Tree water and carbon relations

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Terrestrial plants face a fundamental challenge - how to balance the need for carbon with the need to stay hydrated. During photosynthesis plants absorb carbon dioxide from the atmosphere through tiny pores in their leaves called stomata. Because they are open to the relatively dry air, water inside the leaf evaporates through the stomata via transpiration, creating a negative pressure that pulls water from the soil into a plant's roots and up to the leaves through xylem cells in the wood. The pressure gradient driving water flow through a tree is influenced by tree structure, stomatal behavior, and environmental conditions. In order to avoid desiccation and hydraulic failure, plants must transport a sufficient supply of water from roots to leaves to replace water lost during transpiration. However, if a plant closes its' stomata to reduce water loss then photosynthesis and carbon gain also decline, affecting plant metabolism, growth and survival. Stomata therefore need to operate to maximize photosynthetic production while minimizing water loss.

I have been involved in several projects aimed at deepening our mechanistic understanding of how trees balance this carbon-for-water trade-off, how the uptake, transport and usage of water and carbon are coordinated, and how they are affected by structural properties of trees and by variability in environmental conditions. A large component of this research involves examining how traits regulating water and carbon balance vary throughout individual tree crowns, and quantifying the effects of changing soil and atmospheric conditions on tree physiological performance. This research has documented significant within-crown variability and coordination of leaf- and branch-level morphology and physiology along microclimatic and water potential gradients in tall forest canopies. This research has also documented global maximum daily whole-tree water use in the largest and tallest trees in the world (more than 3000 liters of water can flow through the main trunk of a mature giant sequoia on a typical summer day).

I am currently working with the UC Institute for the Study of Ecological and Evolutionary Climate Impacts in collaboration with scientists at Northern Arizona University to examine environmental controls on the water potential and water use of coast redwood, coast live oak (Quercus agrifolia), southwestern white pine (Pinus strobiformis), and quaking aspen (Populus tremuloides) trees at sites in California (UC Landels-Hill Big Creek Reserve) and Arizona (Hart Prairie Preserve). This project aims to improve our mechanistic understanding of the contrasting hydraulic strategies these different species utilize to accommodate soil and atmospheric moisture deficits.

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