Abstract1. Biased sex ratios among reproductive individuals are common in plants, but the underlying mechanisms, as well as the evolutionary consequences, are not well understood. The classical theory of Düsing and Fisher predicts an equal primary sex ratio at seed production, based on the selective advantage of the rare sex. Biased sex ratios among reproductive plants can arise from sexual dimorphism in survival and flowering. Sex ratio biases can also be present from seed; in these cases, assumptions of Düsing’s and Fisher’s theory, for example, random mating or demographic equilibrium, were not met.2. We investigated mechanisms leading to female-biased sex ratios in the arctic-alpine dwarf willowSalix herbaceaL. We studied sex ratios in three natural populations over three years as well as in 29 crosses (full-sib families) under controlled conditions over four growth periods. We tested whether sex ratio was associated with germination, survival or flowering, and whether females and males differed in habitat, size or flowering.3. We detected a strong and consistent female bias, both in natural populations (sex ratio [proportion of females]: 0.71-0.82) and in our controlled experiment (overall sex ratio: 0.70-0-72). Our data did not support habitat segregation of the sexes or sexual dimorphism in size or flowering. Family sex ratios varied largely (from 0.25 to 1), including many female-biased families, but also unbiased families and two male-biased families. Families with lower germination, seedling establishment, survival or flowering did not have stronger female bias, indicating that intrinsically higher survival or flowering in females does not explain overall female bias.4. Synthesis. Our results suggest that sex ratio bias inS. herbaceais already present in seeds and does not arise through intrinsic differences between sexes. Candidate mechanisms that can lead to both overall female bias and variation in sex ratio among families are meiotic drive or cyto-nuclear interactions. The pioneer habit ofSalixmay lead to non-equilibrium population dynamics that allow for the long-term persistence of variable genetic sex ratio distortion systems that arise from genetic conflict.

AbstractNitrogen enrichment could affect primary consumers by increasing foliar N content (nitrogen-disease hypothesis), by increasing dominance of palatable, fast-growing plants (growth-defence trade-off) or by reducing plant diversity (resource concentration effect). These mechanisms might operate differently at different organizational levels. We tested this in a grassland experiment (PaNDiv), manipulating nitrogen enrichment, plant species richness, fast-slow functional composition, and foliar pathogens. We assessed herbivory and pathogen damage, on focal plant individuals, species, and communities. Plant community characteristics were more important drivers of consumer damage than nitrogen enrichment. Host concentration effects strongly affected pathogens but mostly at the species level. Growth-defence trade-off effects were widespread but frequently emerged from interactions between species (associational effects) or with resource concentration effects, leading to different patterns at species and community levels. Our findings suggest that key mechanisms may operate differently at different organisational levels calling for better understanding of how consumer effects scale across levels.

Abstract The water isotope composition of the winter snow cover on Arctic sea ice is strongly enriched in heavy isotopes near the snow-sea ice interface, incompatible with typical enrichment values through snow metamorphism processes alone. Our stratigraphic investigations from the MOSAiC expedition, using computed tomography combined with isotopic analyses of the snow, highlight that approximately 20% of the snowpack is not of meteoric origin but created from the sea ice. Here, we show that sea ice sublimation under the high-temperature gradients during the Arctic winter produces a snow-like structure and significantly contributes to the total snow water equivalent on Arctic sea ice. This, until now, unaccounted oceanographic source of “snow” furthers our understanding of i) vapor fluxes and gas exchange through the snowpack with biogeochemistry applications, ii) the formation of saline Arctic snow, and contribution to sea salt aerosols, iii) uncertainties in mass balance and physical properties of snow, and iv) additional uncertainties in precipitation estimates when compared to in situ measurements. Ultimately, regional differences in precipitation will result in varying local temperature gradients and therefore different contributions of ocean-sourced snow, and understanding this is essential for improving the accuracy of modeled sea ice predictions.

Global change alters the stability of biological communities by affecting species richness and how species covary through time (i.e., synchrony). There are few large-scale empirical tests of stability-diversity-synchrony relationships and those mostly focus on the terrestrial realm. Moreover, the effect of synchrony is largely unknown when species only covary at either high or low extremes of abundance (i.e., tail-dependent synchrony), a common phenomenon in ecological communities. Here, we synthesized long-term community time-series data (20+ years of species’ abundances/biomass for 2,668 communities across 7 taxonomic groups) from both terrestrial and freshwater realms and explored how the relationships among richness, synchrony, and stability vary across realms. We also investigated the effect of tail-dependent synchrony on stability across 714 freshwater and 1,954 terrestrial communities. For terrestrial communities, we found a positive diversity-stability relationship and that the tail-dependent synchrony was a more important determinant of stability than the traditional measure of overall synchrony (i.e., based on the covariation of all species). For freshwater communities, only overall synchrony explained some variation in stability. Assessing tail-dependent synchrony can improve our ability to understand why stability varies across different ecosystems and thereby our inferences about the causes of human-mediated biodiversity loss.

