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Snow is an important driver of ecosystem processes in cold biomes. Snow accumulation determines ground temperature, light conditions, and moisture availability during winter. It also affects the growing season’s start and end, and plant access to moisture and nutrients. Here, we review the current knowledge of the snow cover’s role for vegetation, plant-animal interactions, permafrost conditions, microbial processes, and biogeochemical cycling. We also compare studies of natural snow gradients with snow experimental manipulation studies to assess time scale difference of these approaches. The number of tundra snow studies has increased considerably in recent years, yet we still lack a comprehensive overview of how altered snow conditions will affect these ecosystems. Specifically, we found a mismatch in the timing of snowmelt when comparing studies of natural snow gradients with snow manipulations. We found that snowmelt timing achieved by snow addition and snow removal manipulations (average 7.9 days advance and 5.5 days delay, respectively) were substantially lower than the temporal variation over natural spatial gradients within a given year (mean range 56 days) or among years (mean range 32 days). Differences between snow study approaches need to be accounted for when projecting snow dynamics and their impact on ecosystems in future climates.

Summary: Targeted removal experiments are a powerful tool to assess the effects of plant species or (functional) groups on ecosystem functions. However, removing plant biomass in itself can bias the observed responses. This bias is commonly addressed by waiting until ecosystem recovery, but this is inherently based on unverified proxies or anecdotal evidence. Statistical control methods are efficient, but restricted in scope by underlying assumptions.We propose accounting for such biases within the experimental design, using a gradient of biomass removal controls. We demonstrate the relevance of this design by presenting (1) conceptual examples of suspected biases and (2) how to observe and control for these biases.Using data from a mycorrhizal association‐based removal experiment, we show that ignoring biomass removal biases (including by assuming ecosystem recovery) can lead to incorrect, or even contrary conclusions (e.g. false positive and false negative). Our gradient design can prevent such incorrect interpretations, regardless of whether aboveground biomass has fully recovered.Our approach provides more objective and quantitative insights, independently assessed for each variable, than using a proxy to assume ecosystem recovery. Our approach circumvents the strict statistical assumptions of, for example, ANCOVA and thus offers greater flexibility in data analysis. [ABSTRACT FROM AUTHOR]

Human‐mediated dispersal of non‐native earthworms can cause substantial changes to the functioning and composition of ecosystems previously earthworm‐free. Some of these earthworm species have the potential to "geoengineer" soils and increase plant nitrogen (N) uptake. Yet the possible consequences of increased plant N concentrations on rodent grazing remains poorly understood. In this study, we present findings from a common garden experiment with two tundra communities, meadow (forb dominated) and heath (shrub dominated), half of them subjected to 4 years of earthworm presence (Lumbricus spp. and Aporrectodea spp.). Within four summers, our earthworm treatment changed plant community composition by increasing graminoid density by, on average, 94% in the heath vegetation and by 49% in the meadow. Rodent winter grazing was more intense on plants growing in soils with earthworms, an effect that coincided with higher N concentrations in plants, indicating a higher palatability. Even though earthworms reduced soil moisture, plant community productivity, as indicated by vegetation greenness (normalized difference vegetation index), was not negatively impacted. We conclude that earthworm‐induced changes in plant composition and trophic interactions may fundamentally alter the functioning of tundra ecosystems. [ABSTRACT FROM AUTHOR]

Northern soils play an important role in Earth’s climate system as they store large amounts of carbon that, if released, could strongly increase greenhouse gas levels in our atmosphere. Most research to date has focused on how the turnover of organic matter in these soils is regulated by abiotic factors and few studies have considered the potential role of biotic regulation. Here, we claim that soil organisms’ presence or absence is key to understanding and predicting future climate feedbacks from northern soils. We propose that the arrival of soil organisms with currently ‘missing traits’, i.e., properties that the present community does not have, can alleviate functional limitation and result in greatly enhanced decomposition rates, in parity with effects predicted due to increasing temperatures. We base this argument on a series of emerging evidence suggesting that the dispersal of until-then absent micro-, meso- and macro-organisms (i.e., microbes and invertebrate soil fauna) into new regions and newly-thawed soil layers can drastically affect soil functioning. These new observations make us question the current view that neglects organism driven ‘alleviation effects’ when predicting the future feedbacks between northern ecosystems and our planets’ climate. We therefore advocate for an updated framework in which soil biota and their traits become essential when predicting the fate of soil functions in warming northern ecosystems. [ABSTRACT FROM AUTHOR]