Vigdis Vandvik

Nitrogen deposition threatens species richness of grasslands across Europe.

Evidence from an international survey in the Atlantic biogeographic region of Europe indicates that chronic nitrogen deposition is reducing plant species richness in acid grasslands. Across the deposition gradient in this region (2-44 kg N ha(-1) yr(-1)) species richness showed a curvilinear response, with greatest reductions in species richness when deposition increased from low levels. This has important implications for conservation policies, suggesting that to protect the most sensitive grasslands resources should be focussed where deposition is currently low. Soil pH is also an important driver of species richness indicating that the acidifying effect of nitrogen deposition may be contributing to species richness reductions. The results of this survey suggest that the impacts of nitrogen deposition can be observed over a large geographical range.

Dispersal limitations matter for microbial morphospecies

In contrast with macroorganisms, whose geographical ranges are typically restricted, many microbial species appear to have cosmopolitan distributions. This observation has been explained as a consequence of ubiquitous dispersal caused by the enormous population sizes of microbial species. Recently, this “everything is everywhere, but, the environment selects” theory has been challenged by the detection of considerable regional genetic variability within microbial morphospecies. We demonstrate that, contrary to what is expected under ubiquitous dispersal, evidence of regional-scale metacommunity processes can be detected in microbial morphospecies. Our results imply that the microbial and macrobial world are structured by analogous processes.

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.

Partitioning floristic variance in Norwegian upland grasslands into within-site and between-site components: are the patterns determined by environment or by land-use?

This study presents a quantitative partitioning of the variance infloristic data from grazed semi-natural vegetation of summer farms inRøldal, western Norway. The data consist of 189 taxa recorded in 1074-m2 sample plots within 10 summer farms with differentland-use histories. Thirty-five environmental variables were recorded,includingaltitude, slope, radiation, geology, soil chemistry, and past and presentland-use. A series of (partial) canonical correspondence analyses (CCAs) wereused to partition the total variation into within-farm and between-farmcomponents, and to investigate the explanatory power of different groups ofenvironmental and land-use variables at the two scales. The results show that:(1) although local gradients are of overriding importance for floristiccomposition, landscape-scale processes also contribute significantly to theobserved patterns; (2) the measured land-use and environmental factors accountfor comparable amounts of compositional variance at the two scales; and (3)evenif the relative contributions of the two classes of explanatory variables arecomparable, details differ, showing that broad-scale environmental and land-usepatterns are not just scaled-up versions of the fine-scale patterns or viceversa. These results support a multi-process view of vegetation patterns.

The impact of nitrogen deposition on acid grasslands in the Atlantic region of Europe

A survey of 153 acid grasslands from the Atlantic biogeographic region of Europe indicates that chronic nitrogen deposition is changing plant species composition and soil and plant-tissue chemistry. Across the deposition gradient (2-44 kg N ha(-1) yr(-1)) grass richness as a proportion of total species richness increased whereas forb richness decreased. Soil C:N ratio increased, but soil extractable nitrate and ammonium concentrations did not show any relationship with nitrogen deposition. The above-ground tissue nitrogen contents of three plant species were examined: Agrostis capillaris (grass), Galium saxatile (forb) and Rhytidiadelphus squarrosus (bryophyte). The tissue nitrogen content of neither vascular plant species showed any relationship with nitrogen deposition, but there was a weak positive relationship between R. squarrosus nitrogen content and nitrogen deposition. None of the species showed strong relationships between above-ground tissue N:P or C:N and nitrogen deposition, indicating that they are not good indicators of deposition rate.

Sources of Diversity in a Grassland Metacommunity: Quantifying the Contribution of Dispersal to Species Richness

Metacommunity theory suggests a potentially important role for dispersal in diversity maintenance at local, as well as regional, scales. In addition, propagule addition experiments have shown that dispersal often limits local diversity. However, actual dispersal rates into local communities and the contribution of immigrants to observed local diversity are poorly known. We present a new approach that partitions the diversity of a target community into dispersal‐maintained and dispersal‐independent components. Specifically, we quantify distances through space and time to the nearest potential seed source for naturally occurring recruits in target communities by using hierarchical data on species pools (local, site, region, and seed bank). Using this “recruit tag” approach, we found that dispersal contributed 29%–57% of the seedling diversity in perennial grasslands with different successional histories. However, both dispersal and seedling mortality remained remarkably constant, in absolute terms, over succession. The considerable loss of diversity over secondary succession (66%), therefore, could be understood only by considering how these processes interact with the decreasing disturbance rate (i.e., frequency of gaps) in later‐successional sites. We conclude that a metacommunity perspective is relevant and necessary to understand the diversity and community assembly of this study system.

