A YSE-led research team found that seedlings sourced from wet sites performed just as well under drought conditions as seedlings of the same species sourced from dry sites, simplifying seed sourcing for reforestation projects with climate change in mind.
Although tropical rainforests may appear perpetually lush and wet, the majority of them experience dry seasons that can last for weeks or months. With climate change predicted to expose tropical tree species to more intense dry seasons, research scientists are searching for ways to increase tropical forest resilience and preserve rainforest biodiversity.
Restoring tropical forests is one method to mitigate both the effects of climate change and biodiversity loss. While selecting more drought resistant species can enhance the resilience of restored forests, whether seeds of those species should be sourced from drier areas to ensure greater drought resistance has been an open question. In an study published in PNAS, a research team led by Yale School of the Environment Professor of Tropical