Nitrogen and calcium additions increase forest growth in northeastern USA spruce-fir forests

We determined responses of red spruce (Picea rubens Sarg.) - balsam fir (Abies balsamea (L.) Mill.) forests to 6 years of nitrogen (N), calcium (Ca), and N + Ca treatments (100, 160, and 260akg center dot ha(-1 center dot)year(-1) of N, Ca, and N + Ca, respectively) in New York (NY) and New Hampshire (NH). Forest responses to Ca treatments were also determined in Vermont (VT). Nitrogen treatments increased aboveground net primary production (ANPP) by 33% and 25% above controls in NY and NH, respectively. Similarly, N + Ca treatments increased ANPP by 27% and 28% in NY and NH, respectively. Calcium treatments increased ANPP by 25% and 21% above controls in NY and VT. Calcium treatment did not increase ANPP in NH, suggesting N, but not Ca limitation. Leaf-litter quantity and quality, and soil C and N storage were greater in treated than in control plots. Fine-root mass and production did not differ among treatments. Trees, therefore, assimilated more soil nutrients without increasing root growth in treated plots. Red spruce ANPP, however, declined or remained unchanged in response to N and Ca additions. The equivalent of 68-102 years of anthropogenic N addition to soils changed forest species composition without decreasing ANPP, and Ca additions did not prevent this change.