Assessing and comparing climatic control on distribution and reproduction of alpine and lowland species in the subalpine habitat of western Norway.

Abstract

Aims and background: Species range shift is among the most well-documented responses to climate change. As a result, a growing number of studies model species climatic niches to predict how species ranges may displace in space and time (SDM studies). These studies are criticised because they do not include reproduction in their predictions. Other studies use empirical data to assess climatic control on reproductive life-stages. However, the climatic niche of reproductive life-stages may not determine the climatic niche of species, limiting the ability of both types of studies to assess the effect of climate change. In this synthesis, I compare the results of a SDM study (Paper I) with the results of two empirical studies focussing on flowering performance (Paper II) and seedling emergence (Paper III). The research focuses on the leading and rear altitudinal edges of lowland and alpine species ranges, respectively, as those are the two delimiting fronts that are expected to be specifically vulnerable to climate change. Reproduction response to climate is a complex process because it involves several sub-stages that can be affected by climate in several ways. Therefore, the results included in this synthesis integrate several direct and size-dependent climatic effects on flowering performance and report on the importance of both the climate conditions occurring at the recruitment sites and those experienced by the source populations for seedling emergence. Study area and species: This thesis makes use of the sub-alpine and alpine landscapes of western Norway to investigate climatic control on species occurrence and reproduction. This study area was chosen because it includes the leading altitudinal edge of lowland species’ ranges and the rear altitudinal edge of alpine species’ ranges. The research uses Viola biflora (alpine), Viola palustris (lowland), Veronica alpina (alpine), and Veronica officinalis (lowland) as study cases because these species are common in the study area and the study sites, and are representative of the alpine and lowland communities occurring at the studied sites. Results and discussion: The climatic control on flowering performances and seedling emergence did not reflect the climatic niches of three out of four species, suggesting ontogenetic niche shift. These mismatches challenge the predictive ability of both SDMs and empirical studies focusing on reproductive life-stages. Papers II and III highlight the complexity of climatic control on reproduction and show species-specific results. Flowering was both climate- and size-dependent for three species, but the way size-dependency was expressed differed between species. Seedling emergence was less species-specific although one species (Veronica officinalis) was found to be affected by the climate experienced by its source populations while the three other species were only responding to the climate of their sowing sites. This illustrates the importance of accounting for the complexity of reproduction to improve climate-change predictions on plant reproduction. Finally, Papers I and II suggest a high importance of biotic interactions from the lowland flora for the two alpine species, stressing the importance of understanding plant-plant interactions to forecast climate-change impacts. Further research and conclusions: Structured population models can resolve some of the problems reported in this synthesis but further methodological developments are necessary to integrate local adaptation patterns and to accurately project the outputs of such models in space. Dispersal has been largely ignored in climate-change studies. Further research should also aim to improve knowledge on dispersal because quantifying dispersal and recruitment rates is required to predict accurately climate-change impacts on plant populations and potential range displacements. Individualistic responses to climate suggest drastic changes in plant communities over the coming years. Given the high importance of biotic interactions, increasing knowledge on species interactions will be an important step to better understand and predict the potential climate-change impacts. Since the climatic control of a given species may not relate to the climatic control of another species, more effort should be given to key and/or endangered species that may have specific value for conservation.