Cold Biomes Research Articles

Decomposition of Abies faxoniana litter varies with freeze–thaw stages and altitudes in subalpine/alpine forests of southwest China

Freeze–thaw events in winter may affect litter decomposition in cold biomes but few reports are available. We characterized the fir (Abies faxoniana) litter decomposition over a whole winter (November 2008 to April 2009) during the late autumn, deep winter, and early spring stages. The mass loss, nutrient release, and quality change of fir litter were determined using the litterbag method at 2700, 3000, 3300, and 3600 m altitude in southwest China. Over the winter an average of 18% mass, 27% C, 50% N, 40% P, 36% K, 30% cellulose, and 14% lignin were lost. Of these total losses, a majority loss of mass (70%), C (65%), N (50%), P (58%), K (42%), cellulose (70%), and lignin (68%) occurred during the deep winter stage. The highest loss rate of mass (19.2%) and lignin (16.4%) but the lowest N loss (47.9%) was at the highest 3600 m altitude. Soil freeze–thaw cycle resulted in significant losses of mass, while mass loss rate did not increase under the higher mean soil temperature during each stage. Our results confirmed that the physical process seemed to be the most important process for cold season decomposition in the cold biome.

Contrasting sensitivity to extreme winter warming events of dominant sub‐Arctic heathland bryophyte and lichen species

Summary1. Climate change in northern high latitudes is predicted to be greater in winter rather than summer, yet little is known about the effects of winter climate change on northern ecosystems. Among the unknowns are the effects of an increasing frequency of acute, short‐lasting winter warming events. Such events can damage higher plants exposed to warm, then returning cold, temperatures after snow melt, and it is not known how bryophytes and lichens, which are of considerable ecological importance in high‐latitude ecosystems, are affected by such warming events. However, even physiological adaptations of these cryptogams to winter environments in general are poorly understood.2. Here we describe findings from a novel field experiment that uses heating from infrared lamps and soil warming cables to simulate acute mid‐winter warming events in a sub‐Arctic heath. In particular, we report the growing season responses of the dominant lichen, Peltigera aphthosa, and bryophyte, Hylocomium splendens, to warming events in three consecutive winters.3. While summertime photosynthetic performance of P. aphthosa was unaffected by the winter warming treatments, H. splendens showed significant reductions in net photosynthetic rates and growth rates (of up to 48% and 52%, respectively). Negative effects were evident already during the summer following the first winter warming event.4. While the lichen develops without going through critical phenological stages during which vulnerable organs are produced, the moss has a seasonal rhythm, which includes initiation of growth of young, freeze‐susceptible shoot apices in the early growing season; these might be damaged by breaking of dormancy during warm winter events.5. Synthesis. Different sensitivities of the bryophyte and lichen species were unexpected, and illustrate that very little is known about the winter ecology of bryophytes and lichens from cold biomes in general. In sharp contrast to summer warming experiments that show increased vascular plant biomass and reduced lichen biomass, these results demonstrate that acute climate events in mid‐winter may be readily tolerated by lichens, in contrast to previously observed sensitivity of co‐occurring dwarf shrubs, suggesting winter climate change may compensate for (or even reverse) predicted lichen declines resulting from summer warming.

Enzymology under global change: organic nitrogen turnover in alpine and sub-Arctic soils

Understanding global change impacts on the globally important carbon storage in alpine, Arctic and sub-Arctic soils requires knowledge of the mechanisms underlying the balance between plant primary productivity and decomposition. Given that nitrogen availability limits both processes, understanding the response of the soil nitrogen cycle to shifts in temperature and other global change factors is crucial for predicting the fate of cold biome carbon stores. Measurements of soil enzyme activities at different positions of the nitrogen cycling network are an important tool for this purpose. We review a selection of studies that provide data on potential enzyme activities across natural, seasonal and experimental gradients in cold biomes. Responses of enzyme activities to increased nitrogen availability and temperature are diverse and seasonal dynamics are often larger than differences due to experimental treatments, suggesting that enzyme expression is regulated by a combination of interacting factors reflecting both nutrient supply and demand. The extrapolation from potential enzyme activities to prediction of elemental nitrogen fluxes under field conditions remains challenging. Progress in molecular '-omics' approaches may eventually facilitate deeper understanding of the links between soil microbial community structure and biogeochemical fluxes. In the meantime, accounting for effects of the soil spatial structure and in situ variations in pH and temperature, better mapping of the network of enzymatic processes and the identification of rate-limiting steps under different conditions should advance our ability to predict nitrogen fluxes.

