KREFT, J. M.**; MCCANDLISH, D. M.; REYNOLDS, L. W.; MERZ, R. A.; Swarthmore College; Swarthmore College; Swarthmore College; Swarthmore College: Trees pumping iron: Seasonal changes in material properties of branch wood in Sugar Maple (Acer saccharum) and Bur Oak(Quercus macrocarpa)

Though structural engineers have developed a large dataset on the material properties of wood, these data are based on lumber (small samples of clear trunk wood). Relatively little has been done to examine the properties of living wood in situ. We examined branches of two temperate deciduous species, Sugar Maple (Acer saccharum) and Bur Oak (Quercus macrocarpa) from early spring, at a point with little or no bud development, through leaf drop in the fall. Using a digital camera and image analysis software we quantified changes in branch position due to a weight series applied to the branch and overall branch movement due to loading as the leaves and branches grew over time. We measured Young�s Modulus of Elasticity (E) of the branch material over the time series. Initial results using elementary beam theory, which reduces the branches to simple cantilever beams, indicate Bur Oak branches are less than half as stiff as Sugar Maple branches. There was wide intraspecific variation in E, probably due to differences in branch microclimate, but individual branches showed little variation in E over time. Though all branches drooped in response to increased water and biomass load, the Bur Oak and Sugar Maple branches moved at different times in the spring, probably because Bur Oaks, unlike Maples, rebuild their vasculature each spring before bud development occurs. Due to the structurally complex nature of tree branches, we use finite element analysis to accommodate the irregular branch shapes and gain a more accurate measure of E over time.