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Abstract:
Recent research in the central Amazon suggests that wind is a major agent of disturbance, however, a mechanistic
understanding of how wind may lead to tree mortality in Amazonian forests remains unclear. Here we
estimated wind speeds necessary to topple central Amazon trees by linking both static and dynamic versions
of two wind speed estimation methods (four methods total) to field data on tree failure derived from a static
winching study. Static versions of these methods assumed invariant wind characteristics as more trees failed,
while dynamic versions updated tree spacing, leaf area index and wind profiles progressively after each tree
failure. First, we used a profile method which estimates wind force on individual trees by segments.We calculated
drag on each segment and converted drag into basal turning moment, and compared the summed
turning moments to the critical turning moment measured in the winching study. Estimated critical wind
speeds from the static profile method varied greatly, from 10.75ms−1 to >120.0ms−1 with a mean of
45.70ms−1. Critical wind speeds estimated with static approaches decreased with tree size but were not
significantly different between two focal genera. Primary drivers of variation in critical wind speed were tree
height and crown size. Second, we used the turning moment coefficient method of Hale, S.E., Gardiner, B.,
Peace, A., Nicoll, B., Taylor, P. and Pizzirani, S. 2015 Comparison and validation of three versions of a forest
wind risk model. Environ. Model. Softw. 68, 27–41. doi:10.1016/j.envsoft.2015.01.016.; the static version of
this method yielded less-variable estimates, ranging from 18.98 to 52.01ms−1, with a mean of 30.88ms−1.
Notably, the two static methods for estimating critical wind speeds differed in the trees they identified as
having the highest and lowest critical wind speeds. Dynamic variants of the above two methods produced
greatly reduced ranges in CWS estimates for our study trees, because after the early tree failures, remaining
trees were subject to greater wind penetration into the stand and thus greater loading for a given abovecanopy
wind speed. CWS estimated with dynamic approaches differed significantly between the focal taxa.
Nevertheless, both estimates suggest that wind speeds commonly observed during Amazon storms are sufficient
to produce widespread tree damage and mortality.
