Aspect modifies the magnitude of edge effects on bryophyte growth in boreal forests

1. The increased length of forest clear-cut edges is considered to be one of the main ecological consequences of silviculture. The effects vary over the landscape, and studies have shown that aspect is one important factor determining the extent of microclimatic edge effects across forest clear-cut boundaries. However, little is known about the relationship between contrasts in microclimate at edges and responses of ecological processes and biodiversity, such as growth, decomposition and species distributions. 2. A field experiment was conducted in the boreal forest of northern Sweden to assess the effect of aspect at north- and south-facing edges using mosses as bioindicators. The growth of two species (Hylocomium splendens and Hylocomiastrum umbratum) was evaluated during one growing season. Samples of each species were planted in pots at eight north- and eight south-facing forest clear-cut edges. 3. Growth increased exponentially with distance from the edge to the interior, and there was a significant effect both in north- and south-facing edges. The percentage decline in growth at the edge was larger in the south- than in the north-facing edges. 4. The spatial extent of the edge effect, when measured at the point of 90% of interior growth, was similar between north- and south-facing edges, although it differed between the two species evaluated. 5. Synthesis and applications. The difference in exposure to sunlight between north- and south-facing edges was shown to modify the magnitude of the growth of a poikilohydric organism at the very edge, but not the depth of the edge influence. Aspect should be taken into account in management plans for conservation of boreal forests. In the northern hemisphere, wider buffers of uncut forest should be left at the south side than at the north side of retained forest patches. Those forest interior species that are most sensitive to alterations in microclimate will, however, need equal protection from edge effects at all aspects.