nema: boreal distriba tains of Centra. in Guatema!a's eral plant coro: ••:üé v N:;.'.y;'n-eN '- .`..n '_6. -, !'w+:^ i o+^ :-.i f 4 . :z i% ni,'.. I. Luna L!sca glaucescrns are abundant en the ground laver. \•Iidstage stands share all the species just mentioned , but pines are les i,r,portant and oaks are more dorninant . In addition, Oreop „ i:az ^nlapcusis and Svnq'Iocos li noncillo are abundart. The matare stage is en oak forest don tinated by Quercus laurina and Q. crass;fo'i a. Pinos are present, but no single species prevaiis . CL-sera theaou'es is ara abundant subcanopy free. Shrubs are chiefiy abre,,^.t (González-Espinosa et al. 1991). This pme-oak seral phase is one of the best examples ef a canopv ]ayer dominated by Holarctic species with a Neotropical understorv (though the degree to t.+'hich this holds depends strongly en geographic location). Intensive grazing, cepping. and logging are common in the Cañadas of Chiapas - Slash and burra agriculture for coro productien is more .'imited, mainly conducted bar indigenous groups. Pinus hamvegii forest. ?tzedowski (1951, 1978) described this vegetation as being rather speciespeor. It is mainly restric:ed to the upp er part of the pedregal laya slopes beiow co.'canic rones or te rather fíat undulating slopes forming plateaus at 3350--3=70 m elevation. Trae relaiive'y >haüow• mis (1 m) are loamv clays with a thin Iitter la, er (5 cm). The phvsiognomv of this forest consists of three lavers: (;) en ove,storv free laver up te 20 m high; Z) a herb-cunchgrass ]ayer with e is chieflv anitic or .500 m elepine domd-:ixed with stñbut ora ,t te manar :ates (Stvles creas laurr;ons (230 e-oak forest .anee ('^1ao a r:. Tbere are are mod.avev loares and matute cocar:", and Q. opy is a sec:::;:,:rea _;r:crrs and 581 Temperare t'ege!arion of ntexico Sergin, Festuca. and Cala' :::;ros::s grass species; and (3) a ground !ayer dominated by.5ldaonaia and Arenara lvcopodioides. The main dia_nostic species of this forest are Pinas Ccrtm ezíi and A'u1.!enberg-,a qundridn:'nta; importar., associated species include ..",Qad:lenbergin nuaaeura, .=:r:r:aria lar:opa:üo:.yes, Fe>tuca tciucensis, and Calara:•-mstis tc.t;ic,sis. The 5t ter rayo bunchgrass species are also diagn:ostic for alpine grassland f.Almei^a, Cleef, Herrera, Veláscuez, and Luna 1994). Trae forest lacks -weil-deti.-:ed s,,. and herb lavers, unlike the Pin:,s i:.r;avri€ forest en Tláloc and Pelado yolcanoes Ln central Mexico, which alw-a ys ha,.e a dense herb :a ver. Pi ': us 'ss r;;rc; ii f ores t is considered by most te be a clímax community; Em (1973), hotvever, s-a _,ested that P.rs:c rsrttrrgii was a successional species in severel'y bu:ned AFies n^'r^:bsn forest, a forest res::leed te yen. steep subaipine slopes at 2900-:23-0 m eleyation, and which nmngro'.+s ir. a matriz tyith and Qacrc:s /eres: (Pvede'.vski ;954, 1975; .Anaya, Hernandez, and Madrigal 1980). Tne least boreal distribution of FM C5 reaches high mountains of Central America arad are best represented in Guatemala's mountains (Islebe et al. 1995). Several plant communities paye been identued asso- ciated with bunchgrassland dominated mainly by ,Liulvler.ber,lia ^ruadridenL^!a or Festucn tolucensis. Rzedowski (1975):eferred te these bunchgrass species, together with C.aL:.niagmstis tolucer,sis, as yen' abundant associates of Pirms l:artwegii forest. Evidence of recent buming and grazing (mainly by sheep) is found in most aseas of this communitv. Wood extraction in central Mexico is practicaliv absent, though in Guatemala ordy a few patches of this forest remain, due to the intensitv of fuelwood cutting (Islebe 1993). ,vtixed pine forest. There are a number of large forested aneas where no single species of pire seems to be dominant. Such a complex pire unit is apparently promoted by intensive logging practices and by fire. It occurs in very heterogeneous landscapes that include flats and steep slopes. Soils are shaiio'.v, acidic, and sandy. Often, there is recentii, deposited Nolcanic ash or lava (e.g., in the Paricutín volcano area and other portions of the Tran versal \eovolcztic Belt). The lava flows hace en irregular topographv, which produces a diversity of microenvironmental stuations with many endemic taxa. Within the 40 m tal) overstorv are Piras