Native shrubs senesce earlier and faster than non-native shrubs in a temperate deciduous woodland in south-eastern Wisconsin, USA.

Shrubs, both native and non-native, play a pivotal role in temperate deciduous forest dynamics by providing food and habitat for a range of organisms, contributing to nutrient and carbon cycling and particularly in the case of non-native shrubs, extending the growing season. Phenological patterns in shrubs differ from trees, including earlier leaf-out and delayed leaf senescence, which prolongs the photosynthetic active period and enhances carbon uptake. Despite their ecological significance shrubs remain understudied. The objective of this research was to quantify variations in autumn leaf senescence timing and rates between native and non-native shrubs using both in situ observations and chlorophyll meter (SPAD) readings. Phenological observations and SPAD values were recorded twice weekly throughout the autumn season (2018-2024) on 5 native and 4 non-native shrub species in a small (4.5 ha) temperate deciduous woodland fragment on the University of Wisconsin-Milwaukee campus. Results show that non-native shrubs retain green leaves roughly 3-weeks longer than native species. Chlorophyll meter readings support this trend, showing prolonged chlorophyll retention and a more abrupt decline in leaf chlorophyll content in non-native shrubs. These findings improve our understanding of how shrubs shape understory forest phenology by quantifying how the pattern of senescence differs between native and non-native shrubs which in turn will have implications for the timing of understory light availability and supply of organic matter for nutrient recycling. In addition, these findings strengthen our knowledge of the ecological role of native and non-native species in temperate deciduous forest understories.