In 2003 Europe suffered a heat wave. Across the continent trees and plants wilted, and only managed to photosynthesize two thirds of the amount of carbon they normally do. The net result was equivalent to losing four years’ worth of carbon storage. As global warming takes hold, extremes of climate like the 2003 European heat wave are expected to become more frequent, leading to concerns that forests may not be able to absorb as much carbon and climate change could accelerate further.

However, at the moment little is known about how much carbon trees can hold and how they respond to climate extremes. To address this knowledge gap Chuixiang Yi’s research group at City University of New York, US, studied forest flux tower and meteorological data from 27 different forests in Europe, North America and South America, each with at least four years of continuous carbon data. The scientists also used satellite data to assess the impact of spatial changes in forestry on carbon absorption.

The team found a strong link between periods of drought and reduced capacity of trees to absorb carbon. In particular, broadleaf (evergreen and deciduous) trees were very sensitive, whereas coniferous trees were less affected. “Most broadleaf trees are adapted to generally well watered and good nutrient conditions, whereas needles are a specific adaptation that evolved to maximize carbon uptake in water- and temperature-limited regions,” said Yi.

The worst-affected trees were in Mediterranean forests, whereas forests in maritime regions experienced less impact. Mediterranean forests have had a relatively short time to adapt to the dry Mediterranean climate (the last glaciation finished just 15,000 years ago). What’s more, droughts tend to be more severe in the Mediterranean than in maritime regions, meaning the trees are already living at the margin of their ecological niche.

Any kind of extreme weather can have a damaging impact on carbon uptake in forests, but drought is particularly worrying because its impact is long-lasting. “Trees absorb carbon dioxide through small openings on the leaves, called stomata, that can open and close, allowing the passage of carbon dioxide into the circulatory system,” Suhua Wei of City University of New York told environmentalresearchweb. “Carbon dioxide, sunlight and water combine to produce sugars, but open stomata also lose water to the atmosphere during this process. In drought conditions, leaf stomata close to prevent water loss and cavitation of sap-conducting vessels in the trunk. Our results demonstrate that drought reduces carbon dioxide uptake immediately in forests but also in following years.”

From this snapshot of forest data, Yi and colleagues show that climate extremes have reduced global forest photosynthesis by 6.3 Pg of carbon (~5.2% of global gross primary production) per growing season over the period 2001–2010, with evergreen broadleaf forests contributing 52% of this total reduction. The findings are published in Environmental Research Letters (ERL).

Looking ahead the scientists are concerned that more frequent droughts could continue this trend. “We don’t know yet if increasing growth in the boreal [high latitude] region will be able to compensate this effect,” said Leonardo Montagnani of the Free University of Bolzano in Italy.

Related links

Related stories