nlicimacarsa. P. n:on!e:un:ac, P. lelepi,^ln, P. pseudostrobus, P. riáis. P. t.'occ:e: and P. iar!:oegi;. A;rus and Quercus species are present but w-ith less cover than Pinus. The free canopv is open, more like that of a woodland than a forest. Beiow is en open shrub laver with Senecto, BuddLfa, Rfbes, and Rubus as charcoteristic genera. The loivest understorv laver contains Satureia, Stcria, Euyaforium, Salvia, and cushion-like species such as Arenaría tiroides, Geranrum seenrar.ii, and r.Id:cn:i!1a p rocunibe?:s. A defínite bunchgrass laver is absent. Fir Forest (Abies) The rnost boreal Mexican yegetation tupe is Abies forest, variously and locally described throughout Mexico bv Leopold (1950), Rzedow•ski (1951), Beaman (1965), Anava (1962), Madrigal (1967), and ?,nava et al. (1980). Rzedo•,c ski (1975) has ó ven a complete description on a natior.al scaie. Fir forests t ricall% occur helow Pie:-s l:.v! a e;ü forest en high yolcanoes, along escarpmenrs, and in giens betv een laya floi,'s. T.nec pret'er canyons or other steep slopes protected from direct sunlight and strong winds. Soils are rich i-n organic master and ash. 1hree fir species uccur in Mexico: Abies corcolor, A. religiosa, and A. gua!rralensis. They usually cooccur in the overstorv with Pimrs, Quercus, Pseu- 582 A. Velazquez, V. Al. Toledo and 1. Luna Temperate Ve; i dotsuga, and Cupressus species. A lower free layer is comprised chiefly of Alnus, Arbutus, Salix, Prunus, and Garrya species. In rare undisturbed stands, there is a ground layer of mosses and cushion plants. T.he Abies religiosa forest is mainly restricted to the Transversal Neovolcanic Belt. It occurs on steep to moder ate (10-30`) outer slopes of volcanic cones at 3000-3500 m elevation. Soils are deep and there is a thick laver of Iitter on the surface. Some evidence of disturbance from grazL-ig, burning, loa ging, and Cree harvest exists (Madrigal 1967; Rzedowski 1975). The forest is dense and tal], reaching 30 m in height (Fig. 15.6). The overstory ]ayer is dominated bv A. religiosa. Below is a ]ayer of shrubs and tal] herbs (0.5-3 m), dominated by Senecio angal folius and Roldana barba-jolutnnis , and a ground layer (5 cm tal]) of rosaceous herhs (e.g., Alchemilla procumbens) and mosses (e.g., Polytrichum juniperir:um). Other diaglostic and common species of this plant corrmunity group are Ser:ecia toluccanus, S. eallosus, S. platnn folius, Sihthorpia rrriens, Salix ox;leps, Festuca amplissima, Alchemi!!a procun:bens, Thuidium delicetvlum, Acaena elongata, Stachvs species, lolium species, Galium, aschenbc nif, Cinna poc:e;ormis, Pernettya prostrata, Dydimaea alsinoides, and Buddleia sessilijora. At lower eievations Chis forest mixes with Mulilenbergia and Calamagrostis grassland and A!rus ^5rm,fotia forest (Velázquez and Cleef 1993). The Abies guatemalensis forest is mainly restricted to the southernmost part of Mexico and into the Guatemalan mountains (Islebe et al. 1995). It occurs on verv steep slopes at 2500.3900 m on deep soils rich in organic matter (Islebe and Velazques 1994; Islebe et al. 1995). It is a dense forest with three layers: an overstory fir layer 30 m tal¡; a shrub layer dominated bv Roldana barba-johannis and Tetragyron orirabensis; and a moss layer dominated by Thuidium delicatulum. Ot-her species present in this forest include Fuchsia microphy:'la, Senecio callosus, Trifollunr amabile, Sabazia piretorum, and Pinus ' l n Mexico, there are no communities where Abies concotor, the other fir species, is dominant. It alwavs occurs mixed with ponderosa pine. At Sierra San Pedro Mártir, in Baja California Sur, Pinus ie eyi coexists with Abies concolor as part of other mixed forest communities (see chapter 5 of this volume). Alpine Bunchgrassland Mexican alpine bunchgrassland is dominated by tussock grasses restricted to steep volcanic slopes at elevations aboye timberline (ca. 3800 m; Fig. 15.7). It has been studied by a large number of researchers throughout this centurv (Standley 1936; Beaman 1962, 1965; Cruz 1969; Delgadillo 1987). Beaman (1962, and 1965) called this vegetation "alpine prairie ," w'hereas \ 8randa and Hernández-X. (1963) related it to .