Sapwood water storage: its contribution to transpiration and effect upon water conductance through the stems of old-growth Douglas-fir

Enough water is stored in the sapwood of largeDouglas-fir to significantly contribute to transpiration.Sapwood water content falls through the season, causingthe wood's conductivity to fall. This leads to low leaf-water potentials, stomatal closure, and reduced photo-synthesis by the trees.
The amount of water stored in the sapwood of Douglas-fir 50-60 m tall, growing in the Cascade Mountains ofOregon, was estimated periodically over two seasonsfrom measurements of sapwood relative water content(R5). The relationship between Rs and volume of watercontained in the sapwood was determined in the labora-tory, and an equation describing the variation of relativeconductivity (K) with Rs was derived from the literature.Stomatal conductance (ks) and leaf water potentialswere measured in the field.
The relative conductivity of the sapwood was calcu-lated from estimates of the flow rate through the treeand differences in water potential between dawn and thetime of comparison. Flow rate was assumed to equaltranspiration rate, calculated from the Penman-Monteithequation using measured k, values. A sixfold decrease inK during the summer was attributed to changes in Rs.The maximum observed diurnal variation in K wouldrequire a change in Rs estimated at 25%.
About 270 m3 ha-1 (27 mm) of water were stored insapwood, and 75% of that was in the stemwood. With-drawal from this store reached 1.7 mm day-' on cleardays after cloudy or rainy weather. Recharge could bealmost as fast (up to 1.6 mm day-1) after rain, but wasvery slow if the foliage remained wet.