Trade-offs in Carbohydrate Allocation and the Distributions of Bornean Tree Species along a Soil Gradient

Background/Question/Methods

The evolutionary responses of plants to trade-offs in allocation of carbohydrates to competing functions have produced a range of strategies that influence beta diversity and species’ distributions along environmental gradients. We conducted a reciprocal transplant experiment across two soil and light environments with seedlings of 13 tree species representing specialists of clay and sandy loam soils, with a defoliation treatment that induced carbon-deficit stress, to investigate how between- and within-species variation in allocation and functional traits relate to species’ distributions along an edaphic gradient in Bornean rain forest.  

Results/Conclusions

We found evidence of a trade-off between allocation of carbohydrates to growth versus storage. First, growth rate and storage were negatively related. Second, both fast growth and storage carried survival costs and benefits, depending on stress. Defoliated seedlings that grew faster before defoliation had reduced survival compared to slower-growing seedlings, but survival of non-defoliated seedlings increased with faster growth.  For non-defoliated seedlings, species with greater storage had reduced survival, but for defoliated seedlings, survival increased with storage. If differences in allocation are a mechanism sorting species between soil types, then allocation strategies of sandy loam and clay specialists should match predictions from the growth-storage trade-off: clay specialists should have faster growth, reduced storage, lower survival rates, and reduced stress tolerance. These predictions did not hold, except for growth. By growing bigger faster, clay specialists take advantage of compounding interest to increase their access to resources and total stored carbohydrate pool. Specialists did not differ in leaf biomass allocation, but clay specialists’ faster growth was associated with greater specific leaf area and total leaf area. Their greater plasticity would also help avoid risks of insufficient storage to survive stress. We conclude that between-species differences in plasticity of allocation and leaf traits are fundamental to how resource-allocation trade-offs affect species’ distributions in this forest.