Phylogenetic conservatism in heat requirement of leaf-out phenology, rather than temperature sensitivity, in Tibetan Plateau

Abstract

Phylogenetic studies on the leaf unfolding date are expected to improve understanding of phenological change. However, it remains unclear whether the response of leaf unfolding date to temperature is phylogenetically conserved among close relatives across differential and harsh habitats. Meanwhile, the phenological mechanism underlying the role of phylogenetic relationship on plant phenology remains unclear. We used Blomberg's K to assess phylogenetic signal in leaf unfolding date, its sensitivities to daily mean, minimum and maximum temperatures, in growing-degree-days and average level of each of daily mean, minimum and maximum temperatures experienced by plants over a given period prior to leaf unfolding date for eighteen species at seventeen sites in the Tibetan Plateau. We showed that leaf unfolding date and its sensitivities to daily mean, minimum and maximum temperatures did not exhibit overall significant phylogenetic signals. Moreover, the sensitivity of leaf unfolding date to daily minimum temperature exhibited phylogenetic antisignal at species level, which indicated the trait difference among close relatives exceed that among distantly related lineages. However, there were overall significant phylogenetic signals in growing-degree-days and the average level of each of daily mean, minimum and maximum temperatures experienced by plants before leaf unfolding date. The signals were stronger for the growing-degree-days and the average level of each of daily mean, minimum and maximum temperatures in the period closer to leaf unfolding date. Our results imply that close relatives trace similar heat requirements for leaf unfolding date in the harsh and spatially heterogeneous environment of the Tibetan Plateau. However, the time when these lineage-specific heat requirements are met varies spatially depending on local conditions, thereby resulting in phylogenetic non-conservatism of leaf unfolding date and its temperature sensitivities. This provides a phenological explanation, for the first time, for phylogenetic non-conservatism in plant leafing phenology.