Virve Ravolainen

Shrub expansion in tundra ecosystems: dynamics, impacts and research priorities

Recent research using repeat photography, long-term ecological monitoring and dendrochronology has documented shrub expansion in arctic, high-latitude and alpine tundra

Local temperatures inferred from plant communities suggest strong spatial buffering of climate warming across Northern Europe

Recent studies from mountainous areas of small spatial extent (<2500 km(2) ) suggest that fine-grained thermal variability over tens or hundreds of metres exceeds much of the climate warming expected for the coming decades. Such variability in temperature provides buffering to mitigate climate-change impacts. Is this local spatial buffering restricted to topographically complex terrains? To answer this, we here study fine-grained thermal variability across a 2500-km wide latitudinal gradient in Northern Europe encompassing a large array of topographic complexities. We first combined plant community data, Ellenberg temperature indicator values, locally measured temperatures (LmT) and globally interpolated temperatures (GiT) in a modelling framework to infer biologically relevant temperature conditions from plant assemblages within <1000-m(2) units (community-inferred temperatures: CiT). We then assessed: (1) CiT range (thermal variability) within 1-km(2) units; (2) the relationship between CiT range and topographically and geographically derived predictors at 1-km resolution; and (3) whether spatial turnover in CiT is greater than spatial turnover in GiT within 100-km(2) units. Ellenberg temperature indicator values in combination with plant assemblages explained 46-72% of variation in LmT and 92-96% of variation in GiT during the growing season (June, July, August). Growing-season CiT range within 1-km(2) units peaked at 60-65°N and increased with terrain roughness, averaging 1.97 °C (SD = 0.84 °C) and 2.68 °C (SD = 1.26 °C) within the flattest and roughest units respectively. Complex interactions between topography-related variables and latitude explained 35% of variation in growing-season CiT range when accounting for sampling effort and residual spatial autocorrelation. Spatial turnover in growing-season CiT within 100-km(2) units was, on average, 1.8 times greater (0.32 °C km(-1) ) than spatial turnover in growing-season GiT (0.18 °C km(-1) ). We conclude that thermal variability within 1-km(2) units strongly increases local spatial buffering of future climate warming across Northern Europe, even in the flattest terrains.

Complementary impacts of small rodents and semi-domesticated ungulates limit tall shrub expansion in the tundra

Summary 1. While shrubs appear to be expanding in Arctic tundra due to climatic warming, patches of tall shrubs in riparian habitats are most likely to colonize new areas. Shrub recruits outside established patches represent the forefront of area expansion, but their dynamics may be sensitive to the action of several herbivore species. The empirical evidence for how different-sized herbivores affect recruits of tall shrubs is lacking. Moreover, although management and natural population dynamics of herbivores happens at landscape and regional scales, field research on herbivore impacts on shrubs seldom covers these scales. 2. Reindeer management and different rodent population dynamics result in regional variation in herbivore abundances in arctic Norway. We conducted an herbivore exclusion experiment, covering three low-arctic river catchments with contrasting herbivore abundances. We assessed the impacts of small rodents and reindeer on growth, and survival of willow Salix spp. recruits after 3 years of herbivore exclusion. 3. As expected, the Salix recruits increased in sizes and had lower mortality when released from herbivores. Both types of herbivores had strong impacts on size and survival of Salix recruits. 4. Spatially contrasting results were consistent with regional differences in the abundance of reindeer and rodents; herbivore impacts on shrubs were found when at least one type of herbivore was abundant. However, the impact was not independent of herbivore species. While both browsing from reindeer and rodents pruned the recruits and prevented them from escaping the field layer, the rodents also inflicted substantial mortality and thus thinned the stand of recruits. 5. Synthesis and applications. Sympatric populations of rodents and reindeer have strongly complementary impacts on shrub recruits and may limit the expansion potential of tall shrubs even in the most productive habitats of arctic tundra. The spatial correspondence between shrub recruits performance and herbivore abundances, found after a short time period, suggests that the extent of tall shrub expansion in tundra is contingent on current variation and future trends in herbivore populations. In areas where humans control large herbivore populations, management may opt to counteract climate-driven shrub expansion also in habitats that are most prone to such expansion.

