Will Biomass Harvesting of Northern Hardwood Forests Lead to Nutrient Depletion In the Northeastern US?.

Forest cover dominates the northeastern USA, with 65% of the Mid-Atlantic region in forest, and Vermont, New Hampshire, and Maine ranging from 78% to 89% forest.  Forest biomass, if not limited by nutrient depletion, could sustainably supply 4-15 Mt per year in excess of current pulp and paper use, amounting to 6% of coal consumption (used for electricity) in the region.  However, it is difficult to predict whether productivity could be sustained under repeated nutrient removals, especially with more intensive harvesting.  We applied a budgetary approach to compare possible rates of nutrient removals in biomass harvesting to nutrient pools in the soil, using 14 northern hardwood stands in the White Mountain region of New Hampshire as a case study.  Calcium and phosphorus were the elements predicted to be depleted first by repeated whole-tree harvest, in as little as one rotation or as many as 4, if only exchangeable and organically bound nutrients were assumed to be available.  Whole-tree harvesting at 30-year intervals would maximize short-term yields, but nutrient removals are also higher per unit biomass in short rotations, because of the higher nutrient concentrations in branches than boles. Bole-only harvesting at 100-year rotation length was much less demanding in terms of nutrient removals, with depletion not predicted for 400-2000 years, depending on site quality.  If plants can obtain Ca and P by accelerating weathering of apatite, then depletion of these elements could be delayed to the point that K or Mg would become limiting.  Although nitrogen is commonly thought to be limiting to forest growth in the region, none of these scenarios resulted in N depletion, because of high current rates of anthropogenic N deposition.