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.     

Winter climate change implications for decomposition in northeastern forests: comparisons of sugar maple litter with herbivore fecal inputs

Forests in northeastern North America are influenced by varying climatic and biotic factors; however, there is concern that rapid changes in these factors may lead to important changes in ecosystem processes such as decomposition. Climate change (especially warming) is predicted to increase rates of decomposition in northern latitudes. Warming in winter may result in complex effects including decreased levels of snow cover and an increased incidence of soil freezing that will effect decomposition. Along with these changes in climate, moose densities have also been increasing in this region, likely affecting nutrient dynamics. We measured decomposition and N release from ¹⁵N‐labeled sugar maple leaf litter and moose feces over 20 months in reference and snow removal treatment (to induce soil freezing) plots in two separate experiments at the Hubbard Brook Experimental Forest in New Hampshire, USA. Snow removal/soil freezing decreased decomposition of maple litter, but stimulated N transfer to soil and microbial biomass. Feces decomposed more rapidly than maple litter, and feces N moved into the mineral soil more than N derived from litter, likely due to the lower C : N ratio of feces. Feces decomposition was not affected by the snow removal treatment. Total microbial biomass (measured as microbial N and C) was not significantly affected by the treatments in either the litter or feces plots. These results suggest that increases in soil freezing and/or large herbivore populations, increase the transfer rate of N from plant detritus or digested plants into the mineral soil. Such changes suggest that altering the spatial and temporal patterns of soil freezing and moose density have important implications for ecosystem N cycling.     

Combined Effects of Soil Freezing and N Addition on Losses and Interception of N Over Winter and Summer

Climate warming is expected to decrease snow cover in many northern temperate regions, potentially increasing the severity of soil freezing. Severe soil freezing can enhance nitrogen (N) losses, but it remains unclear which N pools contribute to these losses, how this may change seasonally and how freezing damage might interact with increased atmospheric N deposition. We added ¹⁵N to grass-dominated old field plots treated with combinations of snow removal and N addition, then resampled the plots at the end of the winter and the next fall to assess N losses from the soil, litter, roots and shoots. In addition, we added ¹⁵N to separate treated plots immediately after snowmelt and at peak biomass and sampled these plots after 1 week to assess the potential for interception of N deposition. Snow removal increased soil and root losses of the ¹⁵N label over the winter, but it had no effect on ¹⁵N losses from these pools over summer. N addition had no effect on ¹⁵N losses from these pools over winter or summer, but N addition decreased the interception of ¹⁵N added at snowmelt. Snow removal decreased plant biomass over summer and reduced the interception of ¹⁵N, whereas N addition enhanced plant biomass and increased the interception of ¹⁵N at this time. Our results confirm that severe soil freezing can both enhance soil and root N losses over winter and reduce the interception of N deposition over the growing season, although the latter can be counteracted by increased plant biomass in response to increased N.     

Increased soil respiration in response to experimentally reduced snow cover and increased soil freezing in a temperate deciduous forest

Winter snowpack in seasonally snow-covered regions plays an important role in moderating ecosystem processes by insulating soil from freezing air temperatures. However, climate models project a decline in snowpack at mid and high latitudes over the next century. We conducted a snow removal experiment in a temperate deciduous forest at Harvard Forest in Massachusetts, USA to quantify the effects of a reduced winter snowpack and increased soil freezing on total soil respiration and its bulk (i.e. heterotrophic) and root-rhizosphere components. Snow removal increased soil freezing severity by more than three-fold, which resulted in a 27.6% increase in annual total soil respiration (p?=?0.058). Across our plots and years of this study, we found that the severity, rather than simply the presence of soil freezing, was the primary driver of the soil respiration response to reduced winter snowpack. Bulk soil respiration made the largest contribution to total soil respiration with root-rhizosphere respiration contributing up to 26.1?±?6.5% of total soil respiration across plot types and years. Snow removal significantly increased fine root mortality (p?=?0.03), which was positively correlated with soil frost depth and duration (p?=?0.068, \({\text{R}}_{{{\text{LMM}}(m)}}^{ 2}\)?=?0.46), rates of total soil respiration (p?=?0.075; \({\text{R}}_{{{\text{LMM}}(m)}}^{ 2}\)?=?0.27) and the contribution of root-rhizosphere respiration to total soil respiration (p?=?0.004; \({\text{R}}_{{{\text{LMM}}(m)}}^{ 2}\)?=?0.58). We conclude that increased rates of soil respiration in response to soil freezing are driven by plant-mediated processes, whereby soil frost-induced root mortality stimulates respiration through decomposition of root necromass with additional enhancements possibly related to priming of soil organic matter decomposition and elevated rates of root respiration associated with growth.     

