Coastal heathland vegetation is surprisingly resistant to experimental drought across successional stages and latitude

In the last decade, several major dwarf-shrub dieback events have occurred in northern European coastal heathlands. These dieback events occur after extended periods with sub-zero temperatures under snow-free conditions and clear skies, suggesting that coastal heathlands have low resistance to winter drought. As climate projections forecast increased drought frequency, intensity, and duration, coastal heathlands are likely to experience more such diebacks in the future. There are, however, few empirical studies of drought impacts and responses on plant communities in humid oceanic ecosystems. We established a drought experiment with two distinct levels of intensified drought to identify responses and thresholds of drought resistance in coastal heathland vegetation. We repeated the experiment in two regions, separated by five degrees latitude, to represent different bioclimatic conditions within the coastal heathlands' wide latitudinal range in Europe. As coastal heathlands are semi-natural habitats managed by prescribed fire, and we repeated the experiment across three post-fire successional phases within each region. Plant community structure, annual primary production, and primary and secondary growth of the dominant dwarf-shrub Calluna vulgaris varied between climate regions. To our surprise, these wide-ranging vegetation- and plant-level response variables were largely unaffected by the drought treatments. Consequently, our results suggest that northern, coastal heathland vegetation is relatively resistant to substantial intensification in drought. This experiment represents the world's wettest (2200 mm year−1) and northernmost (65°8'N) drought experiment to date, thus filling important knowledge gaps on ecological drought responses in high-precipitation and high-latitude ecosystems across multiple phases of plant community succession.publishedVersio

Germination and seedling growth of Calluna vulgaris is sensitive to regional climate, heathland succession, and drought

The coastal heathlands of Northwest Europe are highly valued cultural landscapes, that are critically endangered due to land use and climatic changes, such as increased frequency and severity of drought events. Our study is the first to assess how the germination and early seedling growth of Calluna vulgaris respond to drought. In a factorial design field experiment, we exposed maternal plants to three in-situ drought treatments (control, 60%, 90% roof coverage), across three successional stages after fire (pioneer, building, mature), and two regions (60°N, 65°N). Seeds from 540 plants within the experiment were, weighed, and exposed to five water potentials, ranging from −0.25 to −1.7 MPa, in a growth chamber experiment. We recorded germination (percentage, rate), seedling growth (above- vs. belowground allocation), and seedling functional traits (specific leaf area [SLA], specific root length [SRL]). Overall variation in germination between regions, successional stages, and maternal drought treatments was largely mediated by variation in seed mass. Plants from the northernmost region had higher seed mass and germination percentages. This is indicative of higher investment in seeds, likely linked to the populations' absence of vegetative root sprouting. Seeds from the mature successional stage germinated to lower final percentages than those from earlier successional stages, especially when the maternal plants had been exposed to drought (60% and 90% roof coverage). Exposure to reduced water availability decreased germination percentage and increased the time to 50% germination. Seedlings fully developed in the range −0.25 to −0.7 MPa, with increased root:shoot and lower SRL during reduced water availability, suggesting a resource-conservative response to drought during the early stages of development. Our results thus suggest a sensitivity to drought during the germination and seedling life-history stages that may reduce Calluna's ability to re-establish from seeds as the incidence and severity of droughts are projected to increase under future climates.publishedVersio

Intraspecific trait variability is a key feature underlying high Arctic plant community resistance to climate warming

In the high Arctic, plant community species composition generally responds slowly to climate warming, whereas less is known about the community functional trait responses and consequences for ecosystem functioning. The slow species turnover and large distribution ranges of many Arctic plant species suggest a significant role of intraspecific trait variability in functional responses to climate change. Here we compare taxonomic and functional community compositional responses to a long-term (17-year) warming experiment in Svalbard, Norway, replicated across three major high Arctic habitats shaped by topography and contrasting snow regimes. We observed taxonomic compositional changes in all plant communities over time. Still, responses to experimental warming were minor and most pronounced in the drier habitats with relatively early snowmelt timing and long growing seasons (Cassiope and Dryas heaths). The habitats were clearly separated in functional trait space, defined by 12 size- and leaf economics-related traits, primarily due to interspecific trait variation. Functional traits also responded to experimental warming, most prominently in the Dryas heath and mostly due to intraspecific trait variation. Leaf area and mass increased and leaf δ15N decreased in response to the warming treatment. Intraspecific trait variability ranged between 30% and 71% of the total trait variation, reflecting the functional resilience of those communities, dominated by long-lived plants, due to either phenotypic plasticity or genotypic variation, which most likely underlies the observed resistance of high Arctic vegetation to climate warming. We further explored the consequences of trait variability for ecosystem functioning by measuring peak season CO2 fluxes. Together, environmental, taxonomic, and functional trait variables explained a large proportion of the variation in net ecosystem exchange (NEE), which increased when intraspecific trait variation was accounted for. In contrast, even though ecosystem respiration and gross ecosystem production both increased in response to warming across habitats, they were mainly driven by the direct kinetic impacts of temperature on plant physiology and biochemical processes. Our study shows that long-term experimental warming has a modest but significant effect on plant community functional trait composition and suggests that intraspecific trait variability is a key feature underlying high Arctic ecosystem resistance to climate warming.publishedVersio

