Predicting and assessing climate-change impacts on the population dynamics of alpine and lowland forbs

Background and aims: Global climate change is already affecting plant species worldwide. The global rise in temperatures and regional changes in precipitation are predicted to continue throughout the century. Therefore, studying how climate affects species performance is crucial for understanding the implications that future changes may hold for plants. Recently, there have been several calls for studies addressing the complexities of climate change impacts, by for instance combining and integrating spatial gradient approaches, time-series data, and experiments. Another important challenge is to incorporate precipitation change and its interaction with temperature increase, since the nature of these interactions (positive, negative, additive, non-additive) determines the net effect of the combined change. Moreover, climate change affects plants not only directly, through physiological constraints, but also indirectly, through changes in biotic interactions. Disentangling these biotic interactions is crucial for understanding and predicting climate change impacts. Finally, a need for investigating plant population dynamics is needed to gain a better understanding of how and why plants respond to climatic changes on the population, species and community level. In this thesis I examine the combined effects of changes in temperature and precipitation on the population dynamics of sub-dominant forb species. I investigate climate control on the study species both along climatic gradients in space and time, and in turf- and seed-transplant experiments along climatic gradients. To assess the effect of biotic interactions along spatial climatic gradients, I use a removal experiment. Study area and species: The studies presented in this thesis were performed in the fjord-and-mountain landscape of Western Norway. The steep climatic gradients in this area enable the establishment of a ‘climatic grid’ comprising of 12 sites in three levels of temperature and four levels of precipitation. The study species were two alpine / lowland species pairs, Veronica alpina / officinalis and Viola biflora / palustris, which were relatively common and representative for the forb communities at the study sites. The study area includes the rear altitudinal range edge of the two alpine species and the leading altitudinal range edge of the two lowland species. Main findings: All species were under strong climate control. Warmer temperatures had mainly negative effects on both population growth rates and seedling establishment, while precipitation increase had more varying effects with some positive impacts on the Viola species and mainly negative effects on the Veronica species. Seedling establishment increased with increasing precipitation. The effects of temperature and precipitation change were generally not additive in the combined treatment, they either cancelled each other out, or stayed at the level of one of the single-factor effects. The biotic interaction experiment revealed a shift from facilitation to competition as temperature increases, but no overall pattern for precipitation. This suggests that the negative impacts of a warmer and a warmer+wetter climate found in the other studies may be related to changes in biotic interactions, whereas the responses to precipitation may reflect more direct effects. The results were independent of the species’ alpine or lowland habitat affinities, suggesting that also lowland species will and already do face challenges under climate change. In fact, the studies in this thesis, when taken together, indicate that Veronica officinalis already may be on the move upwards, potentially being pushed out of its historic range by increased competition under climatic warming. In conclusion, this thesis highlights the benefits of including higher complexity in ecological climate change research through ‘integrated studies’ that allow for (1) detecting climate control on species where single approaches fail, and (2) revealing underlying mechanisms and time scales for the found effects