Compound Droughts Hit Plant Growth Far Harder Than Expected

A drooping plant often appears to need a drink, and in most cases it does: its roots are surrounded by soil that has become dry and compacted. That familiar image has shaped both our understanding of drought and the way researchers quantify it.

Yet plants can dehydrate even when the ground remains moist, because the atmosphere can itself be drought-stricken. New research shows that when soil and air become dry at the same time, the harm to plant growth is much greater than scientists had anticipated.

Two droughts at once

Scientists distinguish between two forms of dryness that affect plants. The first is a shortage of water in the soil, the moisture on which roots depend.

The second is atmospheric drought, which occurs when air becomes dry enough to draw water directly from leaves.

These conditions generally occur separately, but they may also coincide. Researchers describe this overlap as a compound drought.

A research group headed by Professor Chiyuan Miao, a hydroclimate scientist at Beijing Normal University (BNU), investigated the worldwide effects of that combination on plant life.

Before this research, the relative impact of the two drought types remained unclear: difficult to compare and even more difficult to total.

Previous studies had largely assessed dry soil and dry air independently, meaning their combined effects were not well understood. Miao’s team sought to establish a single global assessment.

The combined toll of compound drought

When the researchers compared all three scenarios, compound drought stood out clearly. One compound-drought event reduced plant growth by nearly four times as much as dry air or dry soil alone.

Individually, dry air and dry soil caused unexpectedly similar levels of damage, with each removing a modest share of growth. However, when they occurred together, the losses did not merely accumulate. They intensified.

This difference was the missing element that the field had not previously quantified. Scientists had long believed dry air and dry soil could amplify one another, but had not measured the scale of that interaction.

Another study had already cautioned that the resulting loss of carbon was being substantially underestimated.

Decades of satellite data

To identify this trend, the team used satellite observations from 1982 to 2018. These records measure the amount of carbon plants absorb through photosynthesis from season to season.

Using satellites enabled the researchers to observe vegetation across the entire globe, rather than relying on a limited number of field sites.

The lengthy dataset allowed them to identify individual droughts as they developed and ended, before monitoring the response of plant life.

Every event was classified according to its type and severity, then compared with the subsequent decline in growth. Analysis at this scale reduced the influence of small local variations and produced a clearer global view.

Forests react differently

Forests did not all respond in the same manner. Broad-leaved forests, which have wide, flat leaves, experienced the greatest effects from both compound droughts and dry air alone.

Needle-leaved forests, including pines and spruces, showed another pattern. They were affected most severely when soil dried out and proved considerably more resilient to dry air.

The difference is probably related to leaf structure. Broad leaves lose water rapidly as the air dries, whereas needles retain moisture more effectively and depend more heavily on the water their roots can still access.

Heat and rainfall drivers

Weather conditions underpinned all three forms of drought. Temperature and rainfall were the main influences on when and where dryness occurred, as well as on the severity of the resulting decline in plant growth.

Losses caused by dry air were closely associated with heat, since warmer air generally extracts more moisture from plants. The factors driving soil dryness were more complex and differed between regions.

This relationship with heat is not an isolated finding. Other research has shown that as the climate warms, the atmosphere’s capacity to draw moisture from plants becomes even stronger, reinforcing the air-driven losses measured by the team.

Slow to bounce back

The damage continued after rainfall returned. In more than 60% of the regions examined, plants took longer to recover from compound drought than from either type of drought alone.

In many cases, complete recovery did not occur. Before the next period of stress began, numerous areas had regained only some of their lost growth, creating a persistent burden on the landscape.

Locations that have experienced both forms of dryness simultaneously demonstrate this effect clearly.

An assessment of a severe dry period in south-western China during 2023 linked record declines in plant growth to precisely this form of combined stress.

What the findings change

The new element is the scale of the difference. For the first time at a global level, this work quantifies how severely dry air and dry soil reinforce each other, with an impact far beyond that of either condition on its own.

Every year, the world’s plants remove a substantial proportion of human carbon emissions from the atmosphere, slowing the pace at which the planet warms.

If compound droughts reduce that carbon uptake more quickly than existing models assume, projections for future warming may be too low.

The findings give scientists a more precise focus. Climate models can now account for the greater damage caused by overlapping droughts.

Those responsible for managing forests and farmland can also prepare for a prolonged recovery that may never be complete without intervention.