Winter climate change effects on soil C and N cycles in urban grasslands

Despite growing recognition of the role that cities have in global biogeochemical cycles, urban systems are among the least understood of all ecosystems. Urban grasslands are expanding rapidly along with urbanization, which is expected to increase at unprecedented rates in upcoming decades. The large and increasing area of urban grasslands and their impact on water and air quality justify the need for a better understanding of their biogeochemical cycles. There is also great uncertainty about the effect that climate change, especially changes in winter snow cover, will have on nutrient cycles in urban grasslands. We aimed to evaluate how reduced snow accumulation directly affects winter soil frost dynamics, and indirectly greenhouse gas fluxes and the processing of carbon (C) and nitrogen (N) during the subsequent growing season in northern urban grasslands. Both artificial and natural snow reduction increased winter soil frost, affecting winter microbial C and N processing, accelerating C and N cycles and increasing soil : atmosphere greenhouse gas exchange during the subsequent growing season. With lower snow accumulations that are predicted with climate change, we found decreases in N retention in these ecosystems, and increases in N₂O and CO₂ flux to the atmosphere, significantly increasing the global warming potential of urban grasslands. Our results suggest that the environmental impacts of these rapidly expanding ecosystems are likely to increase as climate change brings milder winters and more extensive soil frost.     

Climate Variation and Soil Carbon and Nitrogen Cycling Processes in a Northern Hardwood Forest

We exploited the natural climate gradient in the northern hardwood forest at the Hubbard Brook Experimental Forest (HBEF) to evaluate the effects of climate variation similar to what is predicted to occur with global warming over the next 50–100 years for northeastern North America on soil carbon (C) and nitrogen (N) cycle processes. Our objectives were to (1) characterize differences in soil temperature, moisture and frost associated with elevation at the HBEF and (2) evaluate variation in total soil (TSR) and microbial respiration, N mineralization, nitrification, denitrification, nitrous oxide (N₂O) flux, and methane (CH₄) uptake along this gradient. Low elevation sites were consistently warmer (1.5–2.5°C) and drier than high elevation sites. Despite higher temperatures, low elevation plots had less snow and more soil frost than high elevation plots. Net N mineralization and nitrification were slower in warmer, low elevation plots, in both summer and winter. In summer, this pattern was driven by lower soil moisture in warmer soils and in winter the pattern was linked to less snow and more soil freezing in warmer soils. These data suggest that N cycling and supply to plants in northern hardwood ecosystems will be reduced in a warmer climate due to changes in both winter and summer conditions. TSR was consistently faster in the warmer, low elevation plots. N cycling processes appeared to be more sensitive to variation in soil moisture induced by climate variation, whereas C cycling processes appeared to be more strongly influenced by temperature.     

Carbon limitation of soil respiration under winter snowpacks: potential feedbacks between growing season and winter carbon fluxes

A reduction in the length of the snow‐covered season in response to a warming of high‐latitude and high‐elevation ecosystems may increase soil carbon availability both through increased litter fall following longer growing seasons and by allowing early winter soil frosts that lyse plant and microbial cells. To evaluate how an increase in labile carbon during winter may affect ecosystem carbon balance we investigated the relationship between carbon availability and winter CO₂ fluxes at several locations in the Colorado Rockies. Landscape‐scale surveys of winter CO₂ fluxes from sites with different soil carbon content indicated that winter CO₂ fluxes were positively related to carbon availability and experimental additions of glucose to soil confirmed that CO₂ fluxes from snow‐covered soil at temperatures between 0 and ?3°C were carbon limited. Glucose added to snow‐covered soil increased CO₂ fluxes by 52–160% relative to control sites within 24 h and remained 62–70% higher after 30 days. Concurrently a shift in the δ¹³C values of emitted CO₂ toward the glucose value indicated preferential utilization of the added carbon confirming the presence of active heterotrophic respiration in soils at temperatures below 0°C. The sensitivity of these winter fluxes to substrate availability, coupled with predicted changes in winter snow cover, suggests that feedbacks between growing season carbon uptake and winter heterotrophic activity may have unforeseen consequences for carbon and nutrient cycling in northern forests. For example, published winter CO₂ fluxes indicate that on average 50% of growing season carbon uptake currently is respired during the winter; changes in winter CO₂ flux in response to climate change have the potential to reduce substantially the net carbon sink in these ecosystems.     

