(Table 1) Climate characteristics of the four North American Tundra Experiment (ITEX) sites

Climate warming is expected to differentially affect CO2 exchange of the diverse ecosystems in the Arctic. Quantifying responses of CO2 exchange to warming in these ecosystems will require coordinated experimentation using standard temperature manipulations and measurements. Here, we used the International Tundra Experiment (ITEX) standard warming treatment to determine CO2 flux responses to growing-season warming for ecosystems spanning natural temperature and moisture ranges across the Arctic biome. We used the four North American Arctic ITEX sites (Toolik Lake, Atqasuk, and Barrow [USA] and Alexandra Fiord [Canada]) that span 10° of latitude. At each site, we investigated the CO2 responses to warming in both dry and wet or moist ecosystems. Net ecosystem CO2 exchange (NEE), ecosystem respiration (ER), and gross ecosystem photosynthesis (GEP) were assessed using chamber techniques conducted over 24-h periods sampled regularly throughout the summers of two years at all sites. At Toolik Lake, warming increased net CO2 losses in both moist and dry ecosystems. In contrast, at Atqasuk and Barrow, warming increased net CO2 uptake in wet ecosystems but increased losses from dry ecosystems. At Alexandra Fiord, warming improved net carbon uptake in the moist ecosystem in both years, but in the wet and dry ecosystems uptake increased in one year and decreased the other. Warming generally increased ER, with the largest increases in dry ecosystems. In wet ecosystems, high soil moisture limited increases in respiration relative to increases in photosynthesis. Warming generally increased GEP, with the notable exception of the Toolik Lake moist ecosystem, where warming unexpectedly decreased GEP >25%. Overall, the respiration response determined the effect of warming on ecosystem CO2 balance. Our results provide the first multiple-site comparison of arctic tundra CO2 flux responses to standard warming treatments across a large climate gradient. These results indicate that (1) dry tundra may be initially the most responsive ecosystems to climate warming by virtue of strong increases in ER, (2) moist and wet tundra responses are dampened by higher water tables and soil water contents, and (3) both GEP and ER are responsive to climate warming, but the magnitudes and directions are ecosystem-dependent.

Table 1: Main characteristics of the BIOTA observatories included in this study & Table 3: Occurrence of termite taxa at the observatories

Termites are the most important soil ecosystem engineers of semi-arid and arid habitats. They enhance decomposition processes as well as the subsequent mineralisation of nutrients by bacteria and fungi. Through their construction of galleries, nests and mounds, they promote soil turnover and influence the distribution of nutrients and also alter texture and hydrological properties of soils, thereby affecting the heterogeneity of their ecosystem. The main aim of the present thesis was to define the impact of termites on ecosys-tem functioning in a semi-arid ecosystem. In a baseline study, I assessed the diversity of termite taxa in relation to the amount of precipitation, the vegetation patterns and the land use systems at several sites in Namibia. Subsequently, I focussed on a species that is highly abundant in many African savannas, the fungus growing and mound building species Macro-termes michaelseni (Sjöstedt, 1914). I asked how this species influences the spatial hetero-geneity of soil and vegetation patterns. From repeated samplings at 13 sites in Namibia, I obtained 17 termite taxa of 15 genera. While the type of land use seems to have a minor effect on the termite fauna, the mean annual precipitation explained 96% and the Simpson index of vascular plant diversity 81% of the variation in taxa diversity. The number of termite taxa increased with both of these explanation variables. In contrast to former studies on Macrotermes mounds in several regions of Africa that I reviewed, soil analyses from M. michaelseni mounds in the central Namibian savanna revealed that they contain much higher nitrogen contents when compared to their parent material. Further analyses revealed that nitrate forms a major component of the nitrogen content in termite mounds. As nitrate solves easily in water, evaporation processes are most probably responsible for the transport of solved nitrates to the mound surface and their accumulation there. The analysed mounds in central Namibia contained higher sand propor-tions compared to the mounds of the former studies. Through the higher percentage of coarse and middle sized pores, water moves more easily in sandy soils compared to more clayey soils. In consequence, evaporation-driven nitrate accumulation can occur in the studied mounds at high rates. ff...

Seawater carbonate chemistry, structural and chemical characteristics of the seagrass Thalassia testudinum in an situ CO2 enrichment experiment

Seagrasses commonly display carbon-limited photosynthetic rates. Thus, increases in atmospheric pCO2, and consequentially oceanic CO2(aq) concentrations, may prove beneficial. While addressed in mesocosms, these hypotheses have not been tested in the field with manipulative experimentation. This study examines the effects of in situ CO2(aq) enrichment on the structural and chemical characteristics of the tropical seagrass, Thalassia testudinum. CO2(aq) availability was manipulated for 6 months in clear, open-top chambers within a shallow seagrass meadow in the Florida Keys (USA), reproducing forecasts for the year 2100. Structural characteristics (leaf area, leaf growth, shoot mass, and shoot density) were unresponsive to CO2(aq) enrichment. However, leaf nitrogen and phosphorus content declined on average by 11 and 21 %, respectively. Belowground, non-structural carbohydrates increased by 29 %. These results indicate that increased CO2(aq) availability may primarily alter the chemical composition of seagrasses, influencing both the nutrient status and resilience of these systems.