Higher predation risk for insect prey at low latitudes and elevations

Risky in the tropics It is well known that diversity increases toward the tropics. Whether this increase translates into differences in interaction rates among species, however, remains unclear. To simplify the problem, Roslin et al. tested for predation rates by using a single approach involving model caterpillars across six continents. Predator attack rates were higher toward the equator, but only for arthropod predators. Science, this issue p. 742 Like diversity, predation rates among insects increase toward the equator and at lower altitudes. Biotic interactions underlie ecosystem structure and function, but predicting interaction outcomes is difficult. We tested the hypothesis that biotic interaction strength increases toward the equator, using a global experiment with model caterpillars to measure predation risk. Across an 11,660-kilometer latitudinal gradient spanning six continents, we found increasing predation toward the equator, with a parallel pattern of increasing predation toward lower elevations. Patterns across both latitude and elevation were driven by arthropod predators, with no systematic trend in attack rates by birds or mammals. These matching gradients at global and regional scales suggest consistent drivers of biotic interaction strength, a finding that needs to be integrated into general theories of herbivory, community organization, and life-history evolution.

The role of fire in UK peatland and moorland management: the need for informed, unbiased debate

Fire has been used for centuries to generate and manage some of the UKs cultural landscapes. Despite its complex role in the ecology of UK peatlands and moorlands, there has been a trend of simplifying the narrative around burning to present it as an only ecologically damaging practice. That fire modifies peatland characteristics at a range of scales is clearly understood. Whether these changes are perceived as positive or negative depends upon how trade-offs are made between ecosystem services and the spatial and temporal scales of concern. Here we explore the complex interactions and trade-offs in peatland fire management, evaluating the benefits and costs of managed fire as they are currently understood. We highlight the need for (i) distinguishing between the impacts of fires occurring with differing severity and frequency, and (ii) improved characterization of ecosystem health that incorporates the response and recovery of peatlands to fire. We also explore how recent research has been contextualized within both scientific publications and the wider media and how this can influence non-specialist perceptions. We emphasize the need for an informed, unbiased debate on fire as an ecological management tool that is separated from other aspects of moorland management and from political and economic opinions. This article is part of the themed issue ‘The interaction of fire and mankind’.

Responses of alpine snowbed vegetation to long-term experimental warming

Abstract: In order to assess the influence of experimental warming on individual species response, species composition and richness, and the abundance of ramets in a wet late-melting snowbed, we established 20 open-top chambers (OTCs) permanently for 5 y (six growing seasons) at Finse, southwest Norway. Salix herbacea, Saxifraga stellaris, Omalotheca supina, Cerastium cerastoides, and Epilobium anagallidifolium increased in the experimentally warmed plots, while there was no significant response in Carex lachenalii, Deschampsia alpina, Poa alpina, Juncus biglumis, Saxifraga rivularis, or Veronica alpina. Species composition changed significantly with time both in the OTCs and in the control plots. Although invasion rates slightly increased in the OTCs compared to the controls, differences in overall species composition or richness were not significant between the OTCs and the control plots during the 5-y study. Our results suggest that vegetation change is going on naturally and that the effect of this change overrides the effect of the temperature treatment. We conclude that increased plant growth will result in denser vegetation in a warmer future. Whether the higher net invasion will result in more diverse vegetation is yet unclear, as the processes take more time than allowed for in this study.

From facilitation to competition: temperature-driven shift in dominant plant interactions affects population dynamics in seminatural grasslands.

Biotic interactions are often ignored in assessments of climate change impacts. However, climate-related changes in species interactions, often mediated through increased dominance of certain species or functional groups, may have important implications for how species respond to climate warming and altered precipitation patterns. We examined how a dominant plant functional group affected the population dynamics of four co-occurring forb species by experimentally removing graminoids in seminatural grasslands. Specifically, we explored how the interaction between dominants and subordinates varied with climate by replicating the removal experiment across a climate grid consisting of 12 field sites spanning broad-scale temperature and precipitation gradients in southern Norway. Biotic interactions affected population growth rates of all study species, and the net outcome of interactions between dominants and subordinates switched from facilitation to competition with increasing temperature along the temperature gradient. The impacts of competitive interactions on subordinates in the warmer sites could primarily be attributed to reduced plant survival. Whereas the response to dominant removal varied with temperature, there was no overall effect of precipitation on the balance between competition and facilitation. Our findings suggest that global warming may increase the relative importance of competitive interactions in seminatural grasslands across a wide range of precipitation levels, thereby favouring highly competitive dominant species over subordinate species. As a result, seminatural grasslands may become increasingly dependent on disturbance (i.e. traditional management such as grazing and mowing) to maintain viable populations of subordinate species and thereby biodiversity under future climates. Our study highlights the importance of population-level studies replicated under different climatic conditions for understanding the underlying mechanisms of climate change impacts on plants.