Summer warming accelerates sub‐arctic peatland nitrogen cycling without changing enzyme pools or microbial community structure

The balance of primary production and decomposition in northern peatlands may shift due to climate change, with potential feedbacks to atmospheric CO₂ concentrations. Nitrogen availability will modulate this shift, but little is known about the drivers of soil nitrogen dynamics in these environments. We used a long‐term (9 years) open top chamber (OTC) experiment in an ombrotrophic Sphagnum peat bog in sub‐arctic Sweden, to test for the interactive effects of spring warming, summer warming and winter snow addition on soil nitrogen fluxes, potential activities of nitrogen cycle enzymes, and soil microbial community composition. These simultaneous measurements allowed us to identify the level of organization at which climate change impacts are apparent, an important requirement for developing truly mechanistic understanding. Organic‐N pools and fluxes were an order of magnitude higher than inorganic‐N pools and fluxes. Summer warming approximately doubled fluxes of soil organic nitrogen and ammonia over the growing season. Such a large increase under 1 °C warming is unlikely to be due to kinetic effects, and we propose that it is linked to an observed seasonal decrease in microbial biomass, suggesting that N flux is driven by a substantial late‐season dieback of microbes. This change in N cycle dynamics was not reflected in any of the measured potential peptidase activities. Moreover, the soil microbial community structure was apparently stable across treatments, suggesting a non‐specific microbial dieback. Our results show that in these widespread peat bogs, where many plant species are capable of organic‐N uptake, organic soil N dynamics are quantitatively far more important than the commonly studied inorganic‐N dynamics. Understanding of climate change effects on organic soil N cycling in this system will be advanced by closer investigation of the seasonal dynamics of the microbial biomass and the input of substrates that maintain it.     

Declines in northern forest tree growth following snowpack decline and soil freezing

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.     

Snow depth, soil freezing and nitrogen cycling in a northern hardwood forest landscape

Increases in soil freezing associated with decreases in snow cover have been identified as a significant disturbance to nitrogen (N) cycling in northern hardwood forests. We created a range of soil freezing intensity through snow manipulation experiments along an elevation gradient at the Hubbard Brook Experimental Forest (HBEF) in the White Mountains, NH USA in order to improve understanding of the factors regulating freeze effects on nitrate (NO₃ ^?) leaching, nitrous oxide (N₂O) flux, potential and in situ net N mineralization and nitrification, microbial biomass carbon (C) and N content and respiration, and denitrification. While the snow manipulation treatment produced deep and persistent soil freezing at all sites, effects on hydrologic and gaseous losses of N were less than expected and less than values observed in previous studies at the HBEF. There was no relationship between frost depth, frost heaving and NO₃ ^? leaching, and a weak relationship between frost depth and winter N₂O flux. There was a significant positive relationship between dissolved organic carbon (DOC) and NO₃ ^? concentrations in treatment plots but not in reference plots, suggesting that the snow manipulation treatment mobilized available C, which may have stimulated retention of N and prevented treatment effects on N losses. While the results support the hypothesis that climate change resulting in less snow and more soil freezing will increase N losses from northern hardwood forests, they also suggest that ecosystem response to soil freezing disturbance is affected by multiple factors that must be reconciled in future research.     

