Although there is great diversity among plant species in growth form, leaf size, leaf shape and canopy arrangement, there are also some general relationships, occurring across a wide range of species, in leaf traits central to the carbon fixation ‘strategy’ of plants. For example, in between-species comparisons of outer canopy leaves, specific leaf area (SLA, leaf area per mass) tends to be correlated with leaf nitrogen per unit dry mass (Nmass), photosynthetic (Amass) and foliar dark respiration rates (Rd-mass), but negatively correlated with leaf lifespan (various combinations of these relationships have been reported by Bolstad, Mitchell & Vose 1999; Diemer 1998; Eamus et al. 1999; Field & Mooney 1986; Medina 1984; Mulkey, Kitajima & Wright 1995; Niinemets 1999; Reich et al. 1998; Reich et al. 1999). Consequently, much variation between plant species can be understood as a single spectrum of correlated traits.
Reich et al. (1997, 1999) found that the proportionality of these trait relationships (slopes on log–log axes) did not differ between sites from tropics to tundra, while slope elevations (intercepts) did vary. However, as many environmental factors are confounded in comparisons between widely divergent habitats, we do not know how predictably the relationships vary between different habitat types. Here we address that question by comparing leaf traits of species from sites of contrasting soil nutrient status and rainfall in eastern Australia. Australia is a continent with overwhelmingly ancient, weathered, nutrient-poor soils, on which vegetation types are largely determined by rainfall and soil phosphorus (Beadle 1962; Webb 1968). Thus we also set out to extend the generality of knowledge on leaf trait relationships into a flora largely unlike those studied previously.
We measured leaf traits on a total of 79 perennial species from four sites (nutrient-rich and nutrient-poor sites in each of two rainfall zones), focusing on the proportionality or ‘scaling relationships’ among SLA, leaf N and P concentration, photosynthetic capacity and dark respiration rate, and stomatal conductance to water (Gs). Previously, leaf P has rarely been considered in conjunction with these other traits (Reich, Ellsworth & Uhl 1995). Leaf lifespan is being determined as part of a complementary, longer study. Specific expectations were as follows.
- 1Average leaf traits would differ with site nutrient status, species from nutrient-rich soils having higher mean SLA, Nmass (Chapin 1980; Cunningham, Summerhayes & Westoby 1999), Pmass, Amass and Rd-mass than species from nutrient-poor soils.
- 2Average leaf traits would differ with rainfall, species from drier sites having lower mean SLA (Schulze et al. 1998; Specht & Specht 1989) and higher mean Narea (perhaps due to the lower SLA or as a response to the stronger average irradiance in arid habitats; Cunningham et al. 1999; Mooney, Ferrar & Slatyer 1978).
- 3SLA, Nmass, Pmass, Amass and Rd-mass would scale positively with one another at each site and also across all species, with scaling slopes not differing from site to site (Reich et al. 1998; Reich et al. 1999).
- 4However, slope elevations might differ between site types (Reich et al. 1999).
- 5Both Amass and Rd-mass would scale positively with Nmass at a given SLA, and with SLA at a given Nmass, indicating that both leaf structure and nutrient content independently affect carbon fixation and use (Reich, Ellsworth & Walters 1998a; Peterson et al. 1999).
- 6Leaf P would have significance beyond leaf N in predicting Amass, as Australian soils tend to be particularly P-deficient compared to soils from other continents (Atwell, Kriedemann & Turnbull 1999).