Kari Anne Bråthen

Ecological assembly rules in plant communities-approaches, patterns and prospects

Understanding how communities of living organisms assemble has been a central question in ecology since the early days of the discipline. Disentangling the different processes involved in community assembly is not only interesting in itself but also crucial for an understanding of how communities will behave under future environmental scenarios. The traditional concept of assembly rules reflects the notion that species do not co‐occur randomly but are restricted in their co‐occurrence by interspecific competition. This concept can be redefined in a more general framework where the co‐occurrence of species is a product of chance, historical patterns of speciation and migration, dispersal, abiotic environmental factors, and biotic interactions, with none of these processes being mutually exclusive.

Fifty thousand years of Arctic vegetation and megafaunal diet

Although it is generally agreed that the Arctic flora is among the youngest and least diverse on Earth, the processes that shaped it are poorly understood. Here we present 50 thousand years (kyr) of Arctic vegetation history, derived from the first large-scale ancient DNA metabarcoding study of circumpolar plant diversity. For this interval we also explore nematode diversity as a proxy for modelling vegetation cover and soil quality, and diets of herbivorous megafaunal mammals, many of which became extinct around 10 kyr bp (before present). For much of the period investigated, Arctic vegetation consisted of dry steppe-tundra dominated by forbs (non-graminoid herbaceous vascular plants). During the Last Glacial Maximum (25–15 kyr bp), diversity declined markedly, although forbs remained dominant. Much changed after 10 kyr bp, with the appearance of moist tundra dominated by woody plants and graminoids. Our analyses indicate that both graminoids and forbs would have featured in megafaunal diets. As such, our findings question the predominance of a Late Quaternary graminoid-dominated Arctic mammoth steppe.

Induced Shift in Ecosystem Productivity? Extensive Scale Effects of Abundant Large Herbivores

Abundant large herbivores can strongly alter vegetation composition, shifting the ecosystem into a lasting state of changed productivity. Previous studies of the effects of abundant reindeer on alpine and arctic vegetation have yielded equivocal results, probably due to differing environmental contexts. To overcome context dependency we devised a large-scale survey in the region of Finnmark, northern Norway, possessing some of the most densely stocked reindeer herds in the world. The effects of reindeer abundance on summer pasture vegetation were assessed by employing a quasi-experimental design, including site fertility as a potential modifier of the reindeer–vegetation interaction. The study design comprised ten pairs of neighboring management districts (encompassing 18,003 km2), where over the two last decades a high-density district on average had reindeer densities more than twice as high and calf weights consistently lower than the low-density district. The abundance of different plant functional groups, ranging from those having facilitating to retarding effects on ecosystem productivity, were quantified by the point intercept method on plots selected according to a hierarchical, stratified random sampling design. Species with strong retarding effects on ecosystem productivity (for example, ericoids) were by far the most abundant. However, we found no consistent effects of reindeer density on their abundance. The most consistent differences between high- and low-density districts were found in plant functional groups with facilitating to neutral effects on ecosystem productivity. In particular, the abundance of N-facilitators, large dicotyledons and grasses were substantially reduced in the high-density districts. However, this reduction was restricted to fertile sites. Thus, reindeer when present at high densities have homogenized the biomass of palatable plants across environmental productivity gradients according to predictions from exploitation ecosystem models. Such reduction of plants with facilitating to neutral effects on ecosystem productivity indicates a reduced state of ecosystem productivity in high-density districts.

