Mass Mortality as a Way of Structuring Amazonian and Alpine Tree Populations: Evidence After Storm Vaia

Abstract

By logging in the past, humans can determine current tree population structures, but fast stump decomposition makes difficult to falsify that for Amazonian Rainforests. We reconstructed land-use histories and surveyed trees ≥ 10 cm diameter at breast height on three 1-ha plots (K1, K2, and K6) in Kühbergl, South Tyrolean Alps as we did for four plots in Atacapi, Ecuador (plots A, B, C, and D). Storm Vaia (October 27 –November 1, 2018) stroke Kübergl providing dated evidence of mass tree-mortality on plot K6. We used K6 as control for comparing its pre- and post-storm population structures with the ones of four Amazonian, and three Alpine species where Vaia did not kill trees (Kolmogorov-Smirnov tests). When compared with K6’s Picea abies, the following species had size distributions similar to post-storm, but not to pre-storm situation. Amazonian: Piptocoma discolor, Vochysia bracelineae (plots B and D), Miconia decurrens (plots B and C), and Pseudobombax sp (plot C). Alpine: Larix decidua (plot K1) and Picea abies (plot K2). Storms do not occur in Atacapi, where logging is a common practice. That makes plausible that discrete events of compulsive logging during secondary succession made Amazonian population structures to look similar to K6’s P. abies. Logging is forbidden in Kühbergl, but storms are common there. Thus, the current population structures of Larix decidua (plot K1) and Picea abies (plot K2) should be legacies of storms before Vaia. Looking into tree populations’ history can impulse research for answering some basic questions of Ecology: what alters population structures, and which population structuring processes are more influential than others.