Strategy shifts in leaf physiology, structure and nutrient content between species of high- and low-rainfall and high- and low-nutrient habitats

1. Relationships were examined among photosynthetic capacity (A(mass) and A(area)), foliar dark respiration rate (Rd-mass and Rd-area ), stomatal conductance to water (G(s)), specific leaf area (SLA), and leaf nitrogen (N) and phosphorus (P) across 79 perennial species occurring at four sites with contrasting rainfall levels and soil nutrients in eastern Australia. We hypothesized that the slope of log-log 'scaling' relationships between these traits would be positive and would not differ between sites, although slope elevations might shift between habitat types. 2. A(mass), Rd-mass SLA N-mass and P-mas, were positively associated in common slopes fitted across sites or rainfall zones, although rather weakly within individual sites in some cases. The relationships between A(mass) (and Rd-mass) with each of N-mass and SLA were partially independent of each other, with A(mass) (or Rd-mass) increasing with SLA at a given N-mass or with N as, at a given SLA (only weakly in the case of A(mass)). These results improve the quantification and extend the generalization of reported patterns to floras largely unlike those studied previously, with the additional contribution of including phosphorus data. 3. Species from drier sites differed in several important respects. They had (i) higher leaf N and P (per dry mass or area); (ii) lower photosynthetic capacity at a given leaf N or P; (iii) higher Rd-mass at a given SLA or A(mass); and (iv) lower G(s) at a given A(area) (implying lower internal CO2 concentration). 4. These trends can be interpreted as part of a previously undocumented water conservation strategy in species from dry habitats. By investing heavily in photosynthetic enzymes, a larger drawdown of internal CO2 concentration is achieved, and a given photosynthetic rate is possible at a lower stomatal conductance. Transpirational water use is similar, however, due to the lower-humidity air in dry sites. The benefit of the strategy is that dry-site species reduce water loss at a given A(area), down to levels similar to wet-site species, despite occurring in lower-humidity environments. The cost of high leaf N is reflected in higher dark respiration rates and, presumably, additional costs incurred by N acquisition and increased herbivory risk.