Canopy gap edge effects on overstory sap flux in a temperate mixed-hardwood forest

Canopy openings create gradients in microclimate and can increase transpiration of individual trees bordering the gaps. This study evaluates overstory tree water use in a mature, temperate, upland hardwood forest three and four years after the establishment of 0.1 and 1 ha experimental gaps. Sap flux density (J_s) was measured using thermal dissipation probes in 63 trees of dominant species groups, red maple (Acer rubrum), hickory (Carya spp.), tulip-poplar (Liriodendron tulipifera), and white oak and red oak groups (Quercus spp.), with increasing distance from experimental canopy gap edge (0 – 30 m). Diurnal, daily, and seasonal patterns of J_s showed no change with distance from gap edge and no difference between gap sizes, despite increased edge tree diameter growth. High tree-to-tree variability in J_s was prevalent within species, potentially intensified by steep slopes and rocky soils. There were clear differences in summer J_s between species with mesic species 43 % higher than oak and hickory species on average. As such, short-term effects of gap creation on local-scale water availability and stand-scale water use in mixed-hardwood forests will be largely driven by interactions between microclimate and vegetation dynamics within the gap, but not by canopy trees at the edge of the gap.