Intraspecific morphological variation of Bellidiastrum michelii (Asteraceae) along a 1,155 m elevation gradient in the Tatra Mountains

Introduction

Plants that grow across mountain slopes experience different stresses caused by the variable environmental conditions found along elevation gradients. The most notable changes associated with increasing elevation in mountain regions include decreases in the mean air temperature, shortening of the growing season, increases in light intensity, UV radiation and wind velocity, reductions in carbon dioxide and oxygen concentrations, and decreases in nutrient availability (Billings, 1974; Körner, 2003; Nagy & Grabherr, 2009). Soil microbial activity and resource competition intensity also decrease at higher elevations (Körner, 2003). Meanwhile, competition for pollinator services increases with elevation as the number of pollinators declines at higher elevations (Maad, Armbruster & Fenster, 2013; Zhao & Wang, 2015; Arroyo, Pacheco & Dudley, 2017). Elevation therefore encompasses many abiotic and biotic components. Thus, elevation likely acts as a key summary variable that relates to the phenotypic variation observed in plants growing on mountains (Kelly, 1998; Hautier et al., 2009; Scheepens & Stöcklin, 2013; He et al., 2017; Miljković et al., 2019; Paudel et al., 2019).

Plants growing along large elevational ranges can adapt to the local conditions at the different elevations by adjusting their morphology and allocation of biomass to different vegetative and reproductive structures (Guo et al., 2012; Takahashi & Matsuki, 2016). For example, plant height is a trait that is related to fitness through vegetative competitiveness and ability to use available resources. Plant height has commonly been found to decrease within species with increasing elevation (Nishizawa et al., 2001; Alexander et al., 2009; Zhu et al., 2010; Maad, Armbruster & Fenster, 2013; He et al., 2017; Paudel et al., 2019), possibly as a response to limiting climatic conditions (Körner, 2003). Moreover, flowers, which are organs that are relatively invariant within species, have been found to vary quantitatively in plants of the same species across mountain elevation gradients (Herrera, 2005; Maad, Armbruster & Fenster, 2013; Seguí et al., 2018). Intraspecific flower size can gradually increase (Kudo & Molau, 1999; Herrera, 2005; Maad, Armbruster & Fenster, 2013; He et al., 2017) or steadily decrease (Totland, 2001; Zhao & Wang, 2015; Hattori et al., 2016) with increasing elevation. Increased intraspecific flower size at high elevations, as observed in some entomophilous species, has been explained by pollinator selection for larger flowers at high elevations. This is because, at high elevations, pollinators are rare but generally have a greater size (Malo & Baonza, 2002; Maad, Armbruster & Fenster, 2013). At high elevations, the reproductive success of outcrossing insect-pollinated plants is expected to be limited by pollination (Zhao & Wang, 2015; Theobald, Gabrielyan & HilleRisLambers, 2016; Arroyo, Pacheco & Dudley, 2017). An increased flower size corresponds to an increased insect visitation rate (Totland, 2004), which, in turn, increases the chances of producing viable seeds and achieving reproductive success (Arroyo, Primack & Armesto, 1982; Ohara & Higashi, 1994; Bingham & Orthner, 1998). Conversely, reductions in flower size at high elevations may result from individual plastic responses to climatic constraints (Zhao & Wang, 2015). Flower size reductions may also result from local adaptation via abiotic selection for smaller flowers in unfavourable environmental conditions. This selection occurs due to the lower development and maintenance costs associated with smaller flowers (Herrera, 2009).

Different selective abiotic and biotic pressures occurring at different elevations (Frei et al., 2014) may further generate non-linear responses in plant morphological traits along elevation gradients (Malo & Baonza, 2002). Such non-linear responses have been reported for floral traits; a unimodal relationship was found between flower size and elevation, with the maximum flower size of Cytisus scoparius (L.) Link (Malo & Baonza, 2002), Solidago minuta L. (Kiełtyk, 2018) and Viola maculata Cav. (Seguí et al., 2018) occurring in the middle of the elevational range. Such unimodal relationships may result from the trade-off between pollinator selection for larger flowers (Totland, 2001; Totland, 2004; Malo & Baonza, 2002; Maad, Armbruster & Fenster, 2013) and climatic selection for flower miniaturisation at higher elevations (Herrera, 2009; Zhao & Wang, 2015).

