Climate change is swiftly altering environmental winter conditions, leading to significant ecological impacts such as phenological shifts in many species. As a result, animals might face physiological mismatches due to longer or earlier activity periods and are at risk of being exposed to late spring freezes. Our study points for the first time to the complex physiological challenges that amphibians face as a result of changing thermal conditions due to winter climate change. We investigated the physiological responses to a period of warmer winter days and sudden spring freeze in the common toad (Bufo bufo) by acclimating them to 4°C or 8°C for 48 h or exposing them to 4°C or -2°C for 6 h, respectively. We assessed the daily energy demands, determined body condition and cold tolerance, explored the molecular responses to freezing through hepatic tissue transcriptome analysis, and measured blood glucose levels. Toads acclimated to higher temperatures showed a higher daily energy expenditure and a reduced cold tolerance suggesting faster depletion of energy stores and the loss of winter acclimation during warmer winters. Blood sugar levels were higher in frozen toads indicating the mobilization of cryoprotective glucose with freezing which was further supported by changed patterns in proteins related to glucose metabolism. Overall, our results emphasize that increased thermal variability incurs physiological costs that may reduce energy reserves and thus affect amphibian health and survival. This might pose a serious threat to breeding adults and may have subsequent effects at the population level.

Ongoing climate change and anthropogenic pressure are having a profound influence on insects, causing species diversity to decline and populations to shrink. Insect pests invade new areas and cause economic and human health problems. Low temperatures in winter are thought to be one of the main barriers to the successful colonization of higher latitudes. Climate models predict that winter temperatures will increase more than summer temperatures in temperate and polar regions, potentially allowing species from warmer climates to colonize higher latitudes. Understanding how climate change will affect the distribution of insects is critical to many areas of human activity. One possible but seldom used way to predict likely range shifts of insects due to climate change is through simulation experiments. Here, I present and test a method to assess the potential of insect species from warmer regions to survive winters in colder regions under a warming winter scenario. The method is based on laboratory simulations of warming winters. The applicability of the method is demonstrated using the example of a Mediterranean pest, Sesamia nonagrioides, whose ability to survive Central European winters under a warming winter scenario is assessed. The method presented here is relatively simple, with potentially high accuracy of estimates.

Climate change is transforming ecosystems and affecting ecosystem goods and services. Along the Gulf of Mexico and Atlantic coasts of the southeastern United States, the frequency and intensity of extreme freeze events greatly influence whether coastal wetlands are dominated by freeze-sensitive woody plants (mangrove forests) or freeze-tolerant grass-like plants (salt marshes). In response to warming winters, mangroves have been expanding and displacing salt marshes at varying degrees of severity in parts of north Florida, Louisiana, and Texas. As winter warming accelerates, mangrove range expansion is expected to increasingly modify wetland ecosystem structure and function. Because there are differences in the ecological and societal benefits that salt marshes and mangroves provide, coastal environmental managers are challenged to anticipate the effects of mangrove expansion on critical wetland ecosystem services, including those related to carbon sequestration, wildlife habitat, storm protection, erosion reduction, water purification, fisheries support, and recreation. Mangrove range expansion may also affect wetland stability in the face of extreme climatic events and rising sea levels. Here, we review the current understanding of the effects of mangrove range expansion and displacement of salt marshes on wetland ecosystem services in the southeastern United States. We also identify critical knowledge gaps and emerging research needs regarding the ecological and societal implications of salt marsh displacement by expanding mangrove forests. One consistent theme throughout our review is that there are ecological trade-offs for consideration by coastal managers. Mangrove expansion and marsh displacement can produce beneficial changes in some ecosystem services, while simultaneously producing detrimental changes in other services. Thus, there can be local-scale differences in perceptions of the impacts of mangrove expansion into salt marshes. For very specific local reasons, some individuals may see mangrove expansion as a positive change to be embraced, while others may see mangrove expansion as a negative change to be constrained.

Climate change is disproportionately impacting mountain ecosystems, leading to large reductions in winter snow cover, earlier spring snowmelt and widespread shrub expansion into alpine grasslands. Yet, the combined effects of shrub expansion and changing snow conditions on abiotic and biotic soil properties remains poorly understood. We used complementary field experiments to show that reduced snow cover and earlier snowmelt have effects on soil microbial communities and functioning that persist into summer. However, ericaceous shrub expansion modulates a number of these impacts and has stronger belowground effects than changing snow conditions. Ericaceous shrub expansion did not alter snow depth or snowmelt timing but did increase the abundance of ericoid mycorrhizal fungi and oligotrophic bacteria, which was linked to decreased soil respiration and nitrogen availability. Our findings suggest that changing winter snow conditions have cross-seasonal impacts on soil properties, but shifts in vegetation can modulate belowground effects of future alpine climate change.

