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Medium-height trees survive drought best

04 Sep 2018
Photo of green leaves against blue sky
(Courtesy: iStock/MilosJokic)

Forests with canopy heights of around 18 metres are more resistant to the effects of severe drought than those with shorter and taller trees, according to researchers in China and the US.

In the past, studies have disagreed on whether forests with lower or higher canopies will be more likely to make it through prolonged spells of hot, dry weather. The discrepancy has made it difficult for forest managers, who need to know which tree heights to encourage to ensure the highest growth and survival rates during extreme drought.

Study leader Peipei Xu at Beijing Normal University in China and her colleagues believe the issue is increasingly pressing. “Climate data indicate that warm areas of land are increasing, and the warmed areas are also drying,” says Chuixiang Yi at the City University of New York, US. “Hot-dry-induced forest mortality poses a significant global concern for the future as carbon dioxide continues to rise and the climate continues to warm.”

Xu, Yi and the rest of the team aimed to quantify the relationships between canopy height, growth and survival rates during drought accurately for the first time. They analysed data gathered during a severe drought in the southwestern US in 2002 that showed the effect on the ring widths of tree trunks, a useful indicator of their yearly growth. In addition, satellite data revealed how the density of vegetation changed over the course of the drought; the team used this to calculate both leaf growth and tree mortality rates.

The results revealed that trunk and leaf growth under drought conditions increased with canopy height for trees shorter than 18 metres but decreased with height for trees taller than 18 metres. “Our results indicate that both high and small trees have relatively low drought resistance,” says Yi.

After establishing these relationships, the researchers could determine the biological mechanisms governing tree growth and survival during drought.

“All organic matter in a tree is formed on the leaves at the top of the tree by photosynthesis,” Yi explains. “Tall trees have a longer water transport path from roots to leaves and [it’s] more difficult to overcome tissue resistance and … gravity, particularly under dry conditions. The roots of small trees are short, and their abilities to access water and nutrient supplies unavailable to the surface soil layer are extremely limited.”

The researchers believe that using their results to inform the active management of canopy structure could safeguard vulnerable forests. As climate models predict hotter, drier droughts becoming more commonplace, this could be essential to combat forest dieback – a phenomenon that will also drive climate change.

“Our findings provide insights into how to manage forests or plant what trees to increase forest drought resistance in facing hot-dry climate conditions to mitigate climate change,” says Yi.

The team reported the findings in Environmental Research Letters (ERL).

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