Nitrate and dissolved organic carbon mobilization in response to soil freezing variability

Reduced snowpack and associated increases in soil freezing severity resulting from winter climate change have the potential to disrupt carbon (C) and nitrogen (N) cycling in soils. We used a natural winter climate gradient based on elevation and aspect in a northern hardwood forest to examine the effects of variability in soil freezing depth, duration, and frequency on the mobilization of dissolved organic carbon (DOC) and nitrate (NO₃ ^?) in soils over the course of 2 years. During a winter with a relatively thin snowpack, soils at lower elevation sites experienced greater freezing and especially variable freeze/thaw cycles, which in turn led to greater leaching of DOC from the organic horizon during the following growing season. In contrast to several previous field manipulation studies, we did not find changes in soil solution NO₃ ^? concentrations related to soil freezing variables. Our results are consistent with a soil matrix disturbance from freezing and thawing which increases leachable C. These results build upon previous laboratory experiments and field manipulations that found differing responses of DOC and NO₃ ^? following soil freezing, suggesting that mobilization of labile C may suppress NO₃ ^? losses through microbial immobilization of N. This research highlights the importance of studying natural variation in winter climate and soil freezing and how they impact soil C and N retention, with implications for surface water runoff quality.     

Ecosystem CO2 production during winter in a Swedish subarctic region: the relative importance of climate and vegetation type

General circulation models consistently predict that regional warming will be most rapid in the Arctic, that this warming will be predominantly in the winter season, and that it will often be accompanied by increasing snowfall. Paradoxically, despite the strong cold season emphasis in these predictions, we know relatively little about the plot and landscape‐level controls on tundra biogeochemical cycling in wintertime as compared to summertime. We investigated the relative influence of vegetation type and climate on CO₂ production rates and total wintertime CO₂ release in the Scandinavian subarctic. Ecosystem respiration rates and a wide range of associated environmental and substrate pool size variables were measured in the two most common vegetation types of the region (birch understorey and heath tundra) at four paired sites along a 50 km transect through a strong snow depth gradient in northern Sweden. Both climate and vegetation type were strong interactive controls on ecosystem CO₂ production rates during winter. Of all variables tested, soil temperature explained by far the largest amount of variation in respiration rates (41–75%). Our results indicate that vegetation type only exerted an influence on respiration when snow depth was below a certain threshold (～1 m). Thus, tall vegetation that enhanced snow accumulation within that threshold resulted in more effective thermal insulation from severe air temperatures, thereby significantly increasing respiratory activity. At the end of winter, within several days of snowmelt, gross ecosystem photosynthesis rates were of a similar magnitude to ecosystem respiration, resulting in significant net carbon gain in some instances. Finally, climate and vegetation type were also strong interactive controls on total wintertime respiration, suggesting that spatial variations in maximum snowdepth may be a primary determinant of regional patterns of wintertime CO₂ release. Together, our results have important implications for predictions of how the distribution of tundra vegetation types and the carbon balances of arctic ecosystems will respond to climate change during winter because they indicate a threshold (～1 m) above which there would be little effect of increased snow accumulation on wintertime biogeochemical cycling.     

Effects of experimental warming of air,soil and permafrost on carbon balance in Alaskan tundra

The carbon (C) storage capacity of northern latitude ecosystems may diminish as warming air temperatures increase permafrost thaw and stimulate decomposition of previously frozen soil organic C. However, warming may also enhance plant growth so that photosynthetic carbon dioxide (CO₂) uptake may, in part, offset respiratory losses. To determine the effects of air and soil warming on CO₂ exchange in tundra, we established an ecosystem warming experiment – the Carbon in Permafrost Experimental Heating Research (CiPEHR) project – in the northern foothills of the Alaska Range in Interior Alaska. We used snow fences coupled with spring snow removal to increase deep soil temperatures and thaw depth (winter warming) and open‐top chambers to increase growing season air temperatures (summer warming). Winter warming increased soil temperature (integrated 5–40 cm depth) by 1.5 °C, which resulted in a 10% increase in growing season thaw depth. Surprisingly, the additional 2 kg of thawed soil C m^?2 in the winter warming plots did not result in significant changes in cumulative growing season respiration, which may have been inhibited by soil saturation at the base of the active layer. In contrast to the limited effects on growing‐season C dynamics, winter warming caused drastic changes in winter respiration and altered the annual C balance of this ecosystem by doubling the net loss of CO₂ to the atmosphere. While most changes to the abiotic environment at CiPEHR were driven by winter warming, summer warming effects on plant and soil processes resulted in 20% increases in both gross primary productivity and growing season ecosystem respiration and significantly altered the age and sources of CO₂ respired from this ecosystem. These results demonstrate the vulnerability of organic C stored in near surface permafrost to increasing temperatures and the strong potential for warming tundra to serve as a positive feedback to global climate change.     

