Variation of vegetative and floral traits in the alpine plant Solidago minuta: evidence for local optimum along an elevational gradient

Alpine Botany Variation of vegetative and floral traits in the alpine plant Solidago minuta: evidence for local optimum along an elevational gradient Piotr Kiełtyk 0 0 Faculty of Biology and Environmental Sciences, Cardinal Stefan Wyszyński University in Warsaw , Wóycickiego 1/3, 01-938 Warszawa , Poland 1 Piotr Kiełtyk Alpine plants growing along wide elevational gradients experience very different abiotic and biotic conditions across elevations. As a result of genetic differentiation and/or plastic response, conspecific plants growing in high elevations, as compared to low elevations, generally have shorter stems and lower number of flowers, but larger flower size. However, most often, detailed models of elevational variations were not examined. To reveal the pattern of elevational changes in a set of fitnessrelated morphological traits, tests of linear and unimodal models were performed based on measurements of 1047 Solidago minuta plants collected from 47 sites distributed along a 1000 m elevational gradient in the Tatra Mountains. Nearly all of the investigated floral traits, i.e. inflorescence and flower heads size, and number and size of individual flowers, expressed unimodal relationships with elevation having their maxima in the centre of the elevation range. This pattern suggests the existence of a local optimum with respect to sexual reproduction at the centre of the elevational range. Possible explanations of observed elevational variations are discussed in the context of pollinator selection and the 'resource-cost compromise' hypothesis. Best floral performance in the centre of the elevational range of S. minuta may also support the idea that the favourability of habitat conditions declines from the centre to the margin of the distribution, and species are expected to be more abundant, increase reproduction and perform better in the centre of the range. Altitudinal gradient; 'Centre-periphery' hypothesis; Morphology; Phenotypic variation; Species range Introduction Plants growing along wide elevational gradients in mountains experience very different abiotic and biotic conditions, which change considerably over relatively small vertical distances. Environmental factors associated with the increase in elevation in mountains of temperate seasonal zones include reduction in mean temperature and CO2 partial pressure, decrease in the length of the growth period and prolonged duration of snow cover, reduction in soil microbial activity and resource competition among plants, decrease in nutrient availability, increase in light intensity and UV radiation, increase in precipitation and wind velocity (Billings 1974; Körner 2003, 2007) . Elevation in mountains is a complex ecological factor comprised of many abiotic and biotic components which are vital for plant growth and cannot be easily separated. Therefore, the elevational gradient has been considered as the most important factor in studies on phenotypic variation of plant species in mountains (e.g. Stöcklin et al. 2009; Št’astná et al. 2012; Kim and Donohue 2013; Scheepens and Stöcklin 2013; Gugger et al. 2015; Hamann et al. 2016; Stöcklin and Armbruster 2016; Sakurai and Takahashi 2016; Takahashi and Matsuki 2016) . Plants can respond to elevational changes based on their morphological and physiological plasticity and through local adaptations (Hirano et al. 2017) . Alpine plant species, compared to lowland plants, have distinctly smaller overall size (e.g. Körner and Renhardt 1987; Blionis and Vokou 2002; Guo et al. 2010) , allocate more biomass to underground organs (Körner and Renhardt 1987; Ma et al. 2010) and locate more of their aboveground biomass to flowers (Fabbro and Körner 2004). Morphological differences along elevational gradients are also visible at the intraspecific level in species distributed along wide elevational ranges. Plants growing in high elevation, as compared to low elevation, have shorter stems (Nishizawa et al. 2001; Alexander et al. 2009; Takahashi and Yoshida 2009; Maad et al. 2013) , lower numbers of flowers (Kelly 1998; Baret et al. 2004; Maad et al. 2013) and lower numbers of flower heads (Asteraceae family) (Kelly 1998; Alexander et al. 2009) , but larger flower sizes (Kudo and Molau 1999; Malo and Baonza 2002; Herrera 2005; Maad et al. 2013) . In addition, along with an increase in elevation, plants possess higher plasticity of flower longevity (Trunschke and Stöcklin 2017) , increase clonality, tend to be older (Št’astná et al. 2012) and produce more flowering shoots (von Arx et al. 2006) . Although numerous studies have documented elevational variation in plant morphological traits, the nature of the relationship between traits and elevation has rarely been examined. Often, only the existence of linear relationships (i.e. straight line) between traits and elevation is tested by means of simple linear regression (e.g. Kelly 1998; von Arx et al. 2006; Alexander et al. 2009; Wirth e (...truncated)