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.