Impact of winter climate change on nitrogen biogeochemistry in forest ecosystems: A synthesis from Japanese case studies

Nitrogen (N) is a critical ecological and environmental indicator under changing environments. The impact of winter climate change on N biogeochemical processes in forest ecosystems has gained increasing recognition. Decreasing snowfall has caused a decrease in the heat insulation properties of the snowpack, resulting in an increase in the frequency and magnitude of freezing and thawing cycles in surface soil, where biological processes are most active. Here I synthesize recent research findings from integrated field observations and experiments conducted in northern Japan and compare these results with previous research outcomes from other regions to identify current research gaps and develop the next research agenda to further advance our understanding of this complex problem. Japanese case studies indicated that net ammonium production (ammonification) was mostly dominant in terms of available soil N fertility in cold environments and was sensitive to the increase in soil freezing and thawing cycles because of the decreased snowpack. On the other hands, nitrate dynamics were more stable or conservative than those of ammonium. The soil characteristics (i.e., N pool and microbial activities) were significant explanatory factors of the responses of soil N dynamics and N leakage among different soils to increased freezing–thawing cycles at watershed and national scale. This synthesis indicates that winter climate change had significant impacts on soil N biogeochemistry (such as soil N pool size and microbial N transformation) during the winter and snowmelt season and also during the following growing season. Several research gaps and possible research topics (path dependency and soil microbial community composition) are also presented by synthesizing the current research findings. Further field experiments and observations quantifying the pools and fluxes of inorganic N with modeling analysis under freeze–thaw environments would contribute to increase the understandings of N transformation processes under winter climate change.     

Disentangling the mechanisms behind winter snow impact on vegetation activity in northern ecosystems

Although seasonal snow is recognized as an important component in the global climate system, the ability of snow to affect plant production remains an important unknown for assessing climate change impacts on vegetation dynamics at high‐latitude ecosystems. Here, we compile data on satellite observation of vegetation greenness and spring onset date, satellite‐based soil moisture, passive microwave snow water equivalent (SWE) and climate data to show that winter SWE can significantly influence vegetation greenness during the early growing season (the period between spring onset date and peak photosynthesis timing) over nearly one‐fifth of the land surface in the region north of 30 degrees, but the magnitude and sign of correlation exhibits large spatial heterogeneity. We then apply an assembled path model to disentangle the two main processes (via changing early growing‐season soil moisture, and via changing the growth period) in controlling the impact of winter SWE on vegetation greenness, and suggest that the “moisture” and “growth period” effect, to a larger extent, result in positive and negative snow–productivity associations, respectively. The magnitude and sign of snow–productivity association is then dependent upon the relative dominance of these two processes, with the “moisture” effect and positive association predominating in Central, western North America and Greater Himalaya, and the “growth period” effect and negative association in Central Europe. We also indicate that current state‐of‐the‐art models in general reproduce satellite‐based snow–productivity relationship in the region north of 30 degrees, and do a relatively better job of capturing the “moisture” effect than the “growth period” effect. Our results therefore work towards an improved understanding of winter snow impact on vegetation greenness in northern ecosystems, and provide a mechanistic basis for more realistic terrestrial carbon cycle models that consider the impacts of winter snow processes.     

