The effect of whole-tree harvest on phosphorus cycling in a northern hardwood forest

A small watershed ecosystem at the Hubbard Brook Experimental Forest in New Hampshire was whole, tree harvested in the dormant season; all trees greater than 10 cm diameter were removed. This harvest removed 50 kg P/ha, five times more P than the bole-only clearcut of an adjacent watershed. The P content of branches and twigs was nearly twice that of stem wood and bark, which contributed to the intensity of P removal. The amount of P removed in harvest was small compared to total P in the mineral soil (1600 kg/ha) but large compared to the pools of P in the living vegetation (70 kg/ha) and the forest floor (85 kg/ha). Negligible P was lost in streamwater and sediment (0.2 kg P/ha over 3 years), although export of other nutrients increased dramatically. Although leaching of P from the forest floor to the mineral soil in the first 2 years after logging was higher in harvested sites than in undisturbed forest, the increase in P leaching (0.7 kg P/(ha yr)) was much less than the estimated decrease in P uptake from the forest floor (4.8 kg/(ha yr)), suggesting a 70% decline in net P mineralization in the forest floor. Even in areas where P uptake by regrowing vegetation was quite high, calculated net P mineralization was 40% lower in the first 2 years of growth than in the undisturbed forest. Revegetation was rapid: in the first 2 years after logging, P in biomass had accumulated to 3% of the uncut forest; uptake was 12% of that in the uncut forest. Whether intensive biomass removals could induce P deficiency in future rotations is unknown.