Soil properties constrain forest understory plant distributions along an elevation gradient.

Projections of spatial biodiversity dynamics under climate warming are often based on models including only climate variables, and when non-climatic factors (e.g. soil) are included, data are often at much coarser spatial resolutions than those experienced by plants. Field studies along elevation gradients permit the gathering of detailed soil data, while still covering a wide climatic gradient. Here, an intensive field survey of four spring forest herbs along an elevation gradient showed that soil properties had substantial impacts on the occurrence/abundance of all species, and that soil effects were more pronounced at higher elevations. For Trillium erectum and Claytonia caroliniana, very infrequent occurrences at high elevation were strongly associated with rare microsites with high pH or nutrients. In a seven-year transplant experiment with T. erectum, we found that individuals grew to much smaller sizes at high than low elevation, suggesting that environmental factors rather than dispersal limitation constrain the species' upper range limit, despite substantial warming in recent decades. Our study demonstrates that soil factors interact strongly with climate to determine plant range limits along climatic gradients. Unsuitable soils for plants at high elevations or latitudes may represent an important constraint on future plant migration and biodiversity change. This article is part of the theme issue 'Ecological novelty and planetary stewardship: biodiversity dynamics in a transforming biosphere'.

Range expansion is both slower and more variable with rapid evolution across a spatial gradient in temperature.

Rapid evolution in colonising populations can alter our ability to predict future range expansions. Recent theory suggests that the dynamics of replicate range expansions are less variable, and hence more predictable, with increased selection at the expanding range front. Here, we test whether selection from environmental gradients across space produces more consistent range expansion speeds, using the experimental evolution of replicate duckweed populations colonising landscapes with and without a temperature gradient. We found that the range expansion across a temperature gradient was slower on average, with range-front populations displaying higher population densities, and genetic signatures and trait changes consistent with directional selection. Despite this, we found that with a spatial gradient range expansion speed became more variable and less consistent among replicates over time. Our results therefore challenge current theory, highlighting that chance can still shape the genetic response to selection to influence our ability to predict range expansion speeds.

Negative interaction effect of heat and drought stress at the warm end of species distribution.

Geographic range limits of species are often a reflection of their ecological niche limits. In many organisms, important niche limits that coincide with distribution limits are warm and warm-dry conditions. We investigated the effects of heat and drought, as they can occur at the warm end of distribution. In a greenhouse experiment, we raised North American Arabidopsis lyrata from the centre of its distribution as well as from low- and high-latitude limits under average and extreme conditions. We assessed plant growth and development, as well as leaf and root functional traits, and tested for a decline in performance and selection acting on growth, leaf, and root traits. Drought and heat, when applied alone, lowered plant performance, while combined stress caused synergistically negative effects. Plants from high latitudes did not survive under combined stress, whereas plants originating from central and low latitudes had low to moderate survival, indicating divergent adaptation. Traits positively associated with survival under drought, with or without heat, were delayed and slowed growth, though plastic responses in these traits were generally antagonistic to the direction of selection. In line, higher tolerance of stress in southern populations did not involve aspects of growth but rather a higher root-to-shoot ratio and thinner leaves. In conclusion, combined heat and drought, as can occur at southern range edges and presumably more so under global change, seriously impede the long-term persistence of A. lyrata, even though they impose selection and populations may adapt, though under likely interference by considerable maladaptive plasticity.

The Mismatch between Range and Niche Limits due to Source-Sink Dynamics Can Be Greater than Species Mean Dispersal Distance.

AbstractSpecies distribution models assume that at broad spatial scales, environmental conditions determine species ranges and, as such, source-sink dynamics can be ignored. A rationale behind this assumption is that source-sink dynamics manifest at length scales comparable to species mean dispersal distance, which is much smaller than length scales of species distribution and variation in climate. Using a two-dimensional reaction-diffusion model, we show that species can use sink habitats near the niche limit as stepping-stones to occupy sink habitats much further than the mean dispersal distance, thereby extending the distribution far beyond the environmental niche limit. This mismatch between range and niche limits is mediated by the shape (local curvature) of the niche limit. These curvature effects may be significant for a highly dispersive species with low per capita growth rate sensitivity to changes in the environment. These findings underscore the potential importance of stepping-stone dispersal in determining range limits.

The role of demographic compensation in stabilising marginal tree populations in North America.

