Winter temperatures (Thaw, Ice, and Extreme Cold Days) and snow cover (Snow Covered Days, Bare Ground Days) have changed across the northern forest region, with cold and snow covered conditions decreasing over the past 100 years. Few studies have assessed a comparable suite of general winter temperature and snowpack indicators over large spatial (regional to continental) or long temporal (>30–60 yr) scales, particularly with data that span both the northern United States and Canada. Studies that have examined winter climate change since about 1950 at global to regional scales have shown increases in winter (assessed as December‐February) temperatures, particularly nighttime temperatures, that exceeded increases in temperatures in other seasons (Vincent and Mekis 2006, Hayhoe et al. 2007, Donat et al. 2013, Vincent et al. 2015). Prior research has also demonstrated decreases in snow cover duration since about 1950, both in the northeastern United States and Canada (Burakowski et al. 2008, Vincent et al. 2015). Our findings support this general pattern of warming winter temperatures and reduced snow cover, but add further insight into potential societal and ecological impacts of such changes through the analysis of a unique suite of winter climate change indicators. The overall trend we observed toward increasing numbers of Thaw Days and Bare Ground Days and decreasing numbers of Frost Days and Snow Covered Days suggest changes to ecohydrology, soil microclimate and soil biological process, fine root dynamics, predator–prey dynamics, herbivory, and other ecological dynamics in a northern forest that is historically adapted to cold, snowy conditions. Some of the potential effects of changing winter across the region may even counterbalance one another. For example, lower numbers of days with coldness and/or snow, whether Frost Days, Extreme Cold Days, Snowmaking Days, Snow Covered Days or Frozen Ground Days, suggest positive outcomes for tree health as related to reduced fine root mortality and nutrient loss associated with winter frost (Cleavitt et al. 2008, Fuss et al. 2016). At the same time, the overall loss of coldness and snow cover might have negative consequences for tree health as related to the northward advancement and proliferation of forest insect pests. Declines in coldness and snow cover may also carry negative consequences for logging and forest products, vector‐borne diseases and human health, recreation and tourism, and cultural practices, which together represent important social and economic dimensions of the provinces, states, Tribal Nations, First Nations, Indigenous communities, cities, and communities of the northern forest region.

Our result of declining Snow Covered Days (snow depth > 0 mm) fits with previous studies in both North America and across the northern hemisphere showing that the snowpack is thinner, the snow season is shorter, and snow cover is less continuous (Dyer and Mote 2006 , Kreyling 2013 , Vincent et al. 2015 ). This was particularly evident in the east and west subregions, which exhibited median loss rates from −1.2 to −4.5 d/decade that were comparable to the average loss of −3.6 d/decade calculated by Burakowski et al. ( 2008 ) for the northeastern United States over the period of 1965 to 2005. Though not a perfect analog, Durán et al. ( 2016 ) similarly reported that time‐integrated snowpack depth (depth × duration) decreased significantly from 1971 to 2012. Likewise, Vincent et al. ( 2015 ) noted that snow cover duration (as defined by number of days when snow depth ≥2 cm) declined by −1.0 to −3.4 d/decade during 1950 to 2012. The central subregion had the fewest Snow Covered Days of all three subregions in our study, with a median of only 60.5 Snow Covered Days over the 100‐yr time series (Table 3 ). Increases and mixed trends in related indicators such as snowfall and length of snow season in the Great Lakes area, where central sites were primarily located, have previously been reported for the 20th century, driven by lake‐effect snowfalls, particularly since 1970 (Brown 2000 , Kunkel et al. 2009 ). The combination of warming water, reduced ice cover, and increased evaporation may drive these increasing snowfall trends in the Great Lakes (Kunkel et al. 2009 ), though future winter warming may ultimately lead to increased rainfall over this subregion (Notaro et al. 2015 ).

