Cool microrefugia accumulate and conserve biodiversity under climate change

Climate change can disproportionately reduce habitats of stream fishes with restricted ranges in southern South America

Freshwater fishes are among the most threatened taxa worldwide owing to changes in land use, species introductions, and climate change. Although more than half of the freshwater fishes in the Chilean Mediterranean ecoregion are considered vulnerable or endangered, still little is known about their biogeography. Fishes of the family Perciliidae are endemic of this region and ideal cases to study potential implications of global warming given their endangered conservation status, small size, restricted range, and limited dispersal capacity in fragmented habitats. Here, we model the spatial distribution of habitats for Percilia irwini and P. gillissi under current (1970-2000) and future (2050-2080) climatic scenarios (SSP245, SSP585). We implement maximum entropy (MaxEnt) models adapted for stream networks using high-resolution datasets of selected geophysical and climatic variables. At present, both species inhabit relatively low-quality habitats. In the future (SSP585), suitable habitats for P. irwini are predicted to be reduced drastically (99%) with potential local extirpations in its northern range. Similarly, up to 62% of suitable habitats for P. gillissi would also be reduced in the future. Our study provides insights about assessing future threats and vulnerability of endemic, endangered, range-restricted, and small-bodied freshwater species in this region and elsewhere.

Microrefugia and microclimate: Unraveling decoupling potential and resistance to heatwaves

Microrefugia, defined as small areas maintaining populations of species outside their range margins during environmental extremes, are increasingly recognized for their role in conserving species in the face of climate change. Understanding their microclimatic dynamics becomes crucial with global warming leading to severe temperature and precipitation changes. This study investigates the phenomenon of short-term climatic decoupling within microrefugia and its implications for plant persistence in the Mediterranean region of southeastern France. We focus on microrefugia's ability to climatically disconnect from macroclimatic trends, examining temperature and Vapor Pressure Deficit (VPD) dynamics in microrefugia, adjacent control plots, and weather stations. Our study encompasses both "normal" conditions and heatwave episodes to explore the role of microrefugia as thermal and moisture insulators during extreme events. Landscape attributes such as relative elevation, solar radiation, distance to streams, and vegetation height are investigated for their contribution to short-term decoupling. Our results demonstrate that microrefugia exhibit notable decoupling from macroclimatic trends. This effect is maintained during heatwaves, underscoring microrefugia's vital role in responding to climatic extremes. Importantly, microrefugia maintain lower VPD levels than their surroundings outside and during heatwaves, potentially mitigating water stress for plants. This study advances our understanding of microclimate dynamics within microrefugia and underscores their ecological importance for plant persistence in a changing climate. As heatwaves become more frequent and severe, our findings provide insights into the role of microrefugia in buffering but also decoupling against extreme climatic events and, more generally, against climate warming. This knowledge emphasizes the need to detect and protect existing microrefugia, as they can be integrated into conservation strategies and climate change adaptation plans.

Frequent and strong cold-air pooling drives temperate forest composition

Cold-air pooling is an important topoclimatic process that creates temperature inversions with the coldest air at the lowest elevations. Incomplete understanding of sub-canopy spatiotemporal cold-air pooling dynamics and associated ecological impacts hinders predictions and conservation actions related to climate change and cold-dependent species and functions. To determine if and how cold-air pooling influences forest composition, we characterized the frequency, strength, and temporal dynamics of cold-air pooling in the sub-canopy at local to regional scales in New England, USA. We established a network of 48 plots along elevational transects and continuously measured sub-canopy air temperatures for 6-10 months (depending on site). We then estimated overstory and understory community temperature preferences by surveying tree composition in each plot and combining these data with known species temperature preferences. We found that cold-air pooling was frequent (19-43% seasonal occurrences) and that sites with the most frequent inversions displayed inverted forest composition patterns across slopes with more cold-adapted species, namely conifers, at low instead of high elevations. We also observed both local and regional variability in cold-air pooling dynamics, revealing that while cold-air pooling is common, it is also spatially complex. Our study, which uniquely focused on broad spatial and temporal scales, has revealed some rarely reported cold-air pooling dynamics. For instance, we discovered frequent and strong temperature inversions that occurred across seasons and in some locations were most frequent during the daytime, likely affecting forest composition. Together, our results show that cold-air pooling is a fundamental ecological process that requires integration into modeling efforts predicting future forest vegetation patterns under climate change, as well as greater consideration for conservation strategies identifying potential climate refugia for cold-adapted species.

Microclimate reveals the true thermal niche of forest plant species

Species distributions are conventionally modelled using coarse-grained macroclimate data measured in open areas, potentially leading to biased predictions since most terrestrial species reside in the shade of trees. For forest plant species across Europe, we compared conventional macroclimate-based species distribution models (SDMs) with models corrected for forest microclimate buffering. We show that microclimate-based SDMs at high spatial resolution outperformed models using macroclimate and microclimate data at coarser resolution. Additionally, macroclimate-based models introduced a systematic bias in modelled species response curves, which could result in erroneous range shift predictions. Critically important for conservation science, these models were unable to identify warm and cold refugia at the range edges of species distributions. Our study emphasizes the crucial role of microclimate data when SDMs are used to gain insights into biodiversity conservation in the face of climate change, particularly given the growing policy and management focus on the conservation of refugia worldwide.