-ondean ecosvstems and called it "high páramo." Almeida et al. (1994), however, believed that the urique composition of Mexican alpine regetation mide it different from the actual páramo. Rzedowski (1975), in ap eement, treated it as a separate vegetation tupe, which he called "alpine zacatonal." Alpine bunchgrassiand cornmunities occur in six main high-mountain formations: Cerro Potosí, Nevado de Colima, Nevado de Toluca, Sierra Nevada, \4alinche volcano, and Pico de Orizaba volcano. No svstematic survev has described asid compared these communities. Recent surveys conducted at Popocatápetl (Almeida et a;. 1999), lztacdhuatl, and Nevado de Colima voicanoes rrovide a li-nited surrunarv cf Mexican alpine ecos%stems. Near the low'er lirnit, in tse }ic t of Pinus hart-u'egii forest, Lupinus mo':,anua, Festue. tchrcer:=.., Calamagrostis tohucensis, Per:aten=ion gentirnicdes, and Descarair:ia impatier:s are the mest common species. These are considered the diagnostic species of zonal alpine communities . Arrizaría ovoides and Juniperus monticola tvpify the azonal aipLne cornrnunities. Near the upper rival bordee mosses and lichens do=ate (Bartran :ia and B: roe✓throphyl'urn). Intensive grazing asid fires are fast depleting these alpine ecosvstenrs . Tris vege:aticn also harbors a large number of ende:rvc taza. Des ite the smail area covered bv Mexican alpine b unc'r.gr-ssland (0.02% of the whole countrv), five zonal and b.vo azonal plant associations (sensu Braun -Bianquet 1951) have been described by ALmeida et aL (1994). In the southem mcuntains of Mexico, at the bordar with Guatemala, diere are smail patches of alpine grassland dominated bv Lupinus mortanus asid tussocks of Calamagrosts culcar,ica up to 1 m high. There is ¿!so a ground '-ver with mosses such as Breu''elia asid L ptoda:ti;an. Other cornmon species are Luzula -acrorosa , Agrostis telucensis. Draba r:dcanica, Arrn.:r.'a b soldes, Graphalium srliiciloiium, and FoL^::,•i; (a ;cteresep-ala. On rocky outtcrops, Raccmitriunr ^ü?;dum is dominant. This southem alpiste kege:ation grow s on gentle, winiprotected slopes wüh regosois (Islebe and Velázquez 199:). Fire and grazi:g are the majos causes of degradation of alpine bunchgrassland ecosvstem5 \1'hen fires are frrq.:eni, Lupi r.us rucrtanus becores dominant. Hiking pat5 significantly fragment (:sisa vegetation (Almeida el, al. 1994). F..gure 75.6. ,..spe and 1. Luna Temperare Vegetation oí. Iexico ,00 m; Fig. r; her of re1936; délo 1987). don "al.. ,f:ndez-X. i d called of Mexican .:he actual r+ treated it "al.Cctr in six Potcsí, Nevnra Nevada, vc lcano. No 3 ccmpared cnducted at ccir^atl, and e a limited ns. Near the upe; i forest, C:„-:arr:ag ros!is Desnrra:ina es. These are zonal a!pine F cure 15.6. Aso ct ol irt ;crest at Ajusco vo'cano 3 , 1OO.m). ..es. Near the ns dominate .ensive grazLne ecosys- rge number a covered by of che whole ,!ant associa;ce been de- :o, al the bor:at&es of al:+;arta,:us uptolm .,ith mosses roer common to!ucetsfs. .a!iurn salir, rc+.KV out- ,ant. Ihis :entre, :cindand Veláz- ❑ ses of dei ecosystems. : us becomes :ragment this Figure I5.:. Aspect oi alpine bur,chg ^zssland at Izraccihuad cclcano (4i0O m) 583 584- Subalpine Bunchgrassland (Festuca tolucensis) Ibis community is mainly restricted to the fíat valley bottoms within vokanic craters at 3500-3550 m elevation. Soils are very deep and nave a thick surface ]ayer oí litter. The community consists of a dense layer oí bunchgrasses 50 cm tall, doninated by Festuca tolucensis and Cnlan;atirestis to'ucensis. and an open ,round layer dominated bv.A7dumilla procun:bcns. Other diagnostic spectes are Poa annua. Trisetum spicatum, Pinus mnr:fe_u:rae, Pinas gii. Mu'ilenbergia quadridentafa, Muhienbergia aff. yusiha, Oxalis spp., Sicvos yar;i.`,on+s, Potent:lia stanunca, Pedicrdaris ori_abae, Draa iorullensis, and Arenaría brvoides. Beaman (1965), Cruz (1969), Rzedowski (1975), and Almeida et al. (1994) have observed this community in volcanoes along the Transversal Neovolcanic Belt where continuous buming and erazing disturhances take place. This is che reason these authors considered this grassland to be a seral communitv (Velázquez' 994). \Coodcutting could promote this t\-pe oí vegetation. Less Common Communities Megarosettes oí Furcraea beding'rausii indicate a vegetation upe restricted to the rolling , disse ted, rocky lower slopes oí a few volcanoes in central Mexico, such as Pelado (3090-3340 m) and Tláloc. Soils are shallow, gravelly, loam.v clavs (pH 5.36.5). Half-meter - high monocauiescent agavaceous me,arosettes oí the endemic F:;'aaea be.'ingluusü characterize this community . The maximum height ever measured for Furcraea is 53 m. A f oristiallc rich out relatively open herb :aver is common. Other characteristic species incude Senecia angulfolius, Stipa tchu, Sympharicarpos mfcrophvllus. Conv=a sciriedeana , Muhlenbergia n:a:rcura, M. quadridentata, Geranium poterrtiliacfo :.m, Gr:aphalivnt on+yi:vi!u!u. AIchenrilla procunbens , Sibiharpía reyens, and Festuca an: vtissin!a. Stipa ici:u meadow is cornmonly known as a "pradera" oí Potentüla cand ;'eans. It occurs on poorly drained soils (Cruz 1969 ) cn flats surrounciing volcanic cones at 3000 -3300 m elevation. it is restricted to the Valley oí Mexico (Rzedows)<Jand Rzedowsk: 1975; Rzedowski 1981 ). Soils are deep sandv loaras oí pH 5.G-6.2. Vegetation i s a single ground laver (0.15 m height ), consisting oí lo,.v forbs and grasses. Diagnostic and associated species include Stipa iclm. Potentfila ;n:dita; ,, Ast• alas micrantlrus , Reseda 1uh'ola, 3idenc triylfi:er.'ie, Hedeonra yipen!¡tut , Con:meiina alpes!n s , Vulpta myures, Akl:emilla procumbens , Gnayrv!i:mt seemannfi, A. Velázquez, V. Al. Toledo and 1. Luna and Salria species. This vegetation is significantly disturbed by hikers and campers. .An alpine scrub oí Jumpen:s manticnla, codominated by Tortula andico!a, E:3ngiam prctútlorum, and 5enecio n;.ciretfar:us, is restricted to rocks, wet places aboye timberüne on most high Mexican volcanoes, such as Po, oca tépetl, and it is also present in Guatemala (islebe and Velázquez 1994). A narrow ecotonal Cupressus lusitanica forest used to be widely common behveen fir forest and cloud forest at about 2600 m eleyation. This transition has largely been deforested and transformed finto farmland. In the feto remaining patches, Abies is sometiines co-domLnant in the overstorv. 5enecio piata,nfoiius, S. cinc arioides, Fudsia ndcrophvia, and species oí Rubus and Rifes are common Ln an understora shrub layer. BIOGEOGRAPHIC HYPOTHESES Mexico's larse biodiversity has been attributed 'o hvo mala hvpotheses. First, its geographic location - where t.e Neaaic and the Neotropical zones overlap - necessar ilv ,laxes temperate and tropical elements. This is tl e dispersal h}pothesis. Additionaily, a substantial proportion of the total flora oí ibis region is oí autochthonous origin, fo,nnLng a heterogeneous mosaic oí species (e.g., Rzedowski 1978). 5econd, its role as a Pleistocene refu,e could make .a':zeodv-nzrc events oí paramount importance la gocen.rg present vegetation (Toledo 1952 ). T.ne dispersas hvpothesis imagines that the major mountain ran_es mned as bridges connecting Mexican and North American .-oras. As a consequence, Holarctic taza reached Mexico across these :nountzin bridges,.vhich provide a con:inuoaslc similar climatic condition across latitude (e.g.. Graham 1972; Rzedowski 1975E The arrivzl pernnad oí most boreal e!ernents is controversial: So:ne authors believe that it -,vas in the Ter;iarv and Early Quatemary (Graham 1972), whereas Martín and Harrell (1957), among others, suggest a ¡,lora recent arrival. Tne lose affinity behveen Mexican and North Amerian temperate taxa has been commented on by severa! biogeographers (e.g., Islebe and Velázquez 1994). Pczedo•,vski (1993) estimatei the afíinit<• at 9f at ;`. aeneric 'evel and named it the "nlega:nexice p :: ie,eograp ic unit." According to Toledo (1976), hvo aneas of \fevice functiened as Pleistocene refugia: the Lacondona and Soconusco regio-s, both oí rvhich still harhor a larse number of endernic specie s and subsp2c e` Toledo also hopo;he>ized that additionol reos in Guatemala and Belize sen-ed as refuges and tr`'t collectively thev plaved this role several times dar' . Temperare ing the Qua oí glaciers present dise vegetation. There ar (1952), for , between bic convergent extend bici geograpv/-, rente (1993) DYNAMIC Indigenous tlers, and ci tures all h; !andscape. ulation abo: oí pre-corta to rely on Hemandezgeneralsucc malnir,g Inc a, d maintai Ta: ahumara 1994). There that suggest colonial tim tation couid stand-replac (Islebe e: al. Bv the ti 1500), most covered by %Vhen wood demands ( cvere clear-c and near lar sume extent However, lo creasing ove tun•, expiain forestexcept Livestock for tended finto