Herbarium records demonstrate changes in flowering phenology associated with climate change over the past century within the Cape Floristic Region, South Africa

Climate change is affecting species composition and diversity across the globe. Phenological changes provide a sensitive indicator of biological responses to changes in climate. Recent studies using herbarium records in Europe and North America have shown changes in flowering time and other phenological events in response to changing climate conditions, such as warming temperatures and chilling winters, but few studies have been carried out in the southern hemisphere. We examined changes in flowering time from 1901–2009 in South Africa in the widespread, diverse genus Pelargonium. We combined records from more than 6,200 herbarium specimens of 129 species with historical weather data on temperature to examine the impact of climate change on flowering phenology. Data from over 464 weather stations in South Africa was used to estimate historical climate conditions for each of the 4,600 geographic sites included in our sample. During this time period there was a 2.9 ± 0.53 °C increase in mean annual temperature across South Africa. Flowering date advanced by nearly two weeks (11.6 days), with nearly all of the advance associated with the increase in temperature during this time. Thus, Pelargonium species are showing similar phenological responses when compared to species in the northern hemisphere. This study adds more evidence to the limited number of studies of climate change responses within Mediterranean climate regions that assess large-scale climate and phenological patterns. It also illustrates that herbarium records provide an effective method for detecting effects of climate change on flowering phenology across large geographic scales.