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.

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.

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.

Climate is assumed to strongly influence species distribution and abundance. Although the performance of many organisms is influenced by the climate in their immediate proximity, the climate data used to model their distributions often have a coarse spatial resolution. This is problematic because the local climate experienced by individuals might deviate substantially from the regional average. This problem is likely to be particularly important for sessile organisms like plants and in environments where small-scale variation in climate is large. To quantify the effect of local temperature on vital rates and population growth rates, we used temperature values measured at the local scale (in situ logger measures) and integral projection models with demographic data from 37 populations of the forest herb Lathyrus vernus across a wide latitudinal gradient in Sweden. To assess how the spatial resolution of temperature data influences assessments of climate effects, we compared effects from models using local data with models using regionally aggregated temperature data at several spatial resolutions (≥1 km). Using local temperature data, we found that spring frost reduced the asymptotic population growth rate in the first of two annual transitions and influenced survival in both transitions. Only one of the four regional estimates showed a similar negative effect of spring frost on population growth rate. Our results for a perennial forest herb show that analyses using regionally aggregated data often fail to identify the effects of climate on population dynamics. This emphasizes the importance of using organism-relevant estimates of climate when examining effects on individual performance and population dynamics, as well as when modeling species distributions. For sessile organisms that experience the environment over small spatial scales, this will require climate data at high spatial resolutions. 

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Natural selection on traits expressed repeatedly by individuals is usually investigated with a focus on mean values, although within-individual trait distributions often differ also in other aspects, such as their spread and shape. In plants producing multiple flowers during a season, there might not be a single optimal flowering time, but rather an optimal distribution of flower opening dates. This optimal distribution might depend on both resource allocation patterns and interactions with the abiotic and biotic environment. In this study, we quantified mean, variance, skewness and kurtosis of 495 individual flowering schedules (5287 flowers) over 3 years, and assessed phenotypic selection on these aspects of the within-individual distribution of opening dates in the perennial herb Lathyrus vernus. We also explored how selection on within-individual variation in flowering schedules was related to effects on two fitness components: fruit set and the proportion of seeds escaping pre-dispersal predation. Within-individual variation in phenology was larger than, or at least similar to, among-individual variation in all years. We found phenotypic selection on several aspects of individual flowering schedules. In 1 year, selection favoured plants with higher variance in opening dates, and this coincided with a higher fruit set in plants with an increased spread of the flowering schedule. In two of the study years, selection favoured a higher asymmetry of the flowering schedule, and plants with more right-skewed distributions had higher fruit set and higher proportions of seeds escaping predation. Both fruit set and seed predation increased with an earlier mean flowering, resulting in no net selection on mean flowering date. Synthesis: Our results suggest that phenotypic selection on the spread and shape of flower opening date distributions might be at least as important as selection on the mean flowering date. In a broad sense, this implies that we should consider the entire trait distribution if we aim to understand the evolution of traits that are expressed multiple times within individuals.

Questions: Near-ground temperatures can vary substantially over relatively short distances, enabling species with different temperature preferences and geographical distributions to co-exist within a small area. In a forest landscape, the near-ground temperatures may change due to management activities that alter forest density. As a result of such management activities, current species distributions and performances might not only be affected by current microclimates, but also by past conditions due to time-lagged responses.

Location: Sweden.

Methods: We examined the effects of past and current microclimates on the distributions and performances of two northern, cold-favoured, and two southern, warm-favoured, plant species in 53 managed forest sites. Each pair was represented by one vascular plant and one bryophyte species. We used temperature logger data and predictions from microclimate models based on changes in basal area to relate patterns of occurrence, abundance, and reproduction to current and past microclimate.

Results: The two northern species were generally favoured by microclimates that were currently cold, characterised by later snowmelt and low accumulated heat over the growing season. In contrast, the two southern species were generally favoured by currently warm microclimates, characterised by high accumulated heat over the growing season. Species generally had higher abundance in sites with a preferred microclimate both in the past and present, and lower abundance than expected from current conditions, if the past microclimate had changed from warm to cold or vice versa, indicating time-lags in abundance patterns of the species.

Conclusions: Our results show a potential importance of past and present microclimate heterogeneity for the co-existence of species with different temperature preferences in the same landscape and highlight the possibility to manage microclimates to mitigate climate change impacts on forest biodiversity.