Demographic compensation-the opposing responses of vital rates along environmental gradients-potentially delays anticipated species' range contraction under climate change, but no consensus exists on its actual contribution. We calculated population growth rate (λ) and demographic compensation across the distributional ranges of 81 North American tree species and examined their responses to simulated warming and tree competition. We found that 43% of species showed stable population size at both northern and southern edges. Demographic compensation was detected in 25 species, yet 15 of them still showed a potential retraction from southern edges, indicating that compensation alone cannot maintain range stability. Simulated climatic warming caused larger decreases in λ for most species and weakened the effectiveness of demographic compensation in stabilising ranges. These findings suggest that climate stress may surpass the limited capacity of demographic compensation and pose a threat to the viability of North American tree populations.

Environment dependence of the expression of mutational load and species' range limits.

Theoretical and empirical research on the causes of species' range limits suggest the contribution of several intrinsic and extrinsic factors, with potentially complex interactions among them. An intrinsic factor proposed by recent theory is mutational load increasing towards range edges because of genetic drift. Furthermore, environmental quality may decline towards range edges and enhance the expression of load. Here, we tested whether the expression of mutational load associated with range limits in the North American plant Arabidopsis lyrata was enhanced under stressful environmental conditions by comparing the performance of within- versus between-population crosses at common garden sites across the species' distribution and beyond. Heterosis, reflecting the expression of load, increased with heightened estimates of genomic load and with environmental stress caused by warming, but the interaction was not significant. We conclude that range-edge populations suffer from a twofold genetic Allee effect caused by increased mutational load and stress-dependent load linked to general heterozygote deficiency, but there is no synergistic effect between them.

Competition and disturbance affect elevational distribution of two congeneric conifers.

Climatic change will affect elevational vegetation distribution because vegetation distribution is related to thermal conditions. However, how elevational species distributions are determined by biotic and abiotic factors is not clear. The long-term plot census along an elevational gradient is indispensable to clarify mechanisms of elevational distribution of tree species. Two congeneric conifers, the less shade-tolerant Abies veitchii and shade-tolerant A. mariesii, dominate at low and high elevations, respectively, in the subalpine zone in Japan. This study investigated the population dynamics of the two species at three elevations (low, middle, high) for 13 years to examine why the two species dominated the different elevations from the viewpoints of competition and disturbance. This study showed that growth and survival rates were not highest at the most dominant elevations for each species. At the high elevation where A. mariesii dominated and small disturbances frequently occurred, the recruitment rate of A. mariesii was highest among the three elevations and that of A. veitchii was largely decreased by tree competition. However, A. veitchii was dominant earlier than A. mariesii at the low elevation after large disturbances by the high growth rate of individual trees. Therefore, A. mariesii was superior to A. veitchii at the high elevation because of its high recruitment rate and large reduction of recruitment of A. veitchii due to competition, while A. veitchii was superior to A. mariesii at the low elevation after large disturbances because of higher growth rate than A. mariesii. It is suggested that the elevational distributions of the two species were determined by elevational changes in population dynamics in relation to competition and disturbance. Long-term observational studies of forest dynamics among various elevations are indispensable to predict the effects of climatic change on vegetation distribution.

Temporal variation may have diverse impacts on range limits.

Environmental fluctuations are pervasive in nature, but the influence of non-directional temporal variation on range limits has received scant attention. We synthesize insights from the literature and use simple models to make conceptual points about the potentially wide range of ecological and evolutionary effects of temporal variation on range limits. Because organisms respond nonlinearly to environmental conditions, temporal variation can directionally alter long-term growth rates, either to shrink or to expand ranges. We illustrate this diversity of outcomes with a model of competition along a mortality gradient. Temporal variation can permit transitions between alternative states, potentially facilitating range expansion. We show this for variation in dispersal, using simple source-sink population models (with strong Allee effects, or with gene flow hampering local adaptation). Temporal variation enhances extinction risk owing to demographic stochasticity, rare events, and loss of genetic variation, all tending to shrink ranges. However, specific adaptations to exploit variation (including dispersal) may permit larger ranges than in similar but constant environments. Grappling with temporal variation is essential both to understand eco-evolutionary dynamics at range limits and to guide conservation and management strategies. This article is part of the theme issue 'Species' ranges in the face of changing environments (Part II)'.

A review on trade-offs at the warm and cold ends of geographical distributions.

