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

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.