Patterns of Nutrient Release Associated with CWD Following Varying Levels of Retention, and Their Relationship with Foliar Nutrition.

Sites with shallow soils or those with coarse textured soils can be especially susceptible to overstory removal as low carbon and nutrient pools may limit stand productivity in subsequent rotations.  On these site types, CWD, with its slow and steady decomposition, may provide essential nutrition to the developing 2^nd growth stand prior to internal cycling processes stabilizing at crown closure. The current study sites, which are now approaching crown closure, were established in 1994 and were designed to document carbon loss and nutrient fluxes associated with residual CWD remaining after the application of four levels of biomass removal from mature black spruce forested stands in northwestern Ontario. Replicate stands on two soil types (fresh, loamy : dry, sandy) were selected to test if CWD represents a source or sink for nutrients, (especially N and P) and if the decay pattern varied depending on soil type.  N content in CWD (kg ha^-1) exhibited an initial increasing trend (i.e., immobilization) through years 1-4 (from 50 up to 77 kg ha^-1) and a subsequent release in years 5-14 (from 77 down to 27 kg ha^-1).  This trend was most apparent on the dry, sandy sites where N content peaked at almost 100 kg ha^-1 at year 4, but then reduced to 26 kg ha^-1 by year 14.  Above ground net productivity and foliar uptake of N was also tracked; a clear assart flush in year 4, was followed by a steady decline through to year 10, and exhibited a slight rebound by year 15.  This pattern coincided with the documented trend of initial N immobilization and subsequent release from CWD, which may also serve to buffer the initial leaching of nutrients from the site following harvesting (especially on dry, sandy soils) and provide an available source of N to the regenerating stand prior to crown closure.