REGIONAL VARIATION IN FOLIAR CHEMISTRY AND N CYCLING AMONG FORESTS OF DIVERSE HISTORY AND COMPOSITION

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Abstract

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 net 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.

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