Estimating Water Flux through Stems of Scots Pine with Tritiated Water and Phosphorus-32

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Abstract
The radioisotopes of tritium (3H) and phosphorus-32 (32P) were simultaneously injected into the conducting tissue of more than 40 Scots pine trees over a 6 month period. Analysis of core samples indicated that 32P and 3H are transported by mass movement at similar rates. 3H exhibits diffusion as well as mass flow properties. The strong beta emission of 32P at the bark surface was monitored to determine the velocity of the pulse. Core samples provided an estimate of the volume of waterfilled conducting tissue. The calculations of flux were compared with actual uptake by two trees severed at their bases and supported in water-filled containers. Over a 5 d period, periodic estimations of cumulative water uptake showed an excellent linear relationship with measured uptake (r2 ≥ 0.98). Refinements in the model are discussed, emphasizing the importance of assessing diurnal variation in sapwood water content.

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