Feedback loop dominance analysis of two tree mortality models

摘要

Tree mortality is the least understood process of a tree's life-cycle. Two hypotheses on how mortality progresses in a tree are proposed in the literature: Manion's gradual decline hypothesis and Bossel's sudden death hypothesis. Bossel already formulated a mechanism in his model, BAUMTOD, as the cause of sudden death phenomena. His model, however, cannot be used to generate a causal understanding to Manion's hypothesis. Therefore, a causal mechanism for the gradual decline pattern, advocated by Manion, is suggested. BAUMTOD is modified accordingly and the modified model is called BAUMTOD-M. The suggested mechanism concerns the internal imbalance of respiration demand and available photosynthate supply. Then, the eigenvalue elasticity analysis (EEA) is employed to pinpoint how a simulated tree responds to various stresses. It is applied to BAUMTOD and BAUMTOD-M to study the likely structural causes behind tree mortality according to Bossel's and Manion's hypotheses, respectively. The analyses of both models suggest that, in the absence of a significant amount of stress, a tree functions as an integrated organism. Growths in foliage, feeder roots, and respiring permanent biomass are synchronized. However, this coordinated structure is lost if a tree is seriously affected by stress factors. The analyses further suggest that the inability to supply respiration demand plays a crucial role at the onset of mortality in both models. Differences in assumed mechanisms regarding the impact of this inability cause different paths to mortality in each model. This may mean either 1) Bossel's hypothesis is a special case of Manion's hypothesis or 2) there are truly different mechanisms at work in various observed mortality cases. More data and research is needed to clarify these points.