human pop needed, and popular me, grazing prac oasis througiledo 1955). A mote regrow dental canc (Velázquez 1', Most rece d 1. l;:na temperate Ved (0:.?, coto rockv, \texican pros- 94). fc-est etation in, the Quatemarv. The expansion and contraction of g)aciers mav additionally have modified the present dstribution of temrerate humid montane vegetation. There are also alternative h•,potheses. Croizat (1982), for example, finds a ciose correspondence behceen biologic and geoloo:c histories, suggesting convergent evolution aad vicariant patterns that extend back across longer periods. The panbio,eegrapv/vicariance ideas of Espinosa and Llorente (1993) extend back to Laurasian time. DYNAMICS OF VECETATION -,b.,t d to ic location ica1 zones ..: cpkal Addia! flora forming '.ze ow'skl -_. e could _ct imporn (Toledo .aat th.e m onnechrng .s a consecross these 7..ti-:m.:ously (e.g., Graperiod of Sume auand Eariv 'aren and more reMexican roen cem- ?.,., Islebe estlmated named ot,\fexico acondona `arbor •,:bsoecies. al arcas in > and that times dur- Lndigenous Indians, contact-period Spanish settiers, and current mixed pepulations of many cultures all base modified t.e temperate Mexican landscape. There is ve:v little published speculation about reconstruct-.e maps or descriptions of pre-contact landscapes. Largely we have had lo rely on experienced botanists Miranda, Hemandez-X., Rzedowiti, Madrigal) who infer general successional pathr..avs, and ora t-he ferc rem.aining Lidian groups _^a. _tia manase vegetation .ai.tain sera] stages mar cultura! reasens (e.g., and m Tarahumaras, Purépet'ras. TzotzLes) (Toledo 1993). There is some evidente from : ellen analvsis that suggests that there l :as deforestation prior to colonial times (Metcalfe et al. 1991). The deforestation could perhaps have been caused by large stand-replacirg tires prornoted by drought periods (lslebe et al. 1995). By the time of -he am; al of the Spanish (ca. L0 ), rnost arcas in central \fexico viere densely covered by forest in di.`fere.-d successional stages. when woodcutting took pace to fuif311 European demands (_ 1700-1500), most temperate forests were clear-cut on p!ateaus, valleys, around lakes, znd near larse human se:t!ements. Remaining, to some extent, vas montane temperate vegetation. How.+ever. Jogging in montare arcas has been mcreasi.g oven the past 50 vr of the hventieth centurv, explaining the present ratchiness of montane forest except in the highest, most inaccessible arcas. Livestock foraging in the nrieteenth centurv extended finto tírese moun'.a..e envi:onr,ents. N'ith human populatior increase, more meat vas needed, andburning in t:-.e dn' season became a popular rnethod to increase forage. This tregrazing practice is still i nlemented on a veariy bass throughout most of ;Fe temperate rcgion (Toledo 1988).Although the tres are intended to promote regrowth in the understorv vegetation, accidental canopy tires frequently take place (Velázquez 1992). Most researchers (Miranda and Hernández-X. oi Vexico 585 :963; Madrigal 1967; Rzedowski 1978; Velázquez and Ceef 1993) agree that fir forest s the mesic climax tape of Mexican temperate ecosvstems (Fig. 15.S). Ahies co:nm'unities are favored by soils rich in organic matter, humid terrains, and middle to high elevations (Madrigal 1967). Clear-cutting for paper production rransfo-ms :hese forests Lato subalpine bunch rasslar.ds dominated bv Mularrhergia, Calanwgresfis, ard Festuca; selective cutting trzrsforns them into mixed oak forest w•ith manv svmpatric Qaercus species. Fire and grazing may then degrade subaipine bunchgrasslands Lnto a scrabland of Senecio and Ribes, or roto meadows of Fofertilia and Stipa rrhere soils are poorly drained. Scrub and meadow mav regenerate into conifer forest if there s not stror.g human interferente (Fig. 15.5). Mixed oak forest can develop finto mixed alder-oak forest and traen finto either cloud forest (given sufficient moisture) or hito mixed pire-aider 'orest (where soils are sandv and acidic). Mixed pine-alder forest is 'he most w'idely distributed \Iexican temperate ecosvstem at this .ir.e. Pire and grazig favor pine species, transfcrmLne tris vegetation tape finto p•ure stands of pi— forest (Pf s harrargi at Mgh elevations). Mixed pire-aider forest mav, under very- limited G: camstznces, develcp