Boreal tree adaptation to seasonal drought conditions

Climate change is bringing shifting rainfall patterns and warmer temperatures to the boreal forest. At the University of Alaska Fairbanks, Jessie Young-Robertson is studying how boreal trees store and regulate water use.

“I want to talk about the drought response of boreal forest trees over a long period of time,” Young-Robertson said. “By drought response, I mean how the trees use water, how stressed they get and how they respond to environmental changes in air temperature and rainfall

Young-Robertson is presenting a paper on the topic at the 2022 American Geophysical Union meeting this week.

The study is focusing on two tree types: coniferous black spruce and deciduous aspen and birch in the Caribou-Poker Creeks Research Watershed near Fairbanks. In summer 2022, sensors measured environmental variables, sap (water) flux and trunk water content every 30 minutes. Canopy water stress measurements were taken weekly.

“The story so far is that we really need a snowpack,” Young-Robertson said. “We’re seeing the impact of earlier springs. We’re seeing the impact of super-hot summers, and we’re seeing the impact in the changing of the packaging of the precipitation.

The “packaging” of precipitation refers to when and how much rain falls during the summer. Rain that just comes in August has a different effect than rain that falls all summer.

Trees grow in June and July. If it doesn’t rain while the trees are growing, that adds stress, she said.

Ongoing work is revealing that the trees are approaching their climatic limit with increased drought pressure.