Unstressed Trees Research Articles

Diffusion Resistance and Xylem Potential in Stressed and Unstressed Northern Hardwood Trees

In a northern hardwood forest in central New Hampshire, daytime diffusion resistances of abaxial leaf surfaces were independent on light level above 250 @mE/(m2°s), increased as vapor pressure deficit increased, and decreased as temperature increased. High vapor pressure deficit at midday caused slight stomatal closure, which appeared to be independent of leaf H_2O potential. Low plant H2O potential may have produced additional closure at high atmospheric demand even though soil—water potential was > —0.2 bar. At an atmospheric demand of 250 W/m2 xylem potential averaged —16 bar for Betula alleghaniensis, —21 bar for Acer saccharum, and —23 bar for Fagus gradifolia, and decreased only slightly as demand increased further. Daytime diffusion resistance averaged 2.5 s/cm for B. alleghaniensis, 3.1 s/cm for A. Saccharum, and 3.3 s/cm for F. grandifolia when light was adequate. The resistance difference implies the possibility of higher transpiration rates in B. alleghaniensis. Several trees were artificially stressed by trenching around them and covering the soil block with polyethylene. After 2—3 wk with rain excluded, soil—water potential ranged from —0.4 to —0.8 bar, and daytime diffusion resistance averaged 5 s/cm, indicating significant stomatal closure caused by H_2O stress. However daytime xylem potentials on stressed trees were the same as or higher than those on unstressed trees. Stomatal control effectively prevented excessive H_2O loss and prevented xylem potential from declining below normal operating levels.