How phenology interacts with frost tolerance in Southeastern Himalayan <i>Rhododendron</i> species

The frost resistance of new foliage and flowers and their relationship with the phenology of leaf-out and flowering are essential for explaining plant species distribution in seasonally cold climates. In this study, we performed a congeneric, elevational comparison of phenology with frost resistance in evergreen Rhododendron species in the Southeastern Himalayas. A comparison of the microclimate with long-term meteorological records of low temperature extremes permitted the calculation of a realistic, long-term margin of safety for 12 Rhododendron species. Surprisingly, frost resistance and phenological events were matching for leaf-out time (not flowering) in higher elevation species only. Flower-leaf sequence (FLS) and frost resistance were linked for species at higher elevation and the earliest flowering species at lower elevation only. Despite a selection of FLS by elevation, flowers (including petals, filaments and ovaries) were still prone to frost damage during the early growing season at both lower and higher elevations, while new leaves were generally safe on long-term scales, regardless of phenology and elevation. In contrast to lower montane elevation, where severe frost is rare in spring, treeline elevation species maintain safety margins over centennial time-scales by adjusting leaf-out phenology. Our data show an evolutionary priority of leaf survival over flower survival. Both, physiological acclimation and phylogenetic components contribute to these adjustments. Rare extreme frost events restrict the upper range limit of the examined Rhododendron species by affecting new foliage. It is essential to know the actual temperature extremes at organ level rather than relying on weather station records.