The continuing warming of the climate system is reducing snow cover depth and duration worldwide. Changes in snow cover can significantly affect the soil microclimate and the functioning of many terrestrial ecosystems across latitudinal and elevational gradients. Yet, a quantitative assessment of the effects of snow cover change on soil physicochemical and biotic properties at large or regional scales is lacking. Here, we synthesized data of 3286 observations from 99 publications of snow manipulation studies to evaluate the effects of snow removal, addition, and compaction on soil physicochemical and biotic properties in winter and in the following growing season across (sub)arctic, boreal, temperate, and alpine regions. We found that (1) snow removal significantly reduced soil temperature by 2.2 and 0.9 degrees C in winter and in the growing season, respectively, while snow addition increased soil temperature in winter by 2.7 degrees C but only by 0.4 degrees C in the following growing season whereas snow compaction had no effect; (2) snow removal had limited effects on soil properties in winter but significantly affected soil moisture, pH, and carbon (C) and nitrogen (N) dynamics in the growing season; (3) snow addition had significant effects on soil properties both in winter (e.g., increases in soil moisture, soil C and N dynamics, phosphorus availability, and microbial biomass C and N) and in the growing season (e.g., increases in mineral N, microbial biomass C and N, and enzyme activities); and (4) the effects of snow manipulation on soil properties were regulated by moderator variables such as ecosystem type, snow depth, latitude, elevation, climate, and experimental duration. Overall, our results highlight the importance of snow cover-induced warmer microclimate in regulating soil physicochemical and biotic properties at regional scales. These findings are important for predicting and managing changes in snow-covered ecosystems under future climate change scenarios.

Permafrost is experiencing climate warming at a rate that is two times faster than the rest of the Earth's surface. However, it is still lack of a quantitative basis for predicting the functional stability of permafrost ecosystems in carbon (C) and nutrient cycling. We compiled the data of 708 observations from 89 air-warming experiments in the Northern Hemisphere and characterized the general effects of temperature increase on permafrost C exchange and balance, biomass production, microbial biomass, soil nutrients, and vegetation N dynamics through a meta-analysis. Also, an investigation was made on how responses might change with habitat-specific (e.g., plant functional groups and soil moisture status) conditions and warming variables (e.g., warming phases, levels, and timing). The net ecosystem C exchange (NEE) was found to be downregulated by warming as a result of a stronger sensitivity to warming in respiration (15.6%) than in photosynthesis (6.2%). Vegetation usually responded to warming by investing more C to the belowground, as belowground biomass increased much more (30.1%) than aboveground biomass (2.9%). Warming had a minor effect on microbial biomass. Warming increased soil ammonium and nitrate concentrations. What's more, a synthesis of 70 observations from 11 herbs and 9 shrubs revealed a 2.5% decline of N in green leaves. Compared with herbs, shrubs had a stronger response to respiration and had a decline in green leaf N to a greater extent. Not only in dry condition did green leaf N decline with warming but also in wet conditions. Warming in nongrowing seasons would negatively affect soil water, C uptake, and biomass production during growing seasons. Permafrost C loss and vegetation N decline may increase with warming levels and timing. Overall, these findings suggest that besides a positive C cycling-climate feedback, there will be a negative feedback between permafrost nutrient cycling and climate warming.

Numerous studies have reported that treelines are moving to higher elevations and higher latitudes. Most treelines are temperature limited and warmer climate expands the area in which trees are capable of growing. Hence, climate change has been assumed to be the main driver behind this treeline movement. The latest review of treeline studies was published in 2009 by Harsch et al. Since then, a plethora of papers have been published studying local treeline migration. Here we bring together this knowledge through a review of 142 treeline related publications, including 477 study locations. We summarize the information known about factors limiting tree-growth at and near treelines. Treeline migration is not only dependent on favorable growing conditions but also requires seedling establishment and survival above the current treeline. These conditions appear to have become favorable at many locations, particularly so in recent years. The review revealed that at 66% of these treeline sites forest cover had increased in elevational or latitudinal extent. The physical form of treelines influences how likely they are to migrate and can be used as an indicator when predicting future treeline movements. Our analysis also revealed that while a greater percentage of elevational treelines are moving, the latitudinal treelines are capable of moving at greater horizontal speed. This can potentially have substantial impacts on ecosystem carbon storage. To conclude the review, we present the three main hypotheses as to whether ecosystem carbon budgets will be reduced, increased or remain the same due to treeline migration. While the answer still remains under debate, we believe that all three hypotheses are likely to apply depending on the encroached ecosystem. Concerningly, evidence is emerging on how treeline migration may turn tundra landscapes from net sinks to net sources of carbon dioxide in the future.

High-latitude and high-altitude ecosystems store large amounts of carbon (C) and play a vital role in the global C cycle. Soil respiration (R-S) in these ecosystems is believed to be extremely sensitive to climate warming and could potentially trigger positive C-climate feedback. However, this evidence is largely derived from wet ecosystems, with limited observations from dry ecosystems. Here, we explored the responses of R-S, autotrophic (R-A), and heterotrophic (R-H) respiration under experimental warming in a dry ecosystem, an alpine steppe on the Tibetan Plateau. We assessed the effects of soil temperature and moisture dynamics on R-S, R-A, and R-H and performed a meta-analysis to examine whether the warming effects observed were similar to those reported in wet ecosystems, including Tibetan alpine meadow and arctic ecosystem. Experimental warming did not alter R-S, R-A, and R-H in this alpine steppe, likely because decreased soil moisture constrained positive warming effects. In contrast, the meta-analysis revealed that R-S exhibited a significant increase under experimental warming in both the Tibetan alpine meadow and arctic wet tundra. These results demonstrate that R-S exhibits different responses to climate warming between dry and wet ecosystems, suggesting potential more complex C-climate feedback in cold regions.

Global climate change may result in changes in snow cover, which may enhance freeze-thaw phenomena in mid and high latitude and high elevation ecosystems, especially in the northern hemisphere, in the future. As a common non-biological stress, the freeze-thaw process can substantially alter soil carbon and nitrogen cycles. However, a comprehensive understanding of nutrient pools and dynamics in response to freeze-thaw cycles is not available. Here, we evaluated the effect sizes of the responses of 18 variables related to soil carbon and nitrogen cycles to the freeze-thaw effect from 46 papers. Seventeen studies that reported field observations and 28 studies that reported results from laboratory experiments were included, as well as one paper that used both methods to explore freeze-thaw processes. We used a random-effects model to examine whether soil origins, effect phases (including initial and long-term effects), methods and soil horizons affect the magnitudes of the responses to freeze-thaw events. The soil sources include forest, shrubland, grassland/meadow, cropland, tundra and wetland. We used meta regression to explore possible relationships among effect sizes with freezing temperature, soil pH, soil C/N ratios and other factors. Our results suggest that the freeze-thaw process causes microbial N and the microbial C/N ratio to decrease by 12.2% and 8.5%, respectively. Soil solution dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) are enhanced by 27.5% and 373%, respectively. The freeze thaw effect increases the concentrations of NHS4+, NOT and dissolved inorganic nitrogen (DIN) by 84.1%, 29.6% and 35.4%, respectively. N2O emissions are also increased by 95.0% in freeze-thaw treatments. Laboratory measurements resulted in contrasting responses in terms of mineralization, nitrification and respiration. Freeze-thaw events promote turnover of fine roots but have no effect on the long-term aboveground biomass of grassland and heath. The results of this meta-analysis help to achieve a better understanding of the overall effects of freeze-thaw events on soil carbon and nitrogen cycles and their modulation across different environments. (C) 2017 Elsevier Ltd. All rights reserved.