Vulnerability of phenological progressions over season and elevation to climate change: Rhododendrons of Mt. Yulong

Seasonal timing (phenology) of reproduction is a critical dimension of life-history, affecting ecological and evolutionary processes including individual fitness, community interactions, species boundaries and climate change adaptation. Staggered phenological sequences, or progressions, of flowering in plants have long been a topic of interest. Less well studied are multi-dimensional progressions across seasonal time and elevational space, which may be especially vital to understanding montane and alpine environments that are among the ecosystems most vulnerable to climate change. To further our understanding of how phenological progressions are structured and to predict how they may respond to climate change, we collected data from an assemblage of ten co-occurring Himalayan Rhododendron species on Mt. Yulong, China, through two years of phenology monitoring in transects spanning a 1400 m elevation gradient, greenhouse experiments, and in comparison with the results of long-term models of species phenological responses to temperature derived from herbarium specimens. We asked whether we could quantitatively support flowering as a multi-dimensional progression in Mt. Yulong Rhododendron species, whether species that are part of this progression show differential phenological responses to changes in temperature, and how these responses impact reproductive success. We found evidence for a progression of flowering, with Rhododendron species significantly overdispersed in elevation and flowering time and showing significantly less inter-species overlap in flowering time-space niche (2.9%) than expected by chance (8.5%). As a whole, the progression responded to changes in weather (−2.4 days / °C) and to experimentally increased greenhouse temperatures (−9.3 days / °C). However, individual species responses varied in their response (from −26 – 2 days / °C). Models derived from historical herbarium specimens predicted contemporary observed flowering well (>95% of plants flowering within prediction intervals) and showed corresponding species differences. Reproductive output was affected by phenology, with the quantities of flowers and fruits greater in plants which flowered slightly earlier than their population mean (flowers p < 0.05; fruits p < 0.01), and in plants that responded to warmer weather with commensurately earlier flowering (flowers p < 0.01; fruits p < 0.001). The elaborately sequenced progression of flowering over season and elevation in Himalayan Rhododendron highlights the intricacy of species assemblages in time and space. Varying phenological responses among species and the associated reproductive impacts make this progression, like other staggered phenological sequences, vulnerable to disruption with ongoing climate change.