Low temperature range limits of trees explained by current and past microclimate

Explaining the low temperature range limit of trees, requires in situ information on the actual temperature conditions, the trees’ phenology, and physiological responses. Here, we report the seasonal course of freezing resistance and its response to local temperature in species of Larix and Abies forming treeline (4350-4400 m) in the south-eastern Himalayas, and Quercus and tree-size Rhododendron species, 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. We found that phenology is central for explaining the freezing resistance, the timing of bud burst and leaf unfolding in spring in particular. Larix flushes earliest and exhibits the greatest freezing tolerance, leading to a large safety margin even in the coldest years, while Abies compensates occasional freezing damage in new shoots by a second flush. Thus, freezing damage does not appear to constrain these treeline conifers, and their range limit most likely reflects the general growth limit of the life form tree. In contrast, the evergreen broad-leaved Quercusand Rhododendron experience rare, but critical episodes during which new foliage is lost, indicating historical freezing scenarios and the freezing tolerance most likely the range-setting actors for these species-specific range limits. The results offer a mechanistic explanation of the range limit of these high elevation tree species.