Species' range limits are ubiquitous. This suggests that the evolution of the ecological niche is constrained in general and at the edges of distributions in particular. While there may be many ecological and genetic reasons for this phenomenon, here we focus on the potential role of trade-offs. We performed a literature search on evidence for trade-offs associated with geographical or elevational range limits. The majority of trade-offs were reported as relevant at either the cold end of species' distribution (n = 19), the warm or dry end (n = 19) or both together (n = 14). One common type of trade-off involved accelerating growth or development (27%), often at the cost of small size. Another common type involved resistance to or tolerance of climatic extremes that occur at certain periods of the year (64%), often at the cost of small size or reduced growth. Trade-offs overlapped with some of the classic trade-offs reported in life-history evolution or thermal adaptation. The results highlight several general insights about species' niches and ranges, and we outline how future research should better integrate the ecological context and test for the presence of microevolutionary trade-offs. This article is part of the theme issue 'Species' ranges in the face of changing environments (Part II)'.

Adaptation across geographic ranges is consistent with strong selection in marginal climates and legacies of range expansion.

Every species experiences limits to its geographic distribution. Some evolutionary models predict that populations at range edges are less well adapted to their local environments due to drift, expansion load, or swamping gene flow from the range interior. Alternatively, populations near range edges might be uniquely adapted to marginal environments. In this study, we use a database of transplant studies that quantify performance at broad geographic scales to test how local adaptation, site quality, and population quality change from spatial and climatic range centers toward edges. We find that populations from poleward edges perform relatively poorly, both on average across all sites (15% lower population quality) and when compared to other populations at home (31% relative fitness disadvantage), consistent with these populations harboring high genetic load. Populations from equatorial edges also perform poorly on average (18% lower population quality) but, in contrast, outperform foreign populations (16% relative fitness advantage), suggesting that populations from equatorial edges have strongly adapted to unique environments. Finally, we find that populations from sites that are thermally extreme relative to the species' niche demonstrate strong local adaptation, regardless of their geographic position. Our findings indicate that both nonadaptive processes and adaptive evolution contribute to variation in adaptation across species' ranges.

Range edges of North American marine species are tracking temperature over decades.

Understanding the dynamics of species range edges in the modern era is key to addressing fundamental biogeographic questions about abiotic and biotic drivers of species distributions. Range edges are where colonization and extirpation processes unfold, and so these dynamics are also important to understand for effective natural resource management and conservation. However, few studies to date have analyzed time series of range edge positions in the context of climate change, in part because range edges are difficult to detect. We first quantified positions for 165 range edges of marine fishes and invertebrates from three U.S. continental shelf regions using up to five decades of survey data and a spatiotemporal model to account for sampling and measurement variability. We then analyzed whether those range edges maintained their edge thermal niche-the temperatures found at the range edge position-over time. A large majority of range edges (88%) maintained either summer or winter temperature extremes at the range edge over the study period, and most maintained both (76%), although not all of those range edges shifted in space. However, we also found numerous range edges-particularly poleward edges and edges in the region that experienced the most warming-that did not shift at all, shifted further than predicted by temperature alone, or shifted opposite the direction expected, underscoring the multiplicity of factors that drive changes in range edge positions. This study suggests that range edges of temperate marine species have largely maintained the same edge thermal niche during periods of rapid change and provides a blueprint for testing whether and to what degree species range edges track temperature in general.

Prioritized carbon allocation to storage of different functional types of species at the upper range limits is driven by different environmental drivers.

The upper elevational range limit of tree species (including treeline and non-treeline species) is generally considered to result from either carbon limitation or sink limitation. Some evidence also suggests that tree line might reflect preferential carbon allocation to NSC storage at the expense of growth. How might the importance of these potential mechanisms be determined? We used an elevational gradient to examine light-saturated photosynthesis (Asat) and NSC concentrations in plant tissues of three different functional types of tree species. We also examined the effects of consecutive 4 years of in situ defoliation on growth and NSCs at the upper elevational range limit. Declining temperature with increasing elevation did not reduce Asat in any of the species. We found NSC increased with elevation in major storage tissues (e.g., roots and twigs) but not in leaves. The defoliation showed that C storage took priority over growth. Such preferential carbon allocation, directly caused by growth decline, always existed in the deciduous tree species. In the evergreen tree species, however, growth decline resulted from preferential carbon allocation to storage was only detected in 2017 and then disappeared as the intensity of defoliation increased. Our results showed that trees prioritized sustaining stores of C more highly than allocation of growth, regardless of the trees' C or sink limitations. At the cold range limits, the prioritized carbon allocation to storage in deciduous tree species was in response to low temperature stress, while in evergreen tree species, the prioritization of carbon allocation was only a transient physiological response to defoliation disturbances.

