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Key roles for the freezing line and disturbance in driving the low plant species richness of temperate regions

Citation

Rana, Suresh K; White, Alexander E; Price, Trevor D (2022), Key roles for the freezing line and disturbance in driving the low plant species richness of temperate regions, Dryad, Dataset, https://doi.org/10.5061/dryad.7sqv9s4t2

Abstract

Aim: At the macroscale, climate strongly correlates with species richness gradients, resulting from differences in in-situ diversification and dispersal. One historical explanation for the pattern is that regions spanning temperate climates contain few species because past disturbances have generated high extinction rates, and species from tropical regions are unable to easily colonize temperate regions. We test these postulates for Himalayan plants, which span subtropical to temperate climates over steep elevational gradients.

Location: Himalaya

Time period: Present day

Major taxa studied: Angiosperms

Methods: We use a comprehensive survey of 31 floras to document the elevational and geographical distributions of native Himalayan plants, augmented by field studies of trees in both the east and west Himalaya. We use grade of membership models to cluster species according to locations shared and phylogenetic analysis to evaluate diversification rates.

Results: Species fall into four cohesive biotas, organized by climate. Points of turnover between biotas occur where the mean minimum temperature of the coldest month is approximately 0oC (2,000 m - 2,500 m), and at the point of occasional annual freezing (1,000 m - 1,500 m); these boundaries run the length of the Himalaya. The patterns are retained when we consider whole clades rather than species. All plants (and the subsets trees, herbs and shrubs) belonging to the biota above the 2,000 m - 2,500 m line have higher recent speciation rates than those lower down.

Main conclusions: We attribute the high rate of recent speciation in temperate climates to high rates of turnover, creating ecological and geographical opportunity. The high elevation biota has few species, but spans the largest area, implying species numbers are far from any carrying capacity, at least with respect to accumulation of allopatric forms. This study thus links climatic restrictions of clades to differences in diversification rates, and by inference species numbers.

Methods

We compiled the elevational and distribution ranges of 8765 species of angiosperms reported from the Himalayas in 31 floras published either for an individual country (Nepal, Bhutan), states, or smaller local regions.

Data S1 shows the list of 8765 species with the lowest and highest elevational limit, mid-point of the elevational range, average values of climatic parameters, presence and elevational ranges within states, climatic affinities, diversification rate and mean family age of the species.

Data S2 represents the field sampling data of trees along an elevational gradient in the western Himalaya showing the number of individual trees of each species and the location of the sampling sites at seven elevational zones.

Data S3 shows the top 100 contributing angiosperm species in each of the four biogeographic regions (Motifs) with additional information diversification rate, mean family age etc. 

Data S4 shows the top contributing tree species in each of the four biogeographic regions (Motifs) with associated information.