Snow depth manipulation and its influence on soil frost and water dynamics in a northern hardwood forest

Climate change will likelyresult in warmer winter temperatures leading toless snowfall in temperate forests. Thesechanges may lead to increases in soil freezingbecause of lack of an insulating snow cover andchanges in soil water dynamics during theimportant snowmelt period. In this study, wemanipulated snow depth by removing snow for twowinters, simulating the late development of thesnowpack as may occur with global warming, toexplore the relationships between snow depth,soil freezing, soil moisture, and infiltration.We established four sites, each with two pairedplots, at the Hubbard Brook Experimental Forest(HBEF) in New Hampshire, U.S.A. and instrumentedall eight plots with soil and snow thermistors,frost tubes, soil moisture probes, and soillysimeters. For two winters, we removed snowfrom the designated treatment plots untilFebruary. Snow in the reference plots wasundisturbed. The treatment winters (1997/1998 and1998/1999) were relatively mild, withtemperatures above the seasonal norm and snowdepths below average. Results show the treatedplots accumulated significantly less snow andhad more extensive soil frost than referenceplots. Snow depth was a strong regulator ofsoil temperature and frost depth at all sites.Soil moisture measured by time domainreflectometry probes and leaching volumescollected in lysimeters were lower in thetreatment plots in March and April compared tothe rest of the year. The ratio of leachatevolumes collected in the treatment plots tothat in the reference plots decreased as thesnow ablation seasons progressed. Our data showthat even mild winters with low snowfall,simulated by snow removal, will result inincreased soil freezing in the forests at theHBEF. Our results suggest that a climate shifttoward less snowfall or a shorter duration ofsnow on the ground will produce increases insoil freezing in northern hardwood forests.Increases in soil freezing will haveimplications for changes in soil biogeochemicalprocesses.     

雪被去除对川西亚高山云杉林土壤活性氮的影响

气候变化引发的季节性雪被改变可能对高寒森林土壤氮循环产生深刻影响.以遮雪棚去除雪被,研究了雪被去除样方和对照样方在不同关键时期(雪被初期、深雪被期和雪被融化期)土壤氮库和矿化速率的变化.结果表明: 季节性雪被对土壤具有良好的保温作用,雪被去除使得5 cm深度土壤平均温度和最低温度分别降低0.33和1.17 ℃,并明显增加了土壤冻结深度和冻融循环.土壤活性氮在不同雪被时期存在显著差异.雪被去除使得冬季土壤铵态氮、硝态氮和可溶性有机氮增加38.6%、23.5%和57.3%.此外,雪被去除也促进了融化期土壤硝化和矿化速率的增加.因此,未来气候变暖引起的雪被减少可能加快川西亚高山森林冬季土壤氮循环.     

冬季雪被对青藏高原东缘高海拔森林凋落叶P元素释放的影响

雪被是影响高海拔森林凋落物分解的重要生态因子,其是否影响到生长季节与非生长季节凋落物中的P元素释放,尚未量化。为了量化季节性雪被对高海拔森林凋落物分解过程中P元素释放的影响,于2010年10月至2012年10月间,在青藏高原东缘川西高海拔森林不同厚度冬季雪被斑块下,设置凋落物分解袋实验。检测该地区代表性树种岷江冷杉(Abies faxoniana)、红桦(Betula albo-sinensis)、四川红杉(Larix mastersiana)和方枝柏(Sabina saltuaria)凋落叶在雪被覆盖不同关键时期(雪被形成前期、完全覆盖期和消融期)以及生长季节的P元素动态。结果表明,凋落物质量与雪被厚度均显著影响了P元素的释放过程。雪被覆盖时期凋落物P元素释放率表现为有雪被覆盖大于无雪被覆盖,而生长季节中除岷江冷杉外的其他3种凋落物P元素释放率均为无雪被覆盖下最大。相对于无雪被覆盖斑块,冬季雪被的存在提供了保护绝缘层,促进凋落物P元素释放,提高了各物种冬季P元素释放贡献率。这些结果表明,全球变化情景下的雪被减少可能减缓高海拔森林凋落物P元素的释放过程,改变森林土壤P元素水平。所以在研究高寒、高海拔地区全球气候变化下生态系统功能的工作中,应注重雪被这一异质性环境因子对生态系统功能的影响。     

