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Abstract

Arctic climate change is expected to lead to a greater frequency of extreme winter warming events. During these events, temperatures rapidly increase to well above 0 °C for a number of days, which can lead to snow melt at the landscape scale, loss of insulating snow cover and warming of soils. However, upon return of cold ambient temperatures, soils can freeze deeper and may experience more freeze–thaw cycles due to the absence of a buffering snow layer. Such loss of snow cover and changes in soil temperatures may be critical for litter decomposition since a stable soil microclimate during winter (facilitated by snow cover) allows activity of soil organisms. Indeed, a substantial part of fresh litter decomposition may occur in winter. However, the impacts of extreme winter warming events on soil processes such as decomposition have never before been investigated. With this study we quantify the impacts of winter warming events on fresh litter decomposition using field simulations and lab studies.

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... Consequently, the rate and timing of litter decomposition feedbacks to plant productivity and diversity, and drives soil carbon (C) storage in these ecosystems (Cornelissen et al. 2007;Grigulis et al. 2013). In cold biomes, a significant portion of the decomposition process occurs during winter (Gosz et al. 1973;Hobbie and Chapin 1996; but see Bokhorst et al. 2010), with an increase in microbial biomass during this period (Lipson et al. 1999). This is followed by a rapid decline of the winter-dwelling microbial community during snowmelt and a peak of available N (Schmidt and Lipson 2004;Schmidt et al. 2007; Buckeridge and Grogan 2010;Buckeridge et al. 2013). ...
... At the end of October 2016, senescent leaves were collected in each plot near the ground surface. Although most leaves were already senescent at the end of September, they were left in the field for about one month during the leaching phase, which is particularly intense during the first days after senescence (Nykvist 1961;Gessner and Schwoerbel 1989;Taylor and Bärlocher 1996;Bokhorst et al. 2010) and lasts about one month (Gosz et al. 1973;Hobbie and Chapin 1996;Bokhorst et al. 2010). Unlike previous studies which collected freshly fallen leaves (Gosz et al. 1973;Taylor and Jones 1990;Bokhorst et al. 2010), the material collected here had already been subjected to leaching during autumn in order to measure winter decomposition. ...
... At the end of October 2016, senescent leaves were collected in each plot near the ground surface. Although most leaves were already senescent at the end of September, they were left in the field for about one month during the leaching phase, which is particularly intense during the first days after senescence (Nykvist 1961;Gessner and Schwoerbel 1989;Taylor and Bärlocher 1996;Bokhorst et al. 2010) and lasts about one month (Gosz et al. 1973;Hobbie and Chapin 1996;Bokhorst et al. 2010). Unlike previous studies which collected freshly fallen leaves (Gosz et al. 1973;Taylor and Jones 1990;Bokhorst et al. 2010), the material collected here had already been subjected to leaching during autumn in order to measure winter decomposition. ...
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AimsIn cold biomes, snow cover mitigates the harsh winter soil conditions, thereby enhancing overwinter decomposition of organic matter which controls the availability of nutrients for plants and microbial uptake at the beginning of the growing season. Yet, how this buffering effect is modulated by litter traits, soil characteristics and herbivory remains poorly studied.Methods We conducted a litter-bag experiment in four types of subalpine grasslands, two of which being naturally free of snow during most of the winter. Litter bags were filled either by grasses or forbs sampled in plots submitted the preceding summer to grasshopper grazing treatments.ResultsSnow cover strongly increased the decomposition of forbs, but not of grasses. Litter quality (low C:N and polyphenols triggering a priming effect) and soil ammonium content were correlated with litter decomposition rate in the presence of snow only, whereas soil organic matter content was positively associated with decomposition rate under both snow regimes. Herbivory did not affect decomposition.Conclusions Our findings may be explained by the functional differences between copiotrophic and oligotrophic microbes, copiotrophs being more sensitive to harsh abiotic conditions than oligotrophs. As long as copiotrophs are favored by high litter quality and nutrient-rich soils (high ammonium content), litter decomposition is enhanced in the absence of snow only.
... Higher above-ground biomass could result in a thicker litter layer, which will greatly increase soil nutrient supply and also impact species coexistence by inhibiting seed germination and seedling survival (Berg & McClaugherty, 2008;Chapin, Matson, & Mooney, 2002;Suttle, Thomsen, & Power, 2007). Although many studies have assessed how changes in snow depth affect litter decomposition of individual species, the findings are inconsistent, including positive (Blok, Elberling, & Michelsen, 2016;Christenson, Mitchell, Groffman, & Lovett, 2010;Saccone et al., 2013), negative (Baptist et al., 2010) or no significant responses (Bokhorst, Bjerke, Melillo, Callaghan, & Phoenix, 2010;Bokhorst, Metcalfe, & Wardle, 2013) to deepened snow. Litter layer consists of litter from different species, and it remains unknown how changes in winter snow depth affect the turnover rate of the whole litter layer, and whether the responses vary in different steppes. ...
... Warmer soil environment can stimulate microbial activity and accelerate microbial degradation of litter during winter (Blok et al., 2016;Christenson et al., 2010). However, deeper snow also decreases the frequency of soil freeze-thaw cycles (Bokhorst et al., 2010;Brooks et al., 2011;Buckeridge, Cen, Layzell, & Grogan, 2010), which may negatively affect litter decomposition by reducing physical fragmentation of litter. In addition, the effects of winter snow on litter layer turnover may continue into the growing season. ...
... This direct effect might be correlated with some other undetermined potential processes. For example, deepened snow could not only reduce wind abrasion of standing litter by reducing the amount of litter protruding above the snow cover, but also decrease the frequency of freeze and thaw cycles during early spring (Bokhorst et al., 2010;Buckeridge et al., 2010). These processes could significantly slow down litter fragmentation and thus litter turnover rate. ...
Article
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The turnover of litter layer is a biogeochemical process fundamental to carbon and nutrient cycling, influencing seed germination, species coexisting, and carbon storage. Winter snow depth is undergoing increasing trend in Northern China, which has been shown to alter litter decomposition rate of individual species. However, it remains unknown how changes in snow depth affect the turnover rate of the whole litter layer, and whether the responses vary between different steppes. Most current litter decomposition studies are site‐based or short‐term treated, limiting the exploration of the long‐term response of litter layer turnover in regional pattern. In this study, we selected six long‐term (11–13 years) snow fence sites in Inner Mongolia, with three in the dry steppe and another three in the wet steppe, and investigated the responses of community‐weighted litter residence time (LRT) to long‐term increased snow treatment. We found that LRT increased by 0.02 year for every 10 cm increase in snow depth in the wet steppe, but was not affected in the dry steppe. The lack of effect of deepened snow on LRT in the dry steppe was attributed to the offset between the positive effect of the increased plant community‐weighted height possibly via inhibiting photodegradation and the enhanced litter recalcitrance by producing higher proportion of stem litter, and the negative effect of the increased soil moisture via accelerating microbial decomposition. The significantly positive effect of the deepened snow on LRT in the wet steppe was mainly because the deepened snow increased grass biomass but had no effect on forb biomass, which reduced the degradability of the litter layer. Overall, our findings indicated that deepened snow changed plant community, which altered environmental conditions and enhanced litter recalcitrance, thereby increasing LRT in temperate steppes. However, this effect was diminished by enhancing microbial decomposition in the dry but not wet steppe, resulting in different overall responses of LRT to deep‐snow in the two steppes. The slow litter turnover rate in the wet steppe might result in greater litter accumulation under future increased snow depth, which could be unfavorable for seed germination and alter plant diversity.
... Indeed, depending on regions, climate changes may have various consequences on the snowpack depth and on its physical properties leading, for instance, to thinner snowpack due to reduced winter snow fall associated with increasing frequencies of extreme winter warming events (Christensen et al. 2007). The loss of the snow layer insulating effect could lead the under-snow ecosystems to experience harsher winter conditions with increased frequencies of freeze-thaw cycles (Bokhorst et al. 2010). Such rapid temperature changes are likely to impact soil organisms and the processes they are involved in Coulson et al. 2000;Haei et al. 2011;Schimel et al. 2007;Yergeau and Kowalchuk 2008). ...
... Finally, the few studies conducted in the field have evaluated wintertime litter decomposition using a simple differential approach comparing the remaining litter mass after winter with the initial litter mass deposited before winter (see for example Baptist et al. 2010;Hobbie and Chapin 1996). Bokhorst et al. (2010) suspected that such methods lead to misinterpretation by confusing early and fast organic compounds leaching during fall with slower wintertime litter decomposition. ...
... In an arctic experiment, Hobbie and Chapin (1996) found that after the first summer, all the mass loss of their buried litter bags occurred during winter. Conversely, Bokhorst et al. (2010) found that 20 % of mass loss occurred during the cold season in a subarctic heathlands experiment but only~2 % during the winter itself. They considered that most of the cold season litter mass loss happened during the first weeks after the litter fall. ...
... Moreover, nutrients released into soil via litter decomposition and FT disturbance account for a considerable portion of the annual nutrient mineralization budget, which might affect the quantity and quality of DOM in soils during the following growing season (Baptist et al., 2010;Wu et al., 2010;Tan et al., 2015). Bokhorst et al. (2010) reported that increases in FT cycles resulting from winter warming had little effect on litter decomposition in non-growing season. However, it is not clear whether increase in winter FT cycles can affect the dynamic of soil DOM in subsequent growing seasons (Bokhorst et al., 2010). ...
... Bokhorst et al. (2010) reported that increases in FT cycles resulting from winter warming had little effect on litter decomposition in non-growing season. However, it is not clear whether increase in winter FT cycles can affect the dynamic of soil DOM in subsequent growing seasons (Bokhorst et al., 2010). With respect to the sensitivity of soil DOM to changing environmental conditions during litter decomposition, specific UV-visible absorbance (SUVA) and 3D-EEM combined with PARAFAC decomposition, is widely accepted as a rapid and powerful method for characterizing soil DOM traits, such as aromaticity, molecular weight, the degree of humification, and components of fluorescence after PARAFAC decomposition (Chen et al., 2003;Murphy et al., 2013). ...
Article
Future climatic change is likely to increase the occurrence of soil freeze-thaw (FT) events in high latitude and/or high altitude zones, which can substantially influence the dynamics of dissolved organic matter (DOM) released into the soils. However, it is not clear how the quantity and quality of soil DOM respond to changing FT patterns under different soil moisture and litter manipulation conditions in northern temperate forest stands. In this study, litter-amended and non-amended forest soils were incubated for 360 days at three soil moisture levels (30%, 60%, and 90% water-filled pore space) and three intensities of FT disturbance (low, high, and none). We quantified heterotrophic respiration, enzyme activity, microbial biomass, and water-extractable organic matter (WEOM) as a proxy for DOM in soils during the incubation experiment. The quality of WEOM was characterized by biodegradability, UV absorbance and parallel factor analysis modelling of fluorescence excitation emission matrices. Concentrations of water-extractable organic carbon (WEOC) and biodegradable WEOC declined continuously in all treatments over the 360-day incubation period. The dominant component of fluorescent WEOM shifted from humic-and fulvic-like components during the first 108 days of incubation, to protein-like components of microbial origin, characterized by high aromaticity, at the end of the incubation period. Litter amendment, FT disturbance, and their interaction increased WEOC concentrations in soils during the early 108-day incubation period, particularly in soils with low moisture and high FT intensity, but these increases disappeared after 252 days incubation at 15 • C. Litter-derived fluorescent WEOM in soils without FT disturbance was dominated by protein-like components after 14 days of incubation, but these were replaced by humic-and fulvic-like components after 108 days. This replacement effect was weaker in soils with FT disturbance, which we attribute to changes in microbial properties, including enzyme activity, microbial biomass and activity.
... Temperature and litter quality are the two most important factors influencing the rates of plant litter decomposition in terrestrial ecosystems. A variety of artificial warming experiments have been conducted to understand the mechanisms underlying the response of plant litter decomposition to increasing temperatures, but the results varied among different plant species, ecosystems, and experimental sites; neutral (Bokhorst et al. 2010;Rouifed et al. 2010), negative (Cornelissen et al. 2007), and positive (Suseela et al. 2013;Xu et al. 2012) effects have been observed. Understanding how climate-chemistry interactions regulate the decomposition of plant litter in alpine ecosystems is of particular interest because cold areas are predicted to be extremely sensitive to warming, and plant decomposition in these regions is strongly limited by temperature (Bradford et al. 2016;Hobbie et al. 2002). ...
... However, there were some differences among ecosystems and litter types (Table 2). Previous studies have shown that warming increased the mass loss of litter from arctic dwarf shrubs (Verburg et al. 1999) and trees from boreal forests (Rustad and Fernandez 1998), but it did not affect the decomposition rate of plant litter in a Nordic heathland community (Bokhorst et al. 2010) and forest-tundra ecotone (Sjögersten and Wookey 2004). ...