For the central subregion, the positive or mixed trends we detected for temperature indicators such as Ice Days and Extreme Cold Days fits with previous findings of both decreasing or unchanging winter temperatures in the central United States (Andresen et al. 2012 , Mascioli et al. 2017 ). Long‐term phase changes in the ocean–atmosphere circulation modes such as North Atlantic Oscillation may explain an overall cooling or mixed temperature trend in this area, though the causes of the United States “warming hole” are highly uncertain (Mascioli et al. 2017 , Partridge et al. 2018 ). In addition, we note that the weather stations we used to identify trends in this subregion were located toward the southern portion of the study area. Data were absent for the Upper Peninsula of Michigan and for central and eastern Ontario, limiting the inferences we can draw about general changes in winter coldness for this portion of the central subregion.

Due to lack of an analogous indicator, we are unable to directly compare Extreme Cold Days/Hemlock Woolly Adelgid Kill Days ( T min < −18°C) to values previously reported. However, temperature records for 22 sites in the northeastern United States for the period 1951–1993 showed significantly decreasing trends in the number of days with minimum temperatures ≤−15°C (DeGaetano 1996 ), and this agrees with our finding that Extreme Cold Days significantly declined in both the West and the Northeast over the past 100 yr.

For Ice Days ( T max < 0°C), we found fewer significant trends, weaker magnitudes of change, and greater differences among study subregions compared to other indicators. A smaller number of studies have reported trends in Ice Days compared to trends in Frost Days; however, in the northeastern United States, Brown et al. ( 2010 ) noted that for all colder minimum and maximum temperature indices (including Ice Days), more than one‐half of the stations had significantly declining trends from 1870 to 2005. In stations across the globe from 1951 to 2010, 39.4% of sites had a significant decrease in number of Ice Days, while 3.1% had a significant increase in number of Ice Days (Donat et al. 2013 ). These statistics align with our finding that Ice Days generally decreased in the west and east while increasing in the central subregion.

We found overall declining trends in number of days that quantified general “coldness” (Ice Days, Frost Days, and Extreme Cold Days), with Frost Days exhibiting the most coherent, decreasing trends across the study region. Other studies have documented similar responses, with 57%–65% of stations in the northeastern United States (1893–2005 [Brown et al. 2010 ], 1926–2000 [Griffiths and Bradley 2007 ]) and 62% of sites across Canada (1900–2003 [Vincent and Mekis 2006 ]) exhibiting decreases in Frost Days, and ~97% of sites globally declining in “cool nights” (percentage of time when daily minimum temperature <10th percentile”, 1901–2010 [Donat et al. 2013 ]). Prior investigations have also reported similar rates of change over time. Across the northern forest, Frost Days declined by −0.8 to −3.8 days per decade. This trend is consistent with the findings of Brown et al. ( 2010 ), who reported a decline in Frost Days at a rate of −2.1 d/decade across the northeastern United States from 1951 to 2005. It also fits with Anandhi et al. ( 2013 ), who noted a decline of −3.1 to −6.6 d/decade in the number of Frost Days from 1960 to 2000 for five watersheds in the Catskills region of New York.

