Tall trees in tropical forests are less vulnerable to drought

Photos from within the Amazon Rainforest in Tena, Ecuador. Credit: Jay, Flickr.

Photos from within the Amazon Rainforest in Tena, Ecuador. Credit: Jay, Flickr.

The Amazon rainforest is a huge carbon sink that, according to a 2017 study, absorbs as much carbon as all nine Amazon nations emit. Another study, however, discouragingly found that the amount of carbon the Amazon rainforest is able to absorb has declined by a third over the last decade. This decline is equivalent to over twice the UK’s annual emissions. This is partly due to the fact that the rainforest has reached a saturation point for how much carbon it can soak up, and partly due to droughts. But at least there’s some good news — according to a new study published today in Nature, taller trees in tropical forests are more resilient to droughts.

Not all trees convert solar radiation into chemical energy in the same way. Pierre Gentine, a researcher at Columbia University, New York, along with colleagues, found there is a great deal of variation in how tropical trees do photosynthesis. For instance, spatial and temporal patterns of photosynthesis in the Amazon are regulated by complex interactions between the climate, soils, nutrients and biotic factors. One other important factor that impacts photosynthesis seems to be height.

In order to understand how height affects photosynthesis in the Amazon forest, Gentine and colleagues analyzed remote sensing measurements of sun-induced fluorescence — an indicator of photosynthesis performance — as well as biophysical variables sourced from in-field and satellite measurements. The team found that taller forests are less sensitive to variations in precipitation than shorter ones.

Specifically, trees above 30 meters are three times less sensitive to drought than short trees under 20 meters. The scientists think this happens because taller trees have more extensive root systems that allow them to reach deep soil moisture during dry seasons.

However, taller forests are more vulnerable to atmospheric aridity. Leaves on tall trees constantly have a lower water content, making their photosynthesis not only more adaptable to soil drought but also more sensitive to fluctuations in atmospheric water.

The study’s results will prove useful in modeling climate change and performing conservation efforts aimed at mitigating climate change.

“[The findings] mean that the diversity in tropical forests is important to predict the response to droughts. We currently don’t represent this diversity in traits,” Gentine told ZME Science.

“We are now trying to see how forests are responding to droughts across the tropics,” he added.

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