Article
The decomposition of plant litter is a key link in global C budgets and provides strong feedback to changes in climate and biogeochemical cycles. However, the combined effects of global warming and plant litter quality on the rate of plant litter decomposition and nutrient dynamics in alpine ecosystems are still poorly understood. We conducted a warming experiment to investigate the effects of litter quality and temperature on decomposition rates and variations in nutrients of four common herbaceous plants (low-quality litter species Stipa purpurea and Carex moorcroftii and high-quality litter species Astragalus confertus and Leontopodium nanum) during 2011–2016. During the initial stages of decomposition, warming had no significant effect on the mass loss of plant litter for low-quality litter species, but in the later stages of decomposition, it had a negative effect on the mass loss across all species (P < 0.05). Litter quality was the best predictor of N and P release/immobilisation during the decomposition of aboveground plant litter. Low-quality litter had the highest immobilisation of N at about 80% of the initial remaining mass; nutrients were then released in the following stages of decomposition. However, the fraction of initial P decreased with the mass remaining during the initial and later phases of decomposition, but a short period of P immobilisation occurred in the middle phase of decomposition. For high-quality litter, the fraction of initial N and P decreased with the mass remaining during the whole decomposition process. Warming had a marginal influence on the N and P dynamics throughout the decomposition process. Our study showed that the decay of plant litter was strongly suppressed by warming climate and that the N and P dynamics on the investigated Tibetan grassland were mainly regulated by litter quality, providing valuable insights into the biogeochemical cycles of nutrients in alpine ecosystems.
... However, other studies reveal the opposite results; for instance, Boyero et al. (2011) and Cornelissen et al. (2007) found that warming would not accelerate litter decomposition in streams and cold biomes. Additionally, a neutral effect of warming on litter decomposition was also observed by Bokhorst et al. (2010) in a subarctic heathland community. Previous studies have also reported that the direction and magnitude of the response of litter decomposition to warming varied based on the plant species (Bokhorst et al. 2010), vegetation type (Cornelissen et al. 2007), moisture (Christiansen et al. 2017), and forest type . ...
... Additionally, a neutral effect of warming on litter decomposition was also observed by Bokhorst et al. (2010) in a subarctic heathland community. Previous studies have also reported that the direction and magnitude of the response of litter decomposition to warming varied based on the plant species (Bokhorst et al. 2010), vegetation type (Cornelissen et al. 2007), moisture (Christiansen et al. 2017), and forest type . Although Aerts (2006) demonstrated that warming resulted in slightly increased decomposition rates in cold biomes, only 34 site-species combinations from five experimental warming studies were included. ...
Article
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Background and aims Litter decomposition is a fundamental process of biogeochemical cycles and particularly sensitive to global change. However, the overall effects of warming, elevated carbon dioxide and changed precipitation regime on litter decomposition are not well studied. Methods To assess the effects of these three common global change factors on litter decomposition, we performed a meta-analysis using 366 pairwise observations from 103 published articles. We quantified the responses of litter decomposition rate to the effects of warming, elevated CO2 , and changed precipitation regime (increased and decreased). Results At the global scale, warming and precipitation addition significantly stimulated litter decomposition rate by an average of 4.20% and 11.72%, respectively. In contrast, elevated CO2 and precipitation removal showed significant negative effects on litter decomposition rate (-2.99% and -12.60%). In addition, study type, plant functional traits, and climate were consistent mod-erators. These results indicate that warming, elevated CO2 , and changed precipitation regime have significantly affected litter decomposition, but the direction and magnitude of the effects of different factors varied, and were also differently mediated by moderator variables. Conclusions Global cycles of carbon and nutrients via the litter decomposition process can be substantially affected by global change. However, the combined effects of these global change factors on litter decomposition and the different effects between the arid and humid areas cannot be addressed due to the lack of data, indicating the need of more focus on multi-factor ma-nipulative experiments in a wider range of study sites.
... For example, experiments have found that reduced snow cover decreases the litter decomposition rates by 5-47% in forests (Christenson et al. 2010;Kreyling et al. 2013) and tundra (Blok et al. 2016), and a surprising threefold lower decomposition rate was reported under reduced snow cover in a subalpine meadow (Saccone et al. 2013). However, other experiments conducted in subarctic and alpine ecosystems have suggested that reduced snow cover has only a minor (Baptist et al. 2010;Aerts et al. 2012) or even null effect (Bokhorst et al. 2010) on litter decomposition rate. A possible reason should be attributed to the hierarchical decomposability of chemical compounds or C fractions, particularly at the initial status (Adair et al. 2008). ...
Article
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Purpose Winter snow cover is a key control over soil biogeochemical cycles in cold biomes but has decreased with climate warming, particularly in high-latitude and high-altitude areas. Previous studies have found a decline in carbon (C) release from decomposing litter without snow cover, yet partitioning what C fractions respond to snow removal remains not fully understood. Methods Here we incubated fir and birch litter in microcosms over two years of decomposition in an alpine forest to assess the release of various C fractions with and without snow cover using ¹³C nuclear magnetic resonance (NMR). Results We found that 51% and 61% of total organic C was lost from fir and birch litter, respectively, after two years of decomposition, and 59% and 60% of this C was decomposed during winter. More C was lost from both fir and birch litter under snow cover, and the decomposition rate of birch litter C was decreased by 50% after snow removal. However, only O-alkyl C (particularly carbohydrate C) release was reduced with snow removal, and the release of other C fractions (i.e., alkyl C, aromatic C and carbonyl C) did not change significantly after removing snow cover. Delayed O-alkyl C release from fir and birch litter was in accordance with the declines in soil dissolved organic C and microbial respiration rate under removed snow plots, but this delay induced by snow removal did not carry over to the following growing season. Conclusion We conclude that a reduction in winter snow cover regulates early decomposition of plant litter by controlling the rapid release of more labile C fractions (i.e., carbohydrates) in seasonally snow-covered ecosystems.
... For example, experiments found that reduced snow cover decreases the litter decomposition rates by 5-47% in forests [18,46] and tundra [47], and a surprising threefold lower decomposition rate was reported under reduced snow cover in a subalpine meadow [19]. However, other experiments suggested that reduced snow cover has only a minor [28,48] or even null effect [49,50] on litter decomposition in subarctic and alpine ecosystems. Indeed, in our mid-latitudinal alpine forest, we also found that reduced snow cover decreased the loss of litter mass but only during the first two years and not during the subsequent periods. ...
Article
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While carbon loss from plant litter is well understood, the mechanisms by which this carbon is sequestered in the decomposing litter substrate remains unclear. Here we assessed humus accumulations in five foliar litters during four years of decomposition and their responses to reduced snow cover in an alpine forest. In contrast to the traditional understanding (i.e., the three-stage model), we found that fresh litter had a high humus content (8–13% across species), which consistently increased during litter decomposition and such an increase primarily depended on the accumulation of humic acid. Further, reduced snow cover decreased humus accumulation at early stages but increased it at late stages. These results suggested that humification simultaneously occurred with decomposition during early litter decay, but this process was more sensitive to the changing climate in seasonally snow-covered ecosystems, as previously expected.
... For example, experiments have found that reduced snow cover decreases the litter decomposition rates by 5-47% in forests (Christenson et al. 2010; Kreyling et al. 2013) and tundra (Blok et al. 2016), and a surprising 3-fold lower decomposition rate was reported under reduced snow cover in a subalpine meadow (Saccone et al. 2013). However, other experiments conducted in subarctic and alpine ecosystems have suggested that reduced snow cover has only a minor (Baptist et al. 2010;Aerts et al. 2012) or even null effect (Bokhorst et al. 2010) on litter decomposition rate. A possible reason should be attributed to the hierarchical decomposability of chemical compounds or C fractions, particularly at the initial status (Adair et al. 2008). ...
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Purpose Winter snow cover is a key control over soil biogeochemical cycles in cold biomes but has decreased with climate warming, particularly in high-latitude and high-altitude areas. Previous studies have found a decline in carbon (C) release from decomposing litter without snow cover, yet partitioning of what C fractions in response to snow removal remains not fully understood. Methods Here we incubated fir and birch litter in microcosms over two years of decomposition in an alpine forest to assess the release of various C fractions with and without snow cover using ¹³C nuclear magnetic resonance (NMR). Results We found that 51% and 61% of total C were lost from fir and birch litter, respectively, after two years of decomposition, and 59% and 60% of these C were decomposed during winter time. More C was lost from both fir and birch litter under snow cover, and the decomposition rate of birch litter C was decreased by 50% after snow removal. However, only O-alkyl C (particularly carbohydrate C) release was reduced with snow removal, and the release of other C fractions (i.e., alkyl C, aromatic C and carbonyl C) did not change significantly after removing snow cover. Reduction in O-alkyl C release from fir and birch litter was in accordance with the declines in soil dissolved organic C, invertase activity and microbial respiration rate under removed snow plots. Conclusion We conclude that a reduction in winter snow cover regulates early decomposition of plant litter by controlling the rapid release of more labile C fractions in seasonally snow-covered ecosystems.
... Specifically, (1) warming-induced increase in litter decomposition was found only in warm (but not cold) season of the alpine meadow (Fig. 1E). Similarly, litter decomposition was also found to be less affected by winter warming in other cold biomes (Bokhorst et al. 2010); (2) warm season consistently increased litter decomposition across the mesh treatments; however, litter decomposed significantly faster in coarse-than fine-meshed bags in warm but not cold season (Fig. 1F). This is because during a long "dormant" alpine winter, most biological activities (including faunal feeding and microbial respiration) cease in the frozen soils (Gavazov 2010). ...
Article
We investigated the effects of warming on litter decomposition and the contribution of soil organisms (microbes vs. fauna) to it across the cold and warm seasons in an alpine meadow of the Qinghai-Tibetan Plateau. Our results showed that (1) warming profoundly increased litter decomposition by ~ 35%, but this warming effect only occurred in coarse-meshed bags (i.e., in the presence of soil fauna) and in warm season; (2) litter decomposition significantly increased by ~ 2.3-fold from fine- to coarse-meshed bags. However, such a mesh effect was only detected in warm (but not cold) season; (3) litter decomposed ~ 6.7 times faster in warm season than in cold season, and this seasonal effect was consistent across ambient and warming climates. Collectively, warm season may greatly promote the role of both fauna and microbes in litter decomposition and determine the amount of annual decomposition. Nevertheless, climate warming may only profoundly stimulate faunal (but not microbial) decomposition, especially during warm season of the alpine meadow.
... In this experiment, it should be noted that the litter was collected in June of 2009, part of which could be decomposed from November of the previous year to June of the present year. Here, we did not consider the litter mass loss through leaching (Bokhorst et al. 2010;Taylor and Bärlocher 1996) and soil micro-organisms nutrient immobilization (Aerts 2006), thus our results may underestimate the effect of warming and grazing on litter mass loss. ...
Article
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Aims Warming and grazing and the changes in litter quality induced by them co-determine litter decomposition and nutrient releases in grazing ecosystems. However, their effects have previously been studied in isolation. Methods We conducted a two factorial experiment with asymmetric warming using infrared heaters and moderate grazing in an alpine meadow. Litter samples were collected from all plots in each treatment, among which some subsamples were placed in their original plots and other samples were translocated to other treatment plots to test the relative effects of each treatment on litter decomposition and nutrient releases. Important Findings We found that warming rather than grazing alone significantly increased total losses of litter mass, total organic carbon (TOC), total nitrogen (TN) and total phosphorus (TP) per unit area due to increases in both mass loss rates and litter biomass. However, grazing with warming did not affect their total mass losses because increased mass loss was offset by decreased litter biomass compared with the control treatment. Seasonal mean soil temperature better predicted litter decomposition than litter lignin content or C:N ratio. There were interactions between warming and grazing, but there were no interactions between them and litter quality on litter decomposition. The temperature sensitivity of TN loss was higher than that of TP loss per unit area. Our results suggested that increased temperature had a greater effect on litter decomposition and nutrient release than change in litter quality, and that more N release from litter could result in greater P deficiency in the alpine meadow.
... Therefore, most TBMs assume the same quantum yield for all PFTs and do not include any temperature sensitivity for this parameter (Dietze, 2014;Rogers et al., 2017). While this assumption is robust for many PFTs and biomes, it does not account for observations of low quantum yield that are associated with the typically cold growth temperatures experienced by arctic and boreal vegetation (Albert et al., 2012;Bokhorst et al., 2010;Kolari et al., 2014;Marchand et al., 2006;Rogers et al., 2019;Solanki et al., 2019;Wallin et al., 2013). ...
Article
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The Arctic Boreal Region (ABR) has a large impact on global vegetation-atmosphere interactions and is experiencing markedly greater warming than the rest of the planet, a trend that is projected to continue with anticipated future emissions of CO2. The ABR is a significant source of uncertainty in estimates of carbon uptake in terrestrial biosphere models (TBMs) such that reducing this uncertainty is critical for more accurately estimating global carbon cycling and understanding the response of the region to global change. Process representation and parameterization associated with gross primary productivity (GPP) drives a large amount of this model uncertainty, particularly within the next 50 years, where the response of existing vegetation to climate change will dominate estimates of GPP for the region. Here we review our current understanding and model representation of GPP in northern latitudes, focusing on vegetation composition, phenology and physiology, and consider how climate change alters these three components. We highlight challenges in the ABR for predicting GPP, but also focus on the unique opportunities for advancing knowledge and model representation, particularly through the combination of remote sensing and traditional boots-on-the-ground science.