Potential impacts on ecosystems of the northern forest

Water Changing winter temperature, precipitation, and snowpack conditions can impact ecohydrology by altering stream water quality and quantity and the timing of lake ice formation and loss. Regarding stream water quantity, prior studies examining changes in winter air temperatures have reported that warmer conditions, particularly from February through May, result in both more frequent mid‐winter melt events and earlier spring snow melt (Hodgkins et al. 2003, Dudley et al. 2017). While we did not examine linkages among changing air temperatures, snowmelt, and streamflow, the trend we observed toward increasing frequency of Thaw Days, particularly in the west and the east, suggests that the spring freshet may also occur earlier in these subregions and be smaller in magnitude as compared to historical spring snow melt. Such shifts may carry consequences for both forest ecosystem water balance (Hodgkins and Dudley 2006, Creed et al. 2015), and in populated areas may also severely impact water resources (Barnett et al. 2005). In addition to effects on water quantity, changing winter conditions may also alter water quality. Recent studies (Huntington and Billmire 2014, Huntington et al. 2016) have shown that trends in increased winter temperatures, more frequent winter rains, and greater overall winter precipitation have increased winter runoff that, in turn, may increase the rate of leaching of base cations and dissolved organic carbon (Huntington 2005, Rustad et al. 2012). Rain falling on frozen ground or saturated soils may further increase rates of winter runoff (Shanley and Chalmers 1999, McMillan et al. 2018), though we were unable to examine such phenomena with our data set. In addition to these general trends, an increase in the frequency of rain‐on‐snow events could have implications for water quality. While rain‐on‐snow may have been a rare occurrence in the past, it is likely to become more common in the future (Leung et al. 2004, Ye et al. 2008, Casson et al. 2010, 2012), and indeed has already increased in Arctic regions by up to 50% (Williams et al. 2015). While we observed few significant trends toward increased frequency of Rain‐on‐Snow Days, the apparent rarity of these events over our 100‐year time series may have precluded strong inference about changes in their occurrence. Further, the trend we observed of fewer Snow Covered Days may have led to fewer days with snow cover on which rain could fall; Mukundan et al. (2013) suggested this synergy could mask trends in simulated rain‐on‐snow events in the Esopus Creek watershed that acts as a water supply for New York City, USA. Despite their apparent rarity, existing literature demonstrates that when these events do happen, they have a disproportionately large effect on hydrochemistry, including accounting for 12–42% of annual nitrate export (Casson et al. 2012, Kurian et al. 2013, Crossman et al. 2016), and potential acid pulses (Eimers et al. 2007). As for impacts on lakes, prior studies have extensively documented the effects of winter climate change on lake ice. Across both North America and the globe, lake ice‐out dates have been advancing as a result of warmer air temperatures, resulting in a dramatic shortening of the ice‐covered season (Magnuson et al. 2000, Hodgkins et al. 2002, Sharma et al. 2016). Like changes in the timing of snowmelt, our findings of decreased numbers of Frost Days and increased numbers of Thaw Days, particularly in the western and eastern subregions, fit with these prior studies and indicate that warmer air temperatures, particularly above 0°C, may advance lake ice‐out dates throughout our study domain. The loss of lake ice carries consequences for lake ecology (Hampton et al. 2017), fish populations (Brander 2007), nutrient cycling (Powers et al. 2017), and greenhouse gas emissions (Denfeld et al. 2018). Beyond these ecological impacts, frozen lakes are important for winter recreation, including snowmobiling, ice fishing, and hockey, and loss of consistent frozen conditions may negatively impact these activities and the rural economies they support (McBoyle et al. 2007, Scott et al. 2008).

Soil Because the snowpack insulates soil from freezing, many previous studies have considered how a combination of reduced snow cover plus cold temperatures might increase soil frost, which, in turn, affects microbial biomass, fine roots, and soil aggregates, and thus soil nutrient and carbon retention (Fitzhugh et al. 2001, Hardy et al. 2001, Groffman et al. 2001, 2006, 2011, Neilsen et al. 2001, Decker et al. 2003, Cleavitt et al. 2008, Campbell et al. 2014, Comerford et al. 2013, Durán et al. 2014, Fuss et al. 2016, Patel et al. 2018). Our results indicate that the frequency of potential soil freezing days (i.e., Bare Ground Ice Days/Frozen Ground Days when T max < 0°C and snow depth = 0 mm) that might affect soil physical, biological, and biogeochemical processes and properties did not significantly change over our 100‐year time series, and in fact, were somewhat rare occurrences (Table 3). While our results fit with those of other studies (Campbell et al. 2010, Brown and DeGaetano 2011), they should be interpreted with caution as they are based on the presence or absence of modeled snow depth and air temperature data. A modeled snowpack that we counted as a Snow Covered Day may have been too shallow to insulate soils from cold temperatures that could result in soil frost (Brooks et al. 2011), and thus, we may have underestimated long‐term trends in soil freezing. However, a lack of high‐quality snow depth and soil temperature data across the region, particularly over multi‐decadal timescales, precluded our ability to analyze relationships among snow depth, air temperature, and soil temperature, highlighting the need for such a regional network of observations. Further, our analyses were limited to soil freezing and not soil freeze/thaw cycles, which have already increased and are projected to increase over the next century (Hayhoe et al. 2007, Campbell et al. 2010, Brown and DeGaetano 2011). Unlike Frozen Ground Days, the frequency of Bare Ground Thaw Days/Mud Days (T max > 0°C and snow depth = 0 mm) significantly increased in both the west and east subregions of our study domain. These Mud Days can occur within the core of winter (December–February) as mid‐winter thaws, which prior studies have suggested will become more common in the future (Campbell et al. 2010, Sinha and Charkauer 2010). When coupled with increasing trends of winter rainfall, such mid‐winter Mud Days can result in saturated soils. If refrozen, concrete frost formation can occur (Fahey and Lang 1975, Proulx and Stein 1997, Tatariw et al. 2017), altering soil carbon and nitrogen availability (Patel et al. 2018), reducing groundwater permeability, and increasing surface water runoff (Shanley and Chalmers 1999). Mud Days may also become more frequent during the months of March and April as the ecosystem transitions between the growing and dormant seasons and the timing of snowmelt advances earlier in the year. The ecological effects of more numerous Mud Days during the vernal transition are not well understood. Prior studies examining mid‐winter thaws have generally coupled such events with subsequent freezing to determine the impacts of freeze–thaw cycles on soil physical properties, microbial biomass, greenhouse gas emissions, and/or nutrient cycling (Schimel and Clein 1996, Groffman et al. 2001, Edwards et al. 2007, Schimel et al. 2007, Aanderud et al. 2013, Tatariw et al. 2017, Patel et al. 2018). While numerous studies have reported both earlier snowmelt (Dyer and Mote 2006, Vincent et al. 2015) and canopy leaf out (Schwartz et al. 2006, Post et al. 2018) as a result of climate change, the intervening period of relatively warm, snow‐free soils, that is, the vernal window when Mud Days might occur, has received much less attention and is an important area for future research (Contosta et al. 2017). For example, the increases in soil respiration and soil carbon loss that occur during the growing season as a result of warmer soil temperatures (Rustad et al. 2012) may also occur on Mud Days during the vernal transition.

