Effect of gap size, aspect and slope on available light and soil temperature after patch-selection cutting in yellow birch-conifer stands, Quebec, Canada

Solar radiation, photosynthetically active radiation (PAR) and soil temperature were monitored during three summers following establishment of silvicultural gaps (1-ha square patch clearcut, 20- and 30-m diameter circular gaps) in yellow birch-conifer stands, in eastern Quebec, Canada (47 degrees N latitude, 72 degrees W longitude). We assessed the effect of aspect, slope, opening size, and location within the opening on the microclimate in a foothill site characteristic of this forest type. Daily solar radiation averaged 9.7, 14.3, and 15.1 MJ/m(2) in the patch clearcut, in 2001, 2002, and 2003, respectively, as compared to 1.7, 2.2, and 2.8 MJ/m(2) in the understory of the uncut forest. We also determined that the ratio of PAR to broadband solar radiation was quite constant in the patch clearcut (1.67 mol/MJ), while it increased from 0.76 mol/MJ (June-September) to 1.49 mol/MJ (October) in the forest understory, as broadleaved trees in the main canopy lost their foliage. Circular gaps received intermediate levels of solar radiation (3.9, 6.3, and 7.3 MJ/m(2)) that were comparable for the 20- and 30-m gap diameters. Solar radiation was consistently lower in the southern portion of these gaps than in their central and northern portions, a relative shortfall of 40-50% on north-facing aspects, and 25% on east-southeast aspects. This microclimatic diversity among locations was confirmed by PAR measurements and soil temperature monitoring, which suggests that differing ecological niches may promote species coexistence in these mixedwood stands. (C) 2012 Elsevier B.V. All rights reserved.