... Gibson et al. (2018) confirmed that extreme wildfire events are a major driver of permafrost thaw in the boreal peatlands of the Arctic. Extreme winter warming events tend to cause strong negative impacts on the phenology and growth of vegetation , bryophyte and lichen species , soil fauna and litter decomposition (Bokhorst et al., 2010(Bokhorst et al., , 2012. Gross primary productivity (GPP) is very sensitive to extremely high-temperature events, particularly in high-altitude and high-latitude regions (Yuan et al., 2021). ...
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Plain Language Summary Under the influences of frequent extreme warming events with short duration and high intensity, how much is the impact on seasonally thawing process of active layer? Until now, it remains unclear, and some models related to permafrost exist the knowledge gap in describing the physical mechanism scheme for the impacts. Here, we selected the Tanggula permafrost region of Qinghai‐Tibet Plateau, and used in situ meteorological and active layer hydrothermal data to investigate it. Our results showed that the average contribution rates of extreme warming events to thawing depth was 13.5% ± 4.9% from April to October. Higher contribution rates occurred in the initial and final stages of thawing process. In addition, the active layer thawing process mainly controlled its thermal responses to extreme warming events. Moreover, the inter‐annual differences of active layer thermal conditions during thawing period was likely related to the inter‐annual differences of extreme warming events in frequency and intensity. These results will contribute to understand the influence mechanism of extreme warming events on the active layer thawing process, which can provide useful references for further improving models from the perspective of extreme events.
... Likewise, the mountain system of Nepal is also divided into six life Besides vegetation, climate warming has also been found to affect cryospheric processes (such as changes in snow cover and glacier permafrost) and related hydrological processes (such as water cycle and balance and river discharge) [7]. For example, globally, it was reported that during 1990-2018 the number and area of glacial lakes have rapidly increased from 9414 (5930 km 2 ) to 14,394 (8950 km 2 ), and the glacial lake volume has increased from 105.7 to 156.5 km 3 [24]. Similarly, the increasing number and area of glacier lakes were also revealed in the central Himalayas region during 1990-2010 [25]. ...
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The Himalayas, especially the Everest region, are highly sensitive to climate change. Although there are research works on this region related to cryospheric work, the ecological understandings of the alpine zone and climate impacts are limited. This study aimed to assess the changes in surface water including glacier lake and streamflow and the spatial and temporal changes in alpine vegetation and examine their relationships with climatic factors (temperature and precipitation) during 1995–2019 in the Everest region and the Dudh Koshi river basin. In this study, Landsat time-series data, European Commission’s Joint Research Center (JRC) surface water data, ECMWF Reanalysis 5th Generation (ERA5) reanalysis temperature data, and meteorological station data were used. It was found that the glacial lake area and volume are expanding at the rates of 0.0676 and 0.0198 km3/year, respectively; the average annual streamflow is decreasing at the rate of 2.73 m3/s/year. Similarly, the alpine vegetation greening as indicated by normalized difference vegetation index (NDVI) is increasing at the rate of 0.00352 units/year. On the other hand, the annual mean temperature shows an increasing trend of 0.0329 °C/year, and the annual precipitation also shows a significant negative monotonic trend. It was also found that annual NDVI is significantly correlated with annual temperature. Likewise, the glacial lake area expansion is strongly correlated with annual minimum temperature and annual precipitation. Overall, we found a significant alteration in the alpine ecosystem of the Everest region that could impact on the water–energy–food nexus of the Dudh Koshi river basin.
... We suggest that at the beginning of the vegetation season, remaining fructans as well as sucrose were mobilized to sustain rapid early spring growth after snowmelt (Baptist et al. 2013;Benot et al. 2013), and to prevent frost damage to new photosynthetically active biomass. Thinner snowpack and earlier snowmelt under climate change will lead to colder soil with more frequent freeze-thaw events in early spring (Bardgett et al. 2005;Bokhorst et al. 2010;Kreyling et al. 2010). This could increase remobilization of WSC reserves in leaves, as suggested by a similar growth index at the end of the natural snowmelt among climate treatments despite about one additional month of snow free conditions (Bernard 2017), reflecting a strong growth stress/harsh period. ...
Article
•In subalpine grasslands of the central French Alps, cessation of traditional mowing promotes dominance by Patzkea paniculata (L.) G.H.Loos (Poaceae) tussocks with high biomass but low fodder quality. Mowing limits P. paniculata abundance through the depletion of its water‐soluble carbohydrate (WSC) reserves which sustain early spring growth initiation. However, the effectiveness of mowing effects is modulated by grassland functional composition, fertilization and climate change as WSC compounds, and notably fructans, support plant physiological responses to climate stresses like drought or frost. •To characterize the mechanisms underpinning the control of P. paniculata under global change, we tested the effects of climate manipulation (combined snow removal and drought) and management (cutting and fertilization) alone or in combination on P. paniculata WSC storage in assembled grassland communities of varying functional composition. •Management and climate treatments individually decreased seasonal fructan storage, with neither additive nor synergic effects between them, primarily due to dominance of management over climate effects. Fructan amounts were higher in individuals growing in unmanaged exploitative communities compared to unmanaged conservative communities regardless of climate treatments, but management overrode these differences. •Our findings suggest that reduction by combined snow removal and drought of P. paniculata carbon allocation to WSC storage may limit its dominance similarly to current mowing practices.
... The ΔlogK and E4/E6 values of foliar litter were generally higher than those of other litter components ( Figs. 1 and 2), indicating that the degree of humification in foliar litter was lower than that of the other components. The Kumada's classification also indicated that the humus of foliar litter was types Rp1 and Rp2 (Fig. 5); the humus was young because foliar litter was more likely to be physically damaged by environmental factors (Withington and Sanford Jr. 2007;Bokhorst et al. 2010). Newly formed humus, especially the acid-soluble fractions, is more easily leached by strong rainfall during the growing seasons and stronger snowfall during the snowy seasons than at other times, resulting in a decline in the degree of humification Xiang et al. 2017). ...
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Purpose Humification of plant detritus is an important pathway for soil organic matter formation. The long-standing studies considered that humification proceeds at very late stages of litter decomposition, while the degree of humification in newly shed litters before decomposition remains unclear. In this study, we aimed to explore the degree of humification and conjugate structures of humic substances in newly shed litters, and their variations between litter types with different chemical components and between seasons with different environmental conditions. Materials and methods In this study, we collected fresh foliar (evergreen vs. deciduous and tree vs. shrub), twig, flower, and fruit litters by litter traps in a subalpine forest on the eastern Tibetan Plateau monthly from April to November 2016. The humification degrees of these newly shed litters were evaluated by the optical properties (i.e., ΔlogK, E4/E6, and A600/C) of the alkaline-extracted humic acid-like solutions. E4/E6 and ΔlogK are often used to evaluate the degree of humification, and these values were also used to classify the maturity levels of humic substances. Results and discussion We found that the ΔlogK, E4/E6, and A600/C values varied greatly among seasons with a higher degree of humification observed in August than in the other months for foliar litter. Compared with other litter components, foliar litter exhibited higher ΔlogK, E4/E6, and A600/C values, indicating that fresh foliar litter had a lower degree of humification than the other litters. Twig litter had more stable and abundant precursors of soil organic matter with a higher degree of humification. Moreover, higher degrees of humification with lower ΔlogK and E4/E6 values were observed in evergreen and tree litter species than in deciduous and shrub species, indicating that deciduous and shrub species retained strong abilities to sequester carbon (C). Conclusions The degree of humification in newly shed litters was strongly affected by chemical components. Twig litter had more stable and abundant precursors of soil organic matter with high degree of humification. Deciduous species and shrub species retained strong abilities to sequester C in cold biomes. Additionally, the humic substances in newly shed litters were very young (type Rp), as assessed by the ΔlogK and A600/C values according to the Kumada’s classification, implying that these humic substances might have faster turnover times but strongly contribute to soil organic matter formation in this subalpine forest.
... Reduced snow cover can thus decrease soil biotic activity despite warming 23-25 . Reduced snow cover and increased soil freezing may consequently offset any stimulating effect of warming, leading to no net effects on important ecosystem processes such as litter decomposition 26,27 . Winter warming can lengthen the growing season by later senescence in autumn and earlier green-up in spring but can also increase frost damage of plants and soil biota 28 . ...
Article
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Climate change affects all seasons, but warming is more pronounced in winter than summer at mid- and high latitudes. Winter warming can have profound ecological effects, which are rarely compared to the effects of summer warming, and causal explanations are not well established. We compared mild aboveground infrared warming in winter to warming in summer in a semi-natural, cool-temperate grassland in Germany for four years. Aboveground plant biomass increased following winter warming (+18%) and was unaffected by summer warming. Winter warming affected the composition of the plant community more than summer warming, favoring productive species. Winter warming increased soil respiration more than summer warming. Prolonged growing seasons and changes in plant-community composition accounted for the increased aboveground biomass production. Winter warming stimulated ecological processes, despite causing frost damage to plant roots and microorganisms during an extremely cold period when warming reduced the thermal insulation provided by snow. Future warming beyond such intermittent frosts may therefore further increase the accelerating effects of winter warming on ecological processes.
... The perennial grasses reduced the absorption of nutrients and increased the contribution of litter decomposition, which returned more nutrients to the soil (Propheter and Staggenborg, 2010;Kering et al., 2012;Knoll et al., 2012). In addition, the perennial grasses on the ground mitigated the variation in the soil temperature in winter, protecting the soil freeze-thaw and snow melting (different to dry desert), which prevented soil nutrient leaching and erosion from soil freeze-thaw or snow melting in winter (Bokhorst et al., 2010;Turner and Henry, 2010). Finally, the changes in the enzyme activities might be due to the plants or soil nutrients (Plaza et al., 2004;George et al., 2005). ...
... Carbon, nitrogen and phosphorus release from decomposing litter is an important pathway to maintain the soil biogeochemical processes in forest ecosystems (Moore et al. 2006(Moore et al. , 2010Berg and McClaugherty 2014). Releasing of these elements is mainly affected by climate factors (e.g., temperature and moisture), chemistry and type of the litters (Prescott et al. 2003;Moore et al. 2006) as well as abundance and structure of the decomposer communities (Zhang and Zak 1998;Brussaard et al. 2007;González et al. 2014). ...
Article
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Changes in soil microclimate driven by forest gaps have accelerated mass loss and carbon (C), nitrogen (N) and phosphorus (P) release from foliar litter in alpine forests ecosystems. Yet, it is unclear whether the same gap effect occurs in twig litter decomposition. A 4-year decomposition experiment was conducted in an alpine forest to explore the litter mass loss and C, N and P release among four gap treatments, including (1) closed canopy, (2) small gap (< 10 m in diameter), (3) middle gap (10–15 m in diameter) and (4) large gap (15–20 m in diameter). Compared to the closed canopy, thicker snow cover within gaps resulted in higher soil temperatures but lower frequency of the freeze–thaw cycles during winter. Moreover, gaps slowed the decay rate and C, N and P release after 4-year decomposition, and a total 31–62% of mass, 42–91% of C, 21–111% of N and all P were lost across gaps in winter. Statistical analysis indicated that gap-caused changes in soil temperature and freeze–thaw cycle had significant effects on C, N and P release of twig litter. This study highlights the importance of winter decomposition and gap-driven process in alpine forest ecosystems in southwestern China.
... Increased soil temperature variability, in particular soil freezethaw cycles, adversely affects soil microbial biomass and increases available N and fuel N 2 O emissions according to a meta-analysis on 47 available studies (Song et al. 2017). Winter litter decomposition, however, is remarkably unaffected by midwinter warming events leading to snowmelt in arctic ecosystems (Bokhorst et al. 2010a). Methane emissions in response to winter climate change appear less well studied. ...
Chapter
Climate warming is strongest in winter and in northern ecosystems. Ecological and biogeochemical impacts, however, depend mainly on soil temperatures. Climate warming can contrastingly affect winter soil conditions across northern biomes due to the crucial importance of snow cover: Increasing winter precipitation results in soil warming in the arctic, while midwinter snowmelt events can induce more severe soil frost in arctic and boreal ecosystems. Cold-temperate ecosystems are projected to experience increased soil frost due to strongly reduced snow cover no longer insulating the soil against still cold air temperatures. In cool-temperate ecosystems, warming eventually causes the complete loss of soil frost. Both pathways, soil warming and soil cooling, have important implications for ecology and biosphere-atmosphere feedbacks: In arctic and boreal ecosystems, increased decomposition and mineralization allow for enhanced primary production, but midwinter melting followed by frost and/or rain-on-snow events might counteract this trend. More variable surface temperatures can damage primary production, and colder soil temperatures, due to reduced snow cover, can significantly decrease decomposition in cold-temperate ecosystems. For cool-temperate ecosystems, wetter winters could result in nutrient leaching, and altered dormancy patterns could cause increased frost damage despite air warming. In summary, winter processes are clearly relevant for the biosphere-atmosphere feedback, and even the sign of this feedback, i.e., ecosystems acting as carbon sink or as carbon source, depends on winter processes in temperate, boreal, and arctic ecosystems. This review concludes that current knowledge is not sufficient to quantify this feedback with satisfactory certainty. Important processes and the key uncertainties are identified, e.g., synchronicity in above- versus belowground growing season; temporal hierarchies in ecological processes such as the role of root damage and root activity for decomposition of soil organic matter (“priming”); or shifts in plant species composition due to winter climate change determining primary production as well as litter quantity and decomposability. Evidently, sound projections of future ecosystem functioning and biotic feedbacks to climate change require a comprehensive understanding of winter ecological processes, which have so far too often been neglected.
