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- Supporting Information
Leaf mass per area (LMA), nitrogen (N) concentration (on mass and area bases, Nmass and Narea, respectively), photosynthetic capacity (similarly, Amass and Aarea) and photosynthetic nitrogen use efficiency (PNUE, defined as photosynthetic capacity per unit leaf nitrogen) are fundamental leaf traits, playing key roles in plant functioning (Schulze, 1994; Grime et al., 1997; Mooney et al., 1999; Ackerly, 2004). As a reflection of the dry-mass cost of producing new leaves, LMA correlates positively with leaf lifespan (LL) and negatively with leaf N concentration across species (Reich et al., 1997; Westoby et al., 2002; Wright et al., 2004b). Leaf N concentration itself is strongly correlated with photosynthetic capacity (Field & Mooney, 1986; Evans, 1989; Reich et al., 1994), as N is essential for the synthesis of Rubisco, the key enzyme of photosynthesis (Field & Mooney, 1986; Taiz & Zeiger, 1998). This correlation provides a useful link between processes on short-term, leaf-level scales and long-term, plant- and stand-level scales, and has been used to estimate maximum CO2 uptake over a broad range of species (Schulze, 1994; Baldocchi & Harley, 1995; Harley & Baldocchi, 1995; Aber et al., 1996; Williams et al., 1997; Larocque, 2002). Understanding the relationships between these fundamental traits and their large-scale patterns is essential for scaling up ecophysiological processes from the leaf level to the ecosystem level and in predicting ecosystem functioning in response to environmental change (Ehleringer & Field, 1993; Peterson et al., 1999; Norby & Luo, 2004).
Understanding large-scale patterns of leaf functional traits is a challenging issue of great interest to both plant physiologists and ecologists (Körner, 1989; Yin, 1993; Niinemets, 2001; Reich et al., 2003; Chown et al., 2004; Reich & Oleksyn, 2004; Wright et al., 2005a,b). For example, in an examination of a global dataset, Reich et al. (1997) found that leaf traits such as photosynthetic rate and longevity scale predictably with one another, largely irrespective of environment or phylogeny. Wright et al. (2005b) similarly found that the effect of climate on the relationships among Amass, Nmass, LMA, leaf phosphorus (P), dark respiration rate (R) and LL was modest, although some patterns appeared. A recent study by Reich & Oleksyn (2004) further pursued the link between climate and leaf traits, finding that leaf N and P decreased with mean annual temperature (MAT) from the 5–10°C range to the warmest MAT. At very low MATs, however, the relatively scarce data available hindered arrival at any definitive conclusions.
The Tibetan Plateau is an ideal place for large-scale ecological studies, because it provides a unique opportunity to examine trends in a high-altitude, cold climate with very low MAT. The plateau represents one of the largest alpine grasslands in the world, yet its vegetation has been underrepresented in global-scale studies (e.g. Reich & Oleksyn, 2004; Wright et al., 2004b). Arctic and alpine plants have adapted to low temperatures, and thus are expected to have developed unique survival mechanisms (Chapin & Körner, 1995), enhancing the value of regional and global studies that include such plants. As the largest geomorphological unit on the Eurasian continent (Sun & Zheng, 1998), the Tibetan Plateau has a mean elevation of > 4000 m, with altitudes ranging from approx. 3000 to 8844 m. The plateau covers 12° of latitude and 28° of longitude, for a total area of approx. 2.5 × 106 km2, nearly one-quarter of the area of China. As a consequence of uplift in the past several million years (Zheng, 1996; Tapponnier et al., 2001), the Tibetan Plateau has had tremendous impact on the evolution and the development of species and ecosystems (Sun & Zheng, 1998), making it a center of differentiation for new species and a refuge for ancient species (Zhang et al., 1988; Hou & Chang, 1992). In addition, the Plateau is one of the main regions of low-latitude frozen soils in the world (Zhang et al., 1988; Molnar, 1989). Its alpine vegetation remains relatively undisturbed by humans, and thus the Plateau is an ideal region in which to study the responses of natural ecosystems to global climate change.
This study was designed to explore patterns of leaf functional traits in a high-elevation, low-temperature environment. Specifically, our study objectives were (i) to document the leaf functional traits of the flora in an understudied region over broad regional, elevational, and taxonomic ranges, and (ii) to examine how relationships among these traits, measured near the extremes of plant tolerance, compare with global patterns.