Temporal and spatial variability in population traits of an intertidal fucoid reveals local-scale climatic refugia

Global change is imposing significant losses in the functional traits of marine organisms. Although areas of climatic refugia ameliorate local conditions and help them to persist, the extent to which mesoscale effects contribute for intraregional variability on population traits and conservation is uncertain. Here we assess patterns of conservation status of Fucus guiryi, the main intertidal habitat-forming seaweed in the Strait of Gibraltar (southern Spain and northern Morocco). We investigated the demography, reproductive phenology, and morphology at northern and southern side populations. Population traits were compared seasonally within populations from each side, and at spatial scale in early summer 2019. In the last decade three populations became extinct; two marginal populations had dispersed individuals with a narrower fertility season and miniaturized individuals below 3 cm; and five populations showed variable density and cover with more than 20% of reproductive individuals over the seasons. Highest density, cover, morphology, and reproductive potential was detected at one population from each side, suggesting local-scale climatic refugia in upwelling areas located inside marine protected areas. Southern recruits were more warm-tolerant but grew less at colder conditions than northern ones, revealing a mesoscale heterogeneity in thermal affinities. This study evidenced functional losses and distinct reproductive strategies experienced by F. guiryi at peripheral locations and urges to prioritize its conservation and restoration at contemporary climatic refugia.

Early Holocene plant macrofossils indicate cool refugia for subalpine plant taxa in Acadia National Park, Maine

Identifying refugia— specifically places where species can persist during periods of regionally unsuitable climate— is increasingly important for conservation practitioners and land managers charged with protecting biodiversity in a rapidly warming world. Currently, many researchers assist in this process by building models to predict areas of refugia using climate data projected into the future under different climate scenarios; however, the coarse spatial scale of future climate data can be orders of magnitude larger than the scale of refugia on the landscape. Conservation paleobiology is an emerging field that can contribute to the identification of climate refugia by looking at the macrofossil records contained in sediments to better understand the response of species to past climate change within a small area, and allows us to ground-truth hypotheses about specific areas functioning as climate refugia. Here, we present a conservation paleobiology case study to update vulnerability assessments for subalpine plant species in Acadia National Park and locate potential future refugia on the landscape. We analyzed plant macrofossils in a sediment core from Sargent Mountain Pond in Acadia National Park (Maine, United States) at a fine spatiotemporal resolution to test the hypothesis that the area served as a past climate refugium for the subalpine species it currently hosts. We found that, when compared to a pollen record from a forest hollow core collected on Mount Desert Island, the macrofossils reflect a more stable presence of subalpine taxa throughout the Holocene Climatic Optimum (8,000–5,000 BP) than was observed at lower elevations. Our results indicate the importance of a complementary approach that combines plant macrofossils and pollen in addition to modeling to identify refugia and better understand the vulnerability of species and communities to climate change.

Bridging the gap between microclimate and microrefugia: A bottom-up approach reveals strong climatic and biological offsets

In the context of global warming, a clear understanding of microrefugia-microsites enabling the survival of species populations outside their main range limits-is crucial. Several studies have identified forcing factors that are thought to favor the existence of microrefugia. However, there is a lack of evidence to conclude whether, and to what extent, the climate encountered within existing microrefugia differs from the surrounding climate. To investigate this, we adopt a "bottom-up" approach, linking marginal disconnected populations to microclimate. We used the southernmost disconnected and abyssal populations of the circumboreal herbaceous plant Oxalis acetosella in Southern France to study whether populations in sites matching the definition of "microrefugia" occur in particularly favorable climatic conditions compared to neighboring control plots located at distances of between 50 to 100 m. Temperatures were recorded in putative microrefugia and in neighboring plots for approximately 2 years to quantify their thermal offsets. Vascular plant inventories were carried out to test whether plant communities also reflect microclimatic offsets. We found that current microclimatic dynamics are genuinely at stake in microrefugia. Microrefugia climates are systematically colder compared to those found in neighboring control plots. This pattern was more noticeable during the summer months. Abyssal populations showed stronger offsets compared to neighboring plots than the putative microrefugia occurring at higher altitudes. Plant communities demonstrate this strong spatial climatic variability, even at such a microscale approach, as species compositions systematically differed between the two plots, with species more adapted to colder and moister conditions in microrefugia compared to the surrounding area.

Behavioural thermoregulation in cold-water freshwater fish: Innate resilience to climate warming?

Behavioural thermoregulation enables ectotherms to access habitats providing conditions within their temperature optima, especially in periods of extreme thermal conditions, through adjustments to their behaviours that provide a "whole-body" response to temperature changes. Although freshwater fish have been detected as moving in response to temperature changes to access habitats that provide their thermal optima, there is a lack of integrative studies synthesising the extent to which this is driven by behaviour across different species and spatial scales. A quantitative global synthesis of behavioural thermoregulation in freshwater fish revealed that across 77 studies, behavioural thermoregulatory movements by fish were detected both vertically and horizontally, and from warm to cool waters and, occasionally, the converse. When fish moved from warm to cooler habitats, the extent of the temperature difference between these habitats decreased with increasing latitude, with juvenile and non-migratory fishes tolerating greater temperature differences than adult and anadromous individuals. With most studies focused on assessing movements of cold-water salmonids during summer periods, there remains an outstanding need for work on climatically vulnerable, non-salmonid fishes to understand how these innate thermoregulatory behaviours could facilitate population persistence in warming conditions.