Intraspecific morphological variation in plants growing across elevation gradients in mountain regions is gaining increasing research attention (e.g.,  Takahashi & Matsuki, 2016; He et al., 2017; Olejniczak et al., 2018; Seguí et al., 2018). Knowledge of this variation can help us to understand how plants have adapted to different environmental conditions found across steep mountain gradients. In turn, this allows us to predict possible plant responses to climatic change, particularly in cold mountain environments (Theurillat & Guisan, 2001; Frei, Bodin & Walther, 2010). In this study, phenotypic variation was investigated in the high-mountain plant, Bellidiastrum michelii Cass. (Asteraceae), across a 1,155 m elevation gradient in the Tatra Mountains, Central Europe. The variation in B. michelii traits with elevation were assessed using Generalized Additive Models (GAMs). GAMs provide flexible, nonlinear functions that can account for both linear and non-linear phenotypic responses without the need for a priori selection of candidate models. The aim of this study was to contribute to obtaining a better understanding of morphological variation in a single mountain species across elevation gradient. Specifically, the following questions were addressed: do morphological traits of B. michelii significantly change across the elevation gradient? And if so, what is it the variation pattern of these traits in relationship to the elevation gradient?

Material and Methods

Species and study area

Bellidiastrum michelii Cass. (Asteraceae, syn. Arnica bellidiastrum All., Aster bellidiastrum (L.) Scop., Doronicum bellidiastrum L.) is a perennial plant growing in the mountains of Central and Southern Europe, from the Western Carpathians, Alps and Jura to the Apennines and the western part of the Balkan Peninsula (Merxmüller & Schreiber, 1976; Aeschimann et al., 2004). The scape of B. michelii is erect, 10–35 cm long, not leafy, with one flower head at the top (Fig. 1). Leaves are suborbicular, spatulate, obovate or elliptical in shape and gathered in a basal rosette. Flowers, gathered in the flower head, are insect-pollinated, with outer female white or pink ligulate flowers and inner hermaphrodite yellow flowers. The species blooms from May to August (Rostański, 1971; Merxmüller & Schreiber, 1976). B. michelii grows predominantly in grasslands, pastures, on rocks and along springs on calcareous substrate (Rostański, 1971; Aeschimann et al., 2004).

The research was conducted along an elevation gradient in the Tatra Mountains in southern Poland (Fig. 2), within the protected area of the Tatra National Park (study permission of the Tatra National Park: Bot/380 DBN.503/63/16). The Tatras have the highest altitudinal range among Carpathian Mountains (the highest peak, Gerlach, is at 2,655 m a.s.l. in the Slovak part of the mountains), with a well-developed subnival zone at the highest elevations. The general elevational range in the Polish Tatras extends from ca. 900 m to 2,500 m a.s.l; the lower montane belt extends from an elevation of 600 m a.s.l. in the lower regions of the Carpathian Mountains and reaches an elevation of 1,250 m a.s.l. in the Tatra Mountains. The upper montane belt extends from ca. 1,250–1,500 (1,550) m a.s.l., the subalpine belt from ca. 1,500 (1,550)–1,800 m a.s.l., the alpine belt from ca. 1,800–2,300 m a.s.l. and the subnival belt above 2,300 m a.s.l. (Mirek & Piekoś-Mirkowa, 1992). In the literature, B. michelii has been reported in the Polish Tatras at elevations ranging from ca. 800 m a.s.l. in the Tatra foothills to 2,150 m a.s.l. (Rostański, 1971).

Results

The GAMs of the relationships between plant traits and elevation revealed that 9 of the 12 studied traits changed significantly with elevation (Table 3). Plant size decreased considerably with increasing elevation. The plant height, plant mass and total leaf mass decreased by 69%, 64% and 66%, respectively (Figs. 3A–3C), across the 1,155 m elevation gradient. Hereafter, the percentage changes in trait values will refer to the 1,155 m elevation gradient, unless stated otherwise. Flower head mass and total flower mass showed slightly hump-shaped patterns (Figs. 3D and 3E). Initially, from 920 m a.s.l. to 1,225 m a.s.l., the flower head mass increased by 3%, and then from 1,225 m a.s.l. to 2,075 m a.s.l, decreased by 22%. The total flower mass initially increased by 0.5% from 920 m a.s.l. to 1,040 m a.s.l., and then decreased by 23% from 1,040 m a.s.l. to 2,075 m a.s.l. However, it is important to note that these initial slight increases in flower head mass and total flower mass at low elevations may not be statistically significant. This is because the 95% confidence intervals for the fitted model lines were wider than the observed hump-shaped patterns in the fitted lines at low elevations (Figs. 3D and 3E). Meanwhile, the individual flower mass decreased by 20% (Fig. 3F) and the flower head diameter decreased by 25% (Fig. 3G) from the lowest to highest elevation. The number of flowers in the flower head, both ligulate and tubular, showed no elevational tendency. These traits were very variable at all sites across the elevational species range and did not exhibit any elevational trend (Figs. 3H and 3I). Ligulate flower length decreased by 23% between the lowest and highest elevation (Fig. 3J). Ligulate flower width showed high variation at all sites across the elevation gradient and did not exhibit any significant relationship with elevation (Fig. 3K). Tubular flower length decreased by 7% from the lowest to the highest elevation (Fig. 3L).

