Life-history theory generally assumes that reproduction entails a cost in terms of current or future growth; however studies of size-dependent changes in tree functional biology have overwhelmingly emphasized biophysical effects such as limits to water transport as the main mechanisms determining size-dependent patterns of physiology, morphology, and growth. Here I review evidence and present new data indicating strong direct effects of reproductive allocation on tree growth and physiological parameters.
Results/Conclusions
Negative effects of reproduction on tree growth have been found in tree-ring studies and records of crown extension in masting species, with low growth occurring during mast years. Leaf size and shoot extension are also commonly reduced in shoots subtending reproductive structures within tree crowns, with data also suggesting strong effects of reproduction on leaf chemistry. Comparative studies of ontogenetic patterns indicate that reproductive onset is associated with changes in leaf physiology and chemistry during the course of tree development: for example, leaf size, N content, and photosynthetic capacity increase with size prior to reproductive onset, and thereafter decrease. While all of these data suggest strong effects of reproductive allocation on tree growth and physiology, direct experimental studies are lacking, and a number of alternative explanations are plausible. One such alternative to a simple allocation hypothesis is that reproductive allocation exacerbates negative effects of biotic agents. A possible case of this phenomenon is described, in which a species-specific mite causes larger reductions in photosynthetic performance in reproductive than in non-reproductive shoots of canopy trees.
