Species-specific growth response of southern New England hardwood forest canopy trees to experimental gap

Background/Question/Methods The fragmentation of forests in southern New England multiplies the linear quantity of forest-non forest edges and has important consequences on the ecology and biodiversity of these forests. Most studies of the influence of edges on the forests of eastern North America have focused on forest regeneration and dynamics. Comparatively little research has focused on the effects of edge creation on the growth of adult canopy trees. This is important since canopy trees are the source of future regeneration, and growth influences tree survival and fertility. The objective of this study was to determine if the growth response of canopy trees to an experimentally created forest edge was spatially and temporally species-specific by analyzing past growth increments from four tree species prior to and following a 1999-2000 experimental gap creation in the Yale-Myers Experimental Forest, Connecticut. Some published studies have suggested that shade-tolerant tree species exhibit greater growth plasticity than shade-intolerant ones and take advantage of gap creation by expending their crown. We hypothesized that, following gap creation, shade-tolerant trees such as sugar maple (Acer saccharum) would therefore increase their trunk diameter growth more than shade-intolerant species such as red oak (Quercus rubra). Tree cores were collected from 65 trees ≥ 10 cm DBH within four parallel 30×10-m belt transects randomly located from the 100 m-long gap edge into the forest, and annual rings were measured, cross-dated, and analyzed using non-linear generalized regression models and maximum likelihood, controlling for soil water potential.

Results/Conclusions Canopy trees responded to the creation of an edge by increasing their diameter growth by up to a factor of 10, depending on the species, but only within 10 m of the edge, with no response at > 10 m from the edge. Sugar maple (Acer saccharum) and white pine (Pinus strobus) exhibited a monotonic post-gap-creation increase in trunk radial growth toward the forest edge, while red oak (Quercus rubra) and red maple (Acer rubrum) exhibited little radial growth increase except at the very margin (< 3 m) of the gap. These results support the shade-tolerant growth plasticity hypothesis, but contradictory species-specific results among published studies conducted in different regions of the Eastern US and Canada underline the geographical complexity of growth responses of trees to gap disturbance in the Eastern Deciduous Forest vegetation type. Thus similar gap disturbances may have different long-term effects on the regeneration, dynamics and biodiversity of Eastern forests depending on location.