Spring-flowering herbs in North American temperate forests advance their phenology more than trees with warming temperatures

The phenologies of co-occurring trees and spring-blooming understory herbs in northeastern hardwood forests appear to be regulated by different environmental drivers—air temperature and soil temperature/snowpack, respectively. Accordingly, it has been hypothesized that climate change-driven asymmetry in the advancement of canopy leaf-out relative to the timing of understory growth could reduce photosynthetic rates and reproductive success of understory herbs through greater early-season shading.

To determine whether trees and spring-flowering spring-flowering forest herbs are advancing their phenologies at different rates with respect to increasing global temperatures, we examined the phenological responses to warming of 10 species of trees and 11 species of spring-flowering forest herbs (8,045 observations from 965 sites) in northeastern North America using 13 years of data collected by citizen scientists under the auspices of the USA-National Phenology Network.

Contrary to expectation, the timing of leaf-out of spring-flowering forest herbs was more strongly associated with temperature than was timing of tree leaf-out, with a mean response rate of −4.9 days/˚C (95% BCI [−5.2, −4.6]) for spring-flowering forest herbs vs. −3.3 days/℃ (95% BCI [−3.5, −3.1]) for trees. However, the response to temperature was not consistent across the latitudinal range, with spring-flowering forest herbs responding more strongly to warming than trees at middle (40–44˚N) and higher (45–48˚N) latitudes but not at lower latitudes (35–39˚N). 

In contrast to what has been suggested previously, our results suggest that the growing season and carbon uptake of spring-flowering forest herbs could increase as spring temperatures rise. Our study is the first to show spring-flowering forest herbs advancing their phenology at a higher rate than trees with respect to warming.