All populations are affected by multiple environmental drivers, including climatic drivers such as temperature or precipitation and biotic drivers such as herbivory or mutualisms. The relative response of a population to each driver is critical to prioritizing threat mitigation for conservation and to understanding whether climatic or biotic drivers most strongly affect fitness. However, the importance of different drivers can vary dramatically across species and across populations of the same species. Theory suggests that the response to climatic versus biotic drivers can be affected by both the species' fundamental niche breadth and the latitude of the population at which the response is measured. However, we have few tests of how these two factors affect relative response to drivers separately, let alone tests of how niche breadth and latitude together influence responses. Here, we use a meta-analysis of published studies on population response to climatic and biotic drivers in terrestrial plants, combined with estimates of climatic niche breadth and position within climatic niche derived from herbarium records, to show that species' niche breadth is the primary determinant of response to climatic versus biotic drivers. Namely, we find that response to climatic drivers changes only minimally with increasing niche breadth, while response to biotic drivers increases with niche breadth. We see similar relationships when considering range size instead of niche breadth. Surprisingly, we find no effects of latitude on the relative effect of climatic versus biotic drivers. Our work suggests that populations of species with small and large ranges experience similar extirpation risks due to the negative impacts of climate change. By contrast, populations of species with large (but not small) ranges may be highly susceptible to changes in densities or distributions of interacting species.

As the climate is changing, species respond by changing their distributions and abundances. The effects of climate are not only direct, but also occur via changes in biotic interactions, such as competition. Yet, the role of competition in mediating the effects of climate is still largely unclear. To examine how climate influences species performance, directly and via competition with other species, we transplanted two moss species differing in climate niches, alone and together at 59 sites along a climate gradient. Growth was monitored over three growing seasons. In the absence of competition, both species performed better under warmer conditions. Yet, when transplanted together, a warmer climate had negative effects on the northern moss, while the effects remained positive for the southern species. The negative effect of a cold climate on the southern species was larger when both species were transplanted together. Over three growing seasons, the southern species almost outcompeted the northern in warmer climates. Our results illustrate how competitive interactions can modify, and even reverse, the direct effects of climate on organism performance. A broader implication of our results is that species interactions can have important effects on how environmental and climate change influence performance and abundance.

Due to climate change, droughts are increasingly frequent and intense. Yet, their impact on boreal forest fungal communities remains unclear, especially across different fungal functional and taxonomic groups. We induced an experimental rainfall exclusion for 45 summer days, using a paired design of 1 × 1 m treatment and control plots replicated in 25 sites in a boreal forest landscape in Sweden. Immediately after the experiment, we assessed the effects on soil fungal biomass, community composition and, after 2 months, sporocarp production. We did not detect significant effects of the rainfall exclusion on soil fungal biomass, but the fungal community composition was affected. In the rainfall exclusion plots, richness of ectomycorrhizal species with extensive extramatrical mycelia and saprotrophic basidiomycetes was reduced, while richness of ascomycetes was not affected. Sporocarp production of both saprotrophic and ectomycorrhizal fungi was reduced. The clear effects of a small-scale rainfall exclusion demonstrated in our study suggest that belowground fungal communities in boreal forests may be vulnerable to drought.

Background  All plants are influenced by the temperatures they are exposed to and fascinating adaptations to extreme temperatures have been described for many of them. However, the extent to which adaptation to thermal extremes is associated with costs, in terms of reduced performance at less or other stressful temperatures, is poorly known, especially for plants. In Iceland, there are two lineages of Agrostis stolonifera, one that occurs exclusively on geothermally heated soils (> 50 °C) and one that is only found on non-thermal soils. Since Iceland is a subarctic island, non-thermal areas surrounding the geothermal areas can get bitterly cold. This stark contrast in temperatures over short geographic distances provides an excellent system for studying adaptations to thermal extremes and potentially associated trade-offs. To test whether the geothermal lineage is more heat tolerant and whether this heat tolerance is associated with reduced performance under cooler conditions, we compared the heat and cold stress responses of the two lineages experimentally.

Results  No plants survived the hottest treatment (56 °C), only geothermal plants survived the second hottest treatment (49 °C) and geothermal plants also outperformed the non-thermal plants following the 46 °C treatment. In contrast, there were no differences in survival between geothermal and non-thermal plants under intermediate and cold conditions (41 °C, 21 °C and − 4 °C), but non-thermal plants outperformed geothermal plants under these conditions.

