Associate Professor Baruch Institute of Coastal Ecology and Forest Science - Clemson University Georgetown, South Carolina, United States
Abstract: Incoming solar radiation is one of the primary drivers of ecosystem level gross primary productivity (GPP) and evapotranspiration (ET), but light quality (diffuse vs. direct) also affects light and water use efficiencies (LUE, WUE). The amount of carbon sequestered per unit of incoming light or transpired water has been shown to increase under cloudy or hazy conditions. In clear conditions canopy leaves are often light saturated to the point of physiological stress, reducing LUE and WUE, while simultaneously the shaded leaves are receiving suboptimal levels of photosynthetically active radiation (PAR). Conversely, under cloudy conditions or when light scattering aerosols are present the incoming PAR is more evenly dispersed. This allows radiation to penetrate deeper into stratified foliage, resulting in an overall increase in LUE and WUE. Here we have used a combination of global sensitivity and generalized additive model (GAM) statistical analyses on 17 site-years of eddy covariance and diffuse light observations to determine the independent and joint effects of the potential mechanisms driving this diffuse fertilization effect in Southern pine ecosystems. We analyzed observations from two lobollly pine (Pinus taeda) dominant sites in Virginia and two longleaf pine (Pinus palustris) dominant sites in South Carolina. The colocated ecosystems have different stand ages, with one early plantation and one late mature stand in each state. Our analyses evaluate whether changes in LUE and WUE reflect conditions where the amount of absorbed PAR is sensitive to diffuse light, and whether this sensitivity varies with other environmental variables known to influence GPP, such as vapor pressure deficit and air temperature. Additionally, studies in other ecosystems have found that the influence of diffuse light on GPP can covary with vegetation phenology obscuring the actual contribution, but our results indicate that seasonal phenology, as observed by remotely-sensed vegetation indices, had far less influence in Southern pines. Southern pine forests have been largely overlooked in studies of diffuse fertilization, despite the fact that nearly one-third of forested land in the United States can be classified as such. Since this ecoregion contributes significantly to the national scale terrestrial carbon sink and experiences high aerosol loading and cloudiness, increasing understanding of diffuse light fertilization could improve our ability to predict the climate mitigation potential of Southeastern forests.
