Climatically-Similar Non-Alpine (CSNA) species.
Among non-alpine species, the subset whose ranges occupied similar
temperature, precipitation, and niche-breadth space as those found in
the alpine belt (CSNA species, as defined above) provided a possible
first approximation of which non-alpine American seed plant species
might otherwise be able to inhabit the alpine belt and help address a
key question of whether abiotic factors or historical processes (e.g.,
phylogenetic and biogeographic history) plays a larger role in alpine
community assembly (e.g., Hughes and Eastwood, 2006).
CSNA species were distributed broadly across the Americas (Fig. 2d), but
concentrated in the southern US and in Mexico, and tended to have ranges
with a greater proportion in frost-exposed foothills and lower montane
habitat (Fig. 6d). This might suggest that ecological and physiological
factors, rather than general dispersal limitation, constrains the
ability of these species to enter the alpine belt. It is particularly
interesting to note that CSNA species had their greatest richness in the
same areas where alpine species richness was lowest, namely in and
around Mexican mountains (Fig. 2c,d, 6e). This might suggest a role for
ecological factors, such as competitive dynamics, in separating the
distributions of these species along elevation, however finer scale
studies are needed to specifically address such hypotheses since
macro-scale analyses capture mostly biogeographical processes (Webb,
Ackerly, McPeek, Donoghue, 2002).
Apart from ecological interactions, physiological limitations could also
contribute to why these CSNA species have not entered the alpine belt.
Alpine habitats are often at the physiological limits of what plants can
tolerate (Körner, 2003). This strong abiotic filter might impose
significant challenges and therefore prevent a larger number of lineages
from entering the alpine environment. Only ~28% of CSNA
genera overlapped with those of actual alpine species, and ten taxonomic
orders differed between these groups (Table 1). Such disparity in the
taxonomic composition of these groups might be consistent with strong
abiotic filtering that only approximately ¼ of CSNA genera have been
able to overcome at this time.
Although the ‘competition-filtering’ dichotomy presents a reasonable
framework with which to form testable hypotheses concerning the
separation of alpine and CSNA species, these are not the only possible
explanations for why certain species have not entered the alpine belt.
As noted above for montane communities, soil conditions, and pathogen or
dispersal-agent distributions, as well as changes to growth form
(Sklenář, Kučerová, Macková, Romoleroux, 2016) are all additional,
non-mutually exclusive factors that could differ between these groups
and their ranges. Another interesting, but seemingly untested,
possibility is that the American alpine belt is at its current carrying
capacity.
Conclusions .
Higher elevations provide substantial topographic and climatic
heterogeneity that can help promote and maintain biodiversity. By
assembling a large macroecological dataset modelling the ranges of over
70,000 American seed plants, we were able to provide a detailed
investigation of the realized abiotic niche space of this species pool
and characterize climatic niches for these species in a biologically
meaningful manner. Our approach also allowed us to separate distinct and
biologically relevant groups, such as alpine specialists from
generalists, or climatically similar species that seem otherwise capable
of inhabiting the alpine belt. We found that alpine, but not montane,
communities formed a climatically distinct species pool across the
Americas. These results present a detailed assessment of the current
state of knowledge on the distribution of American seed plants, which
would be especially enhanced by greater sampling of range-limited alpine
endemics, and underscores the importance of understanding regional-scale
diversity patterns in relation to climate and elevation.