Belowground carbon allocation in an African forest with a history of different management practices

Abstract

Belowground carbon allocation (BCA) is one of the most important and dynamic components of the forest carbon cycle, but remains the least studied, especially in African secondary and primary forests. This study investigated BCA elements (standing root crop biomass and fine root productivity) in two forest structures (logged and unlogged, and consequently secondary and primary forest structures respectively) of Budongo Central Forest Reserve, using the ingrowth core method in the top 30 cm of soil. The secondary forest structure allocated proportionally more standing root crop biomass to coarse roots than the primary forest structure. Standing root biomass did not significantly differ in the primary and secondary forest structure across the fine, coarse and total standing root crop biomass. For the three years of observation, over 96% of the productivity was allocated to fine roots in both forest structures. Fine root annual net primary productivity was 1.94±0.60 and 1.32±0.31 Mg C ha-1 yr-1 for the primary and secondary forest structures respectively, and significantly higher in the primary than in the secondary forest structure. Overall, there was no difference in fineroot quarterly productivity between the two forest structures. There was a significant seasonal shift, and an interaction between seasons and forest structure. Fine root productivity in both forest structures declined in the dry season and was higher in the wet season, but a much higher increase was found in the primary forest structure. A tendency for increased fine root productivity after a dry season is seen in both structures, with the highest quarterly mean productivity seen in the dry season for the secondary forest structure. The overall pattern of annual productivity had a cyclic tendency, suggesting circadian control, but with this pattern probably disrupted by moisture deficiency. It is postulated that the secondary forest structure productivity could be limited by competition, based on its structural characteristics, relative to the primary forest structure.