back finto fir forest (Hg. 15.5). This happers, for instante, in ver, humid canvons with a thick litter la%er and an absence of F:e and erazing. Puse pire forest can also be repiaced by fir forest but only at elevations where fir species are better adapted tiran unes. High-elevaion fir and pLne forests, when harvested, revert to alpine bunchgrassland dorninated by Ca'armgrostis, Trsetum, Agrrstis, and Festuca. \'exican alpino b•.:nchsrrassland, as well as the tropical alpine gra stands of páramo and puna, expand where deforestation takes place (Baislev and L-atevn 1992). Forest regeneration is suppressed =y E:e- and grazing aovi^es. Nearnal! temperate vegetation !upes of Mexico are in some stage of regression or progression. Fire, srazLng, wind, herbivon-, avalanches, landslides, volcanism, and human disn:rbances are the causes o-` tnis seral lancscape. Natural tire and arson may be the most cornmon disturbznce<_ as evidenced by t'-. e charcoal that is found in most soils throughout \`.exco's :nountains. At retum Lntenals of 20 vr, tire seerns to be a suitahle tool for forest management, but in central Mexico tires recur everv 1-5 vr. Only a few places have remained unbumed >5 vr. The reason for frecuent buming is that the forage becornes less palatable .`or domesticated animas ove, time. A larse amount of oxalates and siliates, which accumulate in the leaves oí grasses, may be the cause of low consumption of forage A. Velázquez, V. Al. Toledo and 1. Luna 586 Vpine bunchgassland Cool bdgh e^eaa ^. o.^s>3:00 n) Fir forest Mesic clima wz.. chesmider:.;n•n_.e.,s Oow re'va^ov cial:_) Cloud forest Figure 7 5 . 8. Sche natic summay of successional relationshios among Mexican temperate vegetaron tvpes. Mesic climax rypes are s,haded darker than sera! s:ages . Thick arrows represen: natural, progressive succession , v.hereas hin arrow5 represen: re:rogreirre success,on caused by human in:e.nerence or by azora1 en, úonmental cond7ons. (Velázquez 1985). Peasants who set fires, however, are ignorant oí the fact that many nutrients released by the fire are leached and eroded awav. Mexican temperate and tropical forests have been more impacted, fragmented, and depleted than anv other vegetation tupe (Toledo 19SS; Masera et al. 1992). Recent estimations by Masera et al. (1992) are that fires (49%), livestock production (28%), and agricuiture (16%) are the main causes oí the depletion oí tem orate forests. How+'erer, these estimates are based on onh• general field obsenatiors and anecd3tal comments from rural people. Lntensive Jogging activities r+'ere common Lhroughout the coun-r 30 vr ago. These activities became regulated by la:v in the 1950s, although ac- Those few mavs oí current Mexican vegetation that have been published (e.g., Leopold 1950; Miranda and Hernández-X. 1963; Flores, Jimenéz, Madrigal, Moncayo, and Takaki 1971; Rzedowski 1978) rapidly became out oí date as extensive human modification oí the landscape continued. Consequentiv, these maps should be considered as depicting potential vegetation ra:.her than actual vegetation (Velázquez and Cleef 1593). Accordir.g to Masera, Ordonez, and Dirzo (1992), temperate forest deforestation (excluding doud forest) has been estimated at 163,000 ha yr , equivalent to O.Jl°b oí the total Mexican surface being deforested every vear. In contrast, reforestation has been attempted en only 13,000 ha, and no: all the attempts have been successful. tual imp'.ementation oí :he regulations ,as limi:ed to central Mexico. ^,e ,+'estem and eastern Sierra: Madres are stil] beng clear-cut where the original forests remain. Clcar-a;t;ing in Mexico is net followed bv reforestation; consequently, the neoztiv e effects oí soil erosion and se¡¡ poductiviry are astordshing. Temperate Ve, In contrast as practiced f, favors both re trees. Minnicf repeat aerial have had a s Aguascaliente oí Aguascalie. ests (Quercus : roxvla, Q. rugí rus deppeana) t slopes oí barr (Fig. 15.9). Ch pungens, with opensis, Garrea abundant on exploitation fc using rudimer tse 1920s dese Repeat aerial t 1953 reveal th pulse of wood oline sawntills 1950 when dic with the introc the city oí Ag: in exploited fe from pollardir broad-scale th_ by rapid incre. establishment change in the 1 declines in Pin: attack c ring a (Siquéir,,s-Def chaparral expe fires behyeen to resprout or lings from seec geners in Caii with open-rang Recent direc caused bv anor. disn:rbance in derstory in pv frequent Ere pr. species of Que' fires v: ith modo an ability to r fines mav have and encoura2ec characteristie In. United States (s reduce the exte S!aphylos punge' aedo and 1. Luna rcession caused bv 'ro.