Abstract The near-surface boundary layer above patchy snow cover in mountainous terrain is characterized by a highly complex interplay of various flows on multiple scales. In this study, we present data from a comprehensive field campaign that cover a period of 21 days of the ablation season in an alpine valley, from continuous snow cover until complete melt out. We recorded near-surface eddy-covariance data at different heights and investigate cospectral decompositions.The topographic setting led to the categorisation of flows into up and down valley, with most flows being thermally driven except for a Föhn event in the middle of the observation period. Our findings reveal that the snow cover fraction is a major driver for the structure and dynamics of the atmospheric layer adjacent to the snow surface. With bare ground emerging, stable internal boundary layers (SIBL) developed over the snow. As the snow coverage decreased, the depth of the SIBL decreased and cospectra of air temperature variance showed a transition towards turbulent time scales, which were caused by the intermittent advection of shallow plumes of warm air over the snow surface.In addition, our analysis of vertical profiles of turbulence kinetic energy indicates that the distribution of eddy size and, thus, the turbulence structure, did not significantly change towards the surface. However, the SIBLs caused a thermal decoupling of the atmospheric layer adjacent to the snow surface. We complement these findings by presenting high spatio-temporal resolution air temperature profiles.

Forests in mountain regions provide an indispensable ecosystem service by protecting people and infrastructure against natural hazards. Thanks to this Nature-based Solution (NbS), costs of engineered technical protection measures can be reduced or even avoided. Numerous studies have proven the high effectiveness of forests in mitigating the negative impacts of natural hazards. However, open questions remain about the long-term and sustainable provision of protective service by mountain forests, which are expected to be increasingly affected by global change. Natural forest dynamics and disturbances can result in temporary or irreversible loss of protective effects of forests, potentially accelerated by climate change. At the same time, rising temperatures and more frequent and severe droughts will lead to shifts in tree species distribution and forest composition, which may in turn impact their protective effect depending on the type of natural hazard. Furthermore, socio-economic changes, such as land-use change or the expansion of settlements, may affect the protective function of forests. The uncertainties related to these changes pose great challenges for the quantification and sustainable management of this key ecosystem service in mountain areas. To improve our understanding of the various effects global change has on protective forests, we summarized current knowledge based on a quantitative review. We conducted a systematic literature search using predefined terms in different databases. We focused on forests in mountain regions protecting against gravitational hazards (i.e., snow avalanches, landslides, rockfall, torrential floods and debris flow). This resulted in 70 peer-reviewed articles, books or book chapters that we systematically assessed. Most studies focused on anthropogenic forest change (i.e., management, de-/afforestation), followed by natural disturbances, whereas climatically induced changes (e.g., clearly linked to drought or rising temperatures) were less often addressed in the literature. The analyzed studies mainly examined the protection against floods, followed by avalanches, landslides and rockfall. Preliminary results indicate that global change had a predominantly negative impact on the protective effect of forests in mountain areas. In a next step, the types of impacts and potential interacting and compound effects will be analyzed in more detail.

Snow plays a crucial role in the heat transfer between the ocean and atmosphere in sea ice due to its insulating properties. However, wind-induced transport causes the snow distribution to be inhomogeneous, as snow forms dunes and accumulates mostly around pressure ridges and, leading to a heterogeneous underlying ice growth and melt. While models can help to understand the complex interactions of snow and sea ice, there is currently no 3D snow cover model for sea ice that considers detailed snow cover properties. This study presents the first application of the 3D-snow cover-atmosphere model ALPINE3D with the drifting snow module to Arctic sea ice. The model was calibrated and validated with measurements from the MOSAiC expedition. Wind fields used by the snow drift routine were generated with OpenFOAM which was forced by observations. A sensitivity analysis showed the impact of an increased fluid threshold on snow redistribution. The model performed well in simulating snow transport and mass fluxes, but underestimated erosion and poorly reproduced dune formation due to the missing dynamic mesh. The density was partially reproduced very well by the model, but uncertainties still exist in some cases. Comparing the surface snow density results with 1-D SNOWPACK simulations, ALPINE3D produced smaller differences but larger temporal variation in between setups. The study also investigated details of deposition and erosion using cross sections, showing good agreements of snow height differences between model and observations and revealing spatially high-resolution parameters such as age of deposited snow, density, and thermal conductivity.

The amount of snow on Arctic sea ice impacts the ice mass budget. Wind redistribution of snow into open water in leads is hypothesized to cause significant wintertime snow loss. However, there are no direct measurements of snow loss into Arctic leads. We measured the snow lost in four leads in the Central Arctic in winter 2020. We find, contrary to the general consensus, that under typical winter conditions, minimal snow was lost into leads. However, during a cyclone that delivered warm air temperatures, high winds, and snowfall, 35.0 ± 1.1 cm snow water equivalent (SWE) was lost into a lead (per unit lead area). This corresponded to a removal of 0.7–1.1 cm SWE from the entire surface—∼6–10% of this site’s annual snow precipitation. Warm air temperatures, which increase the length of time that wintertime leads remain unfrozen, may be an underappreciated factor in snow loss into leads.