Tropical forest response to drought depends on age

In most of the tropics, droughts are becoming more frequent and severe as a result of climate change. All trees are not created equal, however. Research published in New Phytologist suggests that tropical forests in Panama get better at coping with drought as they get older.

Mario Bretfeld and colleagues at the Smithsonian Tropical Research Institute used plant water use data to study how tropical forests coped with drought. Their findings are important for understanding how tropical forests will respond to the increasingly severe and frequent droughts predicted by climate change models.

Tropical forest canopy. flickr/cordyceps. CC BY-NC-SA 2.0.

Tracking transpiration

“Droughts can be really hard on tropical forests. Too much heat, low humidity and not enough water can drastically alter which trees survive,” said Jefferson Hall, staff scientist at STRI and director of the Agua Salud project. “Globally, 2016 registered as the warmest year since climate records have been kept. We took advantage of an especially extended drought during the El Niño event in 2015 and 2016 to measure water use in 76 trees representing more than 40 different species in forests of different ages in the Panama Canal watershed. We found that forest age matters.”

The researchers compared responses to drought in 8, 25 and 80-year-old forest patches in the Agua Salud project, a 700-hectare land use experiment in collaboration with the Panama Canal Authority, Panama’s Ministry of the Environment, and other partners.

Water moves from soil into roots, through stems and branches into tree leaves, where some of it is used for photosynthesis. Most of this water is released into the atmosphere – a process called transpiration. Transpiration, or plant water use, can be measured using sap flow sensors in the stem.

A question of access

“Transpiration is regulated by external factors, for example how dry the atmosphere is and how much water is available in the soil, as well as internal factors, such as differences in the structure and function of wood and leaves.” said Mario Bretfeld, lead author of the study and post-doctoral researcher at the University of Wyoming. “Our results indicate that the factors most important for regulation of transpiration in young forests had to do with their ability to access water in the soil, whereas older forests were more affected by atmospheric conditions.”

During the record drought, water use increased significantly in the oldest forests whose expansive root systems supplied trees with water from deep soil layers and allowed for maintenance of transpiration on typically sunny and hot days. Trees in younger forests suffered from a lack of water, probably because their shallower root systems could not access water stored deeper in the ground. In response, trees in younger forests regulated the amount of water they were using during the dry period.

‘species and age matter’

“All trees are not created equal. Their species and age matters. We are working on designing techniques we’re calling Smart Reforestation, making decisions about which tree species to plant to achieve different land-use objectives,” said Hall. “This study is the perfect example of the link between basic and applied science because it highlights the need to consider drought tolerance as we reforest wet, yet drought-prone areas.”

 

This research was made possible with funds from the U.S. National Science Foundation, Stanley Motta, the Silicon Valley Foundation and the Heising-Simons Foundation.

The Smithsonian Tropical Research Institute, headquartered in Panama City, Panama, is a unit of the Smithsonian Institution. The Institute furthers the understanding of tropical biodiversity and its importance to human welfare, trains students to conduct research in the tropics and promotes conservation by increasing public awareness of the beauty and importance of tropical ecosystems.

This is a slightly edited version of an original press release.

Read the paper: Bretfeld, M., Ewers, B.E., Hall, J.S. (2018) Plant water use responses along secondary forest succession during the 2015/2016 El Niño drought in PanamaNew Phytologist. doi: 10.1111/nph.15071

Mike Whitfield (@mgwhitfield)
Development Coordinator
New Phytologist