Warmer temperatures are linked to widespread phenological mismatch among native and non-native forest plants

Deciduous trees, shrubs and forest wildflowers may be advancing their leaf-out phenology at different rates in response to a warming climate. A mismatch between understory and overstory phenology may lead to a reduction of understory light levels in the early spring, which is a critical period when many spring-blooming wildflowers achieve highest photosynthetic rates. However, the extent of this phenomenon beyond a single site or region is largely unknown. Using 3083 herbarium specimens collected between 1870 and 2019 across eastern North America, we assessed leaf-out and flowering times of 10 tree species (6 native, 4 non-native), 4 shrub species (2 native, 2 non-native) and 7 wildflower species (6 native, 1 non-native). We paired phenological data with historical climate data to quantify differences in phenological sensitivity to spring temperature across canopy strata, across species' geographical ranges and between native and non-native species. Predicted phenological mismatches between native trees and wildflowers differed across large spatial scales, with wildflower populations in warmer regions of North America more likely to be affected. Overall, native tree species leafed out 3.6 days earlier per degrees C spring warming, while native wildflower species advanced their flowering times by 3.2 days per degrees C, resulting in phenological mismatch as wildflowers experience fewer days before tree leaf-out at warmer temperatures. Native trees and wildflowers in the warmer, southern part of their ranges advanced their spring phenology 2 and 1.5 times faster, respectively, than those in colder, northern locations. The phenological sensitivity of non-native plants was less variable across their ranges. Non-native trees and shrubs exhibited greater phenological sensitivity than native wildflowers. Notably, phenological sensitivities differed substantially among wildflower species, suggesting that certain species are more likely to be affected by phenological mismatch as climate warming progresses. Synthesis: Our results provide new insight into novel phenological responses within and among species across a wide geographical range and the potential impact of competition and interactions with non-native invasive species. This research highlights the value of newly-available digitized museum collections in phenological research to cover longer time periods, wider spatial areas and a greater diversity of species than otherwise possible.