Eric Meineri

Strong microsite control of seedling recruitment in tundra.

The inclusion of environmental variation in studies of recruitment is a prerequisite for realistic predictions of the responses of vegetation to a changing environment. We investigated how seedling recruitment is affected by seed availability and microsite quality along a steep environmental gradient in dry tundra. A survey of natural seed rain and seedling density in vegetation was combined with observations of the establishment of 14 species after sowing into intact or disturbed vegetation. Although seed rain density was closely correlated with natural seedling establishment, the experimental seed addition showed that the microsite environment was even more important. For all species, seedling emergence peaked at the productive end of the gradient, irrespective of the adult niches realized. Disturbance promoted recruitment at all positions along the environmental gradient, not just at high productivity. Early seedling emergence constituted the main temporal bottleneck in recruitment for all species. Surprisingly, winter mortality was highest at what appeared to be the most benign end of the gradient. The results highlight that seedling recruitment patterns are largely determined by the earliest stages in seedling emergence, which again are closely linked to microsite quality. A fuller understanding of microsite effects on recruitment with implications for plant community assembly and vegetation change is provided.

Microrefugia: Not for everyone

Microrefugia are sites that support populations of species when their ranges contract during unfavorable climate episodes. Here, we review and discuss two aspects relevant for microrefugia. First, distributions of different species are influenced by different climatic variables. Second, climatic variables differ in the degree of local decoupling from the regional climate. Based on this, we suggest that only species limited by climatic conditions decoupled from the regional climate can benefit from microrefugia. We argue that this restriction has received little attention in spite of its importance for microrefugia as a mechanism for species resilience (the survival of unfavorable episodes and subsequent range expansion). Presence of microrefugia will depend on both the responses of individual species to local climatic variation and how climate-forcing factors shape the correlation between local and regional climate across space and time.

Seedling emergence responds to both seed source and recruitment site climates: a climate change experiment combining transplant and gradient approaches

Seedling recruitment allows genetic recombination and production of dispersal units. Both the climate experienced by the source populations (seed source effect) and the weather experienced by the seeds during germination and seedling emergence (recruitment site effects) are important for seedling recruitment. Separating these effects in the field is essential to assess potential climate change impacts on plant population. We combine experimental seed transplant and gradient analyses to separate the effects of seed source and recruitment site temperature and precipitation for the seedling emergence of two alpine/lowland species pairs (Viola biflora/Viola palustris, Veronica alpina/Veronica officinalis). Combining these approaches allows us to compare local responses versus responses along environmental gradients, but also tests for local adaptation and/or pre-conditioning effects (adaptive seedling emergence responses). Veronica officinalis emergence increased with increasing seed source temperature in both the experimental and the gradient approaches, and showed adaptive seedling emergence. Viola biflora, Viola palustris and Veronica alpina emergence decreased with recruitment site temperature in both approaches. Both Violas emergences increased with recruitment site precipitation, in both approaches for the alpine violet, and in the gradient approach for lowland one. Emergence was primarily affected by the environment of the recruitment site, whereas seed source climate and adaptive seedling emergence impacted recruitment in only one of our species. The responses to recruitment site temperatures were negative, whereas the response to seed source temperature was positive. Ignoring the distinctions between these different mechanisms can lead to erroneous conclusions regarding potential climate change impacts on plant recruitment.

Biotic interaction effects on seedling recruitment along bioclimatic gradients: testing the stress-gradient hypothesis

Questions Is there a shift from positive to negative biotic interaction effects on seedling recruitment along two different stress gradients, temperature and precipitation (the stress-gradient hypothesis); do such interaction effects differ between species with different bioclimatic affinities? Location Boreal, sub-alpine and alpine grassland in southern Norway. Methods We tested the stress-gradient hypothesis by comparing seedling recruitment in bare-ground gaps where vegetation has been removed vs in extant grassland vegetation in 12 boreal, sub-alpine and alpine grassland sites along a precipitation gradient. This was tested in (1) a seed-sowing experiment and (2) in naturally occurring recruitment of alpine, generalist and boreal species. Results Emergence of the sown alpine species was higher in the cold alpine than in the warmer sub-alpine sites, with no effects of precipitation or vegetation removal. The sown generalists also decreased in emergence towards warmer sites, whereas there was no effect of temperature on the sown boreal species. Vegetation removal, interacting with precipitation, increased the emergence of the generalist and boreal species sown at intermediate precipitation levels. In contrast, interactions between temperature and vegetation removal regulated the emergence of all groups of naturally occurring seedlings. Alpine and generalist species emerged at the highest rate in alpine sites, whereas boreal species had highest emergence in the lowlands. Conclusion For all species groups, strong effects of vegetation removal show that competition from the extant vegetation dominates in controlling seedling emergence across all study sites and species. In generalist and boreal species, positive interactions between vegetation removal and temperature show that competitive interactions affect seedling emergence more strongly towards warmer climates, in line with the stress-gradient hypothesis. In contrast, alpine species show no such interactions. This suggests that species’ adaptations to climate, in combination with environmental forcing, control seedling emergence along the bioclimatic gradients. Our results have implications for nature conservation, as we propose that disturbance from grazing animals can be useful to release competition and thereby increase seedling recruitment and biodiversity in boreal and alpine grasslands in a warmer future.