Relationships between Forest Productivity and Water Yield Across Virginia's Forests

Abstract

Landowners and managers are often incentivized solely for maximized forest productivity (i.e., increasing biomass or carbon); however, maximizing forest productivity can result in a decline in downstream water resources, highlighting the need for a better understanding of such tradeoffs to inform incentive programs aimed at influencing land management decisions. In this study, we use remotely sensed products to quantify forest productivity and water yield across the state of Virginia and analyze the relationships between them at differing spatial scales, across physiographic regions, and by forest cover type. Forest cover was delineated using annual National Land Cover Database (NLCD) land cover products. Net primary productivity, or carbon allocated to plant biomass after losses from respiration, was acquired using the MOD17 algorithm applied to Landsat (30-m resolution) and MODIS (250-m resolution) imagery. Water yield (WY), defined as precipitation minus evapotranspiration, was modeled using Mapping EvapoTranspiration at high Resolution with Internalized Calibration (METRIC) along with PRISM precipitation data. Annual rates of WY and NPP were weakly correlated for both Landsat (ρ = 0.095) and MODIS (ρ = -0.088) ; however, ΔNPP and ΔWY, or the change in pixel-level annual WY and NPP over time, were more strongly and negatively correlated for both Landsat (ρ = -0.35) and MODIS (ρ = -0.5), suggesting strong tradeoffs. When observed by physiographic region, tradeoffs were most prominent in the Valley and Ridge and Blue Ridge. Comparing ΔNPP versus ΔWY rates, while excluding stand-varying environmental controls associated with annual rates, better isolates this tradeoff. Leveraging available remotely sensed information on forest productivity and water yield tradeoffs can better guide management decisions and better inform the provisioning of water yield in relation to other ecosystem services more commonly correlated with forest productivity.