Journal of Biogeography (1997) 24, 197±204 Distribution of C3 and C4 grasses along an altitudinal gradient in Central Argentina Marcelo Cabido*,Norma Ateca², Marta E. Astegiano² and Ana M. Anton**IMBIV, UNC-CONICET, CC 495, 5000 CoÂrdoba, Argentina and ²Facultad de Ciencias Agropecuarias, UNC. CC 509, 5000 CoÂrdoba, Argentina Abstract. The distribution pattern of C3 and C4 grasses nine climatic and environmental variables showed the was studied in eight sites located between 350 m and 2100 m highest correlation with July mean temperature, but all along an altitudinal gradient in Central Argentina. Of 139 temperature variables show highly signi®cant correlations taxa ®fty-nine are C3 and eighty C4. Species of the C3 tribes with % C4. Correlation with annual rainfall is lower but (Stipeae, Poeae, Meliceae, Aveneae, Bromeae and Triticeae) also signi®cant. These results are consistent with previous and C3 Paniceae species increase in number at higher research showing the relative importance of C4 grasses as elevations; only one C3 species was found below 650 m. temperature increases. C3 species make a high contribution C4 Aristideae, Pappophoreae, Eragrostideae, Cynodonteae, to relative grass coverage below the C3/C4 ¯oristic crossover Andropogoneae and Paniceae increase at lower altitudes. point but are rare below 1000 m. The ¯oristic crossover point is at about 1500 m; the ground cover cross-over point is at about 1000 m. Analysis of the Key words. C3 and C4 grasses, altitudinal gradient, climate, relationships between % C4 species along the gradient and Argentina. METHODS INTRODUCTION The distribution of C and C grasses were obtained for Physiological diVerences of C and C grass species are 3 4 3 4 eight sites along an altitudinal gradient ranging from 350 m re¯ected in their distribution patterns along environmental to around 2100 m a.s.l. in the CoÂrdoba Mountains (31°60¢S gradients or under varying climatic conditions. C grasses 4 and 65°50¢W). Floristic surveys and Braun-Blanquet cover are generally represented more on warmer environments, abundance data (Braun-Blanquet 1932), provided by while C grasses are represented more on cooler 3 Cabido (1985), Cabido & Acosta (1986), Cabido, Breimer environments. These distribution patterns are remarkably & Vega (1987), Acosta et al. (1989) and Cabido et al. (1994), consistent at a large range of spatial scales (Wentworth, were used to assess the distribution of species. All the sites 1983), and in all parts of the world where they have been were open grasslands where grasses are the dominant species. investigated (Hattersley, 1992; Ehleringer & Monson, 1993). Only the two lower sites had scattered trees. The climate is Usually temperature is the variable mostly correlated with temperate with warm season rainfall (70% of the rain falls the occurrence of C species in grass ¯oras (Teeri & Stowe, 4 from November to March). A detailed description of the 1976; Cavagnaro, 1988), even though some authors have area is given by the authors cited above. stressed the importance of water availability and soil Total number of C and C grass species, % C species moisture indices (Chazdon, 1978; Tieszen et al., 1979). 3 4 4 and%C and C grass cover were calculated for each According to Hattersley (1983) generally both C and C 3 4 3 4 elevational site. Cover values were obtained from Braun- grass species increase in number with increasing rainfall in Blanquet data. The C photosynthetic pathway was their preferred temperature regime. 4 identi®ed by examination of Kranz anatomy in cross- The study reported here is one of the few examining the sections of fresh and herbarium specimens and from distribution of C and C grass species along an elevational 3 4 literature data (Smith & Epstein, 1971; SaÂnchez & Arriaga, gradient for grasslands in a temperate summer-rainfall 1990; Hattersley & Watson, 1992). region of the world (in Central Argentina). Our objective Pearson's correlation coeYcients were calculated for the was to identify those climatic variables most correlated with relationships between distribution parameters (% C4 and C4 grass species composition. numbers of C3 and C4 species) and climatic variables obtained from the National Meteorological Service (Servicio MeteroroloÂgico Nacional 1958, 1962a, 1962b), from records of the Argentine Railways (unpublished data) and from * Corresponding author: Marcelo Cabido, IMBIV, Casilla de Correo 495, stations located in private properties (Palacios & Zamar, 5000 CoÂrdoba, Argentina. 1986) (Table 1). 