An explanation of natural forest dieback based on the “pipe model” analogy

A decade of investigation of stand-level dieback ofmetrosideros in the Hawaiian montane rain forests has led to the theory that one of the major underlying causes is cohort senescence. A literature review shows that while there is little evidence of a senescence mechanism in polycarpic trees, the low vigor associated with the senescent state is correlated with a declining balance of photosynthesis to respiration. Specific Leaf Burden (SLB, cm³ of sapwood per g of leaf mass) is here defined as an index of respiring to photosynthesizing tissue. Using the assumptions of the pipe model theory, it is shown that SLB increases with stem elongation and is independent of tree diameter. Thus, the trees of a cohort that are the same height although variable in diameter will have the same carbon balance, as indexed by the SLB, and could be expected to decline synchronously, as stands ofMetrosideros do. Changes in growth patterns, deviating from the pipe model, that minimize the SLB as trees grow larger are identified. Research undertaken to test the assumptions of the pipe model and the conclusion of uniform SLB in a cohort of agingMetrosideros is described.