Herbarium records reveal early flowering in response to warming in the southern hemisphere

Abstract

Premise of the StudyHerbarium specimens are increasingly used as records of plant flowering phenology, which has advanced for many species in response to climate change. However, most herbarium-based studies on plant phenology focus on taxa from temperate parts of the northern hemisphere. Here, we explore flowering phenologic responses to climate in a temperate/subtropical plant genus Protea (Proteaceae), an iconic group of woody plants with year-round flowering phenology and endemic to sub-Saharan Africa. Protea is widely used in horticulture and is a flagship genus for the flora of the hyperdiverse Cape Floristic Region.\n\nMethodsWe used a database of 2154 herbarium records of 25 Protea species to explore patterns in flowering spanning the past 100 years. We used a circular sliding window analysis to characterize phenological patterns in these aseasonal species, plus a novel linear mixed effects model formulation to test how both site-to-site and year-to-year variation in temperature and precipitation affect flowering date across species.\n\nResultsBoth warmer sites and warmer years were associated with earlier flowering of 3-5 days/{degrees}C. In general, the timing of peak flowering was influenced more strongly by temperature than precipitation. Although species vary widely in when they flower during the year, their phenological responses to temperature are phylogenetically conserved, with closely related species tending to shift flowering time similarly with increasing temperature.\n\nDiscussionTogether, our results point to climate-responsive phenology for this important plant genus. Our results indicate that the subtropical, aseasonally-flowering genus Protea has temperature-driven flowering phenologic responses that are remarkably similar in magnitude to those of better-studied northern temperate plant species, suggesting a generality across biomes that has not been described elsewhere.