Predominant role of water in regulating soil and microbial respiration and their responses to climate change in a semiarid grassland

Climate change can profoundly impact carbon (C) cycling of terrestrial ecosystems. A field experiment was conducted to examine responses of total soil and microbial respiration, and microbial biomass to experimental warming and increased precipitation in a semiarid temperate steppe in northern China since April 2005. We measured soil respiration twice a month over the growing seasons, soil microbial biomass C (MBC) and N (MBN), microbial respiration (MR) once a year in the middle growing season from 2005 to 2007. The results showed that interannual variations in soil respiration, MR, and microbial biomass were positively related to interannual fluctuations in precipitation. Laboratory incubation with a soil moisture gradient revealed a constraint of the temperature responses of MR by low soil moisture contents. Across the 3 years, experimental warming decreased soil moisture, and consequently caused significant reductions in total and microbial respiration, and microbial biomass, suggesting stronger negatively indirect effects through warming‐induced water stress than the positively direct effects of elevated temperature. Increased evapotranspiration under experimental warming could have reduced soil water availability below a stress threshold, thus leading to suppression of plant growth, root and microbial activities. Increased precipitation significantly stimulated total soil and microbial respiration and all other microbial parameters and the positive precipitation effects increased over time. Our results suggest that soil water availability is more important than temperature in regulating soil and microbial respiratory processes, microbial biomass and their responses to climate change in the semiarid temperate steppe. Experimental warming caused greater reductions in soil respiration than in gross ecosystem productivity (GEP). In contrast, increased precipitation stimulated GEP more than soil respiration. Our observations suggest that climate warming may cause net C losses, whereas increased precipitation may lead to net C gains in the semiarid temperate steppe. Our findings highlight that unless there is concurrent increase in precipitation, the temperate steppe in the arid and semiarid regions of northern China may act as a net C source under climate warming.     

Simulated chronic NO3− deposition reduces soil respiration in northern hardwood forests

Chronic N additions to forest ecosystems can enhance soil N availability, potentially leading to reduced C allocation to root systems. This in turn could decrease soil CO₂ efflux. We measured soil respiration during the first, fifth, sixth and eighth years of simulated atmospheric NO₃^? deposition (3 g N m^?2 yr^?1) to four sugar maple‐dominated northern hardwood forests in Michigan to assess these possibilities. During the first year, soil respiration rates were slightly, but not significantly, higher in the NO₃^?‐amended plots. In all subsequent measurement years, soil respiration rates from NO₃^?‐amended soils were significantly depressed. Soil temperature and soil matric potential were measured concurrently with soil respiration and used to develop regression relationships for predicting soil respiration rates. Estimates of growing season and annual soil CO₂ efflux made using these relationships indicate that these C fluxes were depressed by 15% in the eighth year of chronic NO₃^? additions. The decrease in soil respiration was not due to reduced C allocation to roots, as root respiration rates, root biomass, and root turnover were not significantly affected by N additions. Aboveground litter also was unchanged by the 8 years of treatment. Of the remaining potential causes for the decline in soil CO₂ efflux, reduced microbial respiration appears to be the most likely possibility. Documented reductions in microbial biomass and the activities of extracellular enzymes used for litter degradation on the NO₃^?‐amended plots are consistent with this explanation.     

Cold truths: how winter drives responses of terrestrial organisms to climate change

Winter is a key driver of individual performance, community composition, and ecological interactions in terrestrial habitats. Although climate change research tends to focus on performance in the growing season, climate change is also modifying winter conditions rapidly. Changes to winter temperatures, the variability of winter conditions, and winter snow cover can interact to induce cold injury, alter energy and water balance, advance or retard phenology, and modify community interactions. Species vary in their susceptibility to these winter drivers, hampering efforts to predict biological responses to climate change. Existing frameworks for predicting the impacts of climate change do not incorporate the complexity of organismal responses to winter. Here, we synthesise organismal responses to winter climate change, and use this synthesis to build a framework to predict exposure and sensitivity to negative impacts. This framework can be used to estimate the vulnerability of species to winter climate change. We describe the importance of relationships between winter conditions and performance during the growing season in determining fitness, and demonstrate how summer and winter processes are linked. Incorporating winter into current models will require concerted effort from theoreticians and empiricists, and the expansion of current growing‐season studies to incorporate winter.     

