Tree biomechanics: a study of the mechanical stability of broadleaf trees

Wind is a key disturbance agent in forests worldwide, but the interaction between wind and trees is poorly understood in natural forests. Wind damage leads to the formation of canopy gaps, which drive structural and compositional changes in the forest. Trees also adapt to their local wind regime in the absence of any damage, a process known as thigmomorphogenesis. This process alters local height-diameter allometries and so influences forest carbon stocks. Previous work on the wind-tree interaction has focussed on conifer plantations due to their economic importance and the relative simplicity of these systems. In this thesis I use terrestrial laser scanning to map the 3D architecture of broadleaf trees and so simulates their response to wind loading using finite element analysis. I also use field techniques developed in the forestry industry to directly measure the response of broadleaf trees to the wind. I found that the simulations performed well against field data for trees in Wytham Woods, UK, but that they failed to predict the field data from Danum Valley, Malaysia. This is likely due to the more complex topography and canopy structure in the tropical site. In Wytham Woods, I found that the critical wind speed, a measure of wind damage risk, was related to tree species, growth rate and architecture. I also found an interesting link between critical wind speed and the timing of autumn leaf phenology, although this requires further investigation. In Danum Valley I found that understory trees approach their gravitational limits to height in their struggle to reach the canopy but that gravitational risk factors decrease with tree height. In contrast, I found that wind risk factors increased with tree height meaning that tall trees are likely to be mechanically limited by wind. I also found that the natural frequencies of broadleaf trees do not follow the same simple pattern as those of conifers, a difference which is accounted for by tree architecture. Overall, this thesis emphasises the role of wind damage risk in forest ecology and the importance of tree architecture in determining the response to wind loading for broadleaf trees.