• Large-scale satellite data has shown that while large trees expand their crown during the dry season, small trees drop leaves – possibly due to limited light availability in the shaded understory. A new study finds that tree response to dry weather is far more complex, influenced by exposure to the sun and root depth.
  • Detailed measurements of leaf growth and leaf loss during the annual dry season and extreme drought events shows that small trees respond differently to water deprivation depending on their surrounding environment – shaded trees gain leaves but exposed trees tend to lose them, a possible sign of dehydration stress.
  • Two novel study approaches revealed a complex pattern of leaf growth and loss in response to dry weather: ground-based lidar imaging that produced high-resolution 2D image slices of forest structure, and statistical division of data based on an understory tree’s distance from the canopy top, rather than from the ground up.
  • Losing leaves could spell death for individual trees, but these small-scale changes can also impact transpiration and have consequences for regional weather patterns and regional climate change. Also, importantly, degraded forests, with many open clearings, could be less resilient to worsening Amazon drought.

Small trees in the Amazon rainforest understory are more vulnerable to drought than their larger counterparts, but their fate depends on their local environment, according to a study published in New Phytologist.

Marielle Smith from Michigan State University and an international team of researchers used hand-held lidar to complete monthly surveys of the surface area of leaves at different heights in Tapajós National Forest in Pará state in the Brazil Amazon between 2010 and 2017 to obtain their results. The portable lidar instrument uses a laser to map the leaves in the forest canopy in two-dimensional slices up through the forest structure.

Across the whole forest, they found that trees in the upper canopy tended to gain leaves during the dry season and lose them again in the wet season, whereas trees in the lower canopy showed the opposite behavior. This opposing trend between the upper and lower canopies matches the results of a previous satellite-based study of seasonal changes in leaf area, and is thought to be due to limited light availability in the lower canopy.

However, a closer look at the patterns of leaf growth and loss in different micro-environments within the forest ecosystem revealed a more complex pattern.

“What was exciting for us was that because we’re using a ground-based lidar system, the resolution of our data is very high and so we were able to break the lower canopy into two parts,” said Smith: small trees in the lower canopy shaded by larger trees, and small trees in forest clearings exposed directly to the sun. The researchers found that it was actually the small trees exposed to high sunlight levels in forest openings that lost leaves during dry periods, while shaded trees added to their leaf area in synchrony with the larger canopy trees above them.