Climate-change effects on the Zambezi teak forests' productivity in Zambia

Ngoma, Justine


The study aimed at determining the effects of climate change on the productivity of the Zambezi Teak forests along a rainfall gradient in Zambia. To achieve this aim, six research questions guided my study: (1) Are biomass models available for the Zambezi teak forests and if so, what kind of data are needed to develop these models? (2) How are forests’ carbon stocks distributed in the wetter, intermediate and drier sites of the Zambezi teak forests? (3) How do contemporaneously and future climate affect the productivity of the forests in Africa? (4) What is the relationship between forest productivity and climatic variables in the wetter, intermediate and drier sites of the Zambezi teak forests? (5) Can the forests’ carbon stock realistically be reproduced for current climate conditions at the wetter, intermediate and drier sites? (6) How will climate change affect the productivity of the Zambezi teak forests in the wetter, intermediate and drier sites, and what are the main drivers of change? The study was carried out in the Zambian Zambezi teak forests across a south-north climatic gradient with annual rainfall ranging from 700mm in the south to 1100mm in the north. Following this climatic gradient, I conducted my study at the wetter Kabompo, intermediate Namwala and the drier Sesheke sites. At the Kabompo site, the study was carried out in Kabompo and Zambezi Forest Reserves, while the Ila Forest Reserve was surveyed at the Namwala site. I surveyed the Masese forest reserve at the Sesheke site. The aim was achieved by integrating information from biomass measurements, tree-ring analysis and dynamic vegetation modelling. Three above-ground biomass models, one below-ground biomass model and one stump model were developed. The power model fitted well and diameter at breast height (DBH, p < 0.0001) significantly affected tree biomass. Using the newly developed biomass models, I found that the forests’ carbon stock was highest at the wetter Kabompo site (36ton C ha¯¹), followed by the intermediate Namwala site (25ton C ha¯¹). The lowest carbon stock value was recorded at the drier Sesheke site (16ton C ha¯¹). A climate-to-growth study through the tree-rings of the dominant Baikiaea plurijuga species showed that evaporation had the highest influence on tree growth at all the three study sites compared to temperature and rainfall alone. Literature showed that from 1900 to 2011, mean annual Net Primary Productivity (NPP) increased by 4.8% in Africa, and from the year 1950, the combined NPP of all African biomes is projected to reduce by 8% by the end of the 21st century.

We used local soil parameter values to characterize texture and measured local tree parameter values for maximum crown area, wood density, leaf longevity and allometry, and LPJ-GUESS simulated forest carbon values were closest to the measured forest carbon stock values at the wetter Kabompo site, followed by the drier Sesheke site and then the intermediate Namwala site. NPP related positively with mean annual temperatures of current year, previous year and previous two years at all sites and projections showed that NPP would increase by 1.77% and 0.69% at the wetter Kabompo, and by 0.44% and 0.10% at the intermediate Namwala sites under Representative Concentration Pathways 8.5 (RCP8.5) and 4.5 (RCP4.5) respectively, especially caused by the increased carbon dioxide (CO₂ ) concentration by the end of the 21st century. However, at the drier Sesheke site, NPP would respectively decrease by 0.01% and 0.04% by the end of the 21st century under RCP8.5 and RCP4.5. The projected decreased NPP under RCP8.5 at the Sesheke site results from the reduced rainfall coupled with increasing temperature. This distinct response indicates that differences in the amount of rainfall received in a site per year influence the way in which the projected changes in climate and CO₂ will affect forests resources. The projected increase in CO₂ concentration would have more effects on NPP in high rainfall receiving areas, while in arid regions, NPP would be affected more by the changes in rainfall and temperature. CO₂ concentrations would therefore be more important in forests that are generally not temperature or precipitation limited, while precipitation will continue to be the limiting factor in the drier sites.