| dc.description.abstracteng | Globally rising demand for cash crops such as oil palm, rubber, and cacao is driving the extensive conversion of natural rainforests in Southeast Asia with increasing speed. The consequences for humans and nature are immense and until today not thoroughly investigated.
In the present thesis the impact of natural rainforest conversion into intensively managed rubber and oil palm monocultures, and smallholder rubber and cacao agroforests on carbon storage and sequestration, litter dynamics and hydraulic anatomic properties in Indonesia was assessed and quantified. To fulfill the objectives, carbon and nutrient dynamics in phytomass of lowland rainforest transformation systems in two regions of Jambi Province on Sumatra were studied, while estimators of tree productivity and hydraulic anatomy patterns were investigated in cacao agroforests in Central Sulawesi.
The transformation of rainforest in our study area clearly resulted in significantly lower total carbon stocks in all investigated land-use types. According to our estimations, natural forests in this region store three times more carbon in biomass than even the extensively utilized jungle rubber system. The total carbon pools comprising aboveground tree biomass, dead wood debris as well as coarse and fine roots in rubber and oil palm monocultures consist of even less than 20 % of the natural forest C stocks they are replacing. Thus, compensating for the emitted carbon caused by land-use change on mineral soils may take hundreds of years or even be irreversible.
While oil palm monocultures showed the lowest carbon stock, we have measured the highest total net primary production (NPP) values in oil palm plantations followed by natural forest, jungle rubber and rubber monocultures. However, the carbon sequestration potential is reduced by forest conversion in the long term, as removal of major NPP fractions namely oil palm fruits, rubber latex and wood occurs. Since we found the carbon residence time in biomass to be longer in natural forest and jungle rubber, the establishment of oil palm or rubber plantations on former forest land does not enhance carbon sequestration potential despite the high total net primary production of oil palms. With predicted climate change associated with increasing mean temperatures and drought events, land-use intensification with introduced cash-crops is also expected to have influence on seasonal growth responses in moist tropical lowlands even with only moderately dry seasons. We found a higher coefficient of variance (CV) following forest transformation for all examined components namely aboveground litterfall, fine root mortality, and aboveground woody production.
In addition to changes in the annual net primary production, element return to the soil via aboveground litterfall was significantly reduced for C, N, Ca, Fe, Mg, Mn and S in monoculture plantations, particularly for rubber. Due to crop removal, a high demand for fertilizer use was caused, which likely leaded to significantly lower nutrient use efficiencies (NUE) for all major nutrients in oil palm plantations. Decomposition rate was reduced in oil palm plantations compared to natural forest which further decelerated nutrient cycling.
Beside nutrient availability, a permanent water supply from roots to leaves is essential for effective plant growth. We found wood anatomical and derived hydraulic properties to be a good predictor for tree stem growth performance in the study on hydraulic architecture of the root, stem and branch wood in Theobroma cacao and five common shade tree species in agroforestry systems on Sulawesi. In contrast, neither wood density, nor empirically measured branch and root hydraulic conductivity, foliar delta 13C or foliar nitrogen content were good predictors for aboveground growth performance. A similar relationship was found between basal area increment and theoretical hydraulic conductivity in forest and rubber trees on Sumatra. Several wood anatomical patterns were observed to potentially follow drought adaptations strategies based on biogeographic origin. Hence, our results imply that future research on conceptual trade-offs of tree hydraulic architecture should consider that some of the long-established paradigms might not be uniformly applicable to all tree species.
Overall, the results of our study suggest that where natural forest cannot be conserved, agroforestry is the better land-use option in terms of long-term carbon storage and nutrient sustainability compared to rubber and oil palm monocultures. On the other hand, higher yields of monocultures and thus a potentially better income are decreasing the attractiveness of agroforests for local owners. Further depletion of carbon storage potential in the tropical lowlands may only be mitigated in cooperation with political and local institutions in the long term. Therefore, scientific evidence on the ecological costs and benefits of land-use change like presented in this study is aimed to guide political decision-making towards a more sustainable path in forest conservation and land management. | de |