Frost tolerance of young leaves: Implications for climate change, forests and the success of invasive shrubs in Thoreau’s Concord
As warming temperatures cause some temperate woody plant species to leaf out earlier, damage by spring frost becomes an increasingly common risk to young leaves. However, it is currently unknown how the frost tolerance of young leaves differs for an entire community of woody plants, and what environmental and life history factors might result in differences in frost tolerance between species. In early spring 2014 we collected dormant twigs of 66 tree, shrub, and vine species— including 41 species from the plant community Thoreau monitored in the 1850’s and we continue to monitor in the field in Concord, MA. We induced leaf out of these twigs in the lab, and then subjected twigs with 3-5 day-old leaves from each species to three frost scenarios (-1°, -3°, -5° C) and a control treatment and then visually assessed damage on a scale from 0-5. We then assessed the relationships between the frost tolerance of each species and first leaf date in the field, functional group (native tree, native shrub, or invasive shrub), leaf water content, leaf out sensitivity to spring temperatures, and USDA cold hardiness zone.
Species varied widely in their frost tolerance to the three treatments, with the moderate frost treatment (-3°) producing the greatest variation. We found that the earliest species to leaf out in spring are the most tolerant to hard frost (-5°). Non-native invasive shrubs and vines are more frost tolerant than native shrubs and vines, with 13% of invasive shrubs and 91% of native shrubs undergoing more than 50% damage in moderate frost (-3°). Species in Concord that are the most sensitive in their leaf out dates to changes in spring temperature are not more frost tolerant than less sensitive species. These results indicate that in Thoreau’s Concord, native shrubs and vines, and species with leaf out times that are highly sensitive to temperature will see increased risk of frost damage with continued changes in spring climate. Leaf water content and cold hardiness zones did not shed light on the mechanisms of frost tolerance in this community of species; future studies on the mechanisms of frost tolerance in young leaves will be important for modeling community and ecosystem responses to increasingly common spring frost events.