Forest floor properties across sharp compositional boundaries separating trembling aspen and jack pine stands in the southern boreal forest

Abstract

Jack pine and trembling aspen are two early-seral boreal tree species with contrasting nutrient cycling strategies. Both species may form adjacent mono-specific stands separated by sharp compositional boundaries. We hypothesized that such boundaries result in wider functional ecotones. Spatial transitions in humus forms, forest floor chemistry and microbial communities were assessed across 32 m long transects set perpendicular to sharp compositional boundaries separating four jack pine and aspen stands. Split moving window analysis (SMWA) and moving window regression analysis (MWRA) were used to locate functional boundaries and ecotones. We found a gradual transition from moder (aspen) to mor (jack pine) humus spanning 16 m across the compositional boundary. An abrupt increase in forest floor water content at 3 m within jack pine stands was possibly due to aspen roots foraging for water beyond the boundary. The functional boundary and associated ecotone for forest floor pH, C:N ratio, Mg and ammonification were skewed toward jack pine stands, likely the result of aspen leaf dispersal. Low nitrification rates throughout jack pine stands and up to 11 m into aspen stands suggested that jack pine roots might extend far within aspen stands and produce metabolites that suppress nitrification. SMWA performed on the multivariate dataset of microbial fatty acids (FAs) revealed three distinct forest floor microbial communities that were skewed toward jack pine stands. Pine-type communities were associated to fungal FAs, pine-type and transition-type communities to non-fungal eukaryotic FAs, and aspen-type communities to bacterial FAs. Taken collectively, our data delimit a 24 m wide functional ecotone straddling sharp compositional boundaries separating trembling aspen and jack pine stands. We conclude that the functional diversity of boreal landscapes, where adjacent mono-specific stands are prevalent, is related to the patchiness of the landscape.