... In addition, the original vegetation condition in the permafrost region could be seriously damaged by extreme fire events; the surface albedo could also be altered, especially the process of freeze-thaw cycling for permafrost that could suffer strong influences (Forkel et al., 2012). Furthermore, extreme climate events had strong effects on the thermal-moisture state of permafrost by altering the underlying surface features (e.g., land vegetation and microbes) (Bokhorst et al., 2010(Bokhorst et al., , 2012. However, prominent differences exist in the response of different vegetation types to extreme events. ...
Article
The characteristics of long‐term variation for extreme precipitation events were analyzed at the Tanggula site in the continuous permafrost regions of the Qinghai‐Tibetan Plateau (QTP). In addition, the impacts of extreme precipitation events in summer on soil thermal‐moisture dynamics were also investigated. The results showed that local extreme precipitation indices fluctuated significantly and that the trend magnitudes of local very wet days (R95p), annual total wet‐day precipitation (PRCPTOT), number of heavy precipitation days (R10mm), maximum length of dry spell (CDD), and simple daily intensity index (SDII) were larger than those of the western QTP, other regions of China, and even the global average. The freeze‐thaw cycling in the local active layer occurred from October to the next September during 2006 to 2014. The influence of extreme precipitation event in summer on local soil hydrothermal conditions could reach soil depths up to 105 cm or so, and these were more pronounced than with light or moderate precipitation events. Soil temperature reacted more promptly to local extreme precipitation events than did soil moisture. The rate at which local soil temperature fell after an extreme precipitation event was greater than the rate of increasing temperature on nonprecipitation days. Moreover, the amount of precipitation received during extreme precipitation events had a greater effect on local soil moisture and temperature than duration time for these events. Consecutive extreme precipitation events with a longer duration time did not necessarily to have a greater effect than a single precipitation event with a shorter duration. Finally, the thawing process of active layer and local water migration modes could also affect the response of soil hydrothermal conditions to an extreme precipitation event to a large extent.
... This overwinter decomposition is permitted by the insulating properties of the snowpack which maintains near zero temperatures at the snow/soil interface (Zhang 2005;Saccone et al. 2013). Lack of insulation with a thinner snowpack could lead to colder soil conditions with more frequent freeze-thaw events (Bardgett et al. 2005;Bokhorst et al. 2010;Kreyling et al. 2010), especially in early spring due to advanced snowmelt dates. These rapid temperature changes are likely to select soil organisms and impact their activities Feng et al. 2007;Bokhorst et al. 2012), including litter decomposition (Wu et al. 2010;Baptist et al. 2010;Saccone et al. 2013). ...
Article
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Aims In subalpine grasslands, litter decomposition controls soil nutrient availability and is highly sensitive to increasing intensity and frequency of extreme climate events, potentially impacting grasslands diversity and functioning. Here, we assessed the effects of early snowmelt and summer drought on decomposition, and how these were modulated by agricultural management. Methods In a common garden, conservative and exploitative assembled communities were submitted for two years to combined snow removal and drought, and to cutting and fertilization. We measured decomposition rates of standard and native leaf material from each plant communities on their respective soils. Results We observed relatively weak climatic stress effects on decomposition rates. Management increased decomposition rates and remaining litter N contents through leaf quality improvement rather than changed soil biotic activity. Climate events impacted the decomposer community and limited litter N immobilization in conservative communities. Less recalcitrant litter of exploitative species facilitated decomposition and counterbalanced the negative effects of climate stress. Conclusion Our results suggest that drought and earlier snowmelt could decrease N availability in subalpine grasslands due to reduced litter biomass and decomposition. In the long-term climate change may shift subalpine grasslands towards more conservative plant composition and lower soil fertility.
... Future climate warming can affect CO 2 fluxes in multiple ways ( Arens et al., 2008;Biasi et al., 2008;Bokhorst et al., 2010). On the one hand, warming might accelerate soil respiration by heterotrophs, that is, the micro- bial decomposition of organic matter, thus increasing respiratory carbon losses (Grogan and Chapin, 2000;Nadelhoffer et al., 1991). ...
Article
The Arctic tundra plays an important role in the carbon cycle as it stores 50% of global soil organic carbon reservoirs. The processes (fluxes) regulating these stocks are predicted to change due to direct and indirect effects of climate change. Understanding the current and future carbon balance calls for a summary of the level of knowledge regarding chamber-derived carbon dioxide (CO2) flux studies. Here, we describe progress from recently (2000–2016) published studies of growing-season CO2 flux chamber measurements, namely GPP (gross primary production), ER (ecosystem respiration), and NEE (net ecosystem exchange), in the tundra region. We review the study areas and designs along with the explanatory environmental drivers used. Most of the studies were conducted in Alaska and Fennoscandia, and we stress the need for measuring fluxes in other tundra regions, particularly in more extreme climatic, productivity, and soil conditions. Soil respiration and other greenhouse gas measurements were seldom included in the studies. Although most of the environmental drivers of CO2 fluxes have been relatively well investigated (such as the effect of vegetation type and soil microclimate on fluxes), soil nutrients, other greenhouse gases and disturbance regimes require more research as they might define the future carbon balance. Particular attention should be paid to the effects of shrubification, geomorphology, and other disturbance effects such as fire events, and disease and herbivore outbreaks. An improved conceptual framework and understanding of underlying processes of biosphere–atmosphere CO2 exchange will provide more information on carbon cycling in the tundra.
... However we doubt the importance of large DOC pool production under very cold conditions as the main reason of sustained high concentration of DOC at snowmelt suggested by Finlay et al. (2006). Indeed, the plant litter degradation in winter, even at the very warmest scenario, is minimal and does not contribute significantly to annual litter leaching (Bokhorst et al., 2010(Bokhorst et al., , 2013. Instead, we hypothesis fast plant litter and Complimentary Contributor Copy ground vegetation leaching in spring, at the very beginning of the snow melt. ...
Chapter
We measured organic and inorganic carbon (DOC and DIC, respectively), in ~100 large and small rivers (<100 to ≤ 150,000 km²) of western Siberia sampled in winter, spring, summer and autumn over more than 1500 km latitudinal gradient. This allowed establishing main environmental factors controlling the transport of river dissolved components in this environmentally important region, comprising continuous, discontinuous, sporadic and permafrost-free zones. Across this gradient, there was significant latitudinal trend consisting in general decrease of DOC and DIC concentration northward, reflecting the interplay between groundwater feeding (detectable mostly in the permafrost-free zone, south of 60°N for DIC) and surface flux (in the permafrost-bearing zone, for DOC). The trend of DIC was mostly pronounced in winter and less visible in spring, whereas for DOC, the trend of concentration decrease with latitude was absent in winter, and less pronounced in spring flood then in summer baseflow. The latitudinal trends persisted for all river watershed size, from < 100 to > 10,000 km². This suggested, that in addition to groundwater feeding of the river, there was significant role of surface and shallow subsurface flow linked to plant litter degradation and peat leaching. In the order of their effect on DOC, DIC and δ13CDIC the environmental factors ranked as following: watershed area < season < latitude. Unexpectedly, the DOC flux remained stable around 3 t/km²/y until 61°N, decreased two-fold in the discontinuous permafrost zone (62-66°N) and increased again to 3 t/km²/y in the continuous permafrost zone (67°N). The DIC flux generally followed this pattern. The dominating effect of latitude could not however be interpreted solely in terms of permafrost abundance and water flow path (deep versus surface) but has to be considered in the context of different climate, plant biomass productivity, unfrozen peat thickness and peat chemical composition. It can be anticipated that, under climate warming in western Siberia, the maximal change in DOC and DIC concentrations will occur in small (<1000 km² watershed) rivers and this change will be mostly pronounced in summer and autumn. The winter time concentrations and spring flood fluxes and concentrations are unlikely to be appreciably affected by the change of active layer depth and terrestrial biomass productivity. Assuming conservative precipitation scenario and rising the temperature over next centuries, the annual fluxes of DOC in the discontinuous permafrost zone may increase by max two-fold whereas for DIC this increase may achieve a factor of 3. However, in the continuous permafrost zone, the results of this study predict a decrease of DOC export fluxes, if significant peat thaw will occur and mineral soil horizons become involved in subsurface solute transport from the soil to the river.
... Furthermore, extreme climate events had strong effects on the thermal-moisture state of permafrost by altering the underlying surface features (e.g., land vegetation, microbes) [Bokhorst et al., 2010;. ...
Article
Full-text available
The characteristics of long-term variation for extreme precipitation events were analyzed at the Tanggula site in the continuous permafrost regions of the Qinghai-Tibet Plateau (QTP). In addition, the impacts of extreme precipitation events in summer on soil thermal-moisture dynamics were also investigated. The results showed that local extreme precipitation indices fluctuated significantly, and that the trend magnitudes of local very wet days (R95p), annual total wet-day precipitation (PRCPTOT), number of heavy precipitation days (R10mm), maximum length of dry spell (CDD), and simple daily intensity index (SDII) were larger than those of the western QTP, other regions of China and even the global average. The freeze-thaw cycling in the local active layer occurred from October to the next September during 2006 to 2014. The influence of extreme precipitation event in summer on local soil hydrothermal conditions could reach soil depths up to 105 cm or so, and these were more pronounced than with light or moderate precipitation events. Soil temperature reacted more promptly to local extreme precipitation events than did soil moisture. The rate at which local soil temperature fell after an extreme precipitation event was greater than the rate of increasing temperature on non-precipitation days. Moreover, the amount of precipitation received during extreme precipitation events had a greater effect on local soil moisture and temperature than duration time for these events. Consecutive extreme precipitation events with a longer duration time did not necessarily to have a greater effect than a single precipitation event with a shorter duration. Finally, the thawing process of active layer and local water migration modes could also affect the response of soil hydrothermal conditions to an extreme precipitation event to a large extent.
... A deep, well-formed snowpack limits the depth of soil freezing and can keep the soil surface at or above freezing temperatures during the winter. Climate-driven loss of snowpack may result in increased frequency of freezethaw cycles and harsher conditions during the winter for sub-nivean environments (Bokhorst et al. 2010, Saccone et al. 2013) affecting decomposition and nutrient cycling. ...
Article
Full-text available
The role that microclimates play on soil decomposition is poorly understood. Though litter decomposition is controlled by climate and substrate quality at coarse spatial scales, at the watershed scale, microclimates mediated by forest structure and landscape position can influence decomposition rates and in turn affect nitrogen cycling. To evaluate the effects of landscape position and vegetation heterogeneity on decomposition, we employed a two-year litterbag study (2011–2013) using yellow birch leaf litter across the Weimer Run watershed, a cool, humid watershed located near Davis, West Virginia. From our results, we created a spatially explicit empirical model that we tested against both a single-pool and three-pool decomposition model, each based on climate and derived from the Long-Term Intersite Decomposition Experiment Team. Initial litter decomposition varied by elevation, with greater rates of decomposition at locations lower in the watershed. Decomposition rates differed by elevation, except during the first winter of the study. No differences in decomposition rates were seen among elevation levels when snowfall was below average for the first winter (2011–2012). During the second winter (2012–2013), elevation levels showed separation in decomposition rates, with higher elevations exhibiting lower decomposition rate. This suggests important controls on decomposition exerted by the presence or absence of snow, inter-annual climate variability, and the interaction of both with topography. Our empirical model showed greater rates of decomposition during early stages of decomposition (<12 months), but converged with the three-pool decomposition model after 20 months. Plant available nitrogen differed by vegetation cover, largely driven by greater availability of nitrate (NO3−) beneath areas of canopy closure in the forest. Controls on decomposition and nitrogen cycling within the Weimer Run watershed vary spatially by elevation and vegetation cover and are subject to complex interactions and differ from standard models of decomposition. The effect of the inter-annual variance of snow depth on litter decomposition is of note and an important consideration moving forward. Climate-based models of decomposition greatly underestimate initial rates of decomposition, potentially leading to under-accounting and compounded uncertainty.
... However, we doubt the importance of large DOC pool production under very cold conditions with regard to the main reason for sustained high concentration of DOC at snowmelt suggested by Finlay et al. (2006). Indeed, the plant litter degradation in winter, even in the warmest scenario, is minimal and does not contribute significantly to annual litter leaching (Bokhorst et al., 2010(Bokhorst et al., , 2013. Instead, we suggest fast plant litter and ground vegetation leaching in spring, at the very beginning of the snow melt. ...