1997 Blackwell Science Ltd 197 198 Marcelo Cabido et al. TABLE 1. Climatic parameters for the eight sites along an elevational gradient in Central Argentina. Altitude (m) 350 600 1000 1400 1600 1800 1900 2100 AAT 18.1 17.5 13.1 11.6 10.7 9.7 8.8 8.1 ANF 9.6 28.7 41.9 57.3 65.1 74.0 85.8 89.6 JANT 20.0 16.5 14.7 12.3 11.7 10.2 9.1 8.3 JAMT 34.5 30.6 27.5 23.6 22.1 20.0 18.0 15.8 JuANT 4.1 5.1 2.7 1.3 0.9 0.7 0.1 -2.0 JuAMT 18.8 17.3 15.1 13.0 11.8 10.3 8.3 8.8 JMT 27.2 24.0 22.1 17.9 16.6 15.4 12.1 13.3 JuMT 10.8 10.4 8.3 6.7 6.0 5.2 4.2 4.1 AR 680 750 786 887 858 873 840 891 SR 365 432 473 530 597 586 549 644 ALT=altitude; AAT=annual average temperature; AR=annual rainfall; SR=summer rainfall; ANF=average number of days with frost; JANT=January average minimum temperature; JAMT=January average maximum temperature; JuANT=July average minimum temperature; JuAMT=July average maximum temperature; JMT=January mean temperature; JuMT=July mean temperature. FIG. 1. Frequency of occurrence of C3 and C4 grass species by tribe along an altitudinal gradient in Central Argentina. Light bars=C3 species; dark bars=C4 species. Blackwell Science Ltd 1997, Journal of Biogeography, 24, 197±204 Distribution of C3 and C4 grasses in Argentina 199 more prevalent at lower altitudes. Andropogoneae species are more common at intermediate sites (between 1000 m and 1400 m). C3 and C4 grasses show a divergent distribution along the gradient (Fig. 2). The number of C3 species decreases from thirty-seven at 2100 m to four at 600 m, and only one (Stipa sanluisensis Speg.) at 350 m. C4 grasses are more numerous in the lower zone and less so above 1400 m (Table 2). Nevertheless, nine C4 grasses are still found at 2100 m where the January average maximum temperature is only 15.8°C. The ¯oristic crossover point (equal numbers of C3 and C4 species) is at approximately 1500 m but the ground cover cross-over point is at 1000 m (Fig. 2). Although C3 species decrease considerably below 1600 m, their contribution to total grass coverage is well over 50% from above 1000 m (Table 2). Remarkably high positive correlation was found between both summer and winter temperature variables with % C4 species (Table 3). Correlation of % C4 with rainfall is nega- tive and also is signi®cant. Correlations between the number of C4 species and climatic variables are not signi®cantly diVerent (results are not presented). DISCUSSION There are clear diVerences in the distribution of C3 and C4 grasses along the altitudinal gradient studied. Lower altitude grasslands consist mainly of C4 grasses, whereas C3 grasses dominate at higher altitudes. Our ®ndings are comparable with those reported for diVerent regions of the world (Hattersley, 1992; Hattersley & Watson, 1992; Ehleringer & Monson, 1993). All temperature variables tested showed strong correlations with the relative abundance of C4 and C3 species, suggesting that the cooler the winter, the greater the relative success of C3 grasses, and the hotter the summer the greater the relative success of C4 grasses. An equally close relationship was found between % C4 and January average maximum temperature for Australia (Hattersley, 1983). While temperature and related variables are highly correlated with % C4 at continental scales (Hattersley, 1992), good correlations with rainfall are also reported along elevational gradients at more local scales, where precipitation increases with altitude (Chazdon, 1978; Rundel, 1980). In our study area rainfall also shows good correlation with % C4 and when only summer rainfall is correlated the coeYcient is even higher. Our ®nding contrasts FIG. 2. Relative C3/C4 grass species composition (%) and coverage with that of Cavagnaro (1988) who reported non-signi®cant (%) along an altitudinal gradient in Central Argentina. correlation with rainfall for a close but climatically diVerent region in Argentina. The cross-over point for the number of species occurs at RESULTS 1500 m, and is characterized by a mean annual daily Of the 139 grass taxa found along the elevational gradient, minimum temperature of approximately 7°Cto8°C. The ®fty-nine were C3 and eighty C4. Only the tribe Paniceae cross-over point occurs at diVerent elevations in other contains both C3 and C4 species. Most of the tribes show regions of the world but at similar mean minimum trends in their representation with increasing or decreasing temperature ranges (Tieszen et al., 1979; Rundel, 1980). altitude (Fig. 1). The C3 tribes Poeae, Aveneae, Bromeae and Several authors have found that mean minimum and mean Arundineae and C3 Paniceae species are more represented maximum temperatures of the warmest month are the single at higher altitudes. In contrast, the C4 tribes Aristideae, best predictors of C3 and C4 species numbers (Wentworth, Pappophoreae, Cynodonteae and C4 Paniceae species, are 1983). When our results are compared with those reported Blackwell Science Ltd 1997, Journal of Biogeography, 24, 197±204 200 Marcelo Cabido et al. TABLE 2. Number of species, % species and % cover of C3 and C4 grasses for eight locations along an altitudinal gradient in Central Argentina.