Changes in Foliar Nutrient Content throughout Secondary Succession in a Mid-Atlantic Temperate Forest Ecosystem

Abstract:

In order to elucidate how plant-available nutrients control vegetation transitions and primary productivity throughout secondary succession, we examined foliar nitrogen (N) dynamics over successional time, with temporal and spatial covariates of growing season month and leaf canopy position, respectively. We incorporate leaf spectral characteristics into our analysis of leaf nutrient status to investigate the potential for remote sensing of large-scale ecosystem foliar nutrient dynamics. We collected foliar samples from two successional chronosequences at Blandy Experimental Farm in northwestern Virginia, each with fields in three stages of development following agricultural abandonment: early, mid, and late successional (~15, 30, and 100+ years old, respectively). Leaves were collected from 3-4 dominant species in each successional stage at three times throughout the growing season in 2018 (shade leaves) and 2019 (sun leaves). Hyperspectral reflectance between 400 and 1100 nm was recorded for each leaf before analysis of total carbon (C) and N. While percent N of shade leaves increased significantly through successional time, there were no changes for sun leaves. Additionally, shade leaf N has greater temporal variability throughout the growing season than sun leaf N. Variable importance in projection (VIP) values calculated using partial least squares regression (PLSR) indicate several spectral features that are highly correlated with foliar N, including reflectance at ~550 nm (chlorophyll) and reflectance between 700-750 nm (red edge). Additional spectral analysis indicates a strong correlation between the red edge point of inflection and foliar percent N, with a slightly stronger relationship in shade leaves than in sun leaves. The weaker relationships between foliar N of sun leaves and spectral features have implications for the use of large-scale aerial remote sensing, as above-canopy sensing largely captures information on sun-exposed leaves. An understanding of the variance in leaf chemistry over successional time is a critical piece to understanding how total ecosystem nutrient cycling changes throughout succession, and the ability to connect foliar reflectance with nutrient concentrations could help to achieve this over broad spatial scales with the use of remote sensing.