Climate-driven change in Himalayan Rhododendron phenology

Phenology – the seasonal timing of life-history events – is a critical dimension of natural history. It is also one of the earliest and most noticeable traits by which organisms respond to climate change. However, these responses are complex, and only beginning to be understood, especially in the montane and alpine environments that are among the ecosystems most vulnerable to climate change. Drawing from diverse data sets and employing multiple methodologies, I examined how climate affects phenology in Himalayan Rhododendron spp. Comprehensively monitoring flowering phenology over gradients of season and elevation on Mt. Yulong, China – home to a diverse assemblage of Rhododendron spp. that is ecologically and culturally salient – I showed that the species have an elaborate sequenced progression of flowering over season and elevation. Both phenology and the responses to annual temperatures directly impact reproductive success, making this progression vulnerable to the effects of future climate change. A community phylogeny suggests this flowering progression experiences constraints from shared evolutionary history and morphology: the last flowering day of each species shows phylogenetic signal, and is associated with flower size and fruit size. This pattern may be driven by the limited time for fruit development before the onset of cold temperatures in autumn, underscoring the asymmetric pressures on phenology at seasonal extremes. To gain a longer-term perspective, I contextualized Rhododendron phenological progressions within a 125 year proxy record of phenology based on herbarium collections, demonstrating that although increased annual temperatures are associated with earlier flowering, increased fall temperatures are associated with delayed flowering. These contrasting effects have resulted in opposing changes in flowering time, even during rapid recent anthropogenic warming. Finally, interviews with Mt. Yulong villagers revealed a rich knowledge base, with local understandings of changes in and drivers of Rhododendron phenology paralleling the results from ecological studies. In combination, these studies reveal the complexity of phenological responses to climate change. The multiple methodologies allowed a deeper exploration than simple ‘earlier spring’ models of phenological response. Each illuminated distinct details, while agreement among diverse data sources increased confidence in both the methodologies and the conclusions