Transforming even-aged coniferous stands to uneven-aged stands: an opportunity to increase tree species diversity?

In the hope of increasing forest adaptability, there is a growing interest in converting coniferous even-aged stands to uneven-aged stands. While the former silviculture has a long tradition, the latter as well as the conversion between the two systems remain complex and far less documented. Depending on the proximity of seed bearers of different species, the seedling diversity can be important, offering good opportunities to increase future tree diversity. Nevertheless, with continuous cover forestry that limits forest perturbations, there is a fear that only a few late-successional vigorous species and eventually limit future stand diversity. Nine 1-ha permanent plots have been established in even-aged coniferous stands managed to progressively convert them to uneven-aged stands. The stands were chosen to span contrasted forest composition and a gradient of stand ages at the beginning of the conversion from even-aged to uneven-aged. Twelve subplots were installed in each plot to monitor the natural regeneration and light conditions using hemispherical photography. In contrast to the regeneration density and diversity, the height growth rate of dominant saplings was well related to light conditions. In similar light conditions, saplings of Norway spruce, Douglas fir and European silver fir showed relatively similar height increments, while, in some plots, saplings of Western hemlock and larch had higher height growth rate and might threaten future stand diversity.