Xylem vessel traits predict the leaf phenology of native and non-native understorey species of temperate deciduous forests.
Non-native understorey woody species have been shown to extend leaf display and inhabit vacant phenological niches in early spring and late autumn when growing with native counterparts in temperate deciduous forests across the world. Despite the potential competitive advantages, extended leaf duration also subjects non-native species to possible hydraulic risks associated with maintaining leaves during periods of increased frost probability. It remains unclear how non-native species are able to maintain xylem function within this context. Leaf phenology in temperate deciduous trees has been shown to be a function of xylem anatomy, with earlier bud break associated with smaller xylem vessels due to the presumed resistance of smaller vessels to freezing-induced cavitation. We examined relationships between leaf phenology and xylem vessel traits across 82 native and non-native understorey deciduous woody species common to eastern U.S. deciduous forests. We hypothesized that non-native species possess xylem vessel traits associated with maximum hydraulic safety during frost-prone spring and autumn leaf display without compromising rapid growth rate. Larger metaxylem vessels in non-native species were associated with both faster spring growth and delayed autumn leaf fall compared to native species. Non-native species also had smaller latewood vessel diameter, latewood vessel area percentage and a higher proportion of solitary vessels in the entire secondary xylem cross section compared to natives, potentially increasing their resistance to freezing- and/or drought-induced cavitation in autumn, thus allowing for delayed autumn leaf fall. Native and non-native species exhibited similar dates of spring bud break and leaf emergence, consistent with similar xylem vessel size and vessel area percentage within metaxylem and earlywood. Within both groups, species with earlier bud and leaf emergence had a higher total percentage of vessel area within metaxylem and earlywood. This suggests understorey species need sufficient water to support their early spring growth at the risk of freezing-induced cavitation. Our study suggests xylem vessel properties, along with cross-sectional spatial xylem vessel distribution, reflect the capacity of non-native plants to thrive in a new environment and deepen our understanding of the physiological mechanisms of successful invasions of non-native understorey woody plant species.