Higher temperatures and tropical tree physiology: The known unknowns

Background/Question/Methods   How will trees in the wet tropics fare in a warmer world?  Answering this question is critical because of the importance of tropical forests for the global carbon and water cycles and for biodiversity.  However, effective predictions are currently hampered by extremely sparse data on the physiology of tropical trees, the large number of tree species, and lack of a process-level understanding of several aspects of whole-tree physiology.  I review some of the current unknowns in whole tree physiology that knowledge of would help understand how tropical forest trees will respond to a warmer world.

Results/Conclusions   Some of the major unknowns include (1) stored carbohydrates: the role of these in sustaining physiology in lean times, the mechanisms that control storage input and release, and what carbohydrate levels in trees indicate about plant carbon balance; (2) autotrophic respiration: its role in regulating plant carbon balance; (3) sink regulation of tree carbon economy: controls over processes that regulate plant sinks for carbon and  sink regulation of tree photosynthesis; (4) water use and transport: maintenance of water transport under water stress and the role of stored water in maintaining water transport; and (5) the mechanism(s) of size-related decline on wood growth.  Inferences about these processes and the effect of temperature on them will not likely come from a focus on wood growth (generally less than 20% photosynthesis) or short-term measurements of process rates.  Rather, our understanding of mechanisms will increase only if we focus on the whole tree and examine carbon, water and nutrient balances for longer time periods.  The size of trees has made whole-tree studies logistically very difficult and limited them, but we could also be more creative.  Plant physiologists have settled on a very small flowering plant (Arabidopsis) as their model organism to link molecular biology and physiology or plant performance.  If we had a similar, tractable tree model organism that we could easily manipulate and measure, whole-tree physiology could progress rapidly.  Of course, to identify such an organism, we would have to understand the essential components of ‘treeness’, and what would be lost working with a small woody plant compared to a large tree.