Land-use and climate impacts on drought resistance and resilience in coastal heathland ecosystems
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Background: Coastal heathlands of north-western Europe are semi-natural landscapes of high conservation value. These are ancient and historically widespread landscapes, but anthropogenic land-use and climate change are now increasingly threatening coastal heathland biodiversity, structure, and ecosystem functioning. Semi-natural ecosystems depend on management practices to maintain their ecosystem functioning. In coastal heathlands, prescribed burning and grazing have been key management tools for millennia, keeping the vegetation within alternating post-fire successional phases. Abandonment of these practices, leading to degraded heathlands, combined with agricultural intensification and infrastructure development, has reduced coastal heathland area in Norway to a tenth of its former distribution, paralleling heathland loss trends across their European distributional range. Recently, extreme drought events have led to severe diebacks of the key-stone species Calluna vulgaris in heathlands along the Norwegian coast, the northernmost distribution area of European coastal heathlands. These relatively short-term drought events dramatically alter ecosystem functions, and yet we know little of the system’s ability to recover from such events. Extreme climatic events, including droughts, are projected to increase along the Norwegian coast due to anthropogenic climate change, raising concerns about the resistance and resilience of coastal heathland ecosystems to intensified drought, and consequences for future heathland biodiversity and ecosystem functioning. Quantifying ecological drought and its impacts on ecosystems is challenging due to the intrinsic complexity of ecological and ecosystem responses to global changes. To date, most studies aiming at quantifying ecological response to drought are performed in dry climates, leaving a knowledge gap on how intensified drought affects precipitation-rich ecosystems. Moreover, few studies on ecosystem responses to climate change include the interaction with land-use changes, despite the latter being the main driver of biodiversity loss and habitat changes globally. Objectives: The objectives of this thesis are i) to assess how resistant Norwegian coastal heathland ecosystems are to experimental drought, ii) to discover how resilient these heathlands are in recovering ecosystem functioning after a major natural drought event and to what extent this is affected by prescribed burning, and iii) to contribute to the general understanding of ecological responses to increased frequency, duration and intensity of drought in a precipitation-rich ecosystem, in interaction with land-use change. Main results: I find that plant community dynamics and primary production in coastal heathlands are largely resistant to experimental drought. However, I also find that in the late post-fire successional phase, heathland ecosystem carbon dynamics are affected by experimental drought. A natural drought caused variable drought damage and mortality across the study sites, but mortality was highest in the northernmost sites. Whilst the recovery of Calluna in unburnt stands varied from high to very low over the study period, all sites show high recovery after fire. Overall, prescribed fire seems to be an efficient restoration tool in drought-damaged coastal heathland vegetation, regardless of pre-fire damage and recovery rate of unburnt stands. Conclusion: Intensified drought can alter ecosystem functions in coastal heathlands but predicting ecosystem responses and recovery remains difficult. Early post-fire successional heathlands seem to be more resistant to experimental drought than late successional heathlands. Ecological resilience after drought is dependent on more factors than this study uncovers, but prescribed fire seems to be an efficient restoration tool across ecological and climatic gradients. Consequently, continued traditional management can buffer impacts of climate change on the coastal heathlands of north-western Europe, and be an important tool in restoration.
Has partsPaper I: Haugum SV, Thorvaldsen P, Vandvik V, and Velle LG. Coastal heathland vegetation is surprisingly resistant to experimental drought across successional stages and latitude.Oikos; 130(11): 2015-2027. The article is available in the thesis file. The article is also available at: https://doi.org/10.1111/oik.08098
Paper II: Haugum SV, Christiansen CT, Vågenes A, Velle LG, Thorvaldsen P, and Vandvik V. Land-use management with prescribed burning stabilizes soil carbon pools under experimental drought in wet coastal heathlands. The article is not available in BORA.
Paper III: Velle LG, Haugum SV, Telford RJ, Thorvaldsen P, and Vandvik V. Lasting die-back of Atlantic heathlands after an extreme drought event: Can prescribed burning be a tool to promote recovery? The article is not available in BORA.
Paper IV: Haugum SV, Thorvaldsen P, Velle LG, Birkeli K, Thormodsæther R, Christiansen CT, Geange SR, Aamold D, Busca S, Garcia WFE, Grape VH, Guthu H, Mouton L, Nesheim-Hauge E, Pánková K, Sandsten H, Simonsen L, Swat M, Tjoflot H, Vangdal IT, Vestlie T, Vågenes A, and Vandvik V. A comprehensive multi-scale experiment in a coastal heathland: Plant and ecosystem responses to drought across three postfire successional phases and two latitudinal zones. The article is not available in BORA.