Taller, older trees fare better during Amazon droughts, study finds

Droughts aren’t usually a problem for rainforests, which, as their name implies, are generally typified by lots of precipitation. But in the past few years, there’s been a noticeable drying trend in parts of the Amazon, one that scientists fear will only intensify as climate change ramps up.

What exactly this means for the Amazon rainforest isn’t completely clear. But a new study sheds some light on this mystery. It finds that droughts will likely be harder on shorter, younger trees than on taller, older ones. But, conversely, smaller trees seem to be better at coping with higher temperatures.

The study, published this week in Nature Geoscience, was conducted by a team of researchers at institutions around the world. They used remote sensing to detect “solar-induced fluorescence,” which is light that’s emitted from chlorophyll and is used by scientists as a proxy measurement of photosynthetic activity. They also compared precipitation levels, air vapor pressure and forest height, age and biomass amounts.

They discovered that photosynthesis in forests comprised of trees that are shorter than 20 meters is three times more sensitive to rainfall inconsistency than forests where trees average 30 meters or more. The researchers think this is because taller trees are older, and older trees have deeper roots that allow them to tap into moister soil.

However, while taller forests may be better at securing water in times of drought, the study indicates they may not be as effective at coping with higher temperatures and drier air. While this doesn’t appear to completely negate their drought-resistance advantage over shorter forests, the researchers say expected temperature increases and droughts may have damaging effects to rainforests regardless of their height and age.

“Our findings suggest that forest height and age are an important regulator of photosynthesis in response to droughts,” said Pierre Gentine, associate professor of earth and environmental engineering at Columbia University. “Although older and taller trees show less sensitivity to precipitation variations (droughts), they are more susceptible to fluctuations in atmospheric heat and aridity, which is going to rise substantially with climate change.”

The researchers say that their results underline that there isn’t a one-size-fits-all response to global warming in rainforests, and that human pressures like deforestation could make it even harder to predict how rainforests will fare in a changing world.

“Our study shows that the Amazon forest is not uniform in response to climate variability and drought,” Gentine said, “and illuminates the gradient of responses observable across Amazonian forests to water stress, droughts, land use/land cover changes, and climate change.”

But, in general, their results indicate that tall, old and dense (read: undisturbed) forests will likely fare better if, as scientists expect, climate change brings more droughts to the Amazon. And if the Amazon does better, the world does better.

“Our study makes it clear that forest height and age directly impact the carbon cycle in the Amazon,” Gentine said. “This is especially significant given the importance of the Amazon rainforest for the global carbon cycle and climate.”

 

Citation:

Giardina, F., Konings, A. G., Kennedy, D., Alemohammad, S. H., Oliveira, R. S., Uriarte, M., & Gentine, P. (2018). Tall Amazonian forests are less sensitive to precipitation variability. Nature Geoscience, 1.

FEEDBACK: Use this form to send a message to the editor of this post. If you want to post a public comment, you can do that at the bottom of the page.