Conclusions  These results suggest that there is a trade-off between tolerating extreme heat and performance under cooler conditions, possibly indicating that geothermal A. stolonifera represents a specialised thermophilic lineage in Iceland. Our findings provide new empirical data on whole-plant responses to different thermal conditions, further understanding of the consequences of adapting to high and low temperature extremes, and raise new questions about the mechanisms, benefits and costs of thermal specialisation under different climatic conditions.

Herbivores can affect plant population growth both directly through the damage they inflict to a focal species, and indirectly by moderating conditions for plant recruitment, nutrient cycling, competition and other biotic interactions, for example, through trampling, defecation, and grazing on surrounding vegetation. Still, the relative importance of direct versus indirect effects of herbivores on plant vital rates (establishment, survival, fecundity), size, and population growth rate is poorly known. We quantified direct and indirect effects of ungulate grazers on population growth rate of the short-lived perennial herb Primula farinosa, using integral projection models based on demographic data collected over seven years in exclosures and open control plots in nine grassland populations in southern Sweden. Grazers had negative direct effects on P. farinosa population growth rate, but these were on average more than balanced by positive indirect effects. As predicted, the strength of the direct negative effect tended to increase with grazing intensity. The positive indirect effect was mainly linked to improved conditions for plant recruitment, and was strongest in populations where vegetation height differed most between exclosure and control. Simulations indicated that indirect effects of ungulate grazers on population growth rate via interactions with pollinators, seed predators, and small herbivores were weak. Our study illustrates how both the overall, direct, and indirect effects of grazing on plant population growth rate can be identified and quantified, and thereby provide a more complete understanding of how grazers influence plant fitness, abundance, and distribution. Such insight will be crucial for predictions of the effects of environmental change on population viability and the management of declining species.

A common approach to investigating species' niches is to examine relationships between spatial variation in environmental conditions and contemporary species occurrences, using species distribution models (SDM or niche models). The relationships between past species distributions and environmental variation over time are less commonly explored. One way to examine effects on species changes over time is to use paleo-datasets to parameterize niche models, where the use of temporal variation allows for making more direct links between past species and environmental conditions through records of past changes. We examined the impact of five environmental variables (temperature, incidence of external nutrient input, local [within bog] moisture, incidence of regionally dry periods, and fire activity) on temporal variation in peatland species composition, occurrences, and abundances (Sphagnum, Eriophorum, Carex, and Ericaceous dwarf shrubs) using a high-resolution peat macrofossil paleo-record spanning the last ~10,000 years from the Store Mosse bog (south-central Sweden). Our results showed that species composition was affected by external nutrient input, local moisture conditions and incidence of regionally dry conditions. The presence and abundance of different species groups were mainly affected by external nutrient input and the incidence of regionally dry periods. Moreover, hummock Sphagna benefited from external nutrient input and low moisture, and in one species, warmer temperatures. Intermediate Sphagna from cooler temperatures with no external nutrient input, and hollow Sphagna from cooler temperatures and external nutrient input. Lastly, our results showed that environmental effects differed between the successional stages of the peatland in one case. Overall, the observed species' responses imply that peatland carbon dynamics will shift with future changes in climate. By examining links between climate and species responses of the past, this study demonstrates that the paleo-data approach in SDMs can contribute to a better understanding of the environmental effects influencing species distributions on longer time scales, thereby providing a valuable tool to improve predictions of future climate change effects.

Individual plant size often determines the vital rates of growth, survival and reproduction. However, size can be measured in several ways (e.g. height, biomass, leaf length). There is no consensus on the best size metric for modelling vital rates in plants. Demographic datasets are expanding in geographic extent, leading to choices about how to represent size for the same species in multiple ecological contexts. If the choice of size variable varies among locations, inter-population comparative demography increases in complexity. Here, we present a framework to perform size metric selection in large-scale demographic studies. We highlight potential pitfalls and suggest methods applicable to diverse study organisms. We assessed the performance of five different size metrics for the perennial herb Plantago lanceolata, across 55 populations on three continents within its native and non-native ranges, using the spatially replicated demographic dataset PlantPopNet. We compared the performance of each candidate size metric for four vital rates (growth, survival, flowering probability and reproductive output) using generalized linear mixed models. We ranked the candidate size metrics based on their overall performance (highest generalized R2) and homogeneity of performance across populations (lowest total magnitude of, and variance in, population-level error). While all size variables performed well for modelling vital rates, the number of leaves (modelled as a discrete variable, without transformation) was selected as the best size metric, followed by leaf length. We show how to interrogate potential trade-offs between overall explanatory power and homogeneity of predictions across populations in any organism. Synthesis. Size is an important determinant of vital rates. Using a dataset of unprecedented spatial extent, we find (a) consistent size-based models of growth, survival and reproduction across native and non-native populations of this cosmopolitan plant species and (b) that several tested size metrics perform similarly well. This is encouraging for large-scale demographic studies and for comparative projects using different size metrics, as they may be robust to this methodological difference.