Reduced Snow Cover Increases Wintertime Nitrous Oxide (N<Subscript>2</Subscript>O) Emissions from an Agricultural Soil in the Upper U.S. Midwest

Throughout most of the northern hemisphere, snow cover decreased in almost every winter month from 1967 to 2012. Because snow is an effective insulator, snow cover loss has likely enhanced soil freezing and the frequency of soil freeze–thaw cycles, which can disrupt soil nitrogen dynamics including the production of nitrous oxide (N₂O). We used replicated automated gas flux chambers deployed in an annual cropping system in the upper Midwest US for three winters (December–March, 2011–2013) to examine the effects of snow removal and additions on N₂O fluxes. Diminished snow cover resulted in increased N₂O emissions each year; over the entire experiment, cumulative emissions in plots with snow removed were 69% higher than in ambient snow control plots and 95% higher than in plots that received additional snow (P < 0.001). Higher emissions coincided with a greater number of freeze–thaw cycles that broke up soil macroaggregates (250–8000 µm) and significantly increased soil inorganic nitrogen pools. We conclude that winters with less snow cover can be expected to accelerate N₂O fluxes from agricultural soils subject to wintertime freezing.     

Increased nitrogen leaching following soil freezing is due to decreased root uptake in a northern hardwood forest

The depth and duration of snow pack is declining in the northeastern United States as a result of warming air temperatures. Since snow insulates soil, a decreased snow pack can increase the frequency of soil freezing, which has been shown to have important biogeochemical implications. One of the most notable effects of soil freezing is increased inorganic nitrogen losses from soil during the following growing season. Decreased nitrogen retention is thought to be due to reduced root uptake, but has not yet been measured directly. We conducted a 2‐year snow‐removal experiment at Hubbard Brook Experimental Forest in New Hampshire, USA to determine the effects of soil freezing on root uptake and leaching of inorganic nitrogen simultaneously. Snow removal significantly increased the depth of maximal soil frost by 37.2 and 39.5 cm in the first and second winters, respectively (P < 0.001 in 2008/2009 and 2009/2010). As a consequence of soil freezing, root uptake of ammonium declined significantly during the first and second growing seasons after snow removal (P = 0.023 for 2009 and P = 0.005 for 2010). These observed reductions in root nitrogen uptake coincided with significant increases in soil solution concentrations of ammonium in the Oa horizon (P = 0.001 for 2009 and 2010) and nitrate in the B horizon (P < 0.001 and P = 0.003 for 2009 and 2010, respectively). The excess flux of dissolved inorganic nitrogen from the Oa horizon that was attributable to soil freezing was 7.0 and 2.8 kg N ha^?1 in 2009 and 2010, respectively. The excess flux of dissolved inorganic nitrogen from the B horizon was lower, amounting to 1.7 and 0.7 kg N ha^?1 in 2009 and 2010, respectively. Results of this study provide direct evidence that soil freezing reduces root nitrogen uptake, demonstrating that the effects of winter climate change on root function has significant consequences for nitrogen retention and loss in forest ecosystems.     

Effect of snow removal on leaf water potential,soil moisture,leaf and soil nutrient status and leaf peroxidase activity of sugar maple

Effect of removal of snow cover in winter was investigated in an 80-year-old sugar maple (Acer saccharum Marsh.) stand in southern Quebec. We hypothesized that winter soil frost would induce some of the decline symptoms observed in sugar maple stands in southern Quebec in the early 1980's. Snow was continuously removed from around trees for a one week (partial removal) or for a four-month period (complete removal) during the 1990–1991 winter. Foliage and soils were sampled periodically during the summer of 1991. The complete snow removal treated trees showed decreased leaf water potential and increased peroxidase activity over most of the growing season. Foliar Ca was reduced in both snow removal treatments early in the growing season while foliar N was reduced in the complete snow removal trees late in the growing season. Soil NO ₃ ^– and K⁺ were elevated in both snow removal treatments at various times throughout the growing season. Prolonged soil frost in a sugar maple stand can induce lower leaf water potential, higher leaf peroxidase activity and early leaf senescence during the following growing season. Soil frost may have reduced nutrient uptake without affecting significantly the leaf nutrient status.     

