Christian Rixen

Improved water retention links high species richness with increased productivity in arctic tundra moss communities.

A positive relationship between plant species richness and ecosystem functioning has been found in a number of experimental studies. Positive species interactions at high species numbers have been suggested as a cause, but mechanisms driving positive interactions have not often been tested. In this experiment we asked three questions: (1) What is the relationship between species richness and productivity in experimentally constructed moss communities? (2) Is this relationship affected by plant density? and (3) Can changes in moisture absorption and retention explain observed relationships? To answer these questions we exposed arctic tundra moss communities of different species richness levels (1–11 species) and two different densities in the greenhouse to two levels of drought (short and long). Biomass (by the community and individual species), height and community moisture absorption and retention were measured as response variables. High species diversity increased productivity (more so in low-density plots than in high-density plots), but only when plots were watered regularly. Plot moisture retention was improved at high species richness as well, and plant height and variation in height was increased compared to plants in monoculture. Under high-density and short-drought conditions 10 out of 12 species grew better in mixture than in monoculture, but under the long drought treatment only six species did. A positive feedback loop between biomass and improved humidity under high diversity was supported by path analysis. We conclude that in this community the relationship between species richness and productivity depends on moisture availability and density, with improved water absorption and retention likely to be the mechanism for increased plant growth when drought periods are short. Furthermore, since this is the opposite of what has been found for temperate moss communities, conclusions from one system cannot automatically be extrapolated to other systems.

Small-scale patterns in snowmelt timing affect gene flow and the distribution of genetic diversity in the alpine dwarf shrub Salix herbacea

Current threats to biodiversity, such as climate change, are thought to alter the within-species genetic diversity among microhabitats in highly heterogeneous alpine environments. Assessing the spatial organization and dynamics of genetic diversity within species can help to predict the responses of organisms to environmental change. In this study, we evaluated whether small-scale heterogeneity in snowmelt timing restricts gene flow between microhabitats in the common long-lived dwarf shrub Salix herbacea L. We surveyed 273 genets across 12 early- and late-snowmelt sites (that is, ridges and snowbeds) in the Swiss Alps for phenological variation over 2 years and for genetic variation using seven SSR markers. Phenological differentiation triggered by differences in snowmelt timing did not correlate with genetic differentiation between microhabitats. On the contrary, extensive gene flow appeared to occur between microhabitats and slightly less extensively among adjacent mountains. However, ridges exhibited significantly lower levels of genetic diversity than snowbeds, and patterns of effective population size (Ne) and migration (Nem) between microhabitats were strongly asymmetric, with ridges acting as sources and snowbeds as sinks. As no recent genetic bottlenecks were detected in the studied sites, this asymmetry is likely to reflect current meta-population dynamics of the species dominated by gene flow via seeds rather than ancient re-colonization after the last glacial period. Overall, our results suggest that seed dispersal prevents snowmelt-driven genetic isolation, and snowbeds act as sinks of genetic diversity. We discuss the consequences of such small-scale variation in gene flow and diversity levels for population responses to climate change.

Species removal and experimental warming in a subarctic tundra plant community

Neighbor interactions are likely to play an important role in subarctic plant communities. We conducted experiments in Interior Alaska in which we crossed species removal with greenhouse warming manipulations. We examined changes in community biomass, and in plant survival and growth of individual species in response to experimental warming and to: (1) removal of whole species versus an equivalent amount of biomass across many species, and (2) removal of subdominant (locally common) versus minor (locally uncommon) plants. Community biomass indicated compensation in growth after removal of minor species and after biomass removal without elimination of entire species, but under-compensation after removal of subdominants. Growth and survival of individual species showed facilitation between some species. Warming increased growth of dominant vascular plants, but at the same time reduced survival, and these impacts were greater for larger, more mesic species than for the smaller species associated with drier habitats. Growth of mosses was reduced by the warming. Removal effects did not differ between warming and ambient conditions. The results indicate that common species are able to reduce resources for others (competitive effect) and increase their growth after neighbor removal, whereas locally uncommon species are not able to respond rapidly to increased resources made available by neighbor removal. Therefore, the impact of the presence of common species on locally uncommon species was facilitative overall, but not vice versa. The balance between disturbances such as changes in temperature and species losses from the community will likely be crucial in determining shifts in subsequent community composition.

Non-equilibrium in alpine plant assemblages: shifts in Europe’s summit floras

Climate warming has been more pronounced in Arctic and alpine areas, and changes in the mountain flora can be expected as the temperature envelope moves upslope. On the one hand, alpine habitats will shrink due to upward migration of species from lower areas, such as trees and tall plants. On the other hand, extinctions of summit plants may be slowed down considerably by the high diversity of microhabitats, the longevity of alpine plants and positive plant–plant interactions in extreme environments. This review chapter attempts to document and monitor vegetation changes on mountain summits. Vegetation surveys that repeat century-old historical vegetation records show considerable upward migration and subsequent increases in species on summits. This trend apparently has accelerated in recent decades. Detailed monitoring of the last decade in European mountain ranges, however, shows that this vegetation change may be at the cost of rare endemic species and alpine specialists in drier Mediterranean regions. This chapter furthermore reviews other factors than temperature influencing alpine vegetation, namely precipitation and snow, nutrients, atmospheric CO2 concentrations and land use. A subsequent question is how threatened mountain flora is by the ongoing environmental changes. Finally, this chapter discusses options for conservation and land use in high-alpine areas.