Genetic differentiation in traits is assumed to frequently occur in response to divergent natural selection. For example, developmental traits might respond to differences in climate. However, little is known about when and at which spatial scales environmental differences lead to genetic differentiation, and to what extent there is genetic differentiation also in trait plasticity. Using a crossing design and a greenhouse heating experiment, we investigated genetic differentiation in thermal sensitivity of flowering time in a perennial herb along small-scale gradients in geothermal soil heating in Iceland. We found additive genetic variation in both flowering time and thermal plasticity of flowering time. Genetic differentiation in the median flowering date of individuals showed a counter-gradient pattern; flowering being earlier at higher greenhouse temperatures, while at a given temperature individuals originating from warmer soils flowered later than individuals from colder soils. We found no corresponding pattern for plasticity, suggesting that genetic differentiation in phenology in response to soil heating has occurred through changes in trait means rather than in plasticity. Findings such as these identifying genetic trait differentiation along an environmental gradient are key to understand how environmental variation can drive the process of local adaptation, and to predict responses to future environmental changes.

Extreme droughts are globally increasing in frequency and severity. Most research on drought in forests focuses on the response of trees, while less is known about the impacts of drought on forest understory species and how these effects are moderated by the local environment. We assessed the impacts of a 45-day experimental summer drought on the performance of six boreal forest understory plants, using a transplant experiment with rainout shelters replicated across 25 sites. We recorded growth, vitality and reproduction immediately, 2 months, and 1 year after the simulated drought, and examined how differences in ambient soil moisture and canopy cover among sites influenced the effects of drought on the performance of each species. Drought negatively affected the growth and/or vitality of all species, but the effects were stronger and more persistent in the bryophytes than in the vascular plants. The two species associated with older forests, the moss Hylocomiastrum umbratum and the orchid Goodyera repens, suffered larger effects than the more generalist species included in the experiment. The drought reduced reproductive output in the moss Hylocomium splendens in the next growing season, but increased reproduction in the graminoid Luzula pilosa. Higher ambient soil moisture reduced some negative effects of drought on vascular plants. Both denser canopy cover and higher soil moisture alleviated drought effects on bryophytes, likely through alleviating cellular damage. Our experiment shows that boreal understory species can be adversely affected by drought and that effects might be stronger for bryophytes and species associated with older forests. Our results indicate that the effects of drought can vary over small spatial scales and that forest landscapes can be actively managed to alleviate drought effects on boreal forest biodiversity. For example, by managing the tree canopy and protecting hydrological networks.

Premise: In plants, within-individual trait variation might result from mechanisms related to ontogenetic contingency, i.e., to the position of a particular structure within the plant, previous developmental events, and/or the developmental environment. Flower position within inflorescences as well as inflorescence position within plants can influence resource provisioning, phenology, biotic interactions, and reproductive success. Despite the potential implications of within-individual variation in plant reproductive phenotypes, its causes and effects on reproductive success are still little explored. Methods: We assessed how reproductive success, in terms of fruit and seed set, and seed predation of 5883 flowers in Lathyrus vernus were influenced by their position within and among racemes, to what extent relationships between flower position and reproductive success and seed predation were mediated by phenology, and if positional effects on reproductive success depended on the external environment. Results: In three years, basal flowers and racemes opened earlier and had higher fruit set than distal. Basal flowers also experienced higher seed predation. Differences among racemes in fruit and seed set were largely related to phenology, while differences in fruit set, seed set, and seed predation within racemes were not. In one year, differences in fruit set among flowers at different positions depended on flowering duration. Conclusions: Our results highlight the important role of ontogenetic contingency for within-individual variation in phenology and reproductive success. As the spatial distribution of reproductive structures affects both within-plant trait distributions and fitness, it is a likely target for natural selection.