DOI: 10.5061/dryad.wm37pvmhx
title: Tree functional traits as predictors of microburst-associated treefalls in tropical wet forests

description: On 19 May 2018 a microburst caused 600 isolated forest gaps in a Costa
 Rican tropical forest. We surveyed fallen and standing trees within gaps
 to determine if certain variables are associated with treefalls. Our
 results highlight considerations for future research to understand the
 impacts of microbursts in tropical forests. Our results show that
 at the scale and locality of our study, treefall vulnerability to
 microbursts and characteristics of fall events are independent of the
 biomechanical and biophysical measurements that we took. We recommend that
 future studies on microbursts build from the measurements we took in this
 study to include observations at the individual tree, forest community,
 and forest landscape scales.
description: Treefall
 observations were sampeled within 20 m of trails at La Selva Biological
 Staion for safe access. We sampled damaged trees greater than or equal to
 30 cm diameter at breast height (DBH), and only surveyed fallen trees that
 had abundant green leaves in their canopies, to ensure that trees studied
 were alive at the time of the microburst. For each treefall event, we
 noted whether it involved one or multiple trees. For multiple treefalls,
 we measured the largest tree among the fallen. Each treefall was
 identified as uprooted (main root system visible) or snapped (snapped
 trunk with root system in the ground). Because stem size is often a
 predictor of damage probability in strong winds (Everham & Brokaw
 1996), each fallen tree was matched to the closest standing tree within 20
 m, with an equivalent DBH (± 5 cm). DBH was measured at standard height or
 immediately above the buttresses. Trees were identified to the species
 level and assessed for liana presence and buttress size. Lianas were
 measured as either absent or present in the crowns of both fallen and
 standing trees. Buttresses were categorized ordinally from 0 to 2, where 0
 represented absence of buttresses, 1 indicating buttresses less than or
 equal to the width of the main trunk on each side, and 2 indicating
 buttresses wider than the main trunk on each side. We determined wood
 density of each tree species using the Tree Functional Attributes and
 Ecological Database (Harja <i>et al. </i>n.d.).  
 We
 evaluated how liana presence/absence impacts tree fate using Fisher’s
 Exact Test, which calculates an exact <i>P</i>-value for a 2 x
 2 table. We evaluated if buttress size impacts treefall using a
 Cochran-Mantel-Haenszel Test (CMH test), appropriate for comparing ordinal
 vs. categorical variables. The relationships of wood density and tree
 diameter to tree fate were examined using paired t-tests. Two logistic
 generalized linear models within the subset of fallen trees analyzed 1)
 how the same four predictor variables may influence treefall type, and 2)
 whether treefalls were single or multiple events.  Continuous data was
 standardized and z-scores were calculated for wood density (g/cm3) and
 tree diameter (cm) based on their mean and standard deviation. All
 analyses were performed in Program R version 3.5.1 (R Core Team,
 2018).