Flickr/Kathleen Tyler Conklin

Shrinking tree rings point to a worrying future for forests and humans, a new study finds.

As kids, we learn that a tree’s rings tell its age: one for every year.

But scientists also know that tree rings are a ledger for climate data. Their width speaks to temperatures surrounding the tree, the abundance of light, average rainfall and ground moisture. During wet years, rings tend to be wider. In dry times, they tend to shrink, stunting the tree’s overall growth. And according to a new paper in Nature, rings also tell us that most trees don’t immediately bounce back from periods of drought—a fact that may one day seriously compromise forests’ capacity to store carbon.

With a team of scientists, lead author William R.L. Anderegg, a NOAA Climate and Global Change Research Fellow at Princeton University, studied the ring chronologies of groups of trees from 1,338 sites across the globe, plucked from the International Tree Ring Data Bank. The researchers looked only at rings formed between 1948 and 2008, which enabled them to study tree growth alongside reliable, independent climate data. The rings were then matched up to known periods of severe drought in the trees’ respective locales.

Anderegg and colleagues found that, contrary to longstanding assumptions, drought has significant “legacy effects” on tree growth. Not only did rings generally shrink during dry spells, but they also showed reduced growth afterwards. It took one to four years for the trees to completely recover to pre-drought conditions.

There was variation here: Drier ecosystems, like those in the presently drought-locked American Southwest, were particularly vulnerable to these legacy effects. Because of physiological differences in how they take up water, pines (and gymnosperms in general) were hit harder and took longer to bounce back from drought than oaks (and other angiosperms).

What about urban trees? Anderegg says although trees in cities may get watered by humans, they likely feel these legacy effects just as much as their forest-dwelling peers.

Drought-stressed forests in the southwestern United States. (Leander Anderegg)

So long as these trees eventually bounce back, no big deal, right? Not quite. Forests are carbon sinks, absorbing about one-quarter of our emissions every year. “It’s an incredible contribution to society,” says Anderegg. But climate change is driving more potent, frequent droughts the world over. When a tree’s growth is impacted by drought, its capacity to store carbon also lessens. The authors write:

For example, a simple conservative estimate based on forests in the southwestern United States revealed that legacy effects could lead to 3% lower carbon storage in semi-arid ecosystems over a century, equivalent to 1.6 metric gigatons of carbon when considering all semi-arid ecosystems across the globe.

For context, 1.6 metric gigatons is roughly one-third of how much carbon the U.S. emitted from energy production in 2011. That’s pretty significant. Plus, drought can also kill trees, driving them to become carbon sources, rather than carbon sinks.

“That’s where it can get into a vicious cycle,” says Anderegg, “because that speeds up climate change, and kills off more forests.”

Could some trees adapt to a drier climate? Anderegg says science doesn’t have a clear answer yet, though researchers in the American West are working to determine which species will fare better than others during the current drought. However, drought is always relative to what an ecosystem was like before—it’s not easy to make generalizations.

This is a scary, forest-less future we’re talking about. It is also avoidable. The future of these forests rests in our hands,” says Anderegg. “The climate futures that we control with our greenhouse gas emissions determine the risks these forests face. The more effectively and sooner we tackle those challenges, the lower the risks will be.”

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