Tropicalization is a term used to describe the transformation of temperate ecosystems by poleward-moving tropical organisms in response to warming temperatures. In North America, decreases in the frequency and intensity of extreme winter cold events are expected to allow the poleward range expansion of many cold-sensitive tropical organisms, sometimes at the expense of temperate organisms. Although ecologists have long noted the critical ecological role of winter cold temperature extremes in tropical-temperate transition zones, the ecological effects of extreme cold events have been understudied, and the influence of warming winter temperatures has too often been left out of climate change vulnerability assessments. Here, we examine the influence of extreme cold events on the northward range limits of a diverse group of tropical organisms, including terrestrial plants, coastal wetland plants, coastal fishes, sea turtles, terrestrial reptiles, amphibians, manatees, and insects. For these organisms, extreme cold events can lead to major physiological damage or landscape-scale mass mortality. Conversely, the absence of extreme cold events can foster population growth, range expansion, and ecological regime shifts. We discuss the effects of warming winters on species and ecosystems in tropical-temperate transition zones. In the 21st century, climate change-induced decreases in the frequency and intensity of extreme cold events are expected to facilitate the poleward range expansion of many tropical species. Our review highlights critical knowledge gaps for advancing understanding of the ecological implications of the tropicalization of temperate ecosystems in North America.

Winter snow cover is a major driver of soil microbial processes in high-latitude and high-altitude ecosystems. Warming-induced reduction in snow cover as predicted under future climate scenarios may shift soil bacterial communities with consequences for soil carbon and nutrient cycling. The underlying mechanisms, however, remain elusive. In the present study, we conducted a snow manipulation experiment in a Tibetan spruce forest to explore the immediate and intra-annual legacy effects of snow exclusion on soil bacterial communities. We analyzed bacterial diversity and community composition in the winter (i.e., the deep snow season), in the transitional thawing period, and in the middle of the growing season. Proteobacteria, Acidobacteria, and Actinobacteria were dominant phyla across the seasons and snow regimes. Bacterial diversity was generally not particularly sensitive to the absence of snow cover. However, snow exclusion positively affected Simpson diversity in the winter but not in the thawing period and the growing season. Bacterial diversity further tended to be higher in winter than in the growing season. In the winter, the taxonomic composition shifted in response to snow exclusion, while composition did not differ between exclusion and control plots in the thawing period and the growing season. Soil bacterial communities strongly varied across seasons, and the variations differed in specific groups. Both soil climatic factors (i.e., temperature and moisture) and soil biochemical variables partly accounted for the seasonal dynamics of bacterial communities. Taken together, our study indicates that soil bacterial communities in Tibetan forests are rather resilient to change in snow cover, at least at an intra-annual scale.

Northern temperate ecosystems are experiencing warmer and more variable winters, trends that are expected to continue into the foreseeable future. Despite this, most studies have focused on climate change impacts during the growing season, particularly when comparing responses across different vegetation cover types. Here we examined how a perennial grassland and adjacent mixed forest ecosystem in New Hampshire, United States, responded to a period of highly variable winters from 2014 through 2017 that included the warmest winter on record to date. In the grassland, record-breaking temperatures in the winter of 2015/2016 led to a February onset of plant growth and the ecosystem became a sustained carbon sink well before winter ended, taking up roughly 90 g/m2 more carbon during the winter to spring transition than in other recorded years. The forest was an unusually large carbon source during the same period. While forest photosynthesis was restricted by leaf-out phenology, warm winter temperatures caused large pulses of ecosystem respiration that released nearly 230 g C/m2 from February through April, more than double the carbon losses during that period in cooler years. These findings suggest that, as winters continue to warm, increases in ecosystem respiration outside the growing season could outpace increases in carbon uptake during a longer growing season, particularly in forests that depend on leaf-out timing to initiate carbon uptake. In ecosystems with a perennial leaf habit, warming winter temperatures are more likely to increase ecosystem carbon uptake through extension of the active growing season. Our results highlight the importance of understanding relationships among antecedent winter conditions and carbon exchange across land-cover types to understand how landscape carbon exchange will change under projected climate warming.