Article
Full-text available
Analysis of organic and inorganic carbon (DOC and DIC, respectively), pH, Na, K, Ca, Mg, Cl, SO4 and Si in ~ 100 large and small rivers (< 10 to ≤ 150 000 km2) of western Siberia sampled in winter, spring, and summer over a more than 1500 km latitudinal gradient allowed establishing main environmental factors controlling the transport of river dissolved components in this environmentally important region, comprising continuous, discontinuous, sporadic and permafrost-free zones. There was a significant latitudinal trend consisting in a general decrease in DOC, DIC, SO4, and major cation (Ca, Mg, Na, K) concentration northward, reflecting the interplay between groundwater feeding (detectable mostly in the permafrost-free zone, south of 60° N) and surface flux (in the permafrost-bearing zone). The northward decrease in concentration of inorganic components was strongly pronounced both in winter and spring, whereas for DOC, the trend of concentration decrease with latitude was absent in winter, and less pronounced in spring flood than in summer baseflow. The most significant decrease in K concentration from the southern (< 59° N) to the northern (61–67° N) watersheds occurs in spring, during intense plant litter leaching. The latitudinal trends persisted for all river watershed size, from < 100 to > 10 000 km2. Environmental factors are ranked by their increasing effect on DOC, DIC, δ13CDIC, and major elements in western Siberian rivers as follows: watershed area < season < latitude. Because the degree of the groundwater feeding is different between large and small rivers, we hypothesize that, in addition to groundwater feeding of the river, there was a significant role of surface and shallow subsurface flow linked to plant litter degradation and peat leaching. We suggest that plant-litter- and topsoil-derived DOC adsorbs on clay mineral horizons in the southern, permafrost-free and discontinuous/sporadic permafrost zone but lacks the interaction with minerals in the continuous permafrost zone. It can be anticipated that, under climate warming in western Siberia, the maximal change will occur in small (< 1000 km2 watershed) rivers DOC, DIC and ionic composition and this change will be mostly pronounced in summer.
... A study has suggested that most of mass in fresh litter during cold season loss in autumn before the onset of winter and the decomposition during the 'true' wintertime is almost non-existent (Bokhorst et al. 2010). Both the repeated warming and freezing treatments in their laboratory incubation and the simulated winter warming in field had no effect on total mass loss. ...
Article
Full-text available
Litter decomposition is vital for carbon and nutrient turnover in terrestrial ecosystems, and this process has now been thoroughly demonstrated to be regulated by various mechanisms. The total environment has been continuously changing in recent decades, especially in high-latitude regions; these alterations, however, profoundly contribute to the decomposition process, but a comprehensive recognition has not available. Here we reviewed the empirical observations and current knowledge regarding how hydrological leaching and freeze-thaw events modulate early decomposition of plant litter. Leaching contributes a considerable percentage of mass loss and carbon and nutrient release in early stage of decomposition, but the magnitudes are different between species levels depending on the chemical traits. Frequent freezing and thawing events could positively influence decomposition rate in cold biomes but also hamper soil decomposer and there is no general and predictable pattern has been emerged. Further experiments should be manipulated to estimate how the altered freezing and thawing effect on carbon and nutrient release from plant litter to better understanding the changing environment on litter decomposition.
... Therefore, the abundance of bacteria should increase with the change in the season from winter to summer in cold areas (Deslippe et al., 2012;Lipson et al., 2002), in which case we should have observed higher abundances of bacteria during the growing season stages (EGS, LGS). However, the abundance of bacteria at stage TS was higher than at those during the growing season (EGS, LGS) in the present study, but the lowest bacterial abundance was observed on First, the lack of a snowpack on understory habitat in wintertime results in extreme environmental conditions (e.g., diurnal temperature differences), so this habitat cannot provide a relatively stable environment in which to maintain bacterial activity (Bokhorst et al., 2010;Drotz et al., 2010). Second, the lack of snowpack protection on the understory fir needle litter substrate results in increased mechanical disruption in the form of more frequent freeze-thaw cycles events in winter, which may mean that there is a better substrate environment for bacteria in the TS stage (Groffman et al., 2001;Hentschel et al., 2008;Herrmann and Witter, 2002). ...
Article
Accompanying the seasonal soil freeze-thaw cycle, microbial decomposition of litter exhibited different dynamic response to various snow thicknesses. In this study, we used real-time qPCR to investigate the abundance of bacteria, archaea, ammonia-oxidizing archaea (AOA) and bacteria (AOB), and the amoA gene transcripts, during the decomposition of dwarf bamboo (Fargesia nitida) litter under different snow patches at various snow-cover stages in an alpine forest on the eastern Tibetan Plateau in China. The effects of snow thickness were significant, with thicker snow patches resulting in higher microbial abundance and the amoA gene transcripts, while the degree of the effects were different. Compared with AOB, AOA were more abundant on the majority of sampling dates during the freeze-thaw period, and as well as their amoA gene transcripts. AOA are more persistent and abundant than AOB, and the higher AOA/AOB ratios were observed clearly in shrub litter and continued to decrease as the snow thickness increased, meanwhile gradually increased under uniform snow thickness over time. Our results suggested that the reduced seasonal snow cover and shortened freeze-thaw cycle periods caused by winter warming would significantly affect the ammonia oxidizers particularly tied to the ammonia oxidation process, and then could contribute to N cycle as related to litter in alpine forest ecosystems.
... However, as a result of increased winter temperatures, snow cover is significantly declining by c.a. 31,000 km −2 y −1 on average from 1930 to 2005 (Hooper et al. 2013), leaving the vegetation and soil exposed to frost events (Groffman et al. 2001). Although impacts of winter climate change on plants has been confirmed by a number of observations (Bokhorst et al. 2008(Bokhorst et al. , 2009(Bokhorst et al. , 2015Hollesen et al. 2015), its effect on soil microbes remains overlooked, despite the potential for drastic modifications of biogeochemical processes (Bokhorst et al. 2010;Durán et al. 2013Durán et al. , 2014Robroek et al. 2013). ...
Article
Soil microbial communities significantly contribute to global fluxes of nutrients and carbon. Their response to climate change, including winter warming, is expected to modify these processes through direct effects on microbial functions due to osmotic stress, and changing temperature regimes. Using four European peatlands reflecting different frequencies of frost events, we show that peatland testate amoeba communities diverge among sites with different winter climates, and that this is reflected through contrasting functions. We found that exposure to harder soil frost promoted species β-diversity (species turnover) thus shifting the community composition of testate amoebae. In particular, we found that harder soil frost, and lower water-soluble phenolic compounds, induced functional turnover through the decrease of large species (-68%, >80μm) and the increase of small-bodied mixotrophic species (i.e. Archerella flavum; +79%). These results suggest that increased exposure to soil frost could be highly limiting for large species while smaller species are more resistant. Furthermore, we found that β-glucosidase enzymatic activity, in addition to soil temperature, strongly depended of the functional diversity of testate amoebae (R(2)=0.95, ANOVA). Changing winter conditions can therefore strongly impact peatland decomposition process, though it remains unclear if these changes are carried-over to the growing season.
... -1 on average from 1930 to 2005 (Hooper et al., 2013), leaving the vegetation and soil exposed to frost events (Groffman et al., 2001). Although impacts of winter climate change on plants has been confirmed by a number of observations (Bokhorst et al., 2008;Hollesen et al., 2015), its effect on soil microbes remains overlooked, despite the potential for drastic modifications of biogeochemical processes (Bokhorst et al., 2010;Durán et al., 2013;Robroek et al., 2013). ...
... Although the abundance of bacteria in larch and fir litter increased from winter to the growing season throughout the experiment, abundance was observed to decrease in the cypress and birch litters from the 1st winter to the growing season, which may be explained by several possible underlying mechanisms. First, the lack of a snowpack during winter exposed the litter to extreme environmental conditions (e.g., large diurnal temperature changes); therefore, the region did not experience a relatively stable environment required to maintain bacterial activity in winter (Drotz et al. 2010;Bokhorst et al. 2010). Second, the foliar litter substrate on the forest floor, which lacked the protection of the snowpack, experienced increased mechanical disruption from the freeze events and freeze-thaw cycles in the winter, which resulted in an improved substrate environment for bacteria because of the greater availability of microbial substrates in the subsequent growing season (Berg 2000;Groffman et al. 2001;Herrmann and Witter 2002;Hentschel et al. 2008). ...
Article
Bacterial communities are the primary engineers during litter decomposition and related material cycling, and they can be strongly controlled by seasonal changes in temperature and other environmental factors. However, limited information is available on changes in the bacterial community from winter to the growing season as litter decomposition proceeds in cold climates. Here, we investigated the abundance and structure of bacterial communities using real-time quantitative PCR and denaturing gradient gel electrophoresis (DGGE) during a 2-year field study of the decomposition of litter of 4 species in the winter and growing seasons of an alpine forest of the eastern Tibetan Plateau. The abundance of the bacterial 16S rRNA gene was relatively high during decomposition of cypress and birch litter in the first winter, but for the other litters 16S rRNA abundance during both winters was significantly lower than during the following growing season. A large number of bands were observed on the DGGE gels, and their intensities and number from the winter samples were lower than those from the growing season during the 2-year decomposition experiment. Eighty-nine sequences from the bands of bacteria that had been cut from the DGGE gels were affiliated with 10 distinct classes of bacteria and an unknown group. A redundancy analysis indicated that the moisture, mass loss, and elemental content (e.g., C, N, and P) of the litter significantly affected the bacterial communities. Collectively, the results suggest that uneven seasonal changes in climate regulate bacterial communities and other decomposers, thus affecting their contribution to litter decomposition processes in the alpine forest.
... Leaching of soluble organic substances during the first stages of decomposition during winter might have been responsible for the observed high winter mass loss rates, and could decline during later decomposition stages of cellulose and lignin degradation (Berg and McClaugherty 2003). An experiment that mimicked extreme winter warming events and midwinter snowmelt, conducted in Northern Sweden, did not reveal significant effects on decomposition rates despite the large changes in temperature involved during the warming events (Bokhorst and others 2010). The authors concluded that little or no decomposition occurs during the winter season and all litter mass loss is by leaching during fall, although a more recent study suggests that the effects of snow depth on decomposition are species dependent (Bokhorst and others 2013b). ...
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... Therefore, the abundance of bacteria should increase with the change in the season from winter to summer in cold areas (Deslippe et al., 2012;Lipson et al., 2002), in which case we should have observed higher abundances of bacteria during the growing season stages (EGS, LGS). However, the abundance of bacteria at stage TS was higher than at those during the growing season (EGS, LGS) in the present study, but the lowest bacterial abundance was observed on First, the lack of a snowpack on understory habitat in wintertime results in extreme environmental conditions (e.g., diurnal temperature differences), so this habitat cannot provide a relatively stable environment in which to maintain bacterial activity (Bokhorst et al., 2010;Drotz et al., 2010). Second, the lack of snowpack protection on the understory fir needle litter substrate results in increased mechanical disruption in the form of more frequent freeze-thaw cycles events in winter, which may mean that there is a better substrate environment for bacteria in the TS stage (Groffman et al., 2001;Hentschel et al., 2008;Herrmann and Witter, 2002). ...
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Arctic tundra consists of diverse habitats that differ in dominant vegetation, soil moisture regimes, and relative importance of organic vs. inorganic nutrient cycling. The Arctic is also the most rapidly warming global area, with winter warming dominating. This warming is expected to have dramatic effects on tundra carbon and nutrient dynamics. We completed a meta‐analysis of 166 experimental warming study papers to evaluate the hypotheses that warming changes tundra biogeochemical cycles in a habitat‐ and seasonally specific manner and that the carbon (C), nitrogen (N), and phosphorus (P) cycles will be differentially accelerated, leading to decoupling of elemental cycles. We found that nutrient availability and plant leaf stoichiometry responses to experimental warming were variable and overall weak, but that both gross primary productivity and the plant C pool tended to increase with growing season warming. The effects of winter warming on C fluxes did not extend into the growing season. Overall, although warming led to more consistent increases in C fluxes compared to N or P fluxes, evidence for decoupling of biogeochemical cycles is weak and any effect appears limited to heath habitats. However, data on many habitats are too sparse to be able to generalize how warming might decouple biogeochemical cycles, and too few year‐round warming studies exist to ascertain whether the season under which warming occurs alters how ecosystems respond to warming. Coordinated field campaigns are necessary to more robustly document tundra habitat‐specific responses to realistic climate warming scenarios in order to better understand the mechanisms driving this heterogeneity and identify the tundra habitats, communities, and soil pools most susceptible to warming.
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The Arctic is experiencing an increased frequency of extreme events which can cause landscape-scale vegetation damage. Extreme event-driven damage is an important driver of the decline in vegetation productivity (termed ‘Arctic browning’) which has become an increasingly important component of pan-Arctic vegetation change in recent years. A limited number of studies have demonstrated that event-driven damage can have major impacts on ecosystem CO 2 balance, reducing ecosystem carbon sink strength. However, although there are many different extreme events that cause Arctic browning and different ecosystem types that are affected, there is no understanding of how impacts on CO 2 fluxes might vary between these, or of whether commonalities in response exist that would simplify incorporation of extreme event-driven Arctic browning into models. To address this, the impacts of different extreme events (frost-drought, extreme winter warming, ground icing and a herbivore insect outbreak) on growing season CO 2 fluxes of Net Ecosystem Exchange (NEE), Gross Primary Productivity (GPP) and ecosystem respiration (R eco ) were assessed at five sites from the boreal to High Arctic (64°N-79°N) in mainland Norway and Svalbard. Event-driven browning had consistent, major impacts across contrasting sites and event drivers, causing site-level reductions of up to 81% of NEE, 51% of GPP and 37% of R eco . Furthermore, at sites where plot-level NDVI (greenness) data were obtained, strong linear relationships between NDVI and NEE were identified, indicating clear potential for impacts of browning on CO 2 balance to be consistently, predictably related to loss of greenness across contrasting types of events and heathland ecosystems. This represents the first attempt to compare the consequences of browning driven by different extreme events on ecosystem CO 2 balance, and provides an important step towards a better understanding of how ecosystem CO 2 balance will respond to continuing climate change at high latitudes.
