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

Abstract Enough water is stored in the sapwood of large Douglas‐fir to significantly contribute to transpiration. Sapwood water content falls through the season, causing the wood's conductivity to fall. This leads to low leafwater potentials, stomatal closure, and reduced photosynthesis by the trees. The amount of water stored in the sapwood of Douglasfir 50‐60 m tall, growing in the Cascade Mountains of Oregon, was estimated periodically over two seasons from measurements of sapwood relative water content (R_s). The relationship between R_s and volume of water contained in the sapwood was determined in the laboratory, and an equation describing the variation of relative conductivity (K) with R_s was derived from the literature. Stomatal conductance (k_s) and leaf water potentials were measured in the field. The relative conductivity of the sapwood was calculated from estimates of the flow rate through the tree and differences in water potential between dawn and the time of comparison. Flow rate was assumed to equal transpiration rate, calculated from the Penman‐Monteith equation using measured k_s values. A sixfold decrease in K during the summer was attributed to changes in R_s. The maximum observed diurnal variation in K would require a change in R_S estimated at 25%. About 270 m³ ha⁻¹ (27 mm) of water were stored in sapwood, and 75% of that was in the stemwood. Withdrawal from this store reached 1.7 mm day⁻¹ on clear days after cloudy or rainy weather. Recharge could be almost as fast (up to 1.6 mm day⁻¹) after rain, but was very slow if the foliage remained wet.