Microclimate and regeneration discordance in harvest gaps of mixed-species stands

Background/Question/Methods

Harvesting multiple-tree gaps has become a current strategy for natural regeneration in many forests types, especially when both producing forest goods and emulating natural disturbances dynamics are desired. Harvest gaps can be used in different variants of silvicultural systems, such as selection cutting and shelterwood. In northern ecosystems, use of harvest gaps must take into account an important factor: the latitude. Over the 40°N parallel for instance, the distribution of light in gaps is asymmetrical and the zone of maximum light is offset to the northern part compared to the equator. This asymmetry can cause theoretically a discordance between the optimal ecological niches for regeneration and growth. Since gaps are used to satisfy regeneration requirements of desired species, how does it affect regeneration dynamics in practice?

Results drawn from two experiments of patch-selection cutting in the province of Quebec (Canada) will be presented. The Maniwaki study was located in the northern hardwoods forest zone (46°N, 76°W) and aimed at regenerating eastern white pine (Pinus strobus L.) and birches (Betula spp.) in a mixed tolerant hardwoods – white pine stand. The SSAM study (Silvicultural Systems Adapted to Mixedwood forest) was set in the temperate mixedwood forest zone (47°N, 72°W) and conducted in yellow birch (Betula alleghaniensis Britton) – conifer stands for regenerating yellow birch, red spruce (Picea rubens Sarg.) and balsam fir (Abies balsamea [L.] Mill.). Besides regeneration abundance, light and temperature were measured in both experiments for assessing microclimate variation within gaps. 

Results/Conclusions

As expected, the Maniwaki study showed an asymmetry in light distribution in 40-m circular gaps. The effects on microclimate and patterns of natural regeneration were not only observed along a north – south gradient, but on a west – east gradient too. Moreover, there was evidence of discordance between regeneration and growth niches for white pine. The SSAM study has also underlined evidences of niches discordance for yellow birch and conifer species, since borders were important regeneration niches, while the northern part of gaps had the best light conditions.

Instead of being viewed negatively, discordances of regeneration and growth niches can be considered as an opportunity to create different ecological conditions to regenerate a diversity of species. A good understanding of regeneration dynamics in and around gaps is necessary to adapt gap sizes and silvicultural systems to the specificity of mixed stands.