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°N latitude, 72°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/m2 in the patch clearcut, in 2001, 2002, and 2003, respectively, as compared to 1.7, 2.2, and 2.8 MJ/m2 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/m2) 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.

► We monitored solar radiation and PPFD following creation of silvicultural gaps.
► Light availability was related to gap aspect and to location within gaps.
► The southern part of gaps received the lowest light levels at 47° latitude.
► PPFD was closely related to solar radiation in open conditions and inside the forest.
► The PAR fraction of broadband solar radiation was quantified.