Phenological synchrony between the hoary squash bee (Eucera pruinosa) and cultivated acorn squash (Cucurbita pepo) flowering is imperfect at a northern site.

The phenology of crop flowering and pollinator reproduction can become asynchronous at the edge of their respective ranges. At a northern site in Peterborough County, Ontario, we evaluated offspring emergence of Cucurbita pollen specialist hoary squash bees (Eucera pruinosa) from nests in enclosures to determine their phenological synchrony with a squash crop (Cucurbita pepo). For the crop, we evaluated the percentage of bees that emerged in time to provide pollination services during the crop pollination window. For the bees, we compared the period when both male and females were present and could mate to the whole crop flowering period. We found that fewer than half the bees had emerged by the time the crop pollination window closed and only 34.1% of the flowering period of the crop could support the reproductive activities of the bees, suggesting that phenological synchrony was imperfect from the perspective of both the crop and the pollinator at this northern site.

Microbes, mutualism, and range margins: testing the fitness consequences of soil microbial communities across and beyond a native plant's range.

Interactions between plants and soil fungi and bacteria are ubiquitous and have large effects on individual plant fitness. However, the degree to which spatial variation in soil microbial communities modulates plant species' distributions remains largely untested. Using the California native plant Clarkia xantiana ssp. xantiana we paired glasshouse and field reciprocal transplants of plant populations and soils to test whether plant-microbe interactions affect the plant's geographic range limit and whether there is local adaptation between plants and soil microbe communities. In the field and glasshouse, one of the two range interior inocula had a positive effect on plant fitness. In the field, this benefit was especially pronounced at the range edge and beyond, suggesting possible mutualist limitation. In the glasshouse, soil inocula from beyond-range tended to increase plant growth, suggesting microbial enemy release beyond the range margin. Amplicon sequencing revealed stark variation in microbial communities across the range boundary. Plants dispersing beyond their range limit are likely to encounter novel microbial communities. In C. x. xantiana, our results suggest that range expansion may be facilitated by fewer pathogens, but could also be hindered by a lack of mutualists. Both negative and positive plant-microbe interactions will likely affect contemporary range shifts.

Plant-soil interactions limit lifetime fitness outside a native plant's geographic range margin.

Plant species' distributions are often thought to overwhelmingly reflect their climatic niches. However, climate represents only a fraction of the n-dimensional environment to which plant populations adapt, and studies are increasingly uncovering strong effects of nonclimatic factors on species' distributions. We used a manipulative, factorial field experiment to quantify the effects of soil environment and precipitation (the putatively overriding climatic factor) on plant lifetime fitness outside the geographic range boundary of a native California annual plant. We grew plants outside the range edge in large mesocosms filled with soil from either within or outside the range, and plants were subjected to either a low (ambient) or high (supplemental) spring precipitation treatment. Soil environment had large effects on plant lifetime fitness that were similar in magnitude to the effects of precipitation. Moreover, mean fitness of plants grown with within-range soil in the low precipitation treatment approximated that of plants grown with beyond-range soil in the high precipitation treatment. The positive effects of within-range soil persisted in the second, wetter year of the experiment, though the magnitude of the soil effect was smaller than in the first, drier year. These results are the first we know of to quantify the effects of edaphic variation on plant lifetime fitness outside a geographic range limit and highlight the need to include factors other than climate in models of species' distributions.

Coexistence barriers confine the poleward range of a globally distributed plant.

In the study of factors shaping species' poleward range boundaries, climatic constraints are often assigned greater importance than biotic interactions such as competition. However, theory suggests competition can truncate a species' fundamental niche in harsh environments. We test this by challenging a mechanistic niche model - containing explicit competition terms - to predict the poleward range boundaries of two globally distributed, ecologically similar aquatic plant species. Mechanistic competition models accurately predicted the northern range limits of our study species, outperforming competition-free mechanistic models and matching the predictive ability of statistical niche models fit to occurrence records. Using the framework of modern coexistence theory, we found that relative nonlinearity in competitors' responses to temperature fluctuations maintains their coexistence boundary, highlighting the importance of this fluctuation-dependent mechanism. Our results support a more nuanced, interactive role of climate and competition in determining range boundaries, and illustrate a practical, process-based approach to understanding the determinants of range limits.

Dispersal limitation and fire feedbacks maintain mesic savannas in Madagascar.

