Monday, August 6, 2007

PS 10-126: Buttress roots act as tension members to prevent Australian tropical rainforest trees (Argyrodendron spp) from uprooting in a cyclone

Timothy L. Casper, Davidson College, Eric Nomura, Colorado College, and Timothy J. Curran, School for Field Studies.

The role of buttress roots as structural supports for rainforest trees has long been debated. Some studies indicated that buttresses serve as effective tension members to prevent uprooting. Conversely, buttresses may function as compression members, although there is less support for this idea. We examined these hypotheses by measuring the orientation, height, and length of 242 buttresses from 53 Argyrodendron spp. trees in rainforest fragments located in north-east Queensland, Australia that were recently affected by a severe cyclone. Trees were categorized as upright or uprooted and four buttress strength indices were compared between these groups, including highest and longest buttress, and resultant buttress height and length, which is the vector sum of either the height or length of all buttresses of a tree. Hotelling's one-sample T2 tests were used to determine if directionality of vector groups for each buttress strength index differed significantly from random. Upright trees in our study site did not tend to have their longer and taller buttresses oriented towards the direction of annual prevailing winds, a finding contrary to some other studies. Uprooted trees lacked buttress length on the side opposite direction of fall, supporting the tension member hypothesis, which predicts that buttresses prevent trees from uprooting when subjected to asymmetric loads due to external forces, such as wind. Of the indices of buttress strength, the longest buttress of each tree was the best indicator for direction and magnitude of overall buttress support.