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.