Chapter 3 Consistent negative arctic warming effects on lichen diversity and mixed effects on bryophyte diversity on two continents

Little is known about the impact of changing temperature regimes in the (Sub)Arctic on composition and diversity of lichen and bryophyte communities, despite their potentially large role in ecosystem functions with likely feedbacks to climate. Therefore, we investigated changes in diversity and abundance both within in situ long-term (9 -16 yrs) warming experiments, ranging from Swedish subarctic birch forest and subarctic/subalpine tundra to Alaskan arctic tussock tundra, and along their related natural climatic gradients. On both continents, lichen diversity responded negatively to experimental warming (with the exception of a birch forest) and to higher temperatures along climatic gradients. Depending on the length of the gradient, bryophyte diversity decreased both with increasing temperatures and at extremely low temperatures. Bryophytes were less sensitive to experimental warming than lichens. Among bryophytes, Sphagnum mosses were particularly resistant to experimental warming both in terms of abundance and diversity. Temperature, on both continents, was the main driver of plant community composition within experiments and along gradients, with the exception of the Swedish subarctic birch forest, where amount of litter constituted the best explanatory variable. In the Alaskan acidic tussock tundra experiment, temperature together with soil ammonium availability explained 59% of the variation. Overall, mostly dwarf shrubs (deciduous and evergreen) but also the bryophytes Sphagnum girgensohnii, Hylocomium splendens and Pleurozium schreberi were positively related to warming while the majority of other cryptogams showed a negative relationship. This unique combination of intercontinental comparison, natural gradient studies and experimental studies shows that cryptogam diversity, especially within lichens, is likely to decrease under arctic climate warming. Given the many different functions fulfilled by different cryptogam taxa (e.g. Climate change and arctic cryptogam diversity 84 carbon sequestration, hydrological control, N2-fixation, trophic interactions), these changes will have important feedback consequences for ecosystem functions and climate. Introduction Climate change at high latitudes is expected to be greatest and fastester and more rapid at high latitudes (IPCC 2007), where lichens and bryophytes are greater contributors to biodiversity than vascular plants (Matveyeva & Chernov 2000) and in some areas to aboveground biomass as well (Wielgolaski et al. 1981). Moreover, non-vascular cryptogams are a critical control on ecosystem functions (Longton 1997; Cornelissen et al. 2007) such as regulation of hydrology (Beringer et al. 2001), carbon balance (Rydin & Jeglum 2006), nitrogen (N) fixation (Solheim et al. 1996) and preservation of permafrost (Dyrness 1982), their insulation ability minimizing damage to permafrost caused by wildfires (Yoshikawa et al. 2003). Given this wide array of ecosystem controls, changes in their abundance and composition may likely impact regional biogeochemistry and even global climate. Climate change is expected to influence cryptogams through both increased temperature and changing precipitation regimes (Tenhunen et al. 1992; Lang et al. 2009; Trenbirth & Matthews 2010). Being poikilohydric, both lichens and mosses depend strongly on external water supply, which in turn affects photosynthetic rates (Tenhunen et al. 1992). Many moss species exhibit sharp declines in net photosynthesis rates above 20 °C (Oechel & Sveinbjörnsson 1978), Sphagnum being an exception showing high rates up to 30 °C (Harley et al. 1989). Indeed, Sphagnum, in contrast to many other mosses (Hobbie & Chapin 1998), benefits from increasing temperatures as long as moisture availability is sufficient (Gunnarsson 2005; Lang et al. 2009). Sphagnum may therefore be an exception, being resilient or even reacting positively to climate change. Yet also indirect effects may trigger changes in species composition. As a combined effect of both increased temperature and increased soil nutrient mineralization (Rustad et al. 2001), bigger and faster-growing vascular plant species might outcompete lichens and at least some of the bryophytes, leading to drastic changes in plant community composition (cf. Chapin & Shaver 1985; Molau & Alatalo 1998). Indeed, in response to recent warming, shrub expansion and increased growth have been recorded in various arctic ecosystems (Tape et al. 2006; Forbes et al. 2010). For cryptogams, contrasting responses to climate change have been reported. Temporal scale studies showed that abundance of mosses in high arctic tundra either increased over the last decades (Hudson & Henry 2009) or remained relatively stable (Prach et al. 2010). Similarly, lichen biomass