^.n]e0taf condi- pical forests have red, and depleted Toledo 1988; Masby Masera et al. estock production e the main causes forests. However, general tie)d obments from rural ries viere conunon :o. These activities i930s, although ac>tions esas lir ited and eastern Sierra i,here the original Mexico is not tolently, the negafive productivify are 587 Te.rperate Vegeta:ron of Mexico Ln contrast, less intensive fue!w.wood gatr.ering, as practiced for local consumption by rural people, favorsboth re;eneration and de%elopment of adult trees. \finnich et al. (1994 ) provide evidente from repeat aerial photographs that land use mav not have had a severe impact on pine-oak forest in A uascalientes. The Sierra Fria, located in the State of Aguascalientes , is dominated by pine-oak forests ( Quercus potesi na, Q. lacta, Q. eduardii, Q. sidermaa. Q- rugosa, Pinus teocore. P. leiopltylla, Junfpcru; de}>peann ) that forro contiguos forests on steep slopes oí barrancas, and open savannas on mesas (Fig. 15.9). Chaparral dominated by Arcto;tcphvlos pun ens, with scattered Arburus glandalosa , .4. ralcpe:ss, Garrua spp. and Conreresta^ )rylos poiifclia, is abundant on steep slopes . Technologies for forest exploitatien for rirrtber and charcoal production, using rudimentary ground kiins , viere limited until the 1920s due to the inaccessibcity oí the range. Repeat aerial photographs taken between 1942 and 1993 reveal that the range experienced a distinct pulse oí woodo.:tt ng with the introduction of gasoline sawrnilis alter 19-0. This iand use ended in 1950 when the urban demand íor ;uelwood ceased with the irtroduction oí ural gas pipelines into the city oí Aguasca lientes. Oaks viere still common in exploited forests because most species resprout from pollardinz . Since 1942 there has also been broad-scale thickening of pme-oak forests caused by rapid inaeases of Jurti?er s dc,,peana and slow establishment o: Qucrcus spp. There has been little change im the dstrbution oí pues except for local declines in Pms:eiephvüa and P. teocote from insect attack during en El Atto-related drought in 19S4 (Siquéiros- Delgado 1989 ). Ardo;taphylos pr jenn chaparral experienced little change in spite oí largo ;res betweer, 2920 and 1950. Most species appear to resprout or to establish numerous postf:re seedlings from seedbanks (A. punge'a), similar lo congeners in California. Stand-tnickening ceinc`ided with open-range cattle grazing. Recent direcvona: vegetation changes mav be caused by anoma )o'as)v infreauent Eire as a natural dist'srbance ir' the siena . A dense )erhaceous understorv in p--e-oak forests probably supported frequent fire prior to livestock grating . The cariors species oí Qurc:is are adapted to sunive ground tires with moderately thick bark, tal1 canopies, and en ability to resprout from rootcrowns . Surface tires may hale selective : v c'Iimtnated yni2ng Fines and encouraged open o':d-growth forests, as esas characteristic in cellow pire forests in the western L'nited States (see Chapter 5). Recurrent tires mav reduce the extent oí Juniperus deppeana and Arctostaphylos pur.gens because thev are nonsprouters and do not establish abundant postfire seedlings from seedbarls. J. dcpra :a mav have survived in Eire-protected camyers where old-growth stands no occar. Urba:-tization v perhaps the most threatening human aCivity lo temperate forests in central Mexico. Mares oí the maro urban concentrations oí the co^untrv, includv,g Mexico City, are located in (or near) temperate vegetation. According lo the National Censas oí Population of 1990, the area covered by temperate vegetation s inhabited by about 19 ,—n people, or one quarter oí the total Mexican population (Toledo and Rzedowski 1995). Oí these "•temperate" people, 62% live in cities and 3S°'o in che co,:ntn-side. It is Chis latter nonurban population oí approimateiv 7.3 million who live most int mately with temperate vegetation. We estimate that t`re precortact population within temperate veeetation „as - in contrast - only about 2 m indigenous people belonSing te 40 different ethnic groups. CO.NSERVATION IMPORTANCE AND PROBLEMS The tisea ecological regions that constitute the temperate vegetation area oí Mexico are oí great portante from a biodiversity point oí view. Despite its reiativ ely small area , t're humid temperate cioud forest is bioleg caUy very rich. lt harbors a large number o.