Regional variation in foliar chemistry and N cycling among forests of diverse history and composition

Although understanding of nitrogen cycling and nitrification in forest ecosystems has improved greatly over the past several decades, our ability to characterize spatial patterns is still quite limited. A number of studies have shown linkages between canopy chemistry and N cycling, but few have considered the degree to which these trends can provide an indicator of forest N status across large, heterogeneous landscapes. In this study, we examined relationships among canopy chemistry, nitrogen cycling, and soil carbon: nitrogen ratios across 30 forested stands in the White Mountains of New Hampshire. Plots included a range of species (sugar maple, red maple, American beech, yellow birch, paper birch, red spruce, balsam fir, eastern hemlock) and were broadly grouped into two disturbance categories: those that were historically affected by intensive logging and/or fire and those that experienced minimal human disturbance. Across all plots, rates of not N mineralization and net nitrification were correlated with canopy nitrogen concentrations, but the relationships differed between disturbance treatments. In deciduous forests, historically undisturbed stands had significantly higher rates of net N mineralization and net nitrification than previously disturbed stands, but these differences were not clearly reflected in patterns of stand-level canopy chemistry. Although soil C:N ratios also differed between disturbed and undisturbed stands, a relationship between soil C:N ratios and canopy lignin:N ratios did not vary with either forest type or disturbance, suggesting that this trend is more consistent across diverse conditions. Relationships between foliar chemistry and N cycling within individual species revealed interesting differences between species and functional groups. For four out of five deciduous species, foliar N increased with increasing net N mineralization, indicating, that species were responsive to changes in N availability and suggesting a positive feedback between foliar chemistry and soil N status. These patterns led to significant differences in foliar N between disturbance treatments for some species, but at the stand level, these differences were masked by successional changes in species composition. Among coniferous species, foliar N showed no variation across wide N-cycling gradients, suggesting a fundamentally different plant-soil interaction. We also examined the potential for extending observed field relationships to the regional level using a high-quality data set of high spectral resolution remote sensing, obtained from NASA's AVIRIS instrument (Airborne Visible and InfraRed Imaging Spectrometer). Cloud-free AVIRIS data from 56 scenes covering the White Mountain National Forest were calibrated to canopy lignin:N ratios and were applied to prediction of C:N ratios in soils. Validation at 10 independent plots showed good prediction accuracy but suggested some overprediction at the low end of the range. Preliminary regional estimates of soil C:N ratios indicate that 63% of the region's land area falls below a value of 22. This value is significant because our field data identified this as a critical threshold for the onset of nitrification. Below C:N = 22, we expect increasing but variable rates of nitrification, depending on other factors such as disturbance or species composition.