Damaging droughts

Drought-stressed forests in southwestern USA Drought-stressed forests in southwestern USA Leander Anderegg
30 Jul
2015
Drought is having a much more detrimental impact on forest recovery than previously believed, with many trees taking up to four years to return to pre-drought levels of growth

It is generally believed – and certainly assumed by most climate models – that although forest growth around the world is understandably impacted by drought conditions, once a drought is over, a forest will quickly return to normal, healthy growth levels.

However, a new study indicates a significantly different outcome.

‘In most of our current models of ecosystems and climate, drought effects on forests switch on and off like a light,’ says William Anderegg, assistant professor of Biology at the University of Utah, and lead author of the report. ‘When drought conditions go away, the models assume a forest’s recovery is complete and close to immediate. That’s not how the real world works.’

Instead, his study shows that – assuming they survive their respective droughts – most trees take an average of between two and four years after a drought’s end to recover and resume normal growth rates. There are some anomalies, such as around California and the Mediterranean, but otherwise these results have been observed globally.

‘This really matters,’ continues Anderegg, ‘because in the future, droughts are expected to increase in frequency and severity due to climate change. Some forests could be in a race to recover before the next drought strikes.’

Anderegg and his colleagues analysed historical tree ring data from the International Tree Ring Data Bank, with records going back to 1948 and covering 1,338 forest sites (therefore comprising 49,339 ‘site-years’). Measuring the size of tree rings allowed them to calculate wood growth in the years preceeding, during, and following severe drought events.

If forests are not as good at taking up carbon dioxide, this means climate change would speed up

They found that tree growth was around nine per cent less than anticipated during the first year following a drought event, and five per cent less in the second year. These impacts – known as ‘legacy effects’ – were most prevalent in dry, semi-arid conditions.

As Anderegg highlights, this has a significant impact when attempting to model future climactic conditions, since uptake of carbon is a vital role of forests worldwide. Smaller tree rings, indicating a slowdown in wood growth, is also an indicator of a slowdown in carbon sequestration, resulting in less carbon dioxide being removed from the atmosphere than current models tend to predict. Furthermore, the problem is exacerbated by the fact that warmer weather caused by anthropological climate change is likely to create a feedback loop, causing an increase in droughts, and consequently a further slowdown in carbon dioxide uptake.  

‘The fact that temperatures are going up suggests quite strongly that the western regions of the United States are going to have more frequent and more severe droughts, substantially reducing forests’ ability to pull carbon from the atmosphere,’ he explains. ‘If forests are not as good at taking up carbon dioxide, this means climate change would speed up.’ 

This study was co-authored by researchers from Princeton University, Northern Arizona University, University of Nevada-Reno, Pyrenean Institute Of Ecology, University of New Mexico, Arizona State University, U.S. Forest Service Rocky Mountain Research Station, NOAA Geophysical Fluid Dynamics Laboratory, and the Lamont-Doherty Earth Observatory of Columbia University. 

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