Turning northern peatlands upside down: disentangling microclimate and substrate quality effects on vertical distribution of Collembola

Summary 1. Although direct contributions of soil invertebrates to carbon turnover are modest, they have a disproportionally large indirect impact through their control over the activity of microbial decomposers. Shifts in soil invertebrate species distribution might have a substantial effect on the decomposition process because their functional role depends on the species’ vertical position in soils. Gradients in microclimate and substrate quality and structure largely determine the vertical position of soil invertebrates. Because of the possible impact of climate change on soil invertebrate distribution, and consequently on decomposition, it is important to know the relative contributions of microclimate and substrate quality to the vertical distribution patterns of soil invertebrates. 2. We studied this for springtails (Collembola) as a keystone group in cool and cold biomes, by turning peat cores in a subarctic blanket bog upside down, thereby reversing the substrate quality gradient and leaving temperature and moisture gradients intact. 3. Two opposing groups of springtail species could be distinguished with respect to their abundance responses along the vertical gradient: (i) species that remain associated with the stratum they were originally found in (‘stayers’) and (ii) species that re‐establish the original stratification pattern, by remigration either to the top or deeper layers, irrespective of any substrate quality change (‘movers’). Within the ‘mover’ response pattern, the direction of their migration in response to microclimate changes seemed to coincide with their ecomorphological traits. 4. Our results not only demonstrate that springtail species differ in their responses to changes in climate or substrate quality; they also suggest that interspecific faunal trait variation may provide a useful tool to predict animal responses to climatic changes.

Dynamics of alpine plant litter decomposition in a changing climate

Climatic changes resulting from anthropogenic activities over the passed century are repeatedly reported to alter the functioning of pristine ecosystems worldwide, and especially those in cold biomes. Available literature on the process of plant leaf litter decomposition in the temperate Alpine zone is reviewed here, with emphasis on both direct and indirect effects of climate change phenomena on rates of litter decay. Weighing the impact of biotic and abiotic processes governing litter mass loss, it appears that an immediate intensification of decomposition rates due to temperature rise can be retarded by decreased soil moisture, insufficient snow cover insulation, and shrub expansion in the Alpine zone. This tentative conclusion, remains speculative unless empirically tested, but it has profound implications for understanding the biogeochemical cycling in the Alpine vegetation belt, and its potential role as a buffering mechanism to climate change.

Isotopic analysis of cyanobacterial nitrogen fixation associated with subarctic lichen and bryophyte species

Dinitrogen fixation by cyanobacteria is of particular importance for the nutrient economy of cold biomes, constituting the main pathway for new N supplies to tundra ecosystems. It is prevalent in cyanobacterial colonies on bryophytes and in obligate associations within cyanolichens. Recent studies, applying interspecific variation in plant functional traits to upscale species effects on ecosystems, have all but neglected cryptogams and their association with cyanobacteria. Here we looked for species-specific patterns that determine cryptogam-mediated rates of N2 fixation in the Subarctic. We hypothesised a contrast in N2 fixation rates (1) between the structurally and physiologically different lichens and bryophytes, and (2) within bryophytes based on their respective plant functional types. Throughout the survey we supplied 15N-labelled N2 gas to quantify fixation rates for monospecific moss, liverwort and lichen turfs. We sampled fifteen species in a design that captures spatial and temporal variations during the growing season in Abisko region, Sweden. We measured N2 fixation potential of each turf in a common environment and in its field sampling site, in order to embrace both comparativeness and realism. Cyanolichens and bryophytes differed significantly in their cyanobacterial N2 fixation capacity, which was not driven by microhabitat characteristics, but rather by morphology and physiology. Cyanolichens were much more prominent fixers than bryophytes per unit dry weight, but not per unit area due to their low specific thallus weight. Mosses did not exhibit consistent differences in N2 fixation rates across species and functional types. Liverworts did not fix detectable amounts of N2. Despite the very high rates of N2 fixation associated with cyanolichens, large cover of mosses per unit area at the landscape scale compensates for their lower fixation rates, thereby probably making them the primary regional atmospheric nitrogen sink.

Recent Climate Change Impacts on the Boreal Forest of Alaska

AbstractThe vast boreal forest biome is experiencing rapid climate warming with multiple impacts on the ecosystem which include direct vegetation changes; alterations to the ground thermal and hydrological regimes; and increases in disturbance by fire, insects, and disease. This cold biome serves as one of the major carbon storage reservoirs on this planet and there is much concern that soil warming will result in the release of massive amounts of carbon into the atmosphere where it can cause a significant feedback to global climate change. Climate predictions and observations point to interior Alaska as one of the regions experiencing the most drastic warming and can thus serve as an important harbinger of ecosystem response to climate change.

