Anthropogenic activities are increasing atmospheric CO₂ concentrations and significantly altering the global carbon balance. Forest ecosystems occupy one third of the terrestrial surface of the earth, and store approximately 40% of terrestrial biosphere carbon. Despite the importance of forests in the global carbon cycle, the interactive effects of species diversity and elevated atmospheric CO₂ on temperate forest biogeochemistry and productivity remain poorly understood. This study utilised the Bangor Free Air CO₂ Enrichment (Bangor F ACE) facility to investigate how forest ecosystem biogeochemical cycling is altered by atmospheric CO₂ levels predicted for the year 2050. The experiment consisted of eight FACE rings in a 2 x 4 factorial design, four at ambient atmospheric CO₂ and four at elevated (580 µmol mor ^-1 ) atmospheric CO₂. To elucidate the interactive effects of species diversity and elevated atmospheric CO₂, three broadleaved tree species, birch (Betula pendula), alder (Alnus glutinosa) and beech (Fagus sylvatica) were planted in monoculture and three species polyculture within each experimental plot.
Elevated CO₂ enrichment increased above-ground biomass of all species in monoculture, whilst in polyculture the positive effect was approximately four-fold smaller. Conversely, fine root biomass in the elevated CO₂ plots and averaged across all species was two-fold greater in polyculture than in species grown singularly. Detailed examination of fine root morphology revealed a greater to elevated CO₂ response with increasing depth. The effect of growing trees in mixture was accessed by comparing measured data with data predicted from trees grown in monoculture. Above-ground overyielding of trees grown in polyculture was strongly enhanced under ambient conditions, whereas the overyielding response of trees grown in an enriched CO₂ atmosphere was reduced by a third. In contrast to the aboveground results, below ground overyielding was greater in the elevated CO₂ plots suggesting that carbon sequestration may be greater in diverse tree species communities as atmospheric CO₂ concentration increases. Community level physiological profiles were determined using sixteen different low molecular weight substrates. Microbial utilisation kinetics were determined and related to carbon use efficiency and ecosystem function. When species were grown in monoculture elevated CO₂ mediated an increase in catabolic respiration that increased with depth, and was attributed to deeper prolific rooting, and greater mycorrhizal mycelium inputs. Whereas, when species were grown in polyculture a decrease in catabolic respiration and increase in carbon residence time was apparent. Environmental data correlations suggest that
resource limitation may constrain microbial catabolic simulation in communities of higher diversity. Substrate utilisation profiles also indicated a shift in microbial structure and function to improve acquisition of nutrients such as P. P cycling was closely examined using the Hedley fractionation procedure. Elevated CO₂ induced a decrease in the labile P fractions, whilst soil organic P pools increased, and recalcitrant occluded P pools decreased throughout the experiment. A negative correlation with mineral P pool and fungal biomass suggested that enhanced mycorrhizal mediated mineral P dissolution may maintain ecosystem demand.
The results contained within this thesis provide some of the first clear evidence that tree biodiversity will not only mediate the aboveground response to high CO₂, but also make a major contribution to carbon input in to soils through the fine
root biomass in a CO₂ enriched atmosphere. Reductions of tree biodiversity may weaken the ability of forests to sequester carbon, potentially creating a feedback loop with global implications.