Changes in Alpine Plant Population Sizes in Response to Climate Change
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Changes in alpine plant population sizes in response to climate change Evelina Rostö Degree project in biology, Master of science (2 years), 2020 Examensarbete i biologi 30 hp till masterexamen, 2020 Biology Education Centre and SLU Artdatabanken, Uppsala University Supervisors: Brita Svensson, Per Toräng and Mora Aronsson External opponent: Håkan Rydin Contents Abstract .................................................................................................................................................. 2 Introduction ........................................................................................................................................... 2 Vascular plants and climate change ................................................................................................ 2 Trends of alpine plants ...................................................................................................................... 3 Aims .................................................................................................................................................... 5 Hypotheses ......................................................................................................................................... 5 Material and methods ........................................................................................................................... 5 Study species ...................................................................................................................................... 5 Study sites and data collection ......................................................................................................... 8 Statistical analyses ........................................................................................................................... 11 Results .................................................................................................................................................. 12 Changes in population sizes ............................................................................................................ 12 Relationships between environmental variables and population size changes .......................... 14 Discussion ............................................................................................................................................. 19 Species monitored 2019 ................................................................................................................... 19 Population surveyed several years ................................................................................................. 20 Habitat factors and dispersal ......................................................................................................... 24 Conclusion and prediction for the future ...................................................................................... 25 Acknowledgements .......................................................................................................................... 26 References ............................................................................................................................................ 27 Appendix 1 ........................................................................................................................................... 33 1 Abstract Alpine plants are assumed to be in particular danger as the climate changes rapidly worldwide. Specialist alpine species in Norrbotten County, northern Sweden have been surveyed over the last 20 years, providing insight to population dynamics and how the plants might respond to the changing climate. The main current threat to the species is habitat destruction as the climate changes. Variation in the number of plants among populations and years, and correlations with environmental variables were examined. Some species had increased while others had decreased over the years. No uniform relationship for all species and populations were discovered, but some of the species exhibited relationships between population size changes and temperature and precipitation. However, if the future climate in Norrbotten County changes according to the predictions, the habitats of the specialist alpine plants may be severely altered, leaving the species with no alternative places to establish and grow. Introduction Vascular plants and climate change The climate has changed repeatedly since the Earth was formed, but today it is changing at a more rapid pace than before (SGU 2018). Vascular plants have endured naturally changing conditions many times during their evolutionary history, but previous changes have been slow and gradual (Körner 2003, Hunter & Gibbs 2016). Human actions the last couple of centuries have increased the concentrations of greenhouse gases in the atmosphere rapidly. The elevated concentrations of carbon dioxide (CO2) are increasing global temperatures, resulting in changing precipitation patterns, as well as frequency of extreme weather events. The emissions from fossil fuels and use of fertilizers have also increased the amount of soluble nitrogen (N) in the soil and air (Körner 2003, IPCC 2014). Climate change will affect species richness, plant community composition and productivity of vascular plants. Global warming is predicted to affect species richness in different ways in different places. In habitats where temperature is a limiting factor for many vascular plant species, such as north-temperate and alpine habitats, an increase in species richness is predicted in response to increased mean temperatures (Sommer et al. 2010, Venevskaia et al. 2013, Harrison 2020). Near the equator, in arid areas where water-availability is low, the projected increase in precipitation is predicted to increase plant species richness as well (Harrison 2020). In many other tropical and sub-tropical regions, where precipitation is projected to decrease as the temperature increases, a decrease in plant species richness is predicted (Venevskaia et al. 2013, Harrison 2020). The elevated CO2 concentrations in the air caused by global, anthropogenic activity, has been documented to increase plant growth and photosynthesis rates, as well as decrease water use and concentrations of N and proteins in plant tissues in many plant species (Bowes 1993, Ainsworth & Long 2005, Taub 2010, Gray & Brady 2016). Species with C4-photosynthesis (see definition in Ricklefs & Relyea 2014) show less response to increased concentrations of CO2 (Bowes 1993, Ainsworth & Long 2005, Taub 2010, Gray & Brady 2016). Fertilization experiments by Clark et al. (2007) and Midolo et al. (2019) have shown that increased availability of N increases plant growth and biomass but also decreases species richness and diversity. The plants that are adapted to low N-availability are out-competed by the species that are able to utilize the additional nutrients for rapid growth (Körner 2003, Clark et al. 2007, 2 Midolo et al. 2019). The responses in plant communities to additional N are not uniform but depends on several environmental factors such as mean annual temperature, the annual amount of precipitation and cumulative effects of N addition (Midolo et al. 2019). The poles, and particularly the Arctic, are more affected by global warming than the rest of the planet because of various feedback processes. Globally, the temperature has risen 1°C, in areas towards the poles the increase is several degrees centigrade (IPCC 2014). In places where there is less snow and ice than before, the exposed, darker open water and landmasses absorb more solar radiation, which increases the warming even more (ACIA 2005, Screen & Simmonds 2010, IPCC 2014). This already have a large influence on the permafrost in the Arctic, which is an important carbon sink, but as the temperature rises, the thawing of the frozen ground releases even more greenhouse-gases, further accelerating the global warming (Tarnocai et al. 2009, Schuur et al. 2015). The Swedish Scandes are considered as part of the Oroarctic, at the end of the boreal zone and beginning of the arctic tundra, but are not part of the Arctic (CAFF 2013, Ehrich et al. 2016). However, the alpine areas in Norrbotten County, the northernmost situated county in Sweden, is exhibiting similar effects of global warming as the Arctic regions. At Abisko Scientific Research Station, in the northwest of the county, scientists have measured numerous environmental variables since 1913 (Swedish Polar Research Secretariat 2018). The environment has changed dramatically over the last 100 years. For example, the large lake Torneträsk is ice-free 40 more days each year compared to 100 years ago, the tree-line has moved more than 20 m upslope in elevation, the growing season is about four weeks longer and the winter temperatures have increased (Jonasson et al. 2012, CIRC 2020a, CIRC 2020b). The depth of permafrost thawing by Torneträsk has increased by a meter from 1978 (0.48 m) to 2018 (1.46 m) (CIRC 2020a). The Swedish Metrological and Hydrological Institute (SMHI) modelled the future climate in Norrbotten County until 2100, based on the RCP4.5 and RCP8.5 models (IPCC 2014, SMHI 2015). The models predict dramatic changes in yearly mean temperature, annual amount of precipitation and growing season length, compared to the period 1961 to 1990 (Table 1). The county administrative board in Norrbotten County have predicted that 75% of the area that today are above the treeline will be covered with shrubs and trees