Study Shows That Drought Harms Wetter Forests Most

By Art Kabelowsky, DNR Forest Health Communications, Fitchburg;
Arthur.Kabelowsky@wisconsin.gov or 608-3335-0167

The way three California scientists see it, 6.6 million tree rings can’t be wrong.

Their study was published in Science magazine in December 2023 under the title “Drought sensitivity in mesic forests heightens their vulnerability to climate change.”

The scientists studied the annual growth levels revealed by tree rings of 122 species, drawn from 3,775 sites, “to assess trees’ sensitivity to water and energy availability.”

The cover of the Dec. 8, 2023, issue of Science magazine.

The cover of the Dec. 8, 2023, issue of Science magazine.

The study was conducted by Robert Heilmayr and Joan Dudney of the University of California, Santa Barbara and Frances C. Moore of the University of California, Davis.

One of the scientists’ key findings is notable for Wisconsin foresters and land managers. They found that trees that grow in wetter portions of their range wind up showing a greater sensitivity to drought. So, forests that are accustomed to greater availability of water (mesic forests) will suffer more if hit by drought than will their counterparts that are more used to dry (xeric) conditions.

“Basically, you can’t drought a barrens jack pine when every day is a drought, but a small shortage of rain for a sugar maple accustomed to ample water can be a crisis,” summarized DNR research scientist Bob Smail, Ph.D.

However, the scientists also found that drought impacts are reduced in cooler climates, such as Wisconsin’s, because overall potential evaporation rates are lower.

But drought here can be a one-two punch because many Wisconsin forests benefit from a groundwater subsidy. Here, a short-term meteorological drought – coupled with declining groundwater levels – could prove particularly impactful to affected trees.

Among the six steps the scientists used to analyze their data were ring width index measurements and the variables potential evapotranspiration and climatic water deficit. That data was compared with local historical data to contribute to their findings.

One of the findings showed that in the coldest 10% of sites, growth increased from 8.7% to 25.1% in response to an increase in potential evapotranspiration – however, the growth response quickly declined across a gradient of historic potential evapotranspiration.

The study states that “(c)limate change is shifting the structure and function of global forests, underscoring the critical need to predict which forests are most vulnerable to a hotter and drier future.”

The study predicted tree growth through the year 2100, predicting that in arid regions, drought adaptation will partially buffer trees against climate change – although trees growing in the wetter and hotter areas of their climatic range may show unexpectedly large adverse impacts under climate change.

The scientists predicted that the largest growth declines are predicted for wet and hot regions: “A hotter, drier planet will likely lead to major shifts in the health of global forests and their ability to sequester carbon.”

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