Vegetation Prior work examining the direct response of vegetation to winter conditions has largely quantified the impacts of (1) extremely cold air temperatures on foliar tissues of red spruce (Picea rubens Sargent; Hawley et al. 2006, Lazarus et al. 2006, Kosiba et al. 2013, Schaberg et al. 2011), (2) freeze–thaw cycles on the xylem and root tissues of both yellow birch (Betula alleghaniensis Britton; Zhu et al. 2002, Bourque et al. 2005) and paper birch (Betula papyrifera Marshall; Cox and Malcolm 1997) and (3) soil frost on fine roots (Tierney et al. 2001, Cleavitt et al. 2008, Auclair et al. 2010, Comerford et al. 2013, Campbell et al. 2014, Reinmann and Templer 2016), foliage (Comerford et al. 2013), and aboveground growth (Reinmann and Templer 2016; Reinmann et al. 2019) of a variety of hardwood species. Our results indicate that the Extreme Cold Days (T min < −18°C) that would be required to induce red spruce injury have decreased in frequency over the past 100 years. While we did not explicitly examine the freeze‐thaw conditions that would affect the xylem and root tissues of yellow and paper birch, the trend toward more frequent Thaw Days and less frequent Ice and Frost Days across the region suggest reduced risk for winter dehardening followed by freezing injury and dieback. Likewise, the conditions necessary to induce soil frost and fine root mortality across a variety of species, that is, cold temperatures in the absence of snow cover during bare ground ice days or soil frost days, have also become less frequent over the past century. However, projections for increasing soil freeze/thaw cycles suggest potential damage to roots of maple species, affecting their ability to take up and retain nutrients such as nitrogen (Templer et al. 2017, Sanders‐DeMott et al. 2018a). In addition to the changes in winter conditions that could impact vegetation health, longer and warmer growing seasons, as indicated by increased numbers of Thaw Days, could exacerbate or offset these effects (Templer et al. 2017). Numerous other studies have reported earlier growing season onset due to warmer air temperatures (Richardson et al. 2006, Schwartz et al. 2013, Paio et al., 2015) that ultimately may lead to increased forest productivity (Richardson et al. 2009).

