Interactions between tree species, soil, and nitrogen availability in fir-birch-spruce forests

Fir-birch-spruce forests in the Northeastern United States, have evolved in N-limited conditions and are currently receiving high atmospheric N inputs. In this dissertation I examine the role of heart-leaf paper birch and balsam fir in both influencing and responding to N-availability. Rates of N-availability differed between birch and fir dominated forest floor, and between the 1995 and 1996 growing seasons in fir plots. Rates were strongly affected by soil moisture (or related covariate) in fir dominated plots, but were unaffected in birch dominated plots. These results provide new evidence that net N-mineralization may respond differently to abiotic conditions depending on the local canopy species.; To understand the response of birch and fir to increased N-availability, I grew seedlings in pots under controlled light x N treatments (light = 10, 40, 70, 100% ambient; N = 10, 40, 70, 100 kg ha-1 yr-1 - N) for two seasons. Increased N-availability had no effect on fir seedlings, but significantly increased photosynthesis and growth in birch seedlings in 70 and 100% ambient light. Carbon allocation in both species was strongly controlled by light, except root/shoot ratios which decreased in both birch and fir seedlings grown in high N in 40% and 70% ambient light. Birch seedling growth increased as a step-function between 10% and 40% ambient light, whereas fir seedlings growth increased linearly. Fir seedling growth was more inhibited by shade than was birch.; I analyzed foliar nutrient concentrations in these seedlings to look at treatment effects and to examine relationships between nutrients and physiology. Foliar N concentration was unchanged by N-availability in birch seedlings but increased with N-availability in fir seedlings. Foliar N was positively correlated with photosynthesis at light saturation in birch seedlings but not in fir seedlings. Fir seedlings had a higher phosphorus use efficiency than birch seedlings, suggesting that P may have limited their growth.; Understanding the differential influence of co-existing tree species on N-availability and the differential physiological response of their seedlings to N-availability, may improve N-cycling models, considering atmospheric N-deposition.