Simulated global change: contrasting short and medium term growth and reproductive responses of a common alpine/Arctic cushion plant to experimental warming and nutrient enhancement

Juha M Alatalo 0 Chelsea J Little 0 0 Department of Ecology and Genetics, Uppsala University , Campus Gotland, SE-621 67 Visby, Sweden Cushion plants are important components of alpine and Arctic plant communities around the world. They fulfill important roles as facilitators, nurse plants and foundation species across trophic levels for vascular plants, arthropods and soil microorganisms, the importance of these functions increasing with the relative severity of the environment. Here we report results from one of the few experimental studies simulating global change impacts on cushion plants; a factorial experiment with warming and nutrient enhancement that was applied to an alpine population of the common nurse plant, Silene acaulis, in sub-arctic Sweden. Experimental perturbations had significant short-term impacts on both stem elongation and leaf length. S. acaulis responded quickly by increasing stem elongation and (to a lesser extent) leaf length in the warming, nutrient, and the combined warming and nutrient enhancements. Cover and biomass also initially increased in response to the perturbations. However, after the initial positive short-term responses, S. acaulis cover declined in the manipulations, with the nutrient and combined warming and nutrient treatments having largest negative impact. No clear patterns were found for fruit production. Our results show that S. acaulis living in harsh environments has potential to react quickly when experiencing years with favorable conditions, and is more responsive to nutrient enhancement than to warming in terms of vegetative growth. While these conditions have an initial positive impact, populations experiencing longer-term increased nutrient levels will likely be negatively affected. - Introduction Polar and alpine ecosystems are assumed to be particularly vulnerable to climate change as their organisms dwell at temperatures just above the zero degree threshold for a very short summer growing season. Predicted changes in climate over the next 100 years are expected to be substantial in arctic and sub-arctic regions (IPCC 2007), with the potential to perturb these temperature and freezing patterns. While warming is often the focus of climate change projections, Arctic and alpine areas are also affected by other types of global changes. For instance, atmospheric nitrogen input has almost doubled in the arctic during the last century due to acidic depositions (Neftel et al. 1985), and further increase is anticipated because of predicted changes in climate (Van Cleve et al. 1990). Nutrient availability is also often a limiting factor for tundra plant growth, with nitrogen (N) and phosphorus (P) as the key limiting elements (Shaver and Kummerow 1992). Due to predictions of increasing warming and nutrient deposition, a number of experiments have addressed the potential impact of environmental change in Arctic and alpine areas on singular species of vascular plants (Klanderud 2008), bryophytes (Molau and Alatalo 1998; Jgerbrand et al. 2009), lichens (Alatalo 1998; Cornelissen et al. 2001; Jgerbrand et al. 2009), functional groups (Dormann and Woodin 2002), whole plant communities (Alatalo 1998), bacteria (Rinnan et al. 2009), fungi (Olsrud et al. 2010), and arthropods (Bokhorst et al. 2008; Hgvar and Klanderud 2009; Makkonen et al. 2011). Most experimental global change studies in the Arctic have focused on vascular plants (e.g. Arft et al. 1999; Dorji et al. 2013), especially on graminoids and dwarf shrubs, which are commonly the dominant life forms of plants in the alpine and Arctic regions. Factorial experiments combining temperature and nutrient manipulations have found mixed effects, with abundances sometimes increasing and sometimes decreasing at both the species (Klanderud 2008) and functional group (Graglia et al. 2001) levels. Effects have also shifted between the first year of manipulations and longer-term impacts as experiments continued (Arft et al. 1999; Robinson et al. 1998). Amidst this uncertainty, cushion plants have not been given the same attention even though they make up a major part of some alpine and Arctic vegetation communities (but see Stenstrm et al. 1997; Alatalo and Totland 1997). Cushion plants are distributed globally in harsh polar and alpine regions, where they are of great importance as they often function as facilitators, nurse plants and foundation species in the severe environments and have a positive effect on other species across trophic levels (Cavieres and Arroyo 2002; Arroyo and Cavieres 2003; Molenda et al. 2012; Reid and Lortie 2012; Roy et al. 2013). The number of studies on cushion plants has increased rapidly during the last decade (Reid et al. 2010) and cushion plants have been shown to inhibit loss of phylogenetic diversity in severe environments, where they can act as micro-refugia to less stress tolerant species (Butterfield et al. 2013). Similarly, cushion plants have been shown to enhance species richness more in systems with lower local diversity through their role as nurse plants, sustaining diversity under harsh conditions (Cavieres et al. 2014). For instance, Antonsson et al. (2009) found that the cushion plant Silene acaulis had a significant nurse plant effect above a certain elevation threshold in alpine sub-arctic Sweden, indicating that the facilitation is increasingly important at higher elevations with more severe environments. Likewise, Yang et al. (2010) found that positive associations between the cushion plant Arenaria polytrichoides and other plants increased with elevation in the Sino-Himalayas. S. acaulis has also been shown to function as facilitator for arthropods (Molina-Montenegro et al. 2006; Molenda et al. 2012). As predicted global change lessens the environmental severity of these high-elevation and high-latitude habitats, cushion plants may still play an essential role. A combined removal and temperature enhancement experiment showed that removing Azorella madreporica cushion plants that acted as nurse plants decreased survival, biomass, and photochemical efficiency of the grass Hordeum comosum in an alpine site in Los Andes, Chile. In the same study seedling survival was enhanced by cushions, even under warmer conditions, demonstrating their importance as facilitators even under potentially warmer conditions (Cavieres and Sierra-Almeida 2012). However, the study gave no information regarding the impact of the temperature enhancement on the growth, phenology or reproductive performance of the nurse plant A. madreporica. The fact that there have been few experimental studies (to our knowledge only Stenstrm et al. 1997; Alatalo and Totland 1997; Robinson et al. 1998; Le Roux et al. 2005; Day et al. 2009) of the impact of global change on cushion plants themselves represents a significant gap in knowledge, considering their important function as facilitator and nurse plants in alpine, polar and other harsh environments. The rarity of expe (...truncated)