Falmouth, Mass. – It has been estimated that the forests of the world take up nearly 15% of all CO2 emissions each year. This percentage comes from a calculus that assumes forests rebound quickly from the extreme droughts that have become more common in a warming world. A new study co-authored by Woods Hole Research Center scientist Christopher Schwalm and published in the journal Science, finds that droughts impede forests ability to grow and absorb carbon much longer than previously understood, and the effects can linger for up to four years following a drought event. This finding suggests that Earth’s forests are capable of storing less carbon than climate models have calculated.
“As future droughts are expected to become both longer and more severe it’s quite possible that some ecosystems, particularly forests, will no longer be able recover in the near future,” says Dr. Schwalm. “This has dramatic implications for forests worldwide, especially for their ability to buffer the impacts of fossil fuel emissions.”
The rate of recovery from drought is largely unknown for the vast majority of tree species. Dr. Schwalm and colleagues carefully measured the recovery of tree stem growth after severe droughts since 1948 at more than 1,300 forest sites around the earth using records from the International Tree Ring Data Bank. Tree rings provide a convenient history of wood growth and track carbon uptake of the ecosystem in which the tree grew.
The researchers found that a few forests showed positive effects, that is, observed growth was higher than predicted after drought, most prominently in parts of California and the Mediterranean region. But in the majority of the world’s forests, trees struggled for years after experiencing drought.
On average, radial growth in tree trunks took 2 to 4 years to return to normal. Growth was about 9 percent slower than expected during the first year of recovery, and remained 5 percent slower in the second year. Long-lasting effects of drought were most prevalent in dry ecosystems, and among pines and tree species with low hydraulic safety margins.
How drought causes such long-lasting harm remains unknown, but the researchers offered three possible answers: Loss of foliage and carbohydrate reserves during drought may impair growth in subsequent years. Pests and diseases may accumulate in drought-stressed trees. Lasting damage to vascular tissues could impair water transport.
While it remains difficult to predict changes in precipitation, the impact of higher temperatures is certain. “Drought, especially the type that matters to forests, is about the balance between precipitation and evaporation. And evaporation is very strongly linked to temperature,” says William R.L. Anderegg the lead author of the study. “The fact that temperatures are going up suggests quite strongly that the western regions of the U.S. are going to have more frequent and more severe droughts.”
Forests’ slower than expected recovery from drought add up to a significant loss of carbon dioxide storage. The authors estimate that it could amount to 3 percent lower carbon storage in semi-arid ecosystems over a century, which amounts to nearly a third of the total energy-related carbon dioxide emissions produced by the U.S. in a year.
State-of-the-art earth system models treat drought as a simple switch. You either have a drought or you don’t. Furthermore, when the drought is over the models assume immediate recovery,” Schwalm says. “This is clearly not how plants work and is especially problematic for estimates of carbon uptake in forested regions where so much carbon cycling occurs.”
This research was funded by the National Science Foundation (NSF) and the National Oceanic and Atmospheric Administration (NOAA) Climate and Global Change Postdoctoral Fellowship program.
WHRC is an independent research institute where scientists investigate the causes and effects of climate change to identify and implement opportunities for conservation, restoration and economic development around the globe.