The consequences of multiple resource shifts on the productivity and composition of alpine tundra communities: inferences from a long-term snow and nutrient manipulation experiment

Background: Gradients in the amounts and duration of snowpack and resulting soil moisture gradients have been associated with different plant communities across alpine landscapes.

Aims: The extent to which snow additions could alter plant community structure, both alone and in combination with nitrogen (N) and phosphorus (P) additions, provided an empirical assessment of the strength of these variables on structuring the plant communities of the alpine tundra at Niwot Ridge, Colorado Front Range.

Methods: A long-term snow fence was used to study vegetation changes in responses to snowpack, both alone and in conjunction with nutrient amendments, in plots established in dry and moist meadow communities in the alpine belt. Species richness, diversity, evenness and dissimilarity were evaluated after 20 years of treatments.

Results: Snow additions, alone, reduced species richness and altered species composition in dry meadow plots, but not in moist meadow; more plant species were found in the snow-impacted areas than in nearby controls. Changes in plant community structure to N and N + P additions were influenced by snow additions. Above-ground plant productivity in plots not naturally affected by snow accumulation was not increased, and the positive responses of plant species to nutrient additions were reduced by snow addition. Plant species showed individualistic responses to changes in snow and nutrients, and indirect evidence suggested that competitive interactions mediated responses. A Permanova analysis demonstrated that community dissimilarity was affected by snow, N, and P additions, but with these responses differing by community type for snow and N. Snow influenced community patterns generated by N, and finally, the communities impacted by N + P were significantly different than those affected by the individual nutrients.

Conclusions: These results show that changes in snow cover over a 20-year interval produce measureable changes in community composition that concurrently influence and are influenced by soil nutrient availability. Dry meadow communities exhibit more sensitivity to increases in snow cover whereas moist meadow communities appear more sensitive to N enrichment. This study shows that the dynamics of multiple limiting resources influence both the productivity and composition of alpine plant communities, with, species, life form, and functional traits mediating these responses.     

Short-term responses of winter soil respiration to snow removal in a Picea asperata forest of western Sichuan

Aims Seasonal snow cover is one of the most important factors that control winter soil respiration in the cold biomes. The warming-induced decreases in snowpack could affect winter soil respiration of subalpine forests. The aim of this study was to explore the effects of snow removal on winter soil respiration in a Picea asperata forest.Methods A snow removal experiment was conducted in a P. asperata forest stand in western Sichuan during the winter of 2015/2016. The snow removal treatment was implemented using wooden roof method. Soil temperatures, snow depth and soil respiration rate were simultaneously measured in plots of snow removal and controls during the experimental period.Important findings Compared to the control, snow removal increased the fluctuations of soil temperatures. The average daily temperature of the soil surface and that at 5 cm depth were 1.12 °C and 0.34 °C lower, respectively, and the numbers of freeze-thaw cycles of the soil surface and that at 5 cm depth were increased by 39 and 12, respectively, in plots of snow removal than in the controls. The average rate of winter soil respiration and CO₂ efflux were 0.52 μmol·m^-2·s^-1 and 88.44 g·m^-2, respectively. On average, snow removal reduced soil respiration rate by 21.02% and CO₂ efflux by 25.99%, respectively. More importantly, the snow effect mainly occurred in the early winter. The winter soil respiration rate had a significant exponential relationship with soil temperature. However, snow removal significantly reduced temperature sensitivity of the winter soil respiration. Our results suggest that seasonal snow reduction associated with climate change could inhibit winter soil respiration in the subalpine forests of western Sichuan, with significant implications for the carbon dynamics of the subalpine forests.     

