Forest biomass production and utilization have the potential to both increase forest carbon stocks and reduce CO2 emissions from combustion of fossil fuels. Whole-tree harvesting (WTH) is commonly used to extract forest biomass for energy and commercial wood products. However, the removal of tree tops and branches in WTH has led to concerns about long-term sustainability of production, particularly in conifer forests on low fertility sites. Yet there are few studies of sufficient longevity to determine if incremental (stem-only to whole-tree) biomass removal affects long-term productivity in northern mixedwood (conifer-hardwood) forests. To answer this question, we are using a long-term U.S. Forest Service experiment, established in 1964 on the Penobscot Experimental Forest in Maine, to evaluate effects of biomass harvesting and site quality (soil drainage, grouped as poorly and well-drained) on biomass and carbon stocks 50 years after treatment. Data collected in 2014 and 2015 were used to estimate total aboveground live-tree biomass (Mg ha-1) and carbon stocks (Mg ha-1), applying published species-specific allometric equations. Linear mixed-effects models were used to determine treatments effects at both the plot- and species-level.
Results/Conclusions
Total aboveground live-tree biomass and carbon stocks were greater on well-drained than poorly drained soils, but did not differ between treatments 50 years after harvesting. These findings suggest that, over the long-term, WTH does not degrade mixedwood forest productivity as measured in this study. We observed site, rather than harvesting method, to be the primary factor controlling productivity. Future work will include analysis of foliar and soil nutrition across sites and treatments, as well as re-harvesting to determine the effects of repeated biomass removals. Results to date suggest no differences between stem-only and whole-tree harvesting methods with regard to impacts on long-term sustainability of production in northern mixedwood forests.