DNA from soil mirrors plant taxonomic and growth form diversity

Ecosystems across the globe are threatened by climate change and human activities. New rapid survey approaches for monitoring biodiversity would greatly advance assessment and understanding of these threats. Taking advantage of next-generation DNA sequencing, we tested an approach we call metabarcoding: high-throughput and simultaneous taxa identification based on a very short (usually <100 base pairs) but informative DNA fragment. Short DNA fragments allow the use of degraded DNA from environmental samples. All analyses included amplification using plant-specific versatile primers, sequencing and estimation of taxonomic diversity. We tested in three steps whether degraded DNA from dead material in soil has the potential of efficiently assessing biodiversity in different biomes. First, soil DNA from eight boreal plant communities located in two different vegetation types (meadow and heath) was amplified. Plant diversity detected from boreal soil was highly consistent with plant taxonomic and growth form diversity estimated from conventional above-ground surveys. Second, we assessed DNA persistence using samples from formerly cultivated soils in temperate environments. We found that the number of crop DNA sequences retrieved strongly varied with years since last cultivation, and crop sequences were absent from nearby, uncultivated plots. Third, we assessed the universal applicability of DNA metabarcoding using soil samples from tropical environments: a large proportion of species and families from the study site were efficiently recovered. The results open unprecedented opportunities for large-scale DNA-based biodiversity studies across a range of taxonomic groups using standardized metabarcoding approaches.

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.

New environmental metabarcodes for analysing soil DNA: potential for studying past and present ecosystems

Metabarcoding approaches use total and typically degraded DNA from environmental samples to analyse biotic assemblages and can potentially be carried out for any kinds of organisms in an ecosystem. These analyses rely on specific markers, here called metabarcodes, which should be optimized for taxonomic resolution, minimal bias in amplification of the target organism group and short sequence length. Using bioinformatic tools, we developed metabarcodes for several groups of organisms: fungi, bryophytes, enchytraeids, beetles and birds. The ability of these metabarcodes to amplify the target groups was systematically evaluated by (i) in silico PCRs using all standard sequences in the EMBL public database as templates, (ii) in vitro PCRs of DNA extracts from surface soil samples from a site in Varanger, northern Norway and (iii) in vitro PCRs of DNA extracts from permanently frozen sediment samples of late‐Pleistocene age (∼16 000–50 000 years bp) from two Siberian sites, Duvanny Yar and Main River. Comparison of the results from the in silico PCR with those obtained in vitro showed that the in silico approach offered a reliable estimate of the suitability of a marker. All target groups were detected in the environmental DNA, but we found large variation in the level of detection among the groups and between modern and ancient samples. Success rates for the Pleistocene samples were highest for fungal DNA, whereas bryophyte, beetle and bird sequences could also be retrieved, but to a much lesser degree. The metabarcoding approach has considerable potential for biodiversity screening of modern samples and also as a palaeoecological tool.

Reindeer reduce biomass of preferred plant species

. Reduced weights in reindeer that graze in pastures with high reindeer densities have raised the question if coastal summer pastures are modified by grazing. To evaluate this, the impact of reindeer grazing on standing crop was measured by the plant intercept method inside and outside grazing exclosures in the understorey of a coastal mountain birch forest in northern Norway. The understories of coastal birch forests are dominated by vascular plants and are important summer pastures to reindeer. Based on the literature, we made a priori categorization of the vascular plant species into functional groups of preferred forage, less preferred forage and forage of unknown value to reindeer. Intercept frequency was measured within the same plots on three occasions in the summer of 1996. At the end of the grazing season, total standing crop was 33% lower in open plots compared to plots protected by exclosures. However, the reduction varied between the functional groups, with only preferred forage plants being significantly reduced in standing crop (by 49%). Results suggest that reindeer have a strong annual impact on most of the preferred forage species. However, some of the preferred graminoids are tolerant of grazing and dominate the understorey despite decades of high grazing pressure. We suggest that current grazing pressure is favouring the establishment of a few grazing tolerant graminoids, and that this reduces the forage plant variability. The results are discussed in relation to the grazing optimization hypothesis and the potential importance of plant variability for pasture quality.

