| dc.description.abstracteng | The combined effects of global warming and land-use conversion to human-modified systems are threatening biodiversity and ecosystem processes maintained by tropical mountains. To assess and predict the impact of global change on these ecosystems, it is crucial to understand the drivers and mechanisms of biodiversity and ecosystem processes. For instance, the study of carbon (C) and nitrogen (N) cycles is of key importance, as they encompass fundamental ecosystem processes such as carbon sequestration and storage, fluxes of C and N among ecosystem components as well as soil nitrogen turnover, which influence the performance of plant species and activity of soil microorganisms. Fine roots play a major role for most of the abovementioned ecosystem processes, as they represent the plant-soil interface. They are also essential plant organs for water and nutrient uptake. Thus, the study of fine roots’ carbon economy contributes to the understanding of plant strategies in resource limiting environments and their role in ecosystem processes. Mount Kilimanjaro, in northern Tanzania, with its many different ecosystems (both natural and human-modified) across the large elevational gradient, represents a good opportunity to study the effects of climate and land use on the fine root system, as well as on below- and aboveground NPP relations and carbon allocation patterns. Along the present investigation, fine root bio- and necromass, fine root dynamics and fine root morphological traits across elevation and in different land-use systems were estimated. In addition, above and belowground NPP of woody plants along the elevation was quantified and carbon allocation patterns were assessed. Finally, the spatial vegetation heterogeneity of two contrasting natural ecosystems provided the opportunity to assess the strong link between plants and soil microorganisms by studying the effects of vegetation on belowground processes.
In the first study, fine root biomass and dynamics along the entire elevational gradient were investigated to assess plant carbon investment strategies to adapt to different environmental conditions. In addition, focusing on the tropical montane forest, the effects of elevation and associated biotic and abiotic factors on the fine root system were determined and the existence of a root economic spectrum (RES) was assessed. Ecosystems with pronounced resource limitation (savanna: water limitation, alpine heathland: N limitation) showed much higher root: shoot ratios (fine root biomass and production related to aboveground biomass) compared to tropical montane forest ecosystems. Moreover, the root: shoot ratio in the tropical montane forest increased exponentially with elevation but decreased with precipitation and soil nitrogen availability. The variation in root traits across the elevation gradient fits well within the concept of a multi-dimensional RES. In addition, the species identity of the dominant species had a strong effect on the properties of the fine root system. In conclusion, a general belowground shift in carbohydrate partitioning is evident across the elevation in the tropical montane forest, suggesting that plant growth is increasingly limited by nutrient (probably N) shortages towards higher elevations.
In the second study, we aim to broaden our understanding of the effects of elevation on the carbon economy of plants from fine roots to the aboveground components. Focusing on the tropical montane forest, NPP above- and belowground was quantified, carbon allocation patterns were assessed and C and N return to the soil via leaf and fine root litter across elevation was determined. Total NPP-C declined and carbon allocation from above- to belowground tree organs showed a marked shift with increasing elevation. The C and N fluxes to the soil via leaf and fine root litter also diminished along the slope. These findings suggest that the decrease of total NPP across elevation is caused by decreasing carbon gain due to a lower leaf area index towards the subalpine Erica forest. This fact is consequence of increasing N limiting conditions at high elevations. The shift of carbon allocation from above-to belowground tree organs might contribute to acquire the limiting nutrients in these harsh environmental conditions.
In the third study, the effects of land-use change and ecosystem disturbance on fine root bio- and necromass, dynamics, morphological and chemical traits, as well as on the C and N fluxes to the soil via fine root litter were addressed. We found a consistent decrease of nearly all investigated variables with land-use change and disturbance. However, the traditional agroforestry systems (“Chagga homegardens”) maintained similar values as the natural montane forest for some of the fine root properties (e.g. stand fine root production, fine root litter quality) and outstanded for being a high dynamic ecosystem. Podocarpus forest disturbed by fire showed a markedly strong decline of C and N return to the soil via fine root mortality. These results indicate a modification of the fine root C stocks and the C and N supply to the soil from root litter decay with land-use change, which strongly affects the ecosystems' C and N cycle.
In the fourth and fifth studies, the vegetation effects on belowground processes (gross N turnover rates, soil C sequestration, greenhouse gas (GHG) fluxes) in ecosystems with strong harsh environmental conditions and patchy vegetation were determined. In the alpine Helichrysum heathland, gross N mineralization, NH4+ immobilization rates and CO2 emissions were significantly higher on high-covered vegetation plots than on low-covered plots. Gross N turnover increased with vegetation cover, and thus, with supply of plant litter for the microbial community. The high relative soil N retention indicates high competition for N availability in the soil between microbes and plants and a tight N cycle dominated by tightly coupled ammonification-NH4+- immobilization in Helichrysum heathlands. In savanna woodlands, spatial trends (from the tree crown into the open grassland) in soil properties and GHG fluxes and related above- and belowground processes and attributes were determined. Higher soil fertility, soil C and N content, microbial biomass and fine root density were found under the crown, whereas soil respiration rates, microbial and plant litter C:N were higher in open grasslands. Tree leaf litter held lower C:N than C4 grass litter and contributed 15% of SOM. These patterns suggest that in the open grassland, high microbial competition, together with low substrate C:N from C4 grasses lead to a low carbon use efficiency of soil microbial communities and a higher soil respiration. Hence, the spatial structure of the vegetation in savanna ecosystems results in a spatial redistribution of nutrients and thus in C mineralization and sequestration.
The present investigation contributes to a better comprehension of the effects of climate on woody plants carbon economy, with special attention on the role of the fine roots. In addition, it highlights the impacts of land-use change and disturbance on the fine root system and related carbon and nitrogen fluxes to the soil. Further, the acting mechanisms of the vegetation as a driver of belowground processes were determined. Finally, we highlight the importance of including fine root data in carbon studies in order to develop more accurate terrestrial ecosystem models, to better understand ecosystem functioning and to be able to predict ecosystems responses to disturbances. | de |