Historical floras reflect broad shifts in flowering phenology in response to a warming climate

Organisms across the globe are experiencing shifts in phenological events as a result of ongoing climate change. Recently, a variety of novel methods have been applied in order to fill gaps in the phenological data set, in which records often have a patchy temporal, spatial, and/or taxonomic resolution. Here, I tested whether changes in flowering phenology could be detected through the months of flowering stated in 11 guides to the Swedish flora published over a period of 220 yr (1798-2018), focussing on 241 plant species (approximately 8% of the Swedish flora), and accounting for the large increase in herbarium records that have occurred over the same period. Despite the coarse, monthly scale of flowering times reported, historical floras and wildflower guides may hold potential to fill temporal and taxonomic gaps in the plant phenological data set. However, factors other than climate may also influence any apparent phenological shifts over time. Here, flowering was found to start earlier (0.49 d/decade), end later (0.71 d/decade), and carry on longer (1.19 d/decade), with flowering length also associated with increases in the regional temperature anomaly during the 20th century (0.11 months/degrees C). First flowering occurring earlier in 71% of species (14% showing a significant negative trend), 68% of species ceased flowering later (20%), and 80% flowered for longer (29%). Detected phenological shifts also appeared to be related to species' flowering seasonality. Later-flowering species were found to flower later and for longer, while increasing temperatures appeared to drive stronger responses both in flowering onset in early-flowering species and in flowering cessation in later-flowering species. Although potential issues exist regarding the largely unknown ways by which authors have determined flowering times and the coarseness of the data, historical floras may be a useful resource in phenological and climate change research, with the potential to both identify and compare the broad climatic responses of a region's entire flora over long time periods, as well as filling gaps in an otherwise patchy data set.

Automated summary

Organisms around the world are experiencing shifts in phenological events due to climate change. A study in Sweden spanning 220 years found changes in flowering times, with flowers starting earlier, ending later, and lasting longer, potentially influenced by regional temperature increases. Historical floras could be a valuable resource for filling gaps in plant phenological datasets and studying long-term climate responses.