Aboveground biomass and carbon pool estimates of Portulacaria afra (spekboom)-rich subtropical thicket with species-specific allometric models

Abstract

Accurate biomass and carbon flux estimations are essential for global carbon cycle modeling and are useful for setting and evaluating ecological restoration goals. Estimates derived from remote sensing can only be as good as the field data with which they are calibrated. Biomass estimates derived from allometric models are widely used to extrapolate from a plot to a landscape or regional scale, but both species- and/or site- specific models are scarce, and the model development procedure destructive and labour-intensive. Here we present allometric models that estimate biomass for 40 species common in Spekboom Thicket (ST), an arid South African Subtropical Thicket type, and its ecotones. Portulacaria afra (spekboom) is a canopy cover dominant in intact states of ST. Using these simple regression models, based on plant height and canopy diameter parameters, we estimate the aboveground biomass carbon (ABC) and with litter, the total aboveground carbon (AGC) of five sites situated across the Subtropical Thicket biome in South Africa. On each site, adjacent stands were selected that compared two management histories, intact and livestock-degraded, divided by a fence line boundary. Our estimates compared well with results from other studies. The highest AGC for stands in both intact and degraded states were estimated at 43.0 t C ha−1 and 13.0 t C ha−1 respectively, and the lowest at 26.3 t C ha−1 and 2.5 t C ha−1. Large canopy dominant (LCD) species contributed the largest AGC portion at three intact stands. The second largest portion was recorded for spekboom. Overall, the sites to the northeast of the biome recorded the highest AGC pools, consistent with a higher incidence of rainfall, especially in the summer months. In contrast, sites to the west of the biome showed higher mean litter carbon pools. Should intact sites be applied as reference, livestock-degraded subtropical thicket can potentially recover 22 – 27 t C ha−1 in AGC through restoration action.