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Winter is an important period for ecological processes in northern regions; however, compared to other seasons, the impacts of winter climate on ecosystems are poorly understood. In this review we evaluate the influence of winter climate on carbon dynamics based on the current state of knowledge and highlight emerging topics and future research challenges. Studies that have addressed this topic include plot-scale snow cover manipulation experiments that alter soil temperatures, empirical investigations along natural climatic gradients, laboratory temperature incubation experiments aimed at isolating influential factors in controlled environments, and time series of climate and carbon data that evaluate long-term natural variation and trends. Combined, these studies have demonstrated how winter climate can influence carbon in complex ways that in some cases are consistent across studies and in other cases are difficult to predict. Despite advances in our understanding, there is a great need for studies that further explore: (i) carry-over effects from one season to another, (ii) ecosystem processes in the fall–winter and winter–spring shoulder seasons, (iii) the impacts of extreme events, (iv) novel experimental approaches, and (v) improvements to models to include ecological effects of winter climate. We also call for the establishment of an international winter climate change research network that enhances collaboration and coordination among studies, which could provide a more thorough understanding of how the snow-covered period influences carbon cycling, thereby improving our ability to predict future responses to climate change.
Thesis
Les écosystèmes prairiaux alpestres sont supposés être très sensibles au changement climatique. Cependant leur long historique fait de variation du climat et de multiples siècles d'utilisation des terres peut avoir sélectionné des mécanismes de résilience écologique à la variabilité climatique et aux stress climatiques extrêmes. Nous avons utilisé un dispositif expérimental de grande envergure pour explorer les mécanismes de réponses des prairies subalpines à une combinaison d’extrêmes météorologiques hivernaux (fonte des neiges précoce) et estivaux (sécheresse) en fonction de la composition fonctionnelle des communautés végétales et des pratiques de gestion des terres. La composition fonctionnelle des prairies a été manipulée à l’aide d’assemblages de trois espèces de poacées en différentes abondances relatives, représentant in fine un gradient d’utilisation des ressources allant de la conservation à l'exploitation. De manière générale, la composition fonctionnelle a été le principal déterminant de tous les paramètres observés pour la performance individuelle des plantes, les réponses intraspécifiques des plantes, la décomposition de la litière et les processus de recyclage de l'azote. Le fonctionnement des écosystèmes prairiaux dominés par des plantes conservatrices a été remarquablement résistant aux traitements climatiques extrêmes, tandis que les écosystèmes prairiaux dominés par des plantes plus exploitatrices ont été plus résilients. Les pratiques de gestion ont modulé ces réponses et plus particulièrement dans le cas des communautés exploitatrices. Les allocations souterraines aux réserves glucidiques et aux pools d'azote microbiens ont été identifiées comme deux mécanismes clés sous-tendant les réponses des communautés résilientes. Néanmoins, des répercussions à plus long terme du changement climatique pourraient être observées, causées par un épuisement successif des réserves végétales et une diminution des retours azotés vers le sol via les processus de décomposition.
Thesis
(Einleitung) Räumliche Verbreitungsmuster von Pflanzenarten sind assoziiert mit einer Reihe von ökologischen Merkmalen (Pockock et al. 2006). Es wird angenommen, dass diese Pflanzenmerkmale spezifische Umweltzustände reflektieren. Ein Vergleich des Merkmalspektrums von verschiedenen Habitaten könnte bei der Identifizierung von funktionellen Zusammenhängen der Verbreitungsmuster der Pflanzenarten helfen. Diese Beziehungen sind nützlich für die Vorhersage von Reaktionen eines Ökosystems auf den Klimawandel und Landnutzungsänderungen. Tautenhahn et al. (2008) konnte z.B. zeigen, dass die Größe von Saatkörnern positiv korreliert ist mit dem pH-Wert des Bodens und negativ mit der Bodenfeuchte. Andere interessante Pflanzenmerkmale wie Lebensform (nach Raunkiaer), womit die Lage der Überdauerungsknospen entlang der Sprossachse gemeint ist, oder Blattanatomie sollen in dieser Arbeit auf ihre Verbreitung und den Bezug zu Umweltparametern hin untersucht werden. Eine Studie in Nordchina (Meng et al. 2009) konnte bereits ökologisch plausible Trends beim Auftreten der zu untersuchenden Pflanzenmerkmale entlang klimatischer Gradienten zeigen. So fand man die höchste Abundanz von Baumarten in feuchten Umgebungen, wo hin-gegen Sträucher ihre maximale Abundanz in trockenen Gegenden erreichten. In einer Fallstudie über die Verbreitung von Lebensform- und Blattanatomietypen in sechs Bundesländern von Deutschland sollen Merkmalsverbreitungen und ihre Beziehung mit Umweltvariablen analysiert werden. Es soll geklärt werden, ob die o.g. Pflanzenmerkmale abhängig von Umweltfaktoren sind, sich bestehende Verbreitungsmuster in Folge des Klimawandels und der Landnutzung ändern könnten und welche Merkmale vom Klimawandel profitieren oder einen Nachteil haben werden. (Material & Methode) Das Untersuchungsgebiet umfasste die sechs Bundesländer Baden-Württemberg, Bayern, Hessen, Nordrhein-Westfalen, Rheinland-Pfalz und das Saarland mit einer Gesamtfläche von 183.927 km2. Verbreitungsdaten in Form von Gitterzellen auf Messtischblattebene im Verhältnis 1:25000 wurden der FLORKART-Datenbank vom Bundesamt für Naturschutz (BfN) entnommen. Merkmalsdaten stammten aus der BiolFlor-Datenbank des Umweltforschungszentrums Leipzig (UfZ). Für die Analysen des Merkmals Lebensform wurden 3324 Pflanzenarten berücksichtigt. Im Falle der Blattanatomietypen wurden 2742 Pflanzenarten berücksichtigt. Verbreitungskarten wurden mit der Software ArcGis 9.3 von ESRI erstellt. 17 Landnutzungsvariablen basierten auf dem CORINE Land cover 2000 Datensatz. Weitere Variablen umfassten Bodenparameter, Klima, Landschaftsheterogenität und Topografie. Multiple Regressionsmodelle (GLMs) wurden erstellt mit den Merkmalstypen als abhängige Variablen und den Umweltvariablen als Prädiktoren. Bei allen Modellen wurde räumliche Autokorrelation (SAC) berücksichtigt. Um den unabhängigen Beitrag der Umweltfaktoren zu testen, wurde eine Hierarchische Partitionierung durchgeführt. Die Validierung der Modelle erfolgte mittels ANOVA (Χ2-Test). (Resultate) Abhängig von dem untersuchten Merkmal, Lebensform oder Blattanatomie, und dem Merkmalstyp, zeigten die Verbreitungskarten eindeutige Schwerpunkte. So waren z.B. Hydrophyten vor allem entlang des Rheins und in der Nähe von Seen zu finden. Hemiphanerophyten zeigten eine erhöhte Verbreitung in den bayrischen Alpen. Was die Anzahl an Merkmalsvertretern pro Gitterzelle betraf, so waren die Hemikryptophyten der dominierende Lebensformtyp mit bis zu 656 Vertretern, was auch die gleichmäßige Verbreitung dieses Lebensformtyps erklären könnte. Auf die Blattanatomietypen soll nicht näher eingegangen werden, da bei jenen der Erklärungswert der Regressionsmodelle zu gering war (6,97 - 27,87%). Die Regressionsmodelle der Lebensformtypen erklärten zwischen 39.99 und 58.27% der beobachteten Variabilität der Merkmalsverbreitung. Für die Modelle spielte die räumliche Autokorrelation eine deutliche Rolle (15,28 - 82,03%). Besonders Landnutzungsvariablen lieferten bei allen Lebensformtypen einen großen Anteil des Erklärungswerts (8,76 - 77,29%). Im Gegensatz dazu waren Klimavariablen nur bei sechs Typen signifikant, hatten zudem nur einen geringen Anteil im Modell (1,19 - 25,47%). Therophyten und Hydrophyten waren am klimasensitivsten. Bodenparameter und Topographie lieferten ähnliche Ergebnisse wie die Klimavariablen. Landschaftsheterogenität war nur für die Chamaephyten ein signifikanter Faktor, wenn gleich er mit 1,19% einen zu vernachlässigenden Anteil am Modell hatte. (Schlussfolgerung) Lebensformtypen zeigten eine Abhängigkeit von gewissen Umweltfaktoren. Für die Blattanatomietypen konnte kein solcher Zusammenhang festgestellt werden. Für das Pflanzenmerkmal Lebensform waren Landnutzungsklassen bedeuten-der als die klimatischen Variablen. Die Folgen des Klimawandels auf das Verbreitungsmuster werden somit Auswirkungen haben, allerdings weit weniger stark ausgeprägt, als bisher angenommen. Nur die Verbreitungsmuster der Lebensformtypen Hydrophyt und Therophyt, welche mit Temperaturvariablen korreliert sind, dürften sich deutlich ändern. Landnutzungsänderungen hingegen sollten eine Veränderung von Verbreitungsmustern der Lebensformtypen mit sich bringen. Anstrengungen zum Naturschutz sollten nicht nur auf die Reduzierung der CO2 Emissionen abzielen sondern auch den Einfluss von Landnutzung mitberücksichtigen.
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AimsWarming has the potential to alter plant litter mass loss and nutrient release during decomposition. However, a great deal of uncertainty remains concerning how other factors such as litter species or substrate quality might modify the effects of increased temperature on decomposition. Meanwhile, the temperature sensitivity of plant litter decay in tropical and subtropical forest ecosystems remains poorly resolved. Methods This study was designed to assess the effects of experimental warming on litter decomposition and nutrient release of two contrasting tree species (Schima superba and Machilus breviflora) by translocating model forest ecosystems from the high-elevation sites to the lower-elevation sites in subtropical China. Translocating model mountain evergreen broad-leaved forest (MEBF) to the altitude of 300 m and 30 m increased the average monthly soil temperature at 5 cm depth by 0.88 and 1.84 °C, respectively during the experimental period. Translocating model coniferous and broad-leaved mixed forest (CBMF) to the altitude of 30 m increased the average monthly soil temperature at 5 cm depth by 0.85 °C. ResultsWe found that experimental warming accelerated litter decomposition in both model forest types, and the promoting efficiency was greater when the temperature increased. The litter with high quality (Schima superba) had stronger response to warming than low quality litter (Machilus breviflora). Warming accelerated Na, K, Mg, P, N and Ca release from Schima superba litter, but only simulated Ca release from Machilus breviflora litter. Overall, litter decomposition was controlled by the order: soil temperature > litter quality > soil moisture > litter incubation forest type under experimental warming in the subtropical China. Conclusion We conclude that leaf litter decomposition was facilitated by experimental warming in subtropical China. Litter species might modify the effects of increased temperature on litter decomposition; however, forest type has no effect on litter decomposition.
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Aims Winter soil respiration plays a crucial role in terrestrial carbon cycle, which could lose carbon gained in the growing season. With global warming, the average near-surface air temperatures will rise by 0.3 to 4.8 °C. Winter is expected to be warmer obviously than other seasons. Thus, the elevated temperature can significantly affect soil respiration. The coastal wetland has shallow underground water level and is affected by the fresh water and salt water. Elevated temperature can cause the increase of soil salinity, and as a result high salinity can limit soil respiration. Our objectives were to determine the diurnal and seasonal dynamics of soil respiration in a coastal wetland during the non-growing season, and to explore the responses of soil respiration to environmental factors, especially soil temperature and salinity. Methods A manipulative warming experiment was conducted in a costal wetland in the Yellow River Delta using the infrared heaters. A complete random block design with two treatments, including control and warming, and each treatment was replicated each treatment four times. Soil respiration was measured twice a month during the non-growing season by a LI-8100 soil CO² efflux system. The measurements were taken every 2 h for 24 h at clear days. During each soil respiration measurement, soil environmental parameters were determined simultaneously, including soil temperature, moisture and salinity. Important findings The diurnal variation of soil respiration in the warming plots was closely coupled with that in the control plots, and both exhibited single-peak curves. The daily soil respiration in the warming was higher than that in the control from November 2014 to January 2015. Contrarily, from March to April 2015. During the non-growing seasons, there were no significant differences in the daily mean soil respiration between the two treatments. However, soil temperature and soil salt content in the warming plots were significantly higher than those in the control plots. The non-growing season was divided into the no salt restriction period (November 2014 to middle February 2015) and salt restriction period (middle February 2015 to April 2015). During non-growing season, soil respiration in the warming had no significant difference compared with that in control. During the no salt restriction period, soil respiration in the warming was 22.9% (p < 0.01) greater than the control when soil temperature at 10 cm depth in warming was elevated by 4.0 °C compared with that in control. However, experimental warming decreased temperature sensitivity of soil respiration (Q10). During salt restriction period, soil warming decreased soil respiration by 20.7% compared with the control although with higher temperature (3.3 °C), which may be attributed to the increased soil salt content (Soil electric conductivity increased from 4.4 d⋅m⁻¹ to 5.3 d⋅m⁻¹). The high water content can limit soil respiration in some extent. In addition, the Q10 value in the warming had no significant difference compared with that in control during this period. Therefore, soil warming can not only increase soil respiration by elevating soil temperature, but also decrease soil respiration by increasing soil salt content due to evaporation, which consequently regulating the soil carbon balance of coastal wetlands.