An experimental comparison of chemical traits and litter decomposition rates in a diverse range of subarctic bryophyte, lichen and vascular plant species

Summary1. Climate change in the subarctic is expected to influence vegetation composition, specifically bryophyte and lichen communities, thereby modifying litter decomposition rates and carbon (C) dynamics of these systems with possible feedbacks to climate.2. In a 2‐year experiment, we investigated decomposition rates and chemical traits of 27 bryophytes, 17 lichens and 5 vascular plants in litter beds in subarctic Sweden. The majority of the sampled cryptogam species are widespread at higher northern latitudes.3. Average 2‐year litter decomposition rates (exponential mass loss constantk) of lichen (0.44 ± 0.01) and vascular plant (0.56 ± 0.03) species were higher than that of bryophytes (0.11 ± 0.01), while within main cryptogam taxa, species identity was an important determinant of mass loss rates. At cryptogam group level, 2‐year litter mass loss ofSphagnumwas significantly lower than for non‐Sphagnummosses and liverworts. Within lichens, N2‐fixing versus non‐N2‐fixing lichens showed no variation in decomposability.4. In a subset of the large species set, mass loss differed both among incubation environments (reflecting nutrient‐rich and poor birch forest andSphagnumpeatlands, respectively) and species. The pattern of mass loss across incubation environments was not consistent among cryptogam species. N2‐fixing, in contrast to non‐N2‐fixing lichens with lower nitrogen (N) levels displayed similar decomposition rates across incubation environments. Mass loss of non‐Sphagnummosses was correlated with initial N irrespective of incubation environment.5. Litter mass loss of cryptogam taxa could be predicted very well from infrared spectra of the initial chemical composition of the species, by application of Fourier transform infrared using an attenuated total reflectance probe. The initial macronutrient concentrations (N, phosphorus, C and cations) and initial litter pH correlated less well.6. Synthesis. We showed comprehensively that decomposition rates of bryophytes are generally lower than those of lichens and vascular plants. Among bryophyte or lichen species there is also great variation in litter decomposability which depends strongly on species‐specific chemistry. Our data will help predict changing land surface feedback to C cycles and climate in cold biomes by understanding long‐term climate effects on litter decomposability through shifting vegetation composition.

Seasonal climate manipulations result in species‐specific changes in leaf nutrient levels and isotopic composition in a sub‐arctic bog

Summary Climate change in cold biomes not only involves higher summer temperatures, but also warmer springs and more winter precipitation. So far, little is known about species responses to these seasonal components of climate change. We experimentally manipulated spring and summer temperatures and winter snow accumulation and temperatures independently in a peatland in sub‐arctic Sweden. This yielded six climate scenarios and we studied the responses of the peat moss Sphagnum fuscum, the evergreen dwarf shrubs Empetrum hermaphroditum and Andromeda polifolia, the deciduous dwarf shrubs Betula nana and Vaccinium uliginosum, the grass Calamagrostis lapponica and the forb Rubus chamaemorus. We found substantial interspecific differences in leaf nutrient and carbon exchange variables that reflect the response of tundra plants to climate change. S. fuscum had the lowest N and P concentrations, with increasing N and P concentrations (and decreasing C/N and C/P ratios) going from evergreen dwarf shrubs, to the grass, deciduous dwarf shrubs and the forb. Leaf N/P ratios varied between 10 and 14 which points to N‐limited plant growth. The natural abundance of 15N varied very strongly among species and growth forms. These differences corresponded with the presence and type of mycorrhizal association in the plant roots. Leaf carbon isotope discrimination also differed strongly among species and growth forms, but the absolute differences were relatively small (< 5‰). The rank order was: forb and graminoid < moss and evergreen shrubs < deciduous shrubs. After 4 years, the effects of realistic climate change manipulations on leaf nutrient and carbon exchange variables were idiosyncratic with respect to species and generally small compared to the differences among species and growth forms. At the phenotypic level, spring warming or winter snow addition effects occurred as frequently as summer warming effects. This implies that the changes in the species composition and structure of plant communities that have been observed in medium‐term warming studies in cold biomes will have much more impact on plant‐mediated nutrient and carbon cycling pathways and rates than climate‐change induced phenotypic responses, irrespective of the seasonal timing of these climate changes.

