INTRODUCTION
Elevational and topographic gradients have a substantial influence on biodiversity patterns (Antonelli et al., 2018; Hughes and Atchison, 2015; Rahbek, Borregaard, Colwell, et al., 2019), and high-elevation habitats are among the most diverse in the world (Hughes and Eastwood, 2006; Rahbek, Borregaard, Colwell, et al., 2019; Wen et al., 2014; Xing and Ree, 2017). Mountain range uplift creates substantial topographic heterogeneity, providing a wide variety of microclimatic niche space in which plants can become established (Körner, 2003), as well as opportunities for isolation and allopatric speciation, which can promote diversity. Mountain ranges further offer high-elevation corridors for long-range dispersal (Antonelli et al., 2009; Rahbek, Borregaard, Antonelli, et al., 2019), and such routes have potentially exposed plants to low temperatures prior to the onset of Eocene global cooling (Hawkins, Rueda, Rangel, Field, Diniz-Filho, 2014; Qian, 2017). Because tolerance to freezing appears to be a major driver of the biogeographic distributions of plant lineages (Folk, Siniscalchi, Solits, 2020; Hawkins et al., 2014; Qian, 2017; Segovia et al., 2020; Zanne et al., 2014), understanding the relationship between adaptation to high elevation and adaptation to life in the cold could provide crucial insight into the factors shaping modern plant diversity.
In response to the importance of higher elevation habitat in shaping biodiversity patterns, extensive work has been done to delimit different elevational zones across the globe. In particular, Körner, Paulsen, and Spehn (2011) defined seven life thermal belts based on bioclimatic and topographic characteristics: the nival (perpetual snowline), upper alpine, lower alpine (tree line estimate), upper montane, lower montane, remaining mountain area with frost, and remaining mountain area without frost. These thermal zones account for latitudinal differences in the absolute elevation of alpine and montane habitats (Körner et al., 2011) and provide a biologically meaningful and geographically robust assessment of different elevational zones. Further, these elevational belts allow for comparisons of biodiversity patterns across different mountain ranges at large biogeographic scales (Körner et al. 2011).
This classification of the worlds’ mountainous habitat types provides an opportunity to differentiate the diversity and biogeographic patterns of alpine and montane communities at the regional scale. Despite the importance of understanding high elevation habitats for biodiversity questions, relatively little is known about what distinguishes montane and alpine floras across mountain ranges (Körner, 1995, 2004). Within the alpine belt, especially, plant species may be subjected to conditions at their physiological limits (Körner, 2003), which might not occur at lower elevations. A key question for montane and alpine biologists is therefore to understand how different processes, such as abiotic filtering, dispersal limitation, and historical contingency (e.g., phylogenetic and biogeographic history), have jointly acted in shaping community assembly in these more remote biodiversity hotspots (Hughes and Eastwood, 2006; Elsen and Tingley, 2015; Flantua, O’Dea, Onstein, Giraldo, Hooghiemstra, 2019), and how these processes differ with elevation. Observed changes in species richness (Guo et al. 2013) and turnover (McFadden et al. 2019; Smithers et al. 2020) across elevational gradients further highlight the potentially complex interplay of factors that define and distinguish alpine from montane communities.
While previous work estimating regional American floral diversity (e.g., Graham 1999, 2010; Ullola Ullola et al. 2017; ter Steege et al. 2020) has yielded invaluable insight into this regional species pool, these estimates generally have not provided an assessment of how much usable data currently exists to define specific aspects of this regional pool’s realized niche space. For higher elevation plants, which might be significantly impacted by physiological and ecological limits (Körner, 2003), it could be relevant to know which members of these potential species pools actually do, or could, contribute to montane and alpine communities.
One of the key niche dimensions that determine whether species can establish and survive at different elevations relates to their climatic tolerances. Although numerous climatic variables influence how plants are distributed, temperature and precipitation are often considered among the most important (Clarke and Gaston, 2006; Macarthur, 1972; Whittaker, 1970). Temperature influences processes such as plant growth and metabolic rates (Körner, 2003). Mean annual temperature (MAT), in particular, has shown quantitative correlations with ecologically relevant plant traits (Moles et al., 2014), suggesting it may be an important determinant of ecological strategies. Mean annual precipitation (MAP) is important for its relation to drought tolerance (Craine et al., 2013), and changes in global precipitation and temperature have jointly helped to shape the development of modern American plant biomes, such as the emergence of arid grasslands (Graham, 2011).
In this study, we use a species distribution model (SDM) approach that integrates digitized specimen records and climate data to assemble a large dataset describing the abiotic niches of American seed plants. This dataset provides a detailed look at the contemporary occupied (i.e., realized) niche space of this potential regional species pool in order to address questions of higher elevation community assembly. The Americas are particularly well-suited to this analysis because their mountain ranges run north-south in an almost unbroken line from one pole to another, which might have allowed plants to more easily track favorable climate during cycles of glaciation (Bennett, Tzedakis, Willis, 1991; Rahbek, Borregaard, Antonelli, et al., 2019). Here, we model the distributions of 72,372 American seed plants and use these models to characterize their realized abiotic niche space and climatic niche breadth to ask whether or not species occupying montane or alpine habitats constitute measurably distinct biogeographic and/or climatic species pools across the Americas.