Rev. Biol. Trop., 44(2): 581-590,1996 Vegetation-environment relationships based on a life-forms c1assification in a semiarid region of Tropical Mexico Zavala-Hurtado, J.A.!, P.L. Valverde!, A. Díaz-Solís2, F. Vitel and E. Portillal 1 Departamento de Biología, Universidad Autónoma Metropolitana- Iztapalapa. Ap. Postal 55-535, México 09340, D.F. México, e-mail [email protected] Jardín Botánico y Vivero de Cactaceas "Dra. Helia Bravo-Hollis", Zapotitlán, Puebla, México. (Rec. 15-V-1995. Rev. 23-VI-1995. Acep. 28-VI-1995) Abstract: Taking the view that morphological characteristics represent behavioral strategies of plants to cope with environmental pressures, we defined 18 life-forrns, using multivariate classification techniques in a tropical semiarid ecosystem in Central Mexico. A multiple discriminant analysis confirrned the existence of these groups. A nul! model of random rnembership of species to life-forrns was significantly different from our classification. Vegetation-envíron­ ment relationships were exanúned wíth Detrended Canonical Correspondence Analysis (DCCA). Ordínation axes were ínterpreted mainly by altitude and soíl moisture. Response curves of life-forrns along these gradients were explored fit­ ting generalized linear models (OLIM's). We believe that the life-forrns approach for the study of vegetation-envíron­ ment relationships ís a valid alternative to the tradítional specíes approach usually used in phytosociological research because: í) lífe-forrns number was found to be an excellent specíes diversity predictor, ii) this approach enables consid­ erable reduction in the bulk of data without losing ecological inforrnation, and iíi) life-forrns represent ecological strategies per se and, they constitute an index of the number of different ways the desert's resources are utilízed. Key words: Morphological attributes; morphologica1 strategies; ordination; multiple discriminant analysis; response curves; xerophyllous scrub; desert plants. The occurrence of structural similarities ber and nature of the morphological traits used among desert plants, independent of their phy­ and in the number of life-form groups pro­ logenetic relationships and geographical distri­ duced. bution, is well known . There is a preponder­ In this paper we describe a multivariate clas­ ance of certain growth forms in desert floras sification of 107 perennial plant species based that are rare or restricted in other types of on morphological characteristics in a semiarid ecosystems. Nevertheless, the causes of these region of tropical Mexico. The basic assump­ presumed convergences are not well únder­ tion is that plant species would be naturally stood (Solbrig et al. 1977, Bowers and Lowe clumped in a few morphological categories 1986, Cody 1989). A fundamental problem is each of which represents a convergence of to understand how the environment acts as a strategies to cope with the desert environment. selection pressure on the shape and function of Our aim was to describe vegetation-environ­ plants. How do desert plants coexist interacting ment relationships based on these life-form cat­ with each other and with the physical environ­ egories. ment? Vegetation-environment relationships are There have been several attempts to define usually studied along gradients. The concept of morphological strategies of desert plants in environmental gradients has been a comerstone relation to environmental pressures (Shreve in the development of ecological theory 1942, Cody 1989, Leishman and Westoby (Oldand 1992). Austin et al. (1984) recognize 1992). These attempts vary widely in the num- three basic environmental gradients types: (i) 582 REVISTA DE BIOLOGIA TROPICAL indirect gradients, those which do not have a The study sÍte utilized for the vegetation­ direct influence on plant growth (e.g., altitude); envíronment relationships is a protected area (ii) direct gradients, those which have a direct adjacent to the 'Helia Bravo Hollis' Botanic influence on plant growth, but are not resources Garden, located in the middle of the Zapotitlan potentíally subject to competitíon (e.g., pH); Valley, 28.5 Km SW of Tehuacan city. The and (iii) resource gradients, those environmen­ study site occupies an area of 1 Km2 and con­ tal variables which are direct resources influ­ sists of the tetechera vegetation unit. encing plant growth (e.g., soil nutrients). Using Five 10 x 10 m permanent plots have been certain assumptions, direct environmental gra­ established at each site where vegetatíon dients can be regarded as dimensions of the dynamics studies are carried out. In these 15 Hutchinsonian niche concept. Following this, plots we recorded 107 perenníal plants. We species response curves along direct gradients selected ten individuals of each species in wouId represent their realized niches in one every 10 x 10 m plot that it was present, and dimension (Austin et al. 1990) and, hence, recorded the presence or