Deepened snow increases late thaw biogeochemical pulses in mesic low arctic tundra

Pulses of plant-available nutrients to the soil solution are expected to occur during the dynamic winter–spring transition in arctic tundra. Our aims were to quantify the magnitude of these potential nutrient pulses, to understand the sensitivity of these pulses to winter conditions, and to characterize and integrate the environmental and biogeochemical dynamics of this period. To test the hypotheses that snow depth, temperature and soil water—and not snow nutrient content—are important controls on winter and spring biogeochemistry, we sampled soil from under ambient and deepened snow every 3 days from late winter to spring, in addition to the snowpack at the start of thaw. Soil and microbial biogeochemical dynamics were divided into distinct phases that correlated with steps in soil temperature and soil water. Soil solution and microbial pools of C, N and P fluctuated with strong peaks and declines throughout the thaw, especially under deepened snow. Snowpack nutrient accumulation was negligible relative to these biogeochemical peaks. All nutrient and microbial peaks declined simultaneously at the end of snowmelt and so this decline was delayed by 15 days under deepened snow. The timing of these nutrient pulses is critical for plant species nutrient availability and landscape nutrient budgets. This detailed and statistically-based characterisation of the winter–spring transition in terms of environmental and biogeochemical variables should provide a useful foundation for future biogeochemical process-based studies of thaw, and indicate that spring thaw and possibly growing season biogeochemical dynamics are sensitive to present and future variability in winter snow depth.     

Diel patterns of autotrophic and heterotrophic respiration among phenological stages

Improved understanding of the links between aboveground production and allocation of photosynthate to belowground processes and the temporal variation in those links is needed to interpret observations of belowground carbon cycling processes. Here, we show that combining a trenching manipulation with high‐frequency soil respiration measurements in a temperate hardwood forest permitted identification of the temporally variable influence of roots on diel and seasonal patterns of soil respiration. The presence of roots in an untrenched plot caused larger daily amplitude and a 2–3 h delay in peak soil CO₂ efflux relative to a root‐free trenched plot. These effects cannot be explained by differences in soil temperature, and they were significant only when a canopy was present during the growing season. This experiment demonstrated that canopy processes affect soil CO₂ efflux rates and patterns at hourly and seasonal time scales, and it provides evidence that root and microbial processes respond differently to environmental factors.     

Decrease in winter respiration explains 25% of the annual northern forest carbon sink enhancement over the last 30 years

Aim Winter snow has been suggested to regulate terrestrial carbon (C) cycling by modifying microclimate, but the impacts of change in snow cover on the annual C budget at a large scale are poorly understood. Our aim is to quantify the C balance under changing snow depth. Location Non‐permafrost region of the northern forest area. Methods Here, we used site‐based eddy covariance flux data to investigate the relationship between depth of snow cover and ecosystem respiration (R_eco) during winter. We then used the Biome‐BGC model to estimate the effect of reductions in winter snow cover on the C balance of northern forests in the non‐permafrost region. Results According to site observations, winter net ecosystem C exchange (NEE) ranged from 0.028 to 1.53 gC·m^?2·day^?1, accounting for 44 ± 123% of the annual C budget. Model simulation showed that over the past 30 years, snow‐driven change in winter C fluxes reduced non‐growing season CO₂ emissions, enhancing the annual C sink of northern forests. Over the entire study area, simulated winter R_eco significantly decreased by 0.33 gC·m^?2·day^?1·year^?1 in response to decreasing depth of snow cover, which accounts for approximately 25% of the simulated annual C sink trend from 1982 to 2009. Main conclusion Soil temperature is primarily controlled by snow cover rather than by air temperature as snow serves as an insulator to prevent chilling impacts. A shallow snow cover has less insulation potential, causing colder soil temperatures and potentially lower respiration rates. Both eddy covariance analysis and model‐simulated results show that both R_eco and NEE are significantly and positively correlated with variation in soil temperature controlled by variation in snow depth. Overall, our results highlight that a decrease in winter snow cover restrains global warming as less C is emitted to the atmosphere.     

