Climate change can influence phenological responses in interacting organisms that rely on temperature cues, sometimes causing phenological mismatch between species. Most work on phenological mismatches has focused on cases of mutualistic interactions across trophic levels, such as plants and pollinators or migratory birds and insects, but less on phenological mismatches between functional groups within a trophic level. Leaf-out cues in both trees and spring-blooming understory herbaceous perennials are strongly influenced by winter and spring temperatures. Many herbaceous species in deciduous forests leaf out early in spring before overstory trees leaf out – a strategy to take advantage of high light availability before being shaded by tree canopies. If functional groups respond differently, phenological mismatch between forest layers can lead to significant ecological impacts, but this pattern has not been widely tested. Further, it is unknown whether introduced species respond similarly. In this study, we assessed over 2,700 Carnegie Museum of Natural History herbarium specimens (including 5 native tree, 4 native herb, 3 nonnative tree, and 3 nonnative herb species) collected in Pennsylvania from 1838 to 2017. For each specimen, we scored phenophase as leaf-out, flowering, fruiting, vegetative, or senescent. We regressed collection date of specimens scored as leaf-out with historical climate data from the PRISM database.
Results/Conclusions
We found that for every 1°C in spring warming, native trees leaf out, on average, 3.49 days earlier, while native herbs leaf out, on average, 2.96 days earlier. These results support other studies indicating that trees may be more responsive to warming spring temperatures than wildflowers, resulting in earlier tree leaf-out and a shorter window of opportunity for wildflowers to take advantage of early springtime light availability. We also found that native plants are more responsive to warming spring temperatures than nonnative plants, with native plants responding to every 1°C in warming spring temperatures at a rate of about 1 day earlier than nonnative plants. However, comparisons of native and nonnative herbs using herbarium specimens may be confounded by phenological collector bias. While all herbs were most likely to be collected while flowering, native herbs were next most likely to be collected while leafing out (60%, n=1002) and nonnative herbs are next most likely to be collected while fruiting (56%, n=720). These results indicate that herbariums are a promising source for future study of phenological mismatch between plant functional groups, although native-nonnative comparisons may be more difficult to separate from collector bias.