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Water transport in trees

Tuesday, August 23, 2011, by Alena Tofte
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Multitudes of tightly packed rings in an old, sturdy tree hide a secret – not only do they elucidate to a discerning viewer a historical record of how much the tree grew each year for the course of its life, but these rings also contain the remnants of its once-functional woody vascular tissue, the xylem. Xylem once threaded thin streams of water and vital nutrients throughout the growing tree’s roots, trunk and crown.

Water transport in trees is a process ruled by a multitude of factors, including the porosity of the wood, the size of the vessels which comprise the vasculature, the species' inherent differential ability to tolerate variation in climate and water availability, and the surrounding changing environmental and climatic conditions. Diurnal variation in the amount of water available to a tree, due to soil moisture and atmospheric conditions, can induce cavitation, or the formation of gaseous embolisms within xylem conduits. These bubbles decrease the overall efficiency of water transport, and are previously thought to permanently eliminate functionality of those xylem vessels. However, some evidence has implied that cavitated conduits refill on a diurnal cycle, by possibly pressurizing the embolisms on a localized scale and forcing the gaseous bubbles in to solution and reestablishing continuity within the xylem's transpirational stream. 

To address the ongoing debate surrounding refilling and investigate a little about what is going on within the trunk of a tree, we measured the water transport efficiency, or conductivity, of white ash, red maple, and paper birch stems on a diurnal cycle. We used Bucky, the cherry picker, at our site to cut canopy samples in the afternoon and again the following morning.  There’s nothing quite like the smell of diesel exhaust at 6:00 a.m.! Our measurements show preliminary evidence of refilling in ash, but not in maple or birch. Further data collection of may corroborate these conclusions. This summer, I’ve gained a better understanding of how species-specific physiological capacities like diurnal embolism and refilling may determine rates of photosynthesis and carbon sequestration, tolerance of water stress, and associated geographic distribution  of different trees with respect to their surrounding water availability.

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