Extreme weather events can injure plants, causing decreased survival. However, we may underestimate the ecological importance of extreme events if they have strong sublethal effects that manifest after several months. We tested the hypothesis that late-winter extreme-cold events decrease the ability of woody plants to grow and tolerate stem removal in summer.
Seedlings from four temperate tree species (Abies balsamea, Pinus resinosa, P. strobus, Quercus rubra) were acclimated to winter conditions in growth chambers, and experienced 1 week of warm temperatures before being exposed to one of three 24-h extreme-cold events (minimum temperature: 8°C control, -8°C, or -16°C). Seedlings were then transferred to a greenhouse where we monitored survival and growth. Three months after the extreme-cold event, we mimicked an herbivore attack by removing either 25% or 75% of new stem growth from seedlings of two species (P. resinosa, Q. rubra).
While extreme cold had no immediate effect on seedling survival, the coldest temperature treatment reduced stem growth 51% relative to controls. Stem removal decreased P. resinosa survival in the -16°C treatment, but stem removal treatment had no effect on P. resinosa survival in the intermediate -8°C treatment or 8°C control. Stem removal did not alter Q. rubra survival.
Ephemeral late-winter cold temperatures can have unappreciated effects on growing-season seedling dynamics, including growth and herbivory. For predicting how extreme-cold events might alter large-scale patterns of tree distribution, seedlings should be monitored throughout the growing season following extreme late-winter frosts.

Changes in growing season climate are often the foci of research exploring forest response to climate change. By contrast, little is known about tree growth response to projected declines in winter snowpack and increases in soil freezing in seasonally snow-covered forest ecosystems, despite extensive documentation of the importance of winter climate in mediating ecological processes. We conducted a 5-year snow-removal experiment whereby snow was removed for the first 4-5 weeks of winter in a northern hardwood forest at the Hubbard Brook Experimental Forest in New Hampshire, USA. Our results indicate that adverse impacts of reduced snowpack and increased soil freezing on the physiology of Acer saccharum (sugar maple), a dominant species across northern temperate forests, are accompanied by a 40 ± 3% reduction in aboveground woody biomass increment, averaged across the 6 years following the start of the experiment. Further, we find no indication of growth recovery 1 year after cessation of the experiment. Based on these findings, we integrate spatial modeling of snowpack depth with forest inventory data to develop a spatially explicit, regional-scale assessment of the vulnerability of forest aboveground growth to projected declines in snowpack depth and increased soil frost. These analyses indicate that nearly 65% of sugar maple basal area in the northeastern United States resides in areas that typically experience insulating snowpack. However, under the RCP 4.5 and 8.5 emissions scenarios, we project a 49%-95% reduction in forest area experiencing insulating snowpack by the year 2099 in the northeastern United States, leaving large areas of northern forest vulnerable to these changes in winter climate, particularly along the northern edge of the region. Our study demonstrates that research focusing on growing season climate alone overestimates the stimulatory effect of warming temperatures on tree and forest growth in seasonally snow-covered forests.

Recent climate warming has led to asynchronous species migrations, with major consequences for ecosystems worldwide. In woody communities, localized microclimates have the potential to create feedback mechanisms that can alter the rate of species range shifts attributed to macroclimate drivers alone. Mangrove encroachment into saltmarsh in many areas is driven by a reduction in freeze events, and this encroachment can further modify local climate, but the subsequent impacts on mangrove seedling dynamics are unknown. We monitored microclimate conditions beneath mangrove canopies and adjacent open saltmarsh at a freeze-sensitive mangrove-saltmarsh ecotone and assessed survival of experimentally transplanted mangrove seedlings. Mangrove canopies buffered night time cooling during the winter, leading to interspecific differences in freeze damage on mangrove seedlings. However, mangrove canopies also altered biotic interactions. Herbivore damage was higher under canopies, leading to greater mangrove seedling mortality beneath canopies relative to saltmarsh. While warming-induced expansion of mangroves can lead to positive microclimate feedbacks, simultaneous fluctuations in biotic drivers can also alter seedling dynamics. Thus, climate change can drive divergent feedback mechanisms through both abiotic and biotic channels, highlighting the importance of vegetation-microclimate interactions as important moderators of climate driven range shifts.