Cascading Effects of Climate Change on Forest Ecosystems: Biogeochemical Links Between Trees and Moose in the Northeast USA

The relationship between herbivores, plants and nutrient dynamics, has been investigated in many systems; however, how these relationships are influenced by changing climate has had much less attention. In the northeastern USA, both moose populations and winter climate have been changing. Moose, once extirpated from the region, have made a comeback; while locally, snow depth and duration of snow cover have declined. There is considerable uncertainty in how these changes will interact to influence forested systems. We used small experimental plots and transects along with snow removal (to elicit soil freezing and expose potential forage plants), mechanical browsing, and fecal additions (labeled with ¹⁵N) to examine ecosystem responses. We found that snow removal changed moose browsing behavior, with balsam fir more heavily browsed than sugar maple or Viburnum under low snow conditions. Soil freezing alone did not significantly alter N dynamics or selected plant responses, but there were significant interactions with moose activity. The combined effects of moose fecal additions, mechanical browsing, and soil freezing resulted in higher levels of NO₃ ^? leaching under fir and maple, whereas Viburnum had essentially no response to these multiple factors. Our results suggest that declines in snow depth can initiate a cascade of ecosystem responses, beginning with exposure of plants to increased browsing that then triggers a series of responses that can lead to higher N losses, precipitated by decreased N demand in plants compromised by soil freezing damage. Balsam fir may be particularly susceptible to this cascade of multiple stresses.     

Impact of snowpack decrease on net nitrogen mineralization and nitrification in forest soil of northern Japan

Winter climate change is an important environmental driver that alters the biogeochemical processes of forest soils. The decrease in snowpack amplifies soil freeze–thaw cycles and decreases the snowmelt water supply to soil. This study examined how snow decrease affects nitrogen (N) mineralization and nitrification in forest soil in northern Japan by conducting an in situ experimental snowpack manipulation experiment and a laboratory incubation of soil with different moisture, temperature and freeze–thaw magnitudes. For the incubation studies, surface mineral soil (0–10 cm) was collected from a cool-temperate natural mixed forest and incubated using the resin core method during the winter. In the field, there were two treatments: 50 and 100 % snow removal and control plots. The increase in the soil freeze–thaw cycle increased net N mineralization and marginally decreased the net nitrification in soil. The dissolved organic carbon (DOC) and DOC/DON ratio in soil increased with the decrease in snowpack especially during the snow melt period. These results suggested that the change in substrate quality by the increase in freeze–thaw cycles caused the significant enhancement of microbial ammonium production in soil. The lower soil moisture and higher gross immobilization of inorganic N by soil microbes may be maintaining the slow net nitrification and low nitrate leaching in freeze–thaw cycles with less snowpack. The results indicate that winter climate change would strongly impact N biogeochemistry through the increase in ammonium availability in soil for plants and microbes, whereas it would be unlikely that nitrate loss from surface soil would be enhanced.     

Evergreen alpine shrubs have high freezing resistance in spring,irrespective of snowmelt timing and exposure to frost: an investigation from the Snowy Mountains,Australia

Over winter, alpine plants are protected from low-temperature extremes by a blanket of snow. Climate change predictions indicate an overall reduction in snowpack and an earlier thaw; a situation which could expose the tips of shrubs which extend above the snowpack to freezing events in early spring, and cause foliar frost damage during the onset of physiological activity. We assessed the photosynthetic responses of freezing-damaged shrub leaves from an assay of freezing temperatures in the Snowy Mountains in south-eastern Australia, using chlorophyll fluorometery ex situ. We sampled leaves that were exposed early during the spring thaw and leaves that were buried in snow for up to two extra weeks, from four evergreen shrub species at monthly intervals following the period of snowmelt. Freezing resistance (estimated from LT₅₀) was poorest at the earliest spring sampling time, in both exposed above-snow and protected below-snow foliage in all species. Protected foliage in early spring had lower freezing resistance than exposed foliage, but not significantly so. By the third sampling time, freezing resistance was significantly better in the lower protected foliage (LT₅₀ of ? 14) compared with the upper exposed foliage (LT₅₀ of ? 10) in one species. Over the course of spring, freezing resistance improved significantly in all species, with LT₅₀ values of between ? 10 and ? 15 °C by the third sampling time, which is lower than the minimum air temperatures recorded at that time (> ? 5 °C). The results indicate that the dominant evergreen shrub species in this area may only be susceptible to freezing events very early in spring, before a period of frost-hardening occurs after snowmelt. Later in spring, these alpine shrubs appear frost hardy, thus further perpetuating the positive feedbacks surrounding shrub expansion in alpine areas.     

