Low-temperature range limits of trees explained by freezing tolerance, current microclimate and historical extremes

Explaining the low-temperature limit of trees requires in situ information on the actual temperature conditions, the trees' phenology and freezing tolerance. Here, we report the seasonal course of freezing resistance and its response to local temperature in species of Larix and Abies, forming the treeline (4350-4400 m) in the south-eastern Himalayas, and Quercus and tree-size Rhododendron, reaching their upper limit in the immediate neighborhood (4270-4290 m). By linking tree-crown temperatures with on-site air temperature, we can utilize long-term meteorological records to hindcast critical freezing events. Deciduous Larix flushes earliest and exhibits the greatest freezing tolerance and largest safety margin. The evergreen species flush 2 months later and operate near their freezing damage limit. The conifers can compensate for freezing damage by rapid resprouting from small auxiliary buds, not an option for Quercus and Rhododendron with their bigger buds. Freezing damage does not appear to constrain the two treeline conifers. Their range limit most likely reflects the general low-temperature growth limit of the tree life form. Freezing tolerance is most likely controlling the upper limits of the evergreen broad-leaved species. The results offer a mechanistic explanation of the range limit of these high-elevation tree species.