Arctic Small Rodents Have Diverse Diets and Flexible Food Selection

The ecology of small rodent food selection is poorly understood, as mammalian herbivore food selection theory has mainly been developed by studying ungulates. Especially, the effect of food availability on food selection in natural habitats where a range of food items are available is unknown. We studied diets and selectivity of grey-sided voles (Myodes rufocanus) and tundra voles (Microtus oeconomus), key herbivores in European tundra ecosystems, using DNA metabarcoding, a novel method enabling taxonomically detailed diet studies. In order to cover the range of food availabilities present in the wild, we employed a large-scale study design for sampling data on food availability and vole diets. Both vole species had ingested a range of plant species and selected particularly forbs and grasses. Grey-sided voles also selected ericoid shrubs and tundra voles willows. Availability of a food item rarely affected its utilization directly, although seasonal changes of diets and selection suggest that these are positively correlated with availability. Moreover, diets and selectivity were affected by availability of alternative food items. These results show that the focal sub-arctic voles have diverse diets and flexible food preferences and rarely compensate low availability of a food item with increased searching effort. Diet diversity itself is likely to be an important trait and has previously been underrated owing to methodological constraints. We suggest that the roles of alternative food item availability and search time limitations for small rodent feeding ecology should be investigated. Nomenclature Annotated Checklist of the Panarctic Flora (PAF), Vascular plants. Available at: http://nhm2.uio.no/paf/, accessed 15.6.2012.

Niche construction by growth forms is as strong a predictor of species diversity as environmental gradients

Summary 1. We present a conceptual framework that describes how species belonging to a growth form collectively can be niche constructors (i.e. modify niches) and affect species diversity in plant communities. We use an empirical assessment of tundra plant communities to illustrate the framework’s utility. In doing so, we make a first investigation of collective niche construction in ecological communities. 2. In tundra plant communities, growth forms differently affect ecosystem process rates and cause environmental modifications; thus, growth forms are strong candidates for being niche constructors. To assess the impact of growth form niche construction on plant species diversity, we excluded the species of the growth form applied as niche constructor when estimating the community species diversity. 3. We assessed niche construction in 70 tundra meadow communities and 1450 randomly selected tundra plant communities that are distributed along ecological gradients in temperature, resource availability, competitive interference and herbivory. These gradients allowed us to concomitantly assess to what extent the niche construction is independent of environmental conditions. 4. Growth forms varied from strong positive to neutral predictors of both species richness and Simpson index in the order of forbs, grasses, sedges, deciduous shrubs and evergreen shrubs, suggesting that growth forms have important roles as niche constructors in tundra plant communities. Also, the environmental conditions were strong predictors of species diversity, but they did not interact with or confound the effects of growth forms. 5. Forbs and grasses were the least abundant growth forms, yet they were the strongest positive predictors of species diversity. Therefore, our results suggest a particular niche-constructing role of these growth forms for enhancing species diversity in tundra plant communities. 6. Synthesis. In this study, we provide conceptual and empirical evidence for collective niche construction as a powerful ecological process that affects species diversity and that can act independently of environmental conditions. Species sharing a single trait or species belonging to a growth form can act as collective niche constructors, and as exemplified for growth forms in this study, be important predictors of species diversity in ecological communities.

Shrub patch configuration at the landscape scale is related to diversity of adjacent herbaceous vegetation

Background: Patterns of shrub patches reflect the long-term effects of several environmental drivers; the same drivers may also affect adjacent herbaceous vegetation. The identity of such drivers may be indicated by associating patterns of shrubs and adjacent vegetation. However, such associations have rarely been examined. Aim: Assess the hypothesis that configuration of shrub patches is related to the diversity of adjacent herbaceous vegetation, through the impact of variation in herbivory and/or growing conditions. Methods: Multivariate analyses of the relationship between patch configuration of tall shrubs and diversity of adjacent herbaceous vegetation, across two tundra river catchments in Norway. Results: Configuration of shrub patches was clearly related to α and β diversity of adjacent herbaceous vegetation, independent of concurrent herbivore abundance (estimated from faeces during the study), or growing conditions. Highly-shredded shrub patches (i.e. fragmented patches with high edge density) were associated with grazing-tolerant species and low species richness. Conversely, little-shredded tall shrub patches were associated with grazing-sensitive shrub saplings. This indicates change in herbivory on a timescale longer than that covered by our faeces data. Conclusions: Consistent co-variation of shrub and herbaceous patches in tall shrub tundra supports the hypothesis of impacts of a shared driver. Interpretation of this co-variation points to long-term influence of herbivory as the shared driver.

Understanding the drivers of extensive plant damage in boreal and Arctic ecosystems: Insights from field surveys in the aftermath of damage

The exact cause of population dieback in nature is often challenging to identify retrospectively. Plant research in northern regions has in recent decades been largely focussed on the opposite trend, namely increasing populations and higher productivity. However, a recent unexpected decline in remotely-sensed estimates of terrestrial Arctic primary productivity suggests that warmer northern lands do not necessarily result in higher productivity. As large-scale plant dieback may become more frequent at high northern latitudes with increasing frequency of extreme events, understanding the drivers of plant dieback is especially urgent. Here, we report on recent extensive damage to dominant, short, perennial heath and tundra plant populations in boreal and Arctic Norway, and assess the potential drivers of this damage. In the High-Arctic archipelago of Svalbard, we recorded that 8-50% of Cassiope tetragona and Dryas octopetala shoots were dead, and that the ratios of dead shoots increased from 2014 to 2015. In boreal Norway, 38-63% of Calluna vulgaris shoots were dead, while Vaccinium myrtillus had damage to 91% of shoots in forested sites, but was healthy in non-forested sites. Analyses of numerous sources of environmental information clearly point towards a winter climate-related reason for damage to three of these four species. In Svalbard, the winters of 2011/12 and 2014/15 were documented to be unusually severe, i.e. insulation from ambient temperature fluctuation by snow was largely absent, and ground-ice enforced additional stress. In boreal Norway, the 2013/14 winter had a long period with very little snow combined with extremely low precipitation rates, something which resulted in frost drought of uncovered Calluna plants. However, extensive outbreaks of a leaf-defoliating geometrid moth were identified as the driver of Vaccinium mortality. These results suggest that weather and biotic extreme events potentially have strong impacts on the vegetation state of northern lands.

