Responses of sap flux and stomatal conductance of Pinus taeda L. trees to stepwise reductions in leaf area

Herbivory or artificial foliage removal has been shown to affect gasexchange and canopy water relations. In this study, canopy architecture andwater relations in response to progressive defoliation were examined in astand of 8-year-old loblolly pine (Pinus taeda L.)trees, a shade-intolerant, pioneer species common in the south-eastern USA.Sap flux was measured with constant heat sap flow gauges in order toestimate canopy stomatal conductance (Gs) whilefoliage in the 6 m high stand was harvested in 1 m increments from thebottom up. Leaf-level stomatal conductance and water potential data werealso collected. Profiles of silhouette area ratio and specific leaf areashowed no trends with crown height, reflecting an open canopy (leaf areaindex = 1.55). Therefore, short-term changes in Gswith foliage removal were attributed to hydraulic effects rather thaninfluences of changes in mean microclimate conditions onGs of remaining foliage. A large increase inGs was observed during the 6 h pruning period whichfully compensated for the reductions in foliage area down to 45%. Canopystomatal conductance and whole plant liquid phase conductance as calculatedfrom sap flux were both influenced by the rate of growth as indicated bythe annual basal area increment.