Canopy characteristics and growth rates of ponderosa pine and Douglas-fir at long-established forest edges

Abstract

Trees respond to edge-to-interior microclimate differences in fragmented forests. To better understand tree physiological responses to fragmentation, we measured ponderosa pine ( Pinus ponderosa Dougl. ex P. & C. Laws) and Douglas-fir ( Pseudotsuga menziesii (Mirbel) Franco) leaf area, crown ratios, sapwood area, basal area (BA) growth rates, and BA growth efficiency at 23 long-established (>50 year) forest edges in northern Idaho. Trees located at forest edges had more leaf area, deeper crowns, higher BA growth rates, and more sapwood area at breast height than interior trees. Ponderosa pine had significantly higher BA growth efficiency at forest edges than interiors, but Douglas-fir BA growth efficiency did not differ, which may relate to differences in photosynthetic capacity and drought and shade tolerance. Edge orientation affected BA growth efficiency, with higher values at northeast-facing edges for both species. Edge effects were significant even after accounting for variation in stand density, which did not differ between the forest edge and interior. Although edge trees had significantly greater canopy depth on their edge-facing than forest-facing side, sapwood area was evenly distributed. We found no evidence that growing conditions at the forest edge were currently subjecting trees to stress, but higher leaf area and deeper crowns could result in lower tolerance to future drought conditions.