The importance of vegetation functional composition in mediating climate change impacts on ecosystem carbon dynamics in alpine grasslands

Climate is changing around the world, and because temperature and water are key drivers of many ecosystem processes this is expected to have significant effects on ecosystem processes and functioning, including ecosystem carbon cycling. In addition to the direct effects of increased temperature and changes is precipitation, indirect effects of climate-induced shifts in plant dominance can affect ecosystems and their functioning through a complex series of biotic cascades, couplings, and feedbacks (Wookey et al 2009). Alpine ecosystems in particular are expected to be strongly impacted by global warming because of the high temperature-sensitivity of biological and chemical processes and their vulnerability to vegetation shifts. In this thesis, I investigate the direct and indirect effects of climate change on ecosystem carbon dynamics in semi-natural alpine grasslands. The study design makes use of a systematic climate grid in Western Norway that consists of twelve semi-natural grassland selected along natural climate gradients, where temperature and precipitation vary independently. At each site we performed a fully factorial removal experiment, removing different plant functional groups (graminoids, forbs, bryophytes), to determine their effect on ecosystem carbon cycling and soil physical conditions. In addition, several plant functional traits were measured at each site to assess their contribution in determining ecosystem carbon exchange compared to climate and vegetation structure characteristics. I used a static chamber method to measure ecosystem carbon flux and estimate net ecosystem exchange (NEE), gross primary production (GPP) and ecosystem respiration (Reco) across the climatic gradients and removal experiment. Furthermore, I performed a standardized litter bag experiment to investigate the short-term direct effect of annual variability in temperature and precipitation and long-term indirect effect of climate variability along the natural climatic gradients on litter decomposition. The presence and functional composition of vegetation regulated soil temperature and to an extent soil moisture, which are key controls of ecosystem processes. Vegetation cover reduced maximum soil temperature due to the vegetation’s insulating capacity or shading. The strength of this effect depended on vegetation structure, plant functional group cover and vascular and non-vascular vegetation height. Bryophytes had a larger effect on soil temperature than forbs or graminoids, and increased depth of bryophyte mat strengthened the insulating effect of bryophytes. Soil moisture was primarily determined by the amount of precipitation received by a research site. Functional attributes of vegetation will therefore influence ecosystem processes like plant growth and decomposition through their regulating effect on soil temperature and thus influence ecosystem carbon cycling. Gross primary production was largely determined by vascular plant biomass, while respiration was primarily controlled by temperature and was little influenced by biomass of vascular plants. Bryophytes did not have a significant effect on either gross primary production or ecosystem respiration. Compensation of gross primary production after plant functional group loss was dependent on remaining plant functional groups and their interaction, which again was dependent on climate. In alpine sites, compensation capacity of forbs was stimulated when bryophytes were present, while in boreal sites compensation capacity of gramininoids seemed to be limited by bryophytes. For ecosystem respiration there was no difference in compensation capacity between plant functional groups nor effects of climate. We assessed the value of using plant functional traits for predictions of ecosystem C flux in relation to climate change. Climatic effects on gross primary production were related to changes in vegetation structure and plant functional traits, particularly a shift in traits of plant communities from tall, fast-growing species with big, thin leaves and low C:N in warmer drier sites to communities with lower growth, small and thicker leaves and higher leaf C:N cold sites. Plant functional traits were also able to capture additional between-site variation in ecosystem carbon exchange not related to climate, and could even account for appreciable amounts of variability at the within-site scale, which is likely related to smaller-scale driver of vegetation community composition such as topography and soil characteristics. The decomposition experiment revealed that direct effect of annual variation in temperature and precipitation on decomposition processes are modulated by environmental conditions, including plant diversity. Increasing temperature enhanced decomposition rate k and litter stabilization factor S within each climate regime, while this effect was not found across the different climate regimes for k and even had the reverse effect on S, as S decreased with temperature across climate regimes. Increased precipitation reduced k within and across climatic regimes, while increased precipitation decreased S in sub-alpine and alpine sites, but not boreal sites. We speculate that the differences in decomposition between climate regimes can related to differences in microbial community composition and soil structure. Altogether, this thesis highlight the importance of local environmental conditions and the functional composition of vegetation as modulators of climate change impacts on ecosystem carbon dynamics. This knowledge improves our understanding of how climate-induced changes in the functional composition of vegetation can affect ecosystem carbon cycling, and can possibly help improve predictability of ecosystem carbon exchange under global warming