Forests in a Time of Drought

Fig.1. Trees like this Pinus ponderosa will struggle to recover from droughts (Credit: Thayne Tuason via Flickr)

In the past few decades, the world’s forests have absorbed up to 30 percent of annual anthropogenic CO2 emissions. Forests act as carbon sinks, or reservoirs that accumulate and store carbon. Unlike humans who release carbon dioxide into the atmosphere - either through breathing or fossil fuels - trees take in this compound from the atmosphere during photosynthesis to use for energy and growth. The long-term storage of carbon in trunks, branches, litter, or soil is known as carbon sequestration.

This process means that forests can act to offset increased amounts of atmospheric CO2 and, therefore, mitigate climate change fueled by greenhouse gases. Such ecosystems, however, are sensitive to climate extremes, and changes in temperature and precipitation could have major effects on the ability of the forest to store carbon. A recent study published in the journal Science suggests that the increased risk for drought due to climate change could have disastrous effects on trees, both during and after the dry spell.


Fig.2. The weathered growth rings (tree-rings) from a cross section through a tree felled around 1111 CE used for a building complex in archaeological ruins. The distance between the rings shows amount of growth in one year (Credit: Michael Gäbler, 1978)

Between Two Rings

William Anderegg, a researcher at Princeton University, and his colleagues examined tree-ring data from the International Tree-Ring Data Bank to understand how trees grew during dry conditions. Tree-ring (or growth ring) widths from trees around the world were compared to known temperature and precipitation measurements from 1948 to 2008. They found that layers between tree-rings, which show annual growth, were much thinner during drought years. This lack of growth was expected, but the significant drop in growth even when conditions returned to normal was not.

Surprisingly, trees experience tissue damage both during and in the years immediately following a drought. This phenomenon, called drought legacy, was seen the most in trees growing in arid climates. Even more worrisome was the comparison between climate warming models and measured growth rates. Sophisticated models used by Anderegg and his colleagues failed to predict a lack of plant growth after a drought. The disconnect between the models and reality indicate that scientists need to rethink how forests can mitigate climate change.

Written in Wood

These data act as a good proxy for past climate change, and tree-ring collections are among the main types of environmental records. Anderegg et al. used recent records, but repositories, such as the Laboratory of Tree Ring Research (LTRR) at the University of Arizona, contain wood specimens that document the past 9,000 years. Such specimens have been used to study as wide ranging subjects as paleoclimatology to the Black Death. Although the physiological mechanisms of drought response and long-term effects are still unknown, the evidence written in tree rings is clear. Climate change mitigation must take into account how forests respond to increasingly extreme weather and their ability to store carbon. It would seem that trees are now man’s best friend.




References:
Anderegg, W. R. L., et al. (2015, July 31). Pervasive drought legacies in forest ecosystems and their implications for carbon cycle models. Science, 349 (6247): 528-532. doi: 10.1126/science.aab1833

Bellassen, V., & Luyssaert, S. (2014, February 13). Carbon sequestration: Managing forests in uncertain times. Nature, 506: 153-155. Retrieved from: http://www.nature.com/polopoly_fs/1.14687!/menu/main/topColumns/topLeftColumn/pdf/506153a.pdf

Glossary

carbon sinks
A reservoir that accumulates and stores carbon. In forests, carbon is stored in leaves, trunks, and other plant matter, as well as in litter and soil. Carbon is released through decomposition or burning.
photosynthesis
The process by which plants and other organisms convert light energy into chemical energy for fuel. Carbon dioxide is a necessary compound that is taken from the atmosphere during this process.
carbon sequestration
The process by which a carbon sink removes CO2 from the atmosphere.
drought legacy
The departure of observed tree growth (ring-width index) from expected growth (based on the relationship between the tree’s known growth rate and the climate) in the period after a drought (Anderegg et al. 2015)