Definition of sampling units begets conclusions in ecology: the case of habitats for plant communities

In ecology, expert knowledge on habitat characteristics is often used to define sampling units such as study sites. Ecologists are especially prone to such approaches when prior sampling frames are not accessible. Here we ask to what extent can different approaches to the definition of sampling units influence the conclusions that are drawn from an ecological study? We do this by comparing a formal versus a subjective definition of sampling units within a study design which is based on well-articulated objectives and proper methodology. Both approaches are applied to tundra plant communities in mesic and snowbed habitats. For the formal approach, sampling units were first defined for each habitat in concave terrain of suitable slope using GIS. In the field, these units were only accepted as the targeted habitats if additional criteria for vegetation cover were fulfilled. For the subjective approach, sampling units were defined visually in the field, based on typical plant communities of mesic and snowbed habitats. For each approach, we collected information about plant community characteristics within a total of 11 mesic and seven snowbed units distributed between two herding districts of contrasting reindeer density. Results from the two approaches differed significantly in several plant community characteristics in both mesic and snowbed habitats. Furthermore, differences between the two approaches were not consistent because their magnitude and direction differed both between the two habitats and the two reindeer herding districts. Consequently, we could draw different conclusions on how plant diversity and relative abundance of functional groups are differentiated between the two habitats depending on the approach used. We therefore challenge ecologists to formalize the expert knowledge applied to define sampling units through a set of well-articulated rules, rather than applying it subjectively. We see this as instrumental for progress in ecology as only rules based on expert knowledge are transparent and lead to results reproducible by other ecologists.

Evidence of effects of herbivory on Arctic vegetation: a systematic map protocol

BackgroundAlong with climate change, herbivory is considered a main driver of ecosystem change in terrestrial Arctic environments. Understanding how herbivory influences the resilience of Arctic ecosystems to ongoing environmental changes is essential to inform policy and guide sustainable management practices. However, many studies indicate that the effects of herbivores on plants and ecosystem functioning depend on the abiotic and biotic conditions where the interaction takes place, i.e. the ecological context. Yet, the range of ecological contexts in which herbivory has been studied in the Arctic has not been systematically assessed. A lack of such evaluation prevents understanding the robustness and generalizability of our knowledge of Arctic herbivore effects on vegetation and ecosystems. The main objective of our systematic map is to identify the ecological contexts where herbivory is studied in the Arctic. Hence, this systematic map will enable us to assess our ability to make generalizable and robust conclusions regarding the impacts of Arctic herbivory.MethodsWe will search academic and grey literature using databases, search engines and specialist websites, and select studies addressing the response of the plant(s) to herbivory, deemed relevant in terms of (i) population (terrestrial Arctic plants and plant communities), (ii) exposure (herbivory, including disturbance and fertilization effects of herbivores), and (iii) modifier (ecological context being in the terrestrial Arctic including forest-tundra). We will synthesize the results using systematic mapping approaches.

Transferability of biotic interactions: Temporal consistency of arctic plant-rodent relationships is poor

Abstract Variability in biotic interaction strength is an integral part of food web functioning. However, the consequences of the spatial and temporal variability of biotic interactions are poorly known, in particular for predicting species abundance and distribution. The amplitude of rodent population cycles (i.e., peak‐phase abundances) has been hypothesized to be determined by vegetation properties in tundra ecosystems. We assessed the spatial and temporal predictability of food and shelter plants effects on peak‐phase small rodent abundance during two consecutive rodent population peaks. Rodent abundance was related to both food and shelter biomass during the first peak, and spatial transferability was mostly good. Yet, the temporal transferability of our models to the next population peak was poorer. Plant–rodent interactions are thus temporally variable and likely more complex than simple one‐directional (bottom‐up) relationships or variably overruled by other biotic interactions and abiotic factors. We propose that parametrizing a more complete set of functional links within food webs across abiotic and biotic contexts would improve transferability of biotic interaction models. Such attempts are currently constrained by the lack of data with replicated estimates of key players in food webs. Enhanced collaboration between researchers whose main research interests lay in different parts of the food web could ameliorate this.