Sophie Everbach Thesis Defense (N. Michele Holbrook, Advisor)

Title: Tension Wood Tradeoffs: Hydraulics, Storage, and Mechanics

Abstract: Trees are always on the hunt for light. The variety of branching architectures exhibited by forest trees is a testament to this photic need; individual trees must find, occupy, and maintain branches in the temporally and spatially heterogeneous light environment of a forest canopy despite the mechanical stressors of gravity and wind. A tree’s structural ability to respond to mechanical stress, while also balancing hydraulic efficiency, resistance to cavitation, and carbohydrate storage, is therefore key to its resource acquisition success and survival. Yet these mechanical-hydraulic-storage tradeoffs are not well understood, especially in tension wood, the angiosperm tissue that forms in response to mechanical stress. In this dissertation, I use wood anatomy and physiology to explore the hydraulic tradeoffs, storage tradeoffs, and within-tree distribution of tension wood generated in diffuse-porous (Acer rubrum) and ring-porous (Quercus rubra) trees to better understand the biological costs of maintaining productive foraging growth behaviors in forests battling drought, severe windstorms, and other disturbances that have intensified under global climate change.

In Chapter One I answer the question: What is the effect of tension wood on hydraulic conductivity and cavitation resistance, and does it differ between diffuse and ring porous angiosperms? I find that tension wood does not aid or disadvantage the hydraulics of diffuse or ring porous species. However, diffuse and ring porous trees use different mechanisms to achieve this balance of structure and function. Diffuse porous altered space/resource distribution via changes in ring width to accommodate tension wood, while ring porous had temporal separation of function, producing earlywood vessels before the tension wood fibers. In Chapter Two I answer the question: Where is sugar and starch stored in tension wood tissues and are there differences in this storage potential across wood tissue types? I find no tradeoff between producing tension wood and storing non-structural carbohydrates, with ring porous species again exhibiting a temporal separation of function, leaving tension wood tissues with equally resilient sugar reserves as those not subjected to the same mechanical stresses. In Chapter Three I answer the questions: How much of a tree branch is composed of tension wood and where is it distributed? Are branch hydraulics impacted by tension wood amount and is tension wood distribution correlated with other features of branch foraging? I find that tension wood occurs in significant amounts along the length of each branch, with a positive correlation between tension wood amount and branch angle in Quercus rubra only and with asymmetry in Acer rubrum and Quercus rubra. Consistent with Chapter One, I found no impact of tension wood on hydraulic conductivity. Tension wood may therefore have an active role in light foraging that reflects the different canopy needs of Acer rubrum and Quercus rubra. Altogether, this research illustrates how diffuse porous and ring porous angiosperm trees have successfully adapted their wood structure to fulfill competing functional needs, using different strategies to balance vascular transport, carbohydrate storage, and mechanical support to allow them to withstand gravity and forage for light.

Committee: N. Michele Holbrook (Advisor), William Friedman, Elena Kramer, Benton Taylor (Chair)