Allocation to leaf area and sapwood area affects water relations of co-occurring savanna and forest trees
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Water availability is a principal factor limiting the distribution of closed-canopy forest in the seasonal tropics, suggesting that forest tree species may not be well adapted to cope with seasonal drought. We studied 11 congeneric species pairs, each containing one forest and one savanna species, to test the hypothesis that forest trees have a lower capacity to maintain seasonal homeostasis in water relations relative to savanna species. To quantify this, we measured sap flow, leaf water potential (ΨL), stomatal conductance (g s), wood density, and Huber value (sapwood area:leaf area) of the 22 study species. We found significant differences in the water relations of these two species types. Leaf area specific hydraulic conductance of the soil/root/leaf pathway (G t) was greater for savanna species than forest species. The lower G t of forest trees resulted in significantly lower ΨL and g s in the late dry season relative to savanna trees. The differences in G t can be explained by differences in biomass allocation of savanna and forest trees. Savanna species had higher Huber values relative to forest species, conferring greater transport capacity on a leaf area basis. Forest trees have a lower capacity to maintain homeostasis in ΨL due to greater allocation to leaf area relative to savanna species. Despite significant differences in water relations, relationships between traits such as wood density and minimum ΨL were indistinguishable for the two species groups, indicating that forest and savanna share a common axis of water-use strategies involving multiple traits.
KeywordsLeaf area index Huber value Sap flow Brazil Cerrado
We thank IBGE for logistical support; Bruna Diniz, Marina Carvalho, Mirea A. B. Pereira, Inaldo Araujo, Kristen Mckinley and Palmyra Romeo for assistance in the field; and Renee Marchin, On Lee Lau, Alice Wines and Wade Wall and three anonymous reviewers for comments on this manuscript. This material is based on work supported by the National Science Foundation under grant no. DEB-0542912, the A. W. Mellon Foundation, and CNPq, Brazil.
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