Forest carbon trends 2002-2013 at Mt. Elgon, Uganda

Abstract

Previous research based on satellite imagery analysis demonstrated that in the last 35 years about 35% of Mt. Elgon’s Afromontane rainforests have been deforested. Although the tide slowly seems to change, less visible but large scale forest degradation continues. To asses degradation, the forest’s tree biomass carbon stock was measured in 2002 and in 2013 on 44 permanent nested plots. A dataset of 1437 trees was obtained with 387 repeated-, and 643 first- measured trees in 2013 and 407 disappeared trees between ‘02 and ‘13. Carbon stock calculations (using the allometric formula of Chave et al. (2005)), indicate an overall median increase with 17.98 } 12.42 Mg C ha-1 or 1.6 } 1.1 Mg C yr-1 ha-1 of the median carbon stock in 2002: 32.46 } 22.75 Mg C ha-1 to the median carbon stock in 2013: 45.79 } 16.97 Mg C ha-1. Over the 11 years carbon stocks at plot level gained up to 81.32 Mg C ha-1 or lost up to 80.18 Mg C ha-1, evidence of ongoing degradation and recovery processes. Stocks also varied greatly from 16.47 to 279.87 Mg C ha-1. To identify which factors could influence degradation or recovery, plots were clustered based on their 2013 carbon stock and ’02-’13 carbon flux. Although clusters were only statistically different in their carbon stocks, fluxes and average amounts of lost trees, trends in the plot’s other variables could be noticed. These were further investigated by correlation analysis. Clusters and carbon stocks are significantly correlated with altitude differences between-, and distances and walking times from-, the plots and the nearest park border. The ease at which a plot can be reached by local encroachers thus likely influences its degradation or recovery. Since also the plot’s altitudes are positively correlated with their carbon stocks, disturbance effects seem to obfuscate natural trends of decreasing carbon stocks with altitude, expectable at up to 500 m under the tree line. Significant correlations between the area and the plot’s disturbance and carbon stocks suggest that also local socio-economic settings outside the park’s borders and the effectiveness of local ranger activities play a role. Significant correlations between lost trees, stumps and ranger camp zones further hint at links with ranger activity.status: publishe