Global negative vegetation feedback to climate warming responses of leaf litter decomposition rates in cold biomes

Whether climate change will turn cold biomes from large long-term carbon sinks into sources is hotly debated because of the great potential for ecosystem-mediated feedbacks to global climate. Critical are the direction, magnitude and generality of climate responses of plant litter decomposition. Here, we present the first quantitative analysis of the major climate-change-related drivers of litter decomposition rates in cold northern biomes worldwide. Leaf litters collected from the predominant species in 33 global change manipulation experiments in circum-arctic-alpine ecosystems were incubated simultaneously in two contrasting arctic life zones. We demonstrate that longer-term, large-scale changes to leaf litter decomposition will be driven primarily by both direct warming effects and concomitant shifts in plant growth form composition, with a much smaller role for changes in litter quality within species. Specifically, the ongoing warming-induced expansion of shrubs with recalcitrant leaf litter across cold biomes would constitute a negative feedback to global warming. Depending on the strength of other (previously reported) positive feedbacks of shrub expansion on soil carbon turnover, this may partly counteract direct warming enhancement of litter decomposition.

The freezer defrosting: global warming and litter decomposition rates in cold biomes

Summary Decomposition of plant litter, a key component of the global carbon budget, is hierarchically controlled by the triad: climate > litter quality > soil organisms. Given the sensitivity of decomposition to temperature, especially in cold biomes, it has been hypothesized that global warming will lead to increased litter decomposition rates, both through direct temperature effects and through indirect effects on litter quality and soil organisms. A meta‐analysis of experimental warming studies in cold biomes (34 site‐species combinations) showed that warming resulted in slightly increased decomposition rates. However, this response was strongly dependent on the method used: open top chambers reduced decomposition rates, whereas heating lamps stimulated decomposition rates. The low responsiveness was mainly due to moisture‐limited decomposition rates in the warming treatments, especially at mesic and xeric sites. This control of litter decomposition by both temperature and moisture was corroborated by natural gradient studies. Interspecific differences in litter quality and decomposability are substantially larger than warming‐induced phenotypic responses. Thus, the changes in the species composition and structure of plant communities that have been observed in medium‐term warming studies in cold biomes will have a considerably greater impact on ecosystem litter decomposition than phenotypic responses. Soil fauna communities in cold biomes are responsive to climate warming. Moreover, temperature‐driven migration of the, hitherto absent, large comminuters to high‐latitude sites may significantly increase decomposition rates. However, we do not know how far‐reaching the consequences of changes in the species composition and structure of the soil community are for litter decomposition, as there is a lack of data on functional species redundancy and the species’ dispersal ability. Global warming will lead to increased litter decomposition rates only if there is sufficient soil moisture. Hence, climate scenario and experimental studies should focus more on both factors and their interaction. As interspecific differences in potential decomposability and litter chemistry are substantially larger than phenotypic responses to warming, the focus of future research should be on the former. In addition, more light should be shed on the below‐ground ‘darkness’ to evaluate the ecological significance of warming‐induced soil fauna community changes for litter decomposition processes in cold biomes.

Plant performance in a warmer world: general responses of plants from cold, northern biomes and the importance of winter and spring events

During the past three decades the Earth has warmed with a rate unprecedented during the past 1000 years. There is already ample evidence that this fast climate warming has affected a broad range of organisms, including plants. Plants from high-latitude and high-altitude sites (‘cold biomes’) are especially sensitive to climate warming. In this paper we (1) review the response in the phenology of plants, changes in their range and distribution, soil nutrient availability, and the effects on the structure and dynamics of plant communities for cold, northern biomes; and (2) we show, by using data from an ongoing snow and temperature manipulation experiment in northern Sweden, that also winter and spring events have a profound influence on plant performance. Both long-term phenological data sets, experimental warming studies (performed in summer or year-round), natural gradient studies and satellite images show that key phenological events are responsive to temperature increases and that recent climate warming does indeed lead to changes in plant phenology. However, data from a warming and snow manipulation study that we are conducting in northern Sweden show that plants respond differently to the various climatic scenarios that we had imposed on these species and that especially winter and spring events have a profound impact. This indicates that it is necessary to include several scenarios of both summer and winter climate change in experimental climate change studies, and that we need detailed projections of future climate at a regional scale to be able to assess their impacts on natural ecosystems. There is also ample evidence that the range shift of herbs and shrubs to more northern regions is for the vast majority of species mainly caused by changes in the climate. This is in line with the observed ‘up-greening’ of northern tundra sites. These rapid northern shifts in distribution of plants as a result of climate warming may have substantial consequences for the structure and dynamics of high-latitude ecosystems. An analysis of warming studies at 9 tundra sites shows that heating during at least 3 years increased net N-mineralization from 0.32±0.31 (SE) g N m−2 yr−1 in the controls to 0.53±0.31 (SE) g N m−2 yr−1 in the heated plots (p<0.05), an increase of about 70%. Thus, warming leads to higher N availability in high-latitude northern tundra sites, but the variability is substantial. Higher nutrient availability affects in turn the species composition of high-latitude sites, which has important consequences for the carbon and water balance of these systems.