Nutrient balance and availability following woody bioenergy feedstocking in Populus tremuloides forests

Background/Question/Methods

Interest in utilizing forest-derived biomass for bioenergy is growing, and a fundamental concern is the potential for nutrient depletion following repeated harvests. Removals of nutrient-rich branches and foliage may undermine site nutrient availability and productivity if harvesting removes nutrients faster than replenishment via atmospheric deposition and chemical weathering.. We implemented a forest biomass harvesting experiment at four sites in Populus tremuloides Michx.-dominated forests of northern Minnesota, USA.  Each site included 10 harvest treatments: three levels of woody biomass removal (whole-tree harvest, 20% slash retained, and stem-only harvest) factored with three levels of green-tree retention (no trees retained, dispersed, and aggregate), and an untreated control.  In each treatment, we estimated carbon and nutrient stocks in live and dead trees, understory, woody debris and soil both before and after harvest.  In addition, we assessed nutrient availability and soil respiration across all treatments (including control) after harvest. We used the results to ask 1) How do these treatments influence short-term patterns of nutrient availability and soil respiration? 2) What does the balance between nutrients removed during harvest and deposition rates suggest about the potential for long-term nutrient depletion across these treatments?

Results/Conclusions

Although nutrient losses for individual biomass pools or the whole ecosystem did not differ among green tree retention treatments, these treatments did impact soil respiration and nutrient availability.  We observed significant differences in nutrient loss among biomass removal treatments in some pools, notably coarse and fine woody debris.  However, our results indicate substantial intra-treatment variability in carbon and nutrient pools, and suggest that, even in these highly controlled and prescribed harvest events, the impacts of harvest are more appropriately represented as a continuous function of biomass removed, similar to previous work assessing natural disturbances along a continuous gradient. Assessment of the long-term potential for nutrient depletion indicates that, depending on harvest return interval, nutrients removed by woody biomass harvests may be greater than deposition can replenish, especially in isolated pockets within specific treatments. Quantifying the impact of these biomass harvests along a continuous gradient may provide a more effective means for describing harvest intensity, assessing intra-treatment variability, and characterizing the spatial patterns of harvest impacts within a forest stand.