Buntrock, LauraThomas, Valerie A.Strahm, Brian D.Fox, TomHarrison, RobertHimes, AustinLittke, Kim2022-10-262022-10-262022-10-15Buntrock, L.; Thomas, V.A.; Strahm, B.D.; Fox, T.; Harrison, R.; Himes, A.; Littke, K. Patterns in Foliar Isotopic Nitrogen, Percent Nitrogen, and Site Index for Managed Forest Systems in the United States. Forests 2022, 13, 1694.http://hdl.handle.net/10919/112289Patterns in foliar nitrogen (N) stable isotope ratios (δ<sup>15</sup>N) 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 δ<sup>15</sup>N and spectral reflectance for managed Douglas-fir (<i>Pseudotsuga menziesii</i>) and loblolly pine (<i>Pinus taeda</i>) 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 δ<sup>15</sup>N and N concentration. Unlike the prior work for grasslands and shrubland species, our results show that foliar δ<sup>15</sup>N 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 δ<sup>15</sup>N, with some models explaining more than 65% of the variance in the δ<sup>15</sup>N. Additionally, moderate to strong explanations of variance were found between site index and foliar δ<sup>15</sup>N (<i>R</i><sup>2</sup> = 0.49) and reflectance and site index (<i>R</i><sup>2</sup> = 0.84) in the Douglas-fir data set. The development of relationships between foliar spectral reflectance, δ<sup>15</sup>N and measures of site productivity provides the first step toward mapping canopy δ<sup>15</sup>N for these managed forests with remote sensing.application/pdfenCreative Commons Attribution 4.0 Internationalproductivityloblolly pineDouglas-firspectral reflectancespectroscopyhyperspectralPatterns in Foliar Isotopic Nitrogen, Percent Nitrogen, and Site Index for Managed Forest Systems in the United StatesArticle - Refereed2022-10-26Forestshttps://doi.org/10.3390/f13101694