Discussion

Most of the morphological traits of B. michelii varied significantly across the elevation gradient. The plant height, total aboveground mass and total leaf mass decreased considerably from low to high elevations. Similarly, floral traits, such as flower head mass, total flower mass, individual flower mass, flower head diameter, and ligulate and tubular flower length, also decreased with increases in elevation. However, the reductions in these floral traits were not as great as those observed for plant size traits. Meanwhile, the number of ligulate and tubular flowers produced by the plant did not change across the studied elevation gradient. The intraspecific variation observed in this study suggests that variable stress factors correlated with elevation impact plant morphotype. The elevational variation in morphology of B. michelii may result partially from phenotypic plasticity and partially from local adaptations at different elevations.

The considerable decrease in intraspecific plant size with increasing elevation observed in this study is in line with the results of previous studies (e.g., Nishizawa et al., 2001; Alexander et al., 2009; Zhu et al., 2010; Maad, Armbruster & Fenster, 2013; Paudel et al., 2019). These within-species size reductions may result from restricted growth caused by resource limitations, such as low air temperature, short growing seasons, strong winds, and shallow soil, that occur at high altitudes (Körner, 2003; Nagy & Grabherr, 2009). Moreover, these intraspecific plant size reductions may also partly result from local adaptation to high elevation environments via selection. Smaller phenotypes are thought to be more advantageous in limiting environmental conditions due to their lower resource requirements, and thus may be favoured by selection (Herrera, 2005; Zhao & Wang, 2015).

In this study, significant elevational variations were also found in several floral traits. In general, flowers are relatively invariant within a singular species, as this is important for securing the compatibility of the species mating system. Therefore, intraspecific variations in floral traits are often of adaptive significance (Nosil, 2012). Increasing flower size with increasing elevation has been found in several mountain species (e.g., Kudo & Molau, 1999; Maad, Armbruster & Fenster, 2013; He et al., 2017). This increase in flower size is thought to represent an adaptation that enhances flower attractiveness to pollinators (Herrera, 2005; Maad, Armbruster & Fenster, 2013). However, in B. michelii, neither flower nor flower head size increased with increasing elevation. On the contrary, the sizes and masses of the flowers and flower heads of B. michelii decreased as elevation increased. These observed reductions in the floral traits of B. michelii at high elevations are in line with the results of other studies that found that within-species flower size decreased with increasing elevation (e.g., Totland, 2001; Zhao & Wang, 2015; Hattori et al., 2016). Reductions in flower size and mass with increasing elevation can be linked to the more limiting conditions caused by increases in climate severity at high altitudes. Intraspecific flower miniaturisation has been suggested to be advantageous for plants growing under the resource-limited environmental conditions of high mountain elevations, due to the lower cost of flower development and maintenance (Herrera, 2009; Zhao & Wang, 2015).

In conclusion, significant intraspecific morphological variations were found in B. michelii across the elevation gradient. The results of this study suggest that important ecological factors associated with elevation gradients drive intraspecific morphological variation in mountain plants growing across large elevational ranges. It will be possible to identify these drivers by thoroughly analysing local abiotic (such as climatic) and biotic (such as pollinator services) ecological factors across elevation gradients. Notably, the observed elevational variation in B. michelii probably results partially from plastic responses to abiotic (mainly climatic) conditions and partially from genetic adaptation to locally prevailing conditions via selection. However, further study will be required to disentangle the contributions of genetic differences among locally adapted populations, and of plastic responses, to morphological variation across elevation gradients. Such further study will need to be based on experiments involving reciprocal plant transplantation to different mountain elevations (e.g., Gonzalo-Turpin & Hazard, 2009; Hautier et al., 2009; Scheepens, Frei & Stöcklin, 2010; Hamann et al., 2016).

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