Forest insect pests The northern forest features a large and growing number of active and potential forest pests (Dukes et al. 2009, Weed et al. 2013, Lovett et al. 2016, Ayres and Lombardero 2018). Many of these insects experience mortality from lethal winter cold, though the importance of winter temperatures for insect abundance is variable among species and geographic regions (Bale and Hayward 2010, Weed et al. 2015). In some cases, changing winter conditions can affect the distribution and abundance of forest pests by reducing their exposure to lethally cold temperatures. Relaxation of previous constraints from winter cold may add to the number of species for which the region is climatically suitable. We used long‐term meteorological data to calculate indicators that represent relaxation of these constraints. The indicators were based on two insects whose potential for current and future impacts are clearly related to the occurrence of lethally cold winter temperatures. The southern pine beetle is native to North America, especially forests of loblolly pine (Pinus taeda L.) and shortleaf pine (Pinus echinata Miller) in the southeastern United States. It is one of the most aggressive tree‐killing insects in the world, impacting both the forest products industry and forest ecosystems in general (Coulson and Klepzig 2011, Pye et al. 2011). About one‐half of the beetles die from exposure to a single winter night when the air temperature drops to −17°C, and mortality is >90% if the coldest night drops to −22°C (Ungerer et al. 1999, Lombardero et al. 2000, Trân et al. 2007). The warming of the coldest night of the winter (>4°C in 50 years) has facilitated the northern expansion of the beetle about 200 km beyond its historic range into the pinelands of New Jersey, New York, and Connecticut (Dodds et al. 2018). Climate projections for the next 50 years indicate continued warming of the coldest night of the winter will be sufficient to permit further expansion of southern pine beetle into much of the northeastern United States and southeastern Canada (Lesk et al. 2017). The hemlock woolly adelgid (Adelges tsugae Annand) is an invasive sapsucking insect from Japan that feeds in the crown of hemlocks and causes declining vigor followed by death of the host tree a few years after initial infestation (Dukes et al. 2009). Since its introduction to North America in the early 1950s, the adelgid has caused widespread mortality of eastern hemlock (Tsuga canadensis L.) in infestations from northern Georgia to the southern counties of New York, Maine, New Hampshire, and Vermont (Evans and Gregoire 2007). The loss of hemlock from hemlock wooly adelgid has been both direct (trees dying from adelgid attacks) and indirect (preemptive cutting of hemlock by landowners and forest managers in anticipation of its arrival; Orwig et al. 2002). Populations of hemlock woolly adelgid begin to experience mortality when winter air temperatures decline below −20°C and mortality is nearly complete if temperatures reach −30°C (Skinner et al. 2003, Tobin et al. 2017). Similar to southern pine beetles, previous limits on the northern distribution of the adelgid are being relaxed by amelioration of winter cold (Fitzpatrick et al. 2012). Our indicators of Pine Beetle Kill Days and Hemlock Woolly Adelgid Kill Days were based on a simple temperature threshold but nonetheless captured winter warming trends consistent with actual range expansions of both pests (Table 2). This is not only relevant to anticipating future distributions of these particular insects, but may also signal relaxed distribution limits for numerous other plant and animal species. Our analyses add to the spatial extent of knowledge regarding trends in minimum annual air temperature. Previous work has shown similar warming patterns in eastern and western North America of 2°–4°C over 50 years (Trân et al. 2007, Weed et al. 2015). Trends in the reduction of Hemlock Woolly Adelgid in the western and eastern subregions fit with these prior analyses. In central sites, the lack of loss of kill days was likely due to the fact that this geographic area had few of these days to lose over the 100‐year time series (Table 3). Another forest pest that is of great concern to natural resource managers, foresters, municipalities, Tribal Nations, First Nations, and Indigenous communities across this region is the emerald ash borer (Agrilus planipennis Fairmaire), an invasive Eurasian beetle whose larvae bore through the outer bark of otherwise healthy ash trees to feed in the phloem and cambium, essentially girdling the trees’ trunks and branches (Herms and McCullough 2014, Lovett et al. 2016). The prepupae accumulate high concentrations of glycerol and other antifreeze agents in their body fluids, making them extremely cold tolerant, with kill temperatures as low as −35.3°C (Crosthwaite et al. 2011). As such, we did not calculate a separate indicator for the emerald ash borer, whose distribution is ultimately more limited by host tree availability than climatic factors (Sobek‐Swant et al. 2012).