How shrub encroachment under climate change could threaten pollination services for alpine wildflowers: A case study using the alpine skypilot, Polemonium viscosum

Manual Sampling and Video Observations: An Integrated Approach to Studying Flower-Visiting Arthropods in High-Mountain Environments

Despite the rising interest in biotic interactions in mountain ecosystems, little is known about high-altitude flower-visiting arthropods. In particular, since the research in these environment can be limited or undermined by harsh conditions and logistical difficulties, it is mandatory to develop effective approaches that maximize possibilities to gather high-quality data. Here we compared two different methods, manual sampling and video observations, to investigate the interactions between the high-mountain arthropod community and flowers of Androsace brevis (Primulaceae), a vulnerable endemic alpine species with a short flowering period occurring in early season. We manually sampled flower-visiting arthropods according to the timed-observations method and recorded their activity on video. We assessed differences and effectiveness of the two approaches to estimate flower-visiting arthropod diversity and to identify potential taxa involved in A. brevis pollination. Both methods proved to be effective and comparable in describing the diversity of flower visitors at a high taxonomic level. However, with manual sampling we were able to obtain a fine taxonomic resolution for sampled arthropods and to evaluate which taxa actually carry A. brevis pollen, while video observations were less invasive and allowed us to assess arthropod behavior and to spot rare taxa. By combining the data obtained with these two approaches we could accurately identify flower-visiting arthropods, characterize their behavior, and hypothesize a role of Hymenoptera Apoidea and Diptera Brachycera in A. brevis pollination. Therefore, we propose integrating the two approaches as a powerful instrument to unravel interactions between flowering plants and associated fauna that can provide crucial information for the conservation of vulnerable environments such as high-mountain ecosystems.

Drought exacerbates negative consequences of high-intensity cattle grazing in a semiarid grassland

Grasslands managed for grazing are the largest land-use category globally, with a significant proportion of these grasslands occurring in semiarid and arid regions. In such dryland systems, the effect of grazing on native plant diversity has been equivocal, some studies suggesting that grazing reduces native plant diversity, others that grazing increases or has little impact on diversity. One impediment toward generalizing grazing effects on diversity in this region is that high levels of interannual variation in precipitation may obfuscate vegetative response patterns. By analyzing a long-term data set collected over a 20-yr period in a semiarid grassland, we explicitly evaluated the role of climate in regulating the effect of cattle grazing on plant communities, finding that climate interacted with grazing intensity to shape grassland communities. Community composition of plots that were intensively grazed varied considerably in response to climatic variation and native species richness was low relative to ungrazed and moderately grazed plots. Following a severe drought in 2002, exotic species richness rapidly increased in the high-intensity grazing plots. While this pattern was mirrored in the other treatments, exotic species richness increased to a greater extent and was slower to return to pre-drought levels in the high-intensity grazing plots. Overall, moderate grazing, even compared to grazing cessation, stabilized grassland communities through time, increased resilience to drought, and maintained the highest levels of native plant diversity and lowest levels of exotic diversity. These findings suggest that grazing, at moderate levels, may support grassland resilience to climate change in semiarid regions. However, grazing that exceeds tolerances, particularly in combination with extreme climatic events, like drought, can alter plant composition over relatively long timescales and possibly increase invasibility by nonnative species.

Disentangling the effects of driving forces on soil bacterial and fungal communities under shrub encroachment on the Guizhou Plateau of China

Global shrub encroachment (SE) affects the structure and function of grassland ecosystem. The effects of SE on plant and soil abiotic properties have been well studied; however, little is known about the extent to which driving forces structure soil microbes under SE, especially in subalpine regions of the Guizhou Plateau of China, which is undergoing progressive SE. We investigated the plant factors (viz, plant diversity and relative shrub cover), soil physicochemical properties, enzymatic activities, and microbial communities, quantified microbial element limitations under three encroachment stages, and disentangled the effects sizes of the factors that structure the diversity and composition of soil microbial communities. Redundancy analysis showed that soil factors made a greater contribution than plant factors to shaping the diversity and composition of the soil bacterial community, soil chemical factors made a greater contribution than physical factors both to structuring the diversity and composition of the soil bacterial community and to structuring the composition of the soil fungal community; and soil nutrient stoichiometry made a greater contribution than soil nutrient content to shaping soil bacterial community's diversity and fungal community's composition. In contrast, soil nutrient content made a greater contribution than soil nutrient stoichiometry to shaping the soil bacterial community's composition. The decrease in bacterial community's diversity observed under SE was attributable to increases in the carbon and nitrogen limitations consequent to SE, and the nitrogen limitation had a greater contribution to the soil bacterial community's diversity and composition than did the carbon limitation. These findings provide updated knowledge of the driving forces shaping the diversity and composition of soil microbial communities, which could be crucial for improving microbial prediction models and revealing the element cycling that occurs in SE biomes.

How shrub encroachment under climate change could threaten pollination services for alpine wildflowers: A case study using the alpine skypilot, Polemonium viscosum

Under climate change, shrubs encroaching into high altitude plant communities disrupt ecosystem processes. Yet effects of encroachment on pollination mutualisms are poorly understood. Here, we probe potential fitness impacts of interference from encroaching Salix (willows) on pollination quality of the alpine skypilot, Polemonium viscosum. Overlap in flowering time of Salix and Polemonium is a precondition for interference and was surveyed in four extant and 25 historic contact zones. Pollinator sharing was ascertained from observations of willow pollen on bumble bees visiting Polemonium flowers and on Polemonium pistils. We probed fitness effects of pollinator sharing by measuring the correlation between Salix pollen contamination and seed set in naturally pollinated Polemonium. To ascertain whether Salix interference occurred during or after pollination, we compared seed set under natural pollination, conspecific pollen addition, and Salix pollen addition. In current and past contact zones Polemonium and Salix overlapped in flowering time. After accounting for variance in flowering date due to latitude, Salix and Polemonium showed similar advances in flowering under warmer summers. This trend supports the idea that sensitivity to temperature promotes reproductive synchrony in both species. Salix pollen is carried by bumble bees when visiting Polemonium flowers and accounts for up to 25% of the grains on Polemonium pistils. Salix contamination correlates with reduced seed set in nature and when applied experimentally. Postpollination processes likely mediate these deleterious effects as seed set in nature was not limited by pollen delivery.

SYNTHESIS

As willows move higher with climate change, we predict that they will drive postpollination interference, reducing the fitness benefits of pollinator visitation for Polemonium and selecting for traits that reduce pollinator sharing.