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Bacterial community plays an important role in litter decomposition. Although the changes of bacterial community as litter decomposition proceeding can be regulated by frozen temperature and changed litter quality in cold regimes, little information has been available on. Therefore, the structure and diversity of the bacterial community in Minjiang fir (Abies faxoniana) needle litter were measured in an alpine forest in eastern Tibetan Plateau. The litter samples were sampled at the onset of the freezing stage, the deep freezing stage, the thawing stage, the early growing season and the late growing season from December 2010 to November 2011. The methods of real-time polymerase chain reaction (qPCR) coupled with denaturing gradient gel electrophoresis (DGGE) were used. The copy numbers of bacterial 16S rDNA in the fir needle litter changed significantly as litter decomposition proceeding. The abundance of bacterial 16S rDNA was significantly lower at the deep freezing stage but highest at the thawing stage. A large number of bands were observed on the DGGE gel; the intensities and distances of the bands were significantly different among the samples at different stages; the indexes of bacterial diversity at the onset of the freezing and deep freezing stages were lower than them at the other stages. All of the bacterial sequences were affiliated with six distinct classes and an unknown group. Redundancy analysis indicated that moisture, mass loss and the release of litter elements (e.g., C, N, P) exerted obvious influences over the bacterial communities.
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Carbon (C) release from foliar litter is a primary component in C exchange between the atmosphere and terrestrial ecosystems, but little information is currently related to the effects of freezing and thawing dynamics on C release of foliar litter in cold regions. A two-year field litter decomposition experiment was conducted along an altitudinal gradient (∼2700 m to ∼3600 m) to mimic temperature increases in the eastern Tibetan Plateau. C release was investigated for fresh foliar litter of spruce, fir and birch. The onset of the frozen stage, deep frozen stage, and thawing stage were partitioned according to changes in freezing and thawing dynamics of each winter. High C release was observed in lower altitudes during winter stages, but higher altitudes exhibited high C release during growing season stages. The deep frozen stage showed higher rates of C release than other stages in the second year of decomposition. Negative degree-days showing freezing degree were correlated to C release rates for the deep frozen stages in both years, and this relationship continued for the duration of the experiment, indicating that changes in freezing can directly modify C release from foliar litter. The results suggested that climate warming could delay the onset of C release in fresh litter in this cold region.
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Allelopathy is an interference mechanism by which plants release chemicals which affect other plants; while it has often been proposed as a mechanism for influencing plant populations and communities, its acceptance by plant ecologists has been limited because of methodological problems as well as difficulties of relating the results of bioassays used for testing allelopathy to vegetation patterns in the field. Here we argue that the concept of allelopathy is more appropriately applied at the ecosystem-level, rather than the traditional population/community level of resolution. Firstly, we consider the wide ranging effects of secondary metabolites (widely regarded as allelochemicals) on organisms and processes which regulate ecosystem function, including herbivory, decomposition and nutrient mineralization. It is apparent that plants with allelopathic potential against other organisms induce net changes in ecosystem properties, which may in turn impact upon the plant community in the longer term. We then illustrate these concepts using two contrasting examples of how invasive plant species with allelopathic potential may alter ecosystem properties through the production of secondary metabolites, i.e. Carduus nutans (nodding thistle) in New Zealand pastures and Empetrum hermaphroditum (crowberry) in Swedish boreal forests. In both cases the production of secondary metabolites by the invasive species induces important effects on other organisms and key processes, which help determine how the ecosystem functions and ultimately the structure of the plant community. These examples help demonstrate that the concept of allelopathy is most effectively applied at the ecosystem-level of resolution, rather than at the population-level (i.e. plant-plant interference).
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Relationships between patterns of low frequency climate variability including the El Niño‐Southern Oscillation (ENSO), the Arctic Oscillation (AO) and the Quasi‐Biennial Oscillation (QBO), and the occurrence of Canadian winter temperature extremes are examined for the 1950–98 period. Composite analyses reveal that ENSO plays a dominant role in the frequency and duration of both cold and warm spells. In particular, the warm phase of ENSO is associated with a significant increase (decrease) in the occurrence of warm (cold) spells and the number of extreme warm (cold) days across most of Canada. The findings are generally the opposite for the cold ENSO phase. The singular value decomposition (SVD) technique further shows that ENSO‐like interdecadal sea surface temperature variability plays a significant role in the variability of winter cold and warm spells over Canada. Differences between the high and low index phases of the AO show a significantly higher frequency and duration of cold spells over eastern Canada during positive AO winters, while the frequency of winter warm spells increases over the Canadian Prairies. These results are also confirmed by the SVD analysis using northern hemisphere 1000‐hPa circulation. The QBO, as determined by the phase of the stratospheric winds, has an effect on the spatial pattern of Canadian extreme temperatures that is similar to that of the AO. The most significant result involves increases in the frequency of warm spells and the number of extreme warm days over the southern Prairies during the westerly phase of the QBO. The diagnostic results from this study improve the understanding of the imapact of the low frequency variability modes on temperature extremes over Canada. They may also assist in the attribution of past trends and variability in extremes, and potentially, provide insight into future changes to extreme temperature events.
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In a subalpine balsam fir forest in Quebec, Canada, mass losses, respiration rates, and nitrogen and sulphur dynamics were measured on fir needles, birch leaves, lichens (mixed species), and small twigs decomposing under deep (> 1.5 m) winter snow for 6 months. Mass losses ranged from <6% (twigs) to 70% (lichens) and relative decomposition rates of needles and leaves were reversed from those expected at higher temperatures. Isolation of fir needles from direct contact with the snow did not affect decay rate, nor was decay accelerated by spring snowmelt. In situ respiration rates increased from about 1 mg CO 2 /(g∙day)) in February to 3–5 mg CO 2 /(g∙day)) in May, mostly because of rising temperatures. Summer respiration rates were much higher (> 6 mg CO 2 /(g∙day)). Nitrogen and suphur concentrations increased in all nonwoody litter over winter, but only birch leaves and some fir needles appeared to assimilate nutrients from the environment. Melting snow could easily have provided all of the nitrogen and sulphur taken up by decomposing litter. Decomposing lichens released 40 and 60%, respectively, of their initial nitrogen and sulphur contents. A literature review indicates mass losses from leaf litter decomposing under deep snow vary according to the proportion of labile material in the litter and usually constitute 40–60% of total first-year mass losses. Key words: decomposition, winter, balsam fir, snow.
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Winter CO 2 emissions from soil and snow-covered surfaces can represent a significant fraction of the annual carbon cycle in cool-temperate broad-leaved deciduous forests. The sources of this CO 2 production are, however, poorly understood. To clarify the role of saprophytic microorganisms in the carbon cycle during the winter (December–April), mass loss rates of organic substrates (leaf litter and cellulose sheets), and microbial activity and fungal biomass in the litter layer of a cool-temperate broad-leaved deciduous forest in Japan were investigated. The mass loss rate of leaves reached 13%, which accounted for 26% of the annual mass loss. Microbial respiration in the leaf litter, measured in the laboratory, was detected even at À2 8C. The ergosterol content (an indicator of fungal biomass) of the cellulose sheets increased significantly in the snow-covered season. Seven genera of fungi including Varicosporium elodeae, Epicoccum purpurascens and Cylindrocarpon destructans were isolated from leaf litter collected from under snow cover. Microbial respiration during the winter was estimated based on the relationships between the amount of leaf litter available, the temperature dependency of microbial respiration and the temperature in the litter layer. The microorganisms in the litter layer emitted 31 gCO 2 -C m À2 , which accounts for 38% of the total CO 2 efflux during the winter. These results suggest that through soil respiration, microorganisms in the litter layer play an important role in the carbon cycle during the winter. # 2005 Elsevier B.V. All rights reserved. (M. Uchida), yinermo@yahoo.co.jp (W. Mo), kuyakat@hiroshima-u.ac.jp (T. Nakatsubo), tsuchy@lab.tamagawa.ac.jp (Y. Tsuchiya), takao@hiroshima-u.ac.jp (T. Horikoshi), koizumi@green.gifu-u.ac.jp (H. Koizumi).
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Recent work has shown that plant litter inputs fuel microbial growth in autumn and winter resulting in a large increase of microbial biomass under the snow pack in tundra soils. This winter-adapted microbial community can grow at low temperatures (−5 to 3 • C) and depletes the litter of easily degraded constituents, such as simple phen-olic compounds, and immobilizes nitrogen. During snowmelt there is a die-off of this winter microbial community (due to starvation and intolerance to higher soil temperature) resulting in a release of nitrogen that can be utilized by plants and the summer microbial community. The summer microbial community can tolerate higher temperatures (5 to 20 • C) and utilizes mostly plant root exudates for growth. These yearly cycles of microbial growth dynamics have profound implications for both nutrient and alleochemical availability to plants. Firstly, these results show that release (from litter) and degradation of plant phenolic compounds (potential alleochemicals) occurs before plant growth commences in the spring. Secondly, nitrogen (N) immobilized by over-winter microbial growth is released back to the soil during and after snowmelt, thus becoming available to plants. Both of these results need to be incorporated in the design of experiments to explore plant-plant interactions. Many experiments in which chemicals (or fresh litter) are incorporated during plant growth do not reflect the fact that these two events are temporally uncoupled in many natural systems.
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Mass and nitrogen (N) dynamics of leaf litter measured in Alaskan tussock tundra differed greatly from measurements of these processes made in temperate ecosystems. Nearly all litter mass and N loss occurred during the winter when soils were mostly frozen. Litter lost mass during the first summer, but during the subsequent two summers when biological activity was presumably higher than it is during winter, litter mass remained constant and litter immobilized N. By contrast, litter lost significant mass and N over both winters of measurement. Mass loss and N dynamics were unaffected by microsite variation in soil temperature and moisture. Whether wintertime mass and N loss resulted from biological activity during winter or from physical processes (e.g., fragmentation or leaching) associated with freeze-thaw is unknown, but has implications for how future climate warming will alter carbon (C) and N cycling in tundra. We hypothesize that spring runoff over permafrost as soils melt results in significant losses of C and N from litter, consistent with the observed influx of terrestrial organic matter to tundra lakes and streams after snow melt and the strong N limitation of terrestrial primary production.
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Leaching of soluble substances may be an important first step in leaf litter decomposition in small streams, but recent research has suggested that large leaching losses (up to 30% of initial mass in 48 h) may be an artifact created by using air-dried leaves in decomposition experiments. In laboratory experiments, we compared 3 d leaching losses from freshly fallen and air-dried senescent leaves of 27 tree species from different regions across Canada. Air-dried leaves from all species leached measurable amounts of original mass (3.6–32.8% dry mass), but leaching losses from fresh leaves (0–35%) were detectable in all but two species. Air-drying increased leaching losses in many species, but in others it reduced leaching losses or had no measurable effect. Results for leaves of the same species collected in different regions or in different years were generally similar, but species within the same genus often behaved very differently. Neither moisture content (fresh or air-dried), leaf thickness, nor cuticle thickness proved of any value as predictors of leaching losses or the effect of air-drying. The propensity of autumn-fallen leaves to leach, whether fresh or air-dried, appears to be a property of the individual tree species.
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Fluxes of dissolved organic matter (DOM) are an important vector for the movement of carbon (C) and nutrients both within and between ecosystems. However, although DOM fluxes from throughfall and through litterfall can be large, little is known about the fate of DOM leached from plant canopies, or from the litter layer into the soil horizon. In this study, our objectives were to determine the importance of plant-litter leachate as a vehicle for DOM movement, and to track DOM decomposition [including dissolve organic carbon (DOC) and dissolved organic nitrogen (DON) fractions], as well as DOM chemical and isotopic dynamics, during a long-term laboratory incubation experiment using fresh leaves and litter from several ecosystem types. The water-extractable fraction of organic C was high for all five plant species, as was the biodegradable fraction; in most cases, more than 70% of the initial DOM was decomposed in the first 10 days of the experiment. The chemical composition of the DOM changed as decomposition proceeded, with humic (hydrophobic) fractions becoming relatively more abundant than nonhumic (hydrophilic) fractions over time. However, in spite of proportional changes in humic and nonhumic fractions over time, our data suggest that both fractions are readily decomposed in the absence of physicochemical reactions with soil surfaces. Our data also showed no changes in the 13C signature of DOM during decomposition, suggesting that isotopic fractionation during DOM uptake is not a significant process. These results suggest that soil microorganisms preferentially decompose more labile organic molecules in the DOM pool, which also tend to be isotopically heavier than more recalcitrant DOM fractions. We believe that the interaction between DOM decomposition dynamics and soil sorption processes contribute to the 13C enrichment of soil organic matter commonly observed with depth in soil profiles.