` endemic plant species, especially orchids, ferns, and mosses. Given the small area and tie lar,e nwn:ber of specíes, it is floristically the richest zone it \!exico bv unit area (Rzedowski 1993: Stvles 1993). The zone is notable as well for its lar_e rumbee oí endemic mammals, amphibians, reptiles, and butterfiies (Flores 1993; Flores and Gerez 1994) lo such an extent that it is one oí ihe ^ncipal centers oí autochthonous species. Areas abone timberline (>4000 m) are also oí notable biolc r) and biageog aph cal importance. O'•erall, the temperate zone covers the greatest p¿-:t oí the -nountainous arcas oí Mexico. Of special impon tanto s ;.he Transversal Neovolcanic Belt hecause it harbors one of the highest concentratiors c` specíes dicersity and endemism presentiv kno-t.-n (Fa 1959). Rzedocski (1993) estímates that there are 7000 specíes oí flowering plants, oí which 49',J (ca. 75%) are endemic. About 7.3% of the Mexican territorv is under sonx po ic} oí protection (Flore; and Gerca 1994). T.ne criteria for the selection oí protected arcas and their boundaries hace changed from time lo time. Most protected arcas (79 out oí 1660) are within the temperate region and harbor temperate vege- 588 A. Velazquez, V, Al. Toledo and 1. Luna Temperare Veger Figure 15. U Defortional Park 11,900 m' heede !Dendroctonu cation tepes (Fio( naulative area is al] protected area surface. Most of th 'egetation are %-e. that the probabilc into the future s trolling to reduce croachment is lim: pects nave been ta:permanence and c, com 1994; Toledo ÁREAS FOR FUTURE RESEAR( Figure 75.9. Pine-oak ioresi in me Sierra Frs. A,aasca- and savannas. 31 Tvpical trr cure ui ti:e ,ore>c., ood!."' ` tientes. (A) General aspect, sho.+irg mosaic oi v: oodlands ;Photographs Couiesy oi Richa:d htinnioh.l Temperare cegetati mous impon Lance They also have ccc Portante as sources ceutica's, water, erc rent knowledge is. ^lest studies ha\ e c: munities in Central d 1. Luna Dresd,voodlar.d 589 ,emperare Vegeta don oi A7exico Fisure 15.70. Deioregytion :n Desierto de ¡os Leones xa:ionai Park 2900 m7 near M exico Ciry, caused bv bark beet!e rDendroctonus adjuntes!. Air colutants initially }, eakened :he ,ores:: inrestation by bark beedes then caused the death or 15%% oí me «ees. tation tupes (Flores and Gerez 1994), but their cumulative area is modest, accounting for only 4% of all protected area , or oniv 0.32% of the Mexican surface. Most of the protected areas witi^ temperate ye etation are verv small in sine, which implies that the probability of their cor.tinued existente roto the future is low. Financial support for patrolling to reduce poaching and other human encreachment is limited. Furtherrnore, no social aspects have been taken finto account to ensure their permanente and consen'ation (\IcNeely 1989; Alcem 1994; Toledo and Ordoñez 1993). i-^ormatien about northem and southem communities. In addition te the need for mere descriptive studies, ;reat effort should Se given to docurnenting ecosvstem processes such as vegetation dynamics and succession. These ecosvstem processes are the least known aspects of temperate vegetation. The ¿ata are needed to model funare distributions and compositions of these communities.:f Esancial sapport vvere available, such data could be obtained re!aüvely quickly. In the absence of supporz, the most feasible future of tmperate piant comrnunares is their progressive destruction (Fig. 15.10). ÁREAS FOR FUTURE RESEARCH Temperate vegetation tepes of Mexico hace enormous importarte as resenvoirs of biodiversitr. Thev also have economic and hum: n ! calth importance as sources of timber, fuel•.vood, p'r-armaceuticals, water, eroson control, and oxygen. Current know:edge is, hor+ever, far from complete. Most studies have concentrated on describing communities in Central Mexico, leaving larse gaps in REFERENCES -„mrn. 1. 3 . 159,. 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