Terrestrial trophic dynamics in the Canadian Arctic

The Swedish Tundra Northwest Expedition of 1999 visited 17 sites throughout the Canadian Arctic. At 12 sites that were intensively sampled we estimated the standing crop of plants and the densities of herbivores and predators with an array of trapping, visual surveys, and faecal-pellet transects. We developed a trophic-balance model using ECOPATH to integrate these observations and determine the fate of primary and secondary production in these tundra ecosystems, which spanned an 8-fold range of standing crop of plants. We estimated that about 13% of net primary production was consumed by herbivores, while over 70% of small-herbivore production was estimated to flow to predators. Only 9% of large-herbivore production was consumed by predators. Organization of Canadian Arctic ecosystems appears to be more top-down than bottom-up. Net primary production does not seem to be herbivore-limited at any site. This is the first attempt to integrate trophic dynamics over the entire Canadian Arctic.

Can Reindeer Overabundance Cause a Trophic Cascade

A bstractThe region Finnmark, in northernmost Europe, harbors dense populations of semi-domestic reindeer of which some exhibit characteristics of overabundance. Whereas overabundance is evident in terms of density-dependent reductions in reindeer body mass, population growth and abundance of forage plants, claims have been made that this reindeer overabundance also has caused a trophic cascade. These claims are based on the main premise that reindeer overgrazing negatively impacts small-sized, keystone tundra herbivores. We tested this premise by a large-scale study in which the abundance of small rodents, hares and ptarmigans was indexed across reindeer management districts with strong differences in stocking densities. We examined the scale-dependent relations between reindeer, vegetation and these small-sized herbivores by employing a spatially hierarchical sampling design within the management districts. A negative impact of reindeer on ptarmigan, probably as a result of browsing reducing tall Salix, was indicated. However, small rodents (voles and lemmings), which are usually the keystone herbivores in the plant-based tundra food web, were not negatively impacted. On the contrary, there was a strong positive relationship between small rodents and reindeer, both at the scale of landscape areas and local patches, with characteristics of snow-bed vegetation, suggesting facilitation between Norwegian lemmings and reindeer. We conclude that the recent dampening of the vole and lemming population cycle with concurrent declines of rodent predators in northernmost Europe were not caused by large herbivore overgrazing.

More efficient estimation of plant biomass

Abstract Question: The optimal use of the point intercept method (PIM) for efficient estimation of plant biomass has not been addressed although PIM is a commonly used method in vegetation analysis. In this study we compare results achieved using PIM at a range of efforts, we assess a method for calculating these results that are new with PIM and we provide a formula for planning the optimal use of PIM. Location: Northern Norway. Methods: We collected intercept data at a range of efforts, i.e. from one to 100 pins per 0.25 m2 plots, on three plant growth forms in a mountain meadow. After collection of intercept data we clipped and weighed the plant biomass. The relationship between intercept frequency and weighed biomass (b) was estimated using both a weighted linear regression model (WLR) and an ordinary linear regression model (OLR). The accuracy of the estimate of biomass achieved by PIM at different efforts was assessed by running computer simulations at different pin densities. Results: The relationship between intercept frequency and weighed biomass (b) was far better estimated using WLR compared to the normally used OLR. Efforts above 10 pins per 0.25 m2 plot had a negligible effect on the accuracy of the estimate of biomass achieved by PIM whereas the number of plots had a strong effect. Moreover, for a given level of accuracy, the required number of plots varied depending on plant growth form. We achieved similar results to that of the computer simulations when applying our WLR based formula. Conclusion: This study shows that PIM can be applied more efficiently than was done in previous studies for the purpose of plant biomass estimation, where several plots should be analysed but at considerably less effort per plot. Moreover, WLR rather than OLR should be applied when estimating biomass from intercept frequency. The formula we have deduced is a useful tool for planning plant biomass analysis with PIM. Nomenclature: Lid & Lid (1994). Abbreviations: BM = Biomass; CV = Coefficient of variation; IF = Intercept frequency; PIM = Point intercept method; OLR = Ordinary linear regression model; WLR = weighted linear regression model.