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The abundance and micro-stratification of bacteria and fungi inhabiting the organic layers of a Scots pine forest (Pinus sylvestris L.) were investigated. An experiment using stratified litterbags, containing organic material of four degradation stages (fresh litter, litter, fragmented litter and humus) was performed over a period of 2.5 years. Dynamics and stratification of fluorescent stained bacteria and fungi, ratios between bacterial and fungal biomass, and relationships with moisture and temperature are described. Average bacterial counts in litter and fragmented litter were similar, i.e., approximately 5×109 bacteriag–1 (dry weight) organic matter, and significantly exceeded those in humus. The mean bacterial biomass ranged from 0.338 to 0.252mg carbon (C) g–1 (dry weight) organic matter. Lengths of mycelia were significantly below the usually recorded amounts for comparable temperate coniferous forests. The highest average hyphal length, 53mg–1 (dry weight) organic matter, was recorded in litter and decreased significantly with depth. The corresponding mean fungal biomass ranged from 0.050 to 0.009mg Cg–1 (dry weight). The abundance of bacteria and fungi was influenced by water content, that of fungi also by temperature. A litterbag series with freshly fallen litter of standard quality, renewed bimonthly, revealed a clear seasonal pattern with microbial biomass peaks in winter. The mean hyphal length was 104mg–1 (dry weight) and mean number of bacteria, 2.40×109 bacteria g–1 (dry weight). Comparable bacterial and fungal biomass C were found in the freshly fallen litter [0.154 and 0.132mgCg–1 (dry weight) organic material, respectively]. The ratio of bacterial-to-fungal biomass C increased from 1.2 in fresh litter to 28.0 in humus. The results indicate the existence of an environmental stress factor affecting the abundance of fungi in the second phase of decomposition. High atmospheric nitrogen deposition is discussed as a prime factor to explain low fungal biomass and the relatively short lengths of fungal hyphae in some of the forest soil layers under study.
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Repeated cycles of freezing and thawing of leaf litter could accelerate decomposition if they caused physical damage to leaves or chemical changes which improve access or render leaf material more easily degradable by decomposers. Moistened leaf litter of aspen (Populus tremuloides Michx.) and pine (Pinus contorta Loud. × P. banksiana Lamb.), which had been frozen and thawed 14 times, absorbed water faster than litter frozen for the same total duration, before thawing once. Aspen, but not pine, began leaching sooner. Total solubles content of aspen leaves was reduced 8% by repeated freezing and thawing, but leaching loss from pine needles doubled from 4 to 8% of total mass.Previously frozen and thawed litter (14 cycles) decomposing at 10° C in laboratory microcosms lost more mass initially than litter frozen only once, but the difference was not maintained after 2 or 3 months decay. Litter exposed in microcosms to + 10°C (days) and −6°C (nights) for 3 months lost mass no faster or slower than litter maintained at the same mean temperature (+2° C) without variation, once differences in moisture content were allowed for. However, mass losses from frozen and thawed litter were substantially greater than expected if losses are prorated for the fact that these leaves were frozen 12 h each day. This difference reflects either the acceleration of decomposition due to freezing-thawing, possibly arising from weakening of the cuticle, or continuing decomposition at below-zero temperatures. For aspen litter, this effect was much smaller after 1 month of nightly freezing, followed by 2 months at 10° C. Frequency of freeze-thaw cycles in the field was estimated at 2–11 cycles month−1 during spring or fall at the aspen field site, and 0–8 cycles month−1 at the pine site. Evidence indicates that simple freezing (as opposed to freezing-thawing) and decomposer activity beneath snow may be more important factors in winter time litter decomposition than freeze-thaw cycles.
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Decomposition and mineralisation rates generally increase with increasing moisture and temperature. The expected global climate change may enhance precipitation and raise the temperatures at boreal latitudes, but absence of snow together with occasional low temperatures may cause disturbances in soil processes and faunal communities. To test the effects of disturbances such as hard frosts and freeze-thaw cycles on decomposer populations and N mineralisation, we performed two experiments. In the field experiment, carried out in a pine forest, we induced low soil temperatures by preventing snow covering the ground. In the laboratory test we established three “winter” temperature regimes: constant −2 °C, freeze-thaw cycles of −2° to +2 °C, and the latter with two hard frost periods (−16 °C). The microcosm experiment included two decomposer communities; the “simple” community included microbes, protozoa and nematodes, and the “complex” community microbes, protozoa, nematodes, enchytraeids and microarthropods. In the field experiment, the populations of enchytraeids and several microarthropod taxa, as well as microarthropod species richness, decreased in snowfree plots. In the microcosms, virtually all enchytraeids died and microarthropod populations and taxa decreased in the hard frost treatment, but increased in the freeze-thaw treatment. In the laboratory systems with simple decomposer community structure the amount of mineral nitrogen decreased in the freeze-thaw treatment. The experiments revealed that exceptionally low soil temperatures exert a stronger influence on soil fauna than constant benign temperatures or freeze-thaw cycles, but the rapid recovery of populations may counteract the detrimental effect of occasional frost periods.
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Previous work in an alpine dry meadow in the Front Range of the Rocky Mountains has shown that microbial biomass is high during winter and declines rapidly as snow melts in the spring, and that this decline is associated with changes in temperature regime and substrate availability. In this study we tested the hypothesis that the summer and winter microbial communities differ in function and composition. Shifts in species composition between pre- and post-snowmelt communities were detected using reciprocal hybridization of community DNA; DNA extracted from soils sampled at different times was significantly less homologous relative to spatial replicates sampled at the same time. Fungal/bacterial ratios, as measured by direct microscopic counts and by substrate-induced respiration experiments with specific inhibitors, were higher in winter soils. Specific activity of cellulase (absolute cellulase activity per unit microbial biomass C) was higher in the winter soils than in summer soils, while specific amylase activity was not different between winter and summer. Based on most-probable number measurements, the use of the phenolic compound vanillic acid was highest in the winter, while the use of the amino acid glycine was lowest in the winter. Winter and summer soil respiration responded differently to temperature; at 0 degrees C, winter soils respired at a higher proportion of the 22 degrees C rate than did summer soils.
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The loss of biodiversity can have significant impacts on ecosystem functioning, but the mechanisms involved lack empirical confirmation. Using soil microcosms, we show experimentally that functional dissimilarity among detritivorous species, not species number, drives community compositional effects on leaf litter mass loss and soil respiration, two key soil ecosystem processes. These experiments confirm theoretical predictions that biodiversity effects on ecosystem functioning can be predicted by the degree of functional differences among species.
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An assessment of the impacts of changes in climate and UV-B radiation on Arctic terrestrial ecosystems, made within the Arctic Climate Impacts Assessment (ACIA), highlighted the profound implications of projected warming in particular for future ecosystem services, biodiversity and feedbacks to climate. However, although our current understanding of ecological processes and changes driven by climate and UV-B is strong in some geographical areas and in some disciplines, it is weak in others. Even though recently the strength of our predictions has increased dramatically with increased research effort in the Arctic and the introduction of new technologies, our current understanding is still constrained by various uncertainties. The assessment is based on a range of approaches that each have uncertainties, and on data sets that are often far from complete. Uncertainties arise from methodologies and conceptual frameworks, from unpredictable surprises, from lack of validation of models, and from the use of particular scenarios, rather than predictions, of future greenhouse gas emissions and climates. Recommendations to reduce the uncertainties are wide-ranging and relate to all disciplines within the assessment. However, a repeated theme is the critical importance of achieving an adequate spatial and long-term coverage of experiments, observations and monitoring of environmental changes and their impacts throughout the sparsely populated and remote region that is the Arctic.
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Microorganisms have a variety of evolutionary adaptations and physiological acclimation mechanisms that allow them to survive and remain active in the face of environmental stress. Physiological responses to stress have costs at the organismal level that can result in altered ecosystem-level C, energy, and nutrient flows. These large-scale impacts result from direct effects on active microbes' physiology and by controlling the composition of the active microbial community. We first consider some general aspects of how microbes experience environmental stresses and how they respond to them. We then discuss the impacts of two important ecosystem-level stressors, drought and freezing, on microbial physiology and community composition. Even when microbial community response to stress is limited, the physiological costs imposed on soil microbes are large enough that they may cause large shifts in the allocation and fate of C and N. For example, for microbes to synthesize the osmolytes they need to survive a single drought episode they may consume up to 5% of total annual net primary production in grassland ecosystems, while acclimating to freezing conditions switches Arctic tundra soils from immobilizing N during the growing season to mineralizing it during the winter. We suggest that more effectively integrating microbial ecology into ecosystem ecology will require a more complete integration of microbial physiological ecology, population biology, and process ecology.
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The litter bag technique was used to examine the decomposition of six litter types (representing Picea mariana, Cladina stellaris, Betula glendulosa, and Ledum groenlandicum) and standard cellulose. The decomposition was measured over a 2-yr period, with tissue samples placed on a recently burnt site and on top of and beneath a lichen mat in a mature spruce-lichen woodland in northern Quebec. Mass losses after 2 yr were between 10 and 60%, with the largest losses occurring in B. glandulosa and L. groenlandicum leaves and the smallest in C. stellaris; 60-90% of the lst-yr mass loss occurs during the winter (September to June). The exponential decay constant k ranged from -0.05 to -0.53. There were few statistically significant differences in decomposition rate among the three sites. Of the chemical constituents analyzed, original concentrations of Ca, carbohydrate, P.K. and N were closely correlated either with the decomposition parameter k or with mass remaining after 1 or 2 yr. Ca, Mg, and k are rapidly lost from the decomposing tissues, except for C. stellaris. All the tissues, except B. glandulosa, showed an accumulation of N. associated with high C:N ratios. The results emphasize the importance of N to this ecosystem, in that it tends to remain immobilized in the litter, at least over the first 2 yr.
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Changes in mass and quantities of potassium, calcium, magnesium, nitrogen, and phosphorus and extractable phenolics and carbohydrates were monitored over winter in three tissue types: Picea mariana needles, Betula glandulosa leaves, and Cladina stellaris. Samples were set out in litter bags on the lichen mat on 4 September 1981 and retrieved 7 November 1981, and 1 February, 17 April, and 3 June 1982. Over-winter mass losses ranged from 13 to 29% (lichen <spruce<birch) and between 46 and 80% of this loss occurred during the first sampling period, before a permanent snow cover was established. Loss of potassium, magnesium, and phosphorus and phenolics and carbohydrates was also rapid in most tissues during the first sampling period, and over-winter losses of these constituents ranged from 4 to 97%. Nitrogen losses were small in the spruce needles, but the lichen showed an increase in nitrogen over the winter. The results show the importance of winter-time litter decomposition, particularly during the period before a snow cover is established but also under the relatively warm (0 to -3°C) conditions imposed by a thick (>100 cm) snow cover.
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Leaf litter breakdown, a critical ecosystem level process in streams and other aquatic environments, has been conceptualized using models borrowed from terrestrial systems. We argue that current views of the process in freshwaters need to be conceptually improved. Specifically, we think the idea that breakdown proceeds in three distinct temporal stages (leaching, conditioning, fragmentation) has been overemphasized. Leaching, the massive loss of soluble leaf components within 24 h after immersion, is generally considered to constitute a well-defined first stage. Recent evidence suggests, however, that the initial solute losses are largely an effect of the unnatural drying procedures to which experimental leaves are normally subjected. Fresh leaf litter does lose solutes when immersed, but gradually throughout the breakdown process rather than instantly upon wetting. Conditioning, the second breakdown stage, describes the enhancement of leaf palatability for detritivores by microbial colonization, and is thus ultimately targeted towards a group of organisms (which contribute to litter degradation) rather than addressing the breakdown process per se. Furthermore, conditioning implies a key role for detritivorous invertebrates and underrates the established direct degradative activity of microbial decomposers. If, thus, leaching and conditioning are not generally useful operators to describe portions of the litter breakdown process in freshwaters, the traditional concept, which emphasises leaching, conditioning and fragmentation as three sequential stages, loses much of its appeal. Consequently, we propose a new conceptual model, in which the coincidence and interplay of various subprocesses of litter breakdown is more strongly recognized. In this model, we propose to view the process in terms of the products of litter breakdown-as a complement to the usual perspective which focuses on litter mass loss. Six primary breakdown products are considered: bacterial, fungal and shredder biomass; dissolved organic matter; fine-particulate organic matter; and inorganic mineralization products such as CO2, NH4+ and PO43-. We present a scheme illustrating the hypothesized formation of these products throughout breakdown. However, to improve understanding of the process, application of the proposed conceptual framework in experimental work is necessary.
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Extreme value analysis of observed daily temperature anomalies from a new quasi-global data set indicates that extreme daily maximum and minimum temperatures (>98.5 or <1.5 percentile) have warmed for most regions since 1950. Changes in extreme anomalous daily temperatures are determined by fitting extreme value distributions with time-varying parameters. Changes in the distribution of anomaly exceedances above a high threshold are found to be statistically significant at the 10% level for most land areas when compared with a time-invariant distribution and with the unforced natural variability produced by a coupled climate model. The largest positive trends in the location parameter of the extreme distribution are found in Canada and Eurasia where daily maximum temperatures have typically warmed by 1 to 3°C since 1950. The total area exhibiting positive trends is significantly greater than can be attributed to unforced natural variability. For most regions, positive trend magnitudes are larger and cover a greater area for daily minimum temperatures than for maximum temperatures. The comparatively small areas of cooling are found to be consistent with unforced natural climate variability. The North Atlantic Oscillation (NAO) is found to have a significant influence on extreme winter daily temperatures for many areas, with a negative NAO of one standard deviation reducing expected extreme winter daily temperatures by ˜2°C over Eurasia but increasing temperatures over northeastern North America.