How Much Slash Should Remain On Site to Sustain Tree Growth?

A large scale experimental design was set up to test the effect of various levels of slash (forest harvest residues) on soil and tree growth and to elucidate the mechanisms that are operating. The set-up included four sites located under different soil and climatic conditions in Quebec. Based on estimates of the average level of biomass that should remain on site when the stand is harvested by stem-only harvesting (1 load), the experimental design included the following gradient of slash loading: no slash, half a load, full load, double load. A combination of fertilisation, herbicide and plastic mulches were also used as treatments designed to simulate and isolate specific mechanisms by which slash affects plant growth: providing nutrients, shading the soil, and limiting the growth of competing plants. The experimental unit is a single tree on a 9m² plot. The design also included different tree species with contrasting traits, some of the conservative type such as black spruce and others are from the acquisitive type such as hybrid poplar.   Quantifying the effect of slash on soil conditions and tree development may help putting forward recommendations on the amount of slash that should be left on site to maintain ecosystem functions and on mitigation alternatives.