MODELING BIOMASS AND NUTRIENT DYNAMICS IN MAINE BALSAM FIR STANDS: INFLUENCE OF INCREASED SILVICULTURAL INTENSITY ON FOREST SUSTAINABILITY
- Burke, Andrea
University of Maine
Increasing societal demands on our forest resources have generated a need for improved understanding of how to define and maintain forest health and sustainability. Not the least of these societal needs is the increasing interest in bio-products, most notably renewable energy resources. In Maine, where forests represent a major component of the state’s economy, the potential for increased utilization of forest biomass over current levels raises questions about nutrient supply, depletion, and the long-term sustainability of forests. We used the model REMSS, parameterized for balsam fir (Abies balsamea (L.) Mill.), to assess whether increases in forest biomass utilization over current levels could reduce long-term forest productivity. REMSS was run for four simulated silvicultural regimes: unmanaged (CTR), pre-commercial thinning (PCT), commercial thinning only, and pre-commercial thinning followed by commercial thinning (PC). Cumulative nutrient removals associated with thinning in the PC scenario were added to a) nutrients accumulated in the merchantable stem (PCCH) and b) nutrients accumulated in total aboveground biomass (PCCC) at age 100 in the PC scenario to assess potential nutrient removals with stem-only and whole-tree clearcuts at 100 years.
A literature review was conducted for research on biogeochemical cycling in red spruce (Picea rubens Sarg.)-balsam fir forests of the northeastern United States and southeastern Canada. Values for macronutrient fluxes and pools were compiled and averaged. For each nutrient, a simple mass balance was constructed to assess net nutrient fluxes for mature spruce-fir ecosystems in the study region. The magnitude of soil “plant-available” nutrient pools were compared to the most intensive nutrient removal of the simulated silvicultural scenarios. Ratios of nutrients removed with harvest to “plant-available” nutrients were generated for preliminary assessments of the potential risk of soil nutrient depletion with increased forest biomass removal. This analysis indicated that relative risk of depletion of plant-available nutrients was Ca>K>P>Mg. If N remains the most commonly deficient nutrient in forests of the region today, then accelerated rates of N removal were also a potential concern.
A dataset of soil-site data from managed spruce-fir forests in northern, eastern, and western Maine was analyzed in an attempt to characterize soils representative of Maine’s managed spruce-fir forest resource. Cumulative frequency plots were constructed to ascertain differences among regions. A correlation matrix was constructed to evaluate potential correlations between variables. Boosted regression trees were constructed to assess the relative influence of response variable subsets on site index and soils data. This preliminary assessment indicated that the eastern study region was potentially more deficient in soil nutrients than the western or northern regions.
There are few long-term, comprehensive, empirical investigations into ecosystem dynamics of Maine’s managed spruce-fir resource to directly observe the consequences of the intensification of forest management. In the absence of such studies, this research employs modeling and analysis of a geographically distributed database on forest soil nutrients to assess relative risk of soil nutrient depletion with increased biomass removal from Maine’s spruce-fir forests. These results serve to focus our attention on key nutrient concerns, risk assessment needs, and research priorities for the future
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