Correction to: The biogeochemical consequences of litter transformation by insect herbivory in the Subarctic: a microcosm simulation experiment

Northern forest ecosystems are projected to experience warmer growing seasons and increased soil freeze–thaw cycles in winter over the next century. Past studies show that warmer soils in the growing season enhance nitrogen uptake by plants, while soil freezing in winter reduces plant uptake and ecosystem retention of nitrogen, yet the combined effects of these changes on plant root capacity to take up nitrogen are unknown. We conducted a 2-year (2014–2015) experiment at Hubbard Brook Experimental Forest in New Hampshire, USA to characterize the response of root damage, nitrogen uptake capacity, and soil solution nitrogen to growing season warming combined with soil freeze–thaw cycles in winter. Winter freeze–thaw cycles damaged roots, reduced nitrogen uptake capacity by 42%, and increased soil solution ammonium in the early growing season (May–June). During the peak growing season (July), root nitrogen uptake capacity was reduced 40% by warming alone and 49% by warming combined with freeze–thaw cycles. These results indicate the projected combination of colder soils in winter and warmer soils in the snow-free season will alter root function by reducing root nitrogen uptake capacity and lead to transient increases of nitrogen in soil solution during the early growing season, with the potential to alter root competition for soil nitrogen and seasonal patterns of soil nitrogen availability. We conclude that considering interactive effects of changes in climate during winter and the snow-free season is essential for accurate determination of the response of nitrogen cycling in the northern hardwood forest to climate change.     

Uneven winter snow influence on tree growth across temperate China

Winter snow is an important driver of tree growth in regions where growing‐season precipitation is limited. However, observational evidence of this influence at larger spatial scales and across diverse bioclimatic regions is lacking. Here, we investigated the interannual effects of winter (here defined as previous October to current February) snow depth on tree growth across temperate China over the period of 1961–2015, using a regional network of tree ring records, in situ daily snow depth observations, and gridded climate data. We report uneven effects of winter snow depth on subsequent growing‐season tree growth across temperate China. There shows little effect on tree growth in drier regions that we attribute mainly to limited snow accumulation during winter. By contrast, winter snow exerts important positive influence on tree growth in stands with high winter snow accumulation (e.g., in parts of cold arid regions). The magnitude of this effect depends on the proportion of winter snow to pre‐growing‐season (previous October to current April) precipitation. We further observed that tree growth in drier regions tends to be increasingly limited by warmer growing‐season temperature and early growing‐season water availability. No compensatory effect of winter snow on the intensifying drought limitation of tree growth was observed across temperate China. Our findings point toward an increase in drought vulnerability of temperate forests in a warming climate.     

Winter soil frost conditions in boreal forests control growing season soil CO2 concentration and its atmospheric exchange

The impact of changes in winter soil frost regime on soil CO₂ concentration and its atmospheric exchange in a boreal Norway spruce forest was investigated using a field‐scale soil frost manipulation experiment. The experiment comprised three treatments: deep soil frost, shallow soil frost and control plots (n= 3). Winter soil temperatures and soil frost distribution were significantly altered by the different treatments. The average soil CO₂ concentrations during the growing season were significantly lower in plots with deep soil frost than in plots with shallow soil frost. The average CO₂ soil–atmosphere exchange rate exhibited the same pattern, and differences in soil respiration rates among the treatments were statistically significant. Both the variation in soil CO₂ concentration and the CO₂ soil–atmosphere exchange rate could statistically be explained by the differences in the maximum soil frost depth during the previous winter. A response model for growing season soil respiration rates suggests that every 1 cm change in winter soil frost depth will change the emission rates by ca. 0.01 g CO₂ m^?2 day^?1, corresponding to 0.2–0.5% of the estimated net ecosystem productivity (NEP). This suggests that the soil frost regime has a significant influence on the C balance of the system, because interannual variations in soil frost up to 60 cm have been recorded at the site. We conclude that winter climate conditions can be important in controlling C balances in northern terrestrial ecosystems, and also that indirect effects of the winter season must be taken into account, because these can affect the prevailing conditions during the growing season.