Aboveground net primary productivity and littermass estimates
We estimated aboveground net primary productivity (ANPP) for each plot by scaling tree diameter (diameter at breast height, DBH) to total aboveground biomass using species-specific allometric equations and calculating annual increment growth. ANPP includes all aboveground wood and non-woody production. Specifically, for plots within the ForestGEO forest dynamics plots where long-term tree census data are available, we calculated DBH growth using the two most recent censuses. This included all plots at SCBI (2013, 2018 census), SERC (2014, 2019 census), TRC (2013, 2019 census) and WLF (2013, 2018 census), as well as three HF plots (2013, 2019 census). For plots outside the forest dynamics plots, we measured DBH growth over one year (in 2019 for HF, in 2018 for LDW). In all cases, only trees alive and > 10 cm DBH in both censuses were included. In doing so, we recognize we are ignoring any new recruitment or death between measurements. DBH measurements were converted to total aboveground biomass using species-specific allometric equations following Gonzalez-Akre et al. (2016) or, when not available, from the allodb R package (https://github.com/forestgeo/allodb, Gonzalez-Akre, pers. comm) Equations are provided in SI Table 1. Annual increment growth of aboveground biomass C (g C m2yr-1) was then calculated for each tree using a 0.47 conversion factor for biomass to C and summed at the plot-level to estimate annual aboveground net primary productivity (ANPP).
Plot-specific litter mass was measured in 2017 at all plots. Specifically, two to three litter collectors were placed in each plot, covering a total average area of ~0.5-1.0 m-2 per plot. All litter, including leaves, woody debris and reproductive material, was collected two to three times over the course of senescence. Litter was dried at 60 °C for at least 72 hours and then weighed to 0.01g. Plot litter mass C inputs were calculated as the total mass of all litter collected in a given plot in 2017 and converted to units C using a 0.47 conversion factor for biomass to C.