Madagascar is regarded by some as one of the most degraded landscapes on Earth, with estimates suggesting that 90% of forests have been lost to indigenous Tavy farming. However, the extent of this degradation has been challenged: paleoecological data, phylogeographic analysis, and species richness indicate that pyrogenic savannas in central Madagascar predate human arrival, even though rainfall is sufficient to allow forest expansion into central Madagascar. These observations raise a question-if savannas in Madagascar are not anthropogenic, how then are they maintained in regions where the climate can support forest? Observation reveals that the savanna-forest boundary coincides with a dispersal barrier-the escarpment of the Central Plateau. Using a stepping-stone model, we show that in a limited dispersal landscape, a stable savanna-forest boundary can form because of fire-vegetation feedbacks. This phenomenon, referred to as range pinning, could explain why eastern lowland forests have not expanded into the mesic savannas of the Central Highlands. This work challenges the view that highland savannas in Madagascar are derived by human-lit fires and, more importantly, suggests that partial dispersal barriers and strong nonlinear feedbacks can pin biogeographical boundaries over a wide range of environmental conditions, providing a temporary buffer against climate change.

Expressed mutational load increases toward the edge of a species' geographic range.

There is no general explanation for why species have restricted geographic distributions. One hypothesis posits that range expansion or increasing scarcity of suitable habitat results in accumulation of mutational load due to enhanced genetic drift, which constrains population performance toward range limits and further expansion. We tested this hypothesis in the North American plant, Arabidopsis lyrata. We experimentally assessed mutational load by crossing plants of 20 populations from across the entire species range and by raising the offspring of within- and between-population crosses at five common garden sites within and beyond the range. Offspring performance was tracked over three growing seasons. The heterosis effect, depicting expressed mutational load, was increased in populations with heightened genomic estimates of load, longer expansion distance or long-term isolation, and a selfing mating system. The decline in performance of within-population crosses amounted to 80%. Mutation accumulation due to past range expansion and long-term isolation of populations in the area of range margins is therefore a strong determinant of population-mean performance, and the magnitude of effect may be sufficient to cause range limits.

Ability of FFR-CT to detect the absence of hemodynamically significant lesions in patients with high-risk NSTE-ACS admitted in the emergency department with chest pain, study design and rationale.

BACKGROUND: In the era of High-sensitive troponin (hs-Tn), up to 50% of patients with a mild increase of hs-Tn will finally have a normal invasive coronary angiogram. Fractional Flow Reserve (FFR) derived from coronary computed tomographic angiography (FFR-CT) has never been used as a non-invasive tool for the diagnosis of coronary artery disease in patients with high-risk acute coronary syndrome without ST segment elevation (NSTE-ACS). AIMS: The study aims to determine the role of coronary CT angiography and FFR-CT in the setting of high-risk NSTE-ACS. METHODOLOGY: We will conduct a prospective trial, enrolling 250 patients admitted with high-risk NSTE-ACS who will rapidly undergo a coronary CT angiography and then a coronary angiography with FFR measurements. Results of coronary CT, FFR-CT and coronary angiography (± FFR) will be compared. POTENTIAL SIGNIFICANCE: In conclusion, non-invasive identification of patients with high-risk NSTE-ACS who could avoid coronary angiography would reduce procedure related risks and medical costs.

Cold range edges of marine fishes track climate change better than warm edges.

Species around the world are shifting their ranges in response to climate change. To make robust predictions about climate-related colonizations and extinctions, it is vital to understand the dynamics of range edges. This study is among the first to examine annual dynamics of cold and warm range edges, as most global change studies average observational data over space or over time. We analyzed annual range edge dynamics of marine fishes-both at the individual species level and pooled into cold- and warm-edge assemblages-in a multi-decade time-series of trawl surveys conducted on the Northeast US Shelf during a period of rapid warming. We tested whether cold edges show stronger evidence of climate tracking than warm edges (due to non-climate processes or time lags at the warm edge; the biogeography hypothesis or extinction debt hypothesis), or whether they tracked temperature change equally (due to the influence of habitat suitability; the ecophysiology hypothesis). In addition to exploring correlations with regional temperature change, we calculated species- and assemblage-specific sea bottom and sea surface temperature isotherms and used them to predict range edge position. Cold edges shifted further and tracked sea surface and bottom temperature isotherms to a greater degree than warm edges. Mixed-effects models revealed that for a one-degree latitude shift in isotherm position, cold edges shifted 0.47 degrees of latitude, and warm edges shifted only 0.28 degrees. Our results suggest that cold range edges are tracking climate change better than warm range edges, invalidating the ecophysiology hypothesis. We also found that even among highly mobile marine ectotherms in a global warming hotspot, few species are fully keeping pace with climate.