The effect of soil freezing on N cycling: comparison of two headwater subcatchments with different vegetation and snowpack conditions in the northern Hokkaido Island of Japan

Climate change models predict that the snowpacks of temperate forests will develop later and be shallower resulting in a higher propensity for soil freezing. In the northern most island of Japan, Hokkaido, snowpack depth decreases from west to east. This snowpack depth gradient provided a unique opportunity to test the effects of variable snowpack and soil freezing on N biogeochemistry. The Shibecha Northern Catchment in Shibecha Experimental Forest, eastern Hokkaido had deciduous trees and a mean annual snowpack of 0.7 m while the M3 catchment in Uryu Experimental Forest, western Hokkaido had mixed deciduous and coniferous tree species and a mean annual snowpack of 2.0 m. We conducted a field study (October 2004–April 2005) to determine if differences in Shibecha and Uryu soil extractable N, N mineralization, and nitrification were controlled by the variability in soil freezing during winter or tree species composition that affected the quality of the forest floor. The mixed deciduous and coniferous trees forming the Uryu forest floor had a higher C:N ratio (25.0 vs. 22.4 at Shibecha), higher lignin:N ratio (15 vs. 8.8), and higher lignin concentrations (0.28 vs. 0.18 g lignin g⁻¹). These differences in forest floor quality contributed to higher net N mineralization and nitrification in Shibecha compared to Uryu. In Shibecha, soil remained frozen for the entire study. For Uryu, except for an early period with cold temperatures and no snow, the soil generally remained unfrozen. As a result of the early winter cold period and soil freezing, extractable soil NH₄⁺ did not change but NO₃⁻ increased. Reciprocal 0–5 cm mineral soil transplants made between Shibecha and Uryu and incubated during winter at 0, 5, and 30 cm suggested that soil freezing resulted in greater net N mineralization yet lower nitrification regardless of the soil origin. The effect of soil freezing should be considered when evaluating differences in N dynamics between temperate ecosystems having a propensity for soil freezing.     

Experimental Manipulation of Forest Structure: Near-Term Effects on Gap and Stand Scale C Dynamics

Canopy gaps and coarse woody debris are two forest structural features that are more abundant in old-growth forests than in second-growth, even-aged stands. These features directly influence the carbon balance of the ecosystem, yet few studies have quantified their interactive effects. We experimentally manipulated the forest structure of a second-growth northern hardwood forest in north-central Wisconsin (USA) and measured the shift of C between pools of the ecosystem components. Here, we question the longevity of the changes to the aboveground pools and address their implications for total ecosystem C (TEC) and net ecosystem production (NEP) at both the gap and stand scale. At the scale of the gap, the harvest and removal of trees significantly reduced NEP (?3.2 to ?3.5 Mg C ha^?1 for gaps vs 2.2 to 2.5 Mg C ha^?1 for reference conditions), but did not alter heterotrophic respiration. The addition of woody debris without harvest significantly increased heterotrophic respiration, decreasing soil C storage of the gap area (?0.5 to ?1.1 Mg C ha^?1). The combined treatment of gap creation and woody debris addition made the gap area a significant C source to the atmosphere for the 3 years of the study (?4.9 to ?5.1 Mg C ha^?1). We also estimated how these structural features would affect C dynamics at a broader scale. The conversion of 10% of the stand canopy to gap conditions caused only a brief decrease in the stand NEP with the C balance returning to reference conditions by the third year following tree harvest. The woody debris additions caused an increase in both TEC and heterotrophic respiration. When combined the addition of canopy gaps and woody debris caused plots to initially become significant C sources, relative to undisturbed locations that were consistently accumulating C, with an annual NEP ranging from 2.1 to 2.8 Mg C ha^?1 y^?1. Understanding the effects of these structural features on forest C dynamics is highly relevant as the maturing forests of the region transition to more structurally complex forests and the demand for managing ecosystems for long-term C sequestration increases.     

