The Role of Aggregated Forest Harvest Residue in Soil Fertility, Plant Growth, and Pollination Services

While post-harvest residue (slash) is increasingly viewed as a source of biofuel, few studies have considered the potential ecological impact of the spatial distribution of forest harvest residue. We hypothesized that slash piles create islands of high soil fertility and light, leading to greater abundance, growth, and reproduction of plants. In 6-yr-old intensively managed hybrid poplar (Populus balsamifera L. x P. maximowiczii A. Henry) plantations, we showed that soluble organic N, NO3-N, NH4-N, and P decreased as a negative exponential function with distance from the pile. Pin cherry (Prunus pensylvanica L. f.) relative growth rate was fastest near piles in the first few years after harvest. We found significantly greater stem size, flowers per plant, and foliar P near piles for cherry, and this inverse gradient sharpened in a 16-yr-old white spruce [Picea glauca (Moench) Voss] plantation. For the two other flowering species, strawberry (Fragaria virginiana Mill.) and raspberry (Rubus idaeus L.), flower and fruit abundance were also significantly and strongly negatively correlated with distance to the pile. Further, directly correlating soil nutrient availability with plant traits, we found significant positive relationships between plant growth, reproductive output, and N availability. Partial correlation analysis indicated that more of the variance in plant traits was explained by distance than by soil nutrition. We conclude that in industrial forests, piles replace canopy gaps as sites where understory plant species can episodically reproduce and are therefore important for many plant species as well as the pollinators and frugivores dependent on them.