Humidity does not appear to trigger leaf out in woody plants

Posted by Lucy Zipf

Leaf-out phenology in woody trees and shrubs is considered to be strongly controlled by a combination of spring warming, winter chilling requirement, and photoperiod. In 2014 researchers suggested that humidity, rather than temperature itself, might be also be a main trigger of the spring leaf-out of woody plants – with plants leafing out earlier in higher humidity environments. We tested this hypothesis in our recent paper titled “Humidity does not appear to trigger leaf-out in woody plants.”

Gathering dormant twigs from the Hammond Woods in Newton, MA 

We gathered dormant twigs from 15 species of woody plants and exposed twigs from each species to four humidities ranging from 20% to 94% relative humidity. We then monitored the twigs daily for a month to see if twigs in higher humidity treatments leafed out before those exposed to lower humidities.

Twigs of the 15 study species in the highest humidity treatment halfway through the experiment 

The title of the paper gives away our main result: We did not find any consistent, measurable effect of high humidity advancing leaf-out. We also did not see patterns of progressively earlier leaf-out in successively higher humidities. Given our results, humidity does not appear to be a determinant of spring leaf-out in these New England plants.