Photosynthetic and phenological responses of dwarf shrubs to the depth and properties of snow

Snow is known to have a major impact on vegetation in arctic ecosystems, but little is known about how snow affects plants in boreal forests, where the snowpack is uneven due to canopy impact. The responses of two dwarf shrubs, the evergreen Vaccinium vitis‐idaea and the deciduous V. myrtillus, to snow conditions were studied in a snow manipulation experiment in southern Finland. The thermal insulation of the snowpack was expected to decrease with partial removal or compression of the snow, while addition of snow was expected to have the opposite effect. The penetration of light was manipulated by partial removal of snow or by formation of an artificial ice layer in the snowpack. CO₂ exchange measurements that were carried out at the time of maximum snow depth in late March indicated significant photosynthetic activity in the leaves of V. vitis‐idaea under snow. Net gain of CO₂ was observed in the daytime on all the manipulation plots, excluding the snow addition plots, where light intensity was very low. The subnivean photosynthesis compensated for a substantial proportion (up to 80%) of the respiratory CO₂ losses. Chlorophyll fluorescence measurements indicated reduced potential capacity of photosystem II in the leaves of V. vitis‐idaea on those plots where snow cover was thin. Neither V. vitis‐idaea nor V. myrtillus suffered from frost damage (assessed as electrolyte leakage) when thermal insulation was reduced by means of snow manipulations. No phenological responses were observed in V. vitis‐idaea, but in V. myrtillus bud burst, leaf unfolding and flowering were advanced by 1–3 days on the addition plots. The results of the present study show that dwarf shrubs respond to not only the thickness of snow but also the physical properties of snow, both of which are expected to change due to climatic warming.     

Measuring the effects of reduced snow cover on Australia's alpine arthropods

Snow is one of the most important factors in the ecology of alpine ecosystems. In Australia, both the depth and duration of snow cover have declined significantly in recent decades and this trend is projected to continue with global warming. Many small arthropods remain active throughout the winter, within a space beneath the snowpack (subnivean) where the snow's insulation creates a thermally stable environment. Using field surveys and experimental manipulation of snow depth at two locations in the Australian alpine region, we explored the diversity of winter‐active arthropods and their response to reduced snow. Individuals from 18 arthropod Orders were detected beneath the snow during winter, with Collembola, Araneae, Acari and Coleoptera accounting for 95–98% of the individuals collected. The subnivean taxa represented a distinct subset of those active outside the winter months. Removal of the snow layer increased daily temperature fluctuations, increased the number of days below freezing and raised the mean surface temperatures. Community composition was altered by snow removal, driven by changes in the numbers of two abundant springtail taxa at each location. We found a strong reduction in the abundances of both taxa at one study site, and contrasting responses (one strong positive and one strong negative) to snow removal at the second study site. Subnivean arthropod communities in Australia thus appear sensitive to snow conditions at small spatial scales.     

N2O emission in a Norway spruce forest due to soil frost: concentration and isotope profiles shed a new light on an old story

In mountain regions of Central Europe an increase of soil frost periods is predicted for this century due to reduced snow fall. To investigate the effects of freezing and thawing on soil N₂O fluxes in a mature Norway spruce forest in the mountainous Fichtelgebirge, Germany, the natural snow cover on three experimental plots was removed to induce soil frost. Three plots with natural snow cover served as controls. Soil N₂O fluxes were recorded in biweekly to monthly intervals during the frost and subsequent thawing period of the below-average cold winter in 2005/2006 and in the above-average warm winter in 2006/2007. In addition, N₂O concentrations and isotope signatures in soil air were measured along soil profiles in six different depths (from 6 to 70 cm). The soil of the snow removal plots was frozen down to 15 cm depth from January to April 2006 while the soil of control plots remained unfrozen under snow cover. Both soil freezing and thawing resulted in almost tenfold enhanced N₂O fluxes on snow removal plots contributing 84% to annual N₂O emissions. In the subsequent winter without soil frost no effects were observed. Vertical gradients of N₂O concentrations together with isotope abundance suggest that the subsoil of all plots was a probably weak, but continuous N₂O source throughout the year. Isotope signatures and N₂O concentration gradients in the soil profile indicate that microbial N₂O production and reduction of N₂O to N₂ did not or just marginally occur in frozen soil layers of the snow removal plots. Consequently, elevated N₂O fluxes in the late winter were attributed to the release of accumulated N₂O originating from the subsoil. At unfrozen soil, however, N₂O emissions were reduced due to a shift of the N₂O production-consumption ratio towards more consumption in the topsoil of both the control and snow removal plots. These findings contradict the general assumption that N₂O production in the organic layer is responsible for bursts of N₂O due to soil frost.