Print PageThe worldwide leaf economics spectrum has identified a suite of correlated leaf traits that are thought to represent fundamental trade-offs shaping responses to natural selection, principally defined by a spectrum of fast versus slow rates of return on investment in leaf tissue. Using data on leaf functional traits of 31 tree species representing 13 genera, inter- and intraspecific variation was quantified with respect to tree species’ distributions along an edaphic gradient in Bornean rain forest. The edaphic gradient spans nutrient-poor, well-drained, sandy soil to clay-rich soil with greater nutrient concentrations and water content. We tested the hypothesis that distributions along this gradient result in part from sorting of species based on their allocation to fast versus slow rates of return on investment in leaf tissue.
Results/Conclusions
Phylogenetically controlled comparisons between soil specialists showed that, across lineages, few traits shifted consistently with species’ soil specialization. Among functional traits measured, leaf lifespan and specific leaf area showed the strongest soil-related differences between congeneric specialists. Leaf lifespan correlated with construction cost and was the only trait to correlate with species’ growth and mortality rates. Gas-exchange traits related to photosynthetic capacity and water-use efficiency were variable, shifted inconsistently with soil specialization, and showed a stronger response to light availability, as compared with soil resource availability. Within-species variation of populations of soil specialists growing on home and non-home soil types demonstrated that leaf traits varied in their degree of plasticity and that specialists of the more nutrient-poor, well-drained soil type tended to be less plastic than specialists of clay soil. We conclude: (1) Compared to the rate of return, the duration of return on investment in leaf tissue may be a stronger target of natural selection along this edaphic gradient, given the observed physiological plasticity of photosynthetic responses. (2) At least some of the observed interspecific trait variation between soil specialists results from plastic responses to the local environment, but this effect appears weaker for specialists of the nutrient-poor well-drained, compared to the more resource-rich soil type. (3) Individual leaf functional traits respond differently to often negatively covarying environmental resources. (4) There are multiple combinations of functional traits that lead to similar demographic outcomes and distributions on this edaphic gradient.