Global maps of soil temperature

Research in global change ecology relies heavily on global climatic grids derived from estimates of air temperature in open areas at around 2 m above the ground. These climatic grids do not reflect conditions below vegetation canopies and near the ground surface, where critical ecosystem functions occur and most terrestrial species reside. Here, we provide global maps of soil temperature and bioclimatic variables at a 1-km² resolution for 0–5 and 5–15 cm soil depth. These maps were created by calculating the difference (i.e., offset) between in-situ soil temperature measurements, based on time series from over 1200 1-km² pixels (summarized from 8500 unique temperature sensors) across all the world’s major terrestrial biomes, and coarse-grained air temperature estimates from ERA5-Land (an atmospheric reanalysis by the European Centre for Medium-Range Weather Forecasts). We show that mean annual soil temperature differs markedly from the corresponding gridded air temperature, by up to 10°C (mean = 3.0 ± 2.1°C), with substantial variation across biomes and seasons. Over the year, soils in cold and/or dry biomes are substantially warmer (+3.6 ± 2.3°C) than gridded air temperature, whereas soils in warm and humid environments are on average slightly cooler (-0.7 ± 2.3°C). The observed substantial and biome-specific offsets emphasize that the projected impacts of climate and climate change on near-surface biodiversity and ecosystem functioning are inaccurately assessed when air rather than soil temperature is used, especially in cold environments. The global soil-related bioclimatic variables provided here are an important step forward for any application in ecology and related disciplines. Nevertheless, we highlight the need to fill remaining geographic gaps by collecting more in-situ measurements of microclimate conditions to further enhance the spatiotemporal resolution of global soil temperature products for ecological applications.publishedVersio

Land-use and climate impacts on drought resistance and resilience in coastal heathland ecosystems

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.Doktorgradsavhandlin

Land-use and climate impacts on drought resistance and resilience in coastal heathland ecosystems

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

The effect of grazing and hiking on the elevational range shift of vascular plant species in the Scandes during recent decades

Resurvey of historical plant distribution data can provide valuable information about how plant communities have changed, a topic which is of great interest due to recent decades of climate and land use change. Range shifts towards higher elevations have been observed for alpine vegetation in mountainous areas all over the world during the recent decades. A correlation in time between the observed range shift and climatic changes has led to an expectation of climate as the driving factor, while other factors and their interactions with climate have received less attention. One factor which might affect species distribution, both directly and in interaction with climate, is dynamics in the species communities induced by two key factors of disturbance: large herbivores and hikers. Intensity of these factors has changed dramatically over the past century. I investigated the effect of large herbivore grazing and hiking on upward plant species range shift and dynamics in the summit flora species composition, through a resurvey study in three areas along the south-north axis of the Scandes. I demonstrate a buffering effect on the upward range shift from grazing by semi- domestic reindeer and sheep. All mountains have had a turnover in the species composition in the upper elevation range, but there was no detectable difference between mountains exposed to grazing and/or hiking and not. Hiking was not found to affect the upward range shift either. The results from this thesis show that land use, specially grazing, is affecting the observed range shift. Grazers have effects on range shifts through a variety of mechanisms which are undistinguishable in this form of resurvey and should be the focused of experimental work in the future

The northernmost sightings of Humpback whales (<i>Megaptera novaeangliae</i>)

The decrease in sea ice coverage impacts the accessibility of marine areas especially in the Arctic. Areas further north than previously reported can be accessed by marine mammals which are usually restricted by the sea ice. We report sightings of several humpback whales north of 81º North. This is a previously unknown habitat expansion. The recent decrease in Arctic sea ice cover is likely to increase accessibility of marine areas for cetaceans, potentially resulting in northward expansions of species habitat ranges. Here, we report multiple sightings of humpback whales (Megaptera novaeangliae) north and west of Jackson Island in the Franz Josef archipelago in July 2016. The observations are north of 81º North, significantly increasing the northernmost observation of humpback whales. The sea ice loss in the summer of 2016 was larger than previous recorded. Changes in population dynamics, especially increase of population size, might be the most important driver for the habitat expansion, but climate change is opening up new marine habitats in the Arctic. Our observations suggest that cetaceans are already expanding their ranges into ice-free areas