Patterns in Foliar Isotopic Nitrogen, Percent Nitrogen, and Site Index for Managed Forest Systems in the United States

Patterns in foliar nitrogen (N) stable isotope ratios (delta N-15) have been shown to reveal trends in terrestrial N cycles, including the identification of ecosystems where N deficiencies limit forest ecosystem productivity. However, there is a gap in our understanding of within-species variation and species-level response to environmental gradients or forest management. Our objective is to examine the relationship between site index, foliar %N, foliar delta N-15 and spectral reflectance for managed Douglas-fir (Pseudotsuga menziesii) and loblolly pine (Pinus taeda) plantations across their geographic ranges in the Pacific Northwest and the southeastern United States, respectively. Foliage was measured at 28 sites for reflectance using a handheld spectroradiometer, and further analyzed for delta N-15 and N concentration. Unlike the prior work for grasslands and shrubland species, our results show that foliar delta N-15 and foliar %N are not well correlated for these tree species. However, multiple linear regression models suggest a strong predictive ability of spectroscopy data to quantify foliar delta N-15, with some models explaining more than 65% of the variance in the delta N-15. Additionally, moderate to strong explanations of variance were found between site index and foliar delta N-15 (R-2 = 0.49) and reflectance and site index (R-2 = 0.84) in the Douglas-fir data set. The development of relationships between foliar spectral reflectance, delta N-15 and measures of site productivity provides the first step toward mapping canopy delta N-15 for these managed forests with remote sensing.