A feasibility study for determining the mean annual aboveground biomass gain of tropical seasonal forests in Cambodia

To study the feasibility of determining the mean annual aboveground biomass gain of tropical seasonal forests at the national scale, we estimated the gain (i.e., increase due to growth of living trees) and loss (i.e., decrease due to tree death) of aboveground tree stand biomass (stand AGB) using 49 permanent sample plots distributed nationwide for 139 observation periods from 2005 to 2015 in Cambodia. In a linear mixed-effects model, stand AGB gain was predicted to increase with the initial stand AGB: Stand AGB gain=0.0165 Stand AGB+2.20 (n=139, P<0.0001, R=0.4531, RMSE=2.84), where Stand AGB gain is the sum of tree AGB growth (Mg ha year), and Stand AGB is the sum of initial tree AGBs (Mg ha). The mean estimated stand AGB gain was 4.79 Mg ha year for an average initial stand AGB of 155.5 Mg ha. The annual stand AGB loss was<20 % of the initial stand AGB and the influence of stand AGB loss on stand AGB gain was negligible. The 2006 IPCC Guidelines for National Greenhouse Gas Inventories assigned stand-age-dependent values of default annual stand AGB gain for tropical natural forests. However, age is difficult to determine in tropical trees. Our stand-age-independent approach based on the stand AGB offers a practical method for assessing the AGB gain of tropical natural forests.