absence of 30 mor­ would provide measurements of niche ampli­ phological attributes (Table 1). These attributes tude and overIap. have clear adaptive significance in terms of Taxonomical nomenc1ature in this paper fol­ photosynthesis optimization (presence of lows Davila et al. (1993). leaves, photosynthetic stems, etc.), water stor­ age and conservatíon (waxy leaves and stems, succulence, etc.), and thermic regulation (mor­ MATERIAL AND METHODS phology of stems, hairy leaves, etc.). Our study site was located in the semiarid TABLE 1 valley of Zapotitlán (18° 20' N, 9T 28' W), a local basin in the Pueblan- Oaxacan region in Morphological attributes usedjór classification and multiple discriminant analysís the Mexican State of Puebla (Vite et al. 1990). This is a unique regíon because of its biological richness. About 30% of its species are endemic 1 Spines present 16 Branched and it is especially rich in columnar cacti 2 Succulent stem 17 Extensive branching (Villaseñor et al. 1990). 3 Exfoliant cortex 18 Plagiotrophic branching The c1imate is dry with summer rains. 4 Perennial stem 19 Wide base Annual mean temperature is 18-22°C and pre­ 5 Spiny stem (*) 20 Leaves present (*) cipitation is around 400 mm/yr. The soils are 6 Photosynthetic stem 21 Rosetophyllous leaves (*) shallow, stony, and halomorphic (Byers 1967). 7 Waxy stem 22 Simple leaves Arid conditions are produced by the rain shadow 8 Woody stem 23 Microphyllous leaves of the Sierra Madre Oriental. The vegetation has 9 Ereet stem 24 Hairy leaves been c1assified as xerophyIlous scrub 10 Flattened stem 25 Succulent Ieaves (Rzedowski 1978) or as thorn scrub cactus I1 Solitary stem 26 Glabrous leaves (Smith 1965), and is a well-preserved example 12 Globose stem 27 Waxy leaves of this vegetation type, that supposedly covered 13 Candelabrous shape (*) 28 Perennialleaves the region sorne 10,000 years ago (Smith 1967, 14 Ribs present 29 Caudex present Zavala-Hurtado 1982). Zavala-Hurtado (1982) 15 Tubercles present 30 Epiphytic habit described four vegetation units in the Zapotitlan Valley: thorn scrub (dominated by thorny shrubs (*) Attributes not entered by multiple discriminant analysis. and trees, mainly legumes, agavaceae and low cacti), cardonal (thorn scrub with the columnar On the species-attributes matrix (107 x 30) cactus Cephalocereus columna-trajani Weber), we carried out an agglomerative cluster analy­ izotal (thorn scrub with Yucca periculosa Baker sis usíng Ward's method (1963) with the or Beaucarnea gracilis Lem., and tetechera SPSS+Pc package (Norusis 1988). The number (thorn scrub with the columnar cactus of groups in the species c1assification was cho­ Neobuxbaumia spp.). There are permanent sites sen subjectively, based on a visual inspectíon for research on vegetatíon dynamics in the of the dendrogram obtained (Fig. 1). Each of tetechera, izotal and cardonal units. these groups was regarded as a life-form (LF). ZAVALA-HURTADO et al.: Vegetation-environment relationships 583 Rescal.ed Di.stance. Cl.uster CoJablne LIFE-FORK SPECIES o 5 10 1.5 20 [ I I I I 79 101 24 LO' 1. 77 76 90 [ .49 80 81 83 LF 2 106 84 4 82 39 44 35 LO'3 2S 78 f 100 85 65 67 LO'4 60 62 66 43 45 LF5 63 64 61 LF6 f 1� [ 12 l 38 1i LF 7 10 8 48 102 LF 8 [ 103 98 52 LF 9 71 70 S7 96 55 56 LF10 32 3. 28 30 3>- 29 .. 58 40 LF11 69 5 92 27 36 10. LF 12 105 50 [ 94 95 23 88 5' 3 75 LF 13 93 22 99 68 42 2 74 73 91 1 26 19 37 20 34 54 21 72 86 LE' 16 87 41 16 17 LF17 14 18 15 59 97 53 LF18 47 89 �]- 46 Fig. 1. Dendrogram with the agglomerative schedule oC the c1assificationof 107 species from the semíarid valley of Zapotitlan, Mexico into 18life-forms (indicated by LF#) using Ward'smethod. See App. 1 for speciesnames. 584 REVISTA DE BIOLOGIA TROPICAL Results of the classification were evaluated Soil moisture was estimated as a percentage by a stepwise multiple discriminant analysis of the soil dry weight (A very and Bascomb using SPSS+pc. This method was also applied 1974). pH was measured in a water- saturation to a null model of random assignment of percentage preparation (Jackson 1958) using a species to the same number of groups detected pH-meter Chandos type M43. with the cluster analysis. The objective of this Altitude, aspect and slope were measured comparison was to test the nulI hypothesis of using conventional methods. Topography (con­ random membership to life-forms. cave, plain and convex) and stoniness were The null model was constructed permuting estimated visualIy. at random the life- form constituency of each With these data, we built-up two matri­ species (maintaining its own morphological ces: life-forms x samples and environmental attributes). variables x samples. Both matrices were ana­ Analysis of vegetation-environment rela­ lyzed simultaneously with a Detrended tionships was carried out in the protected area Canonical Correspondence Analysis (DCCA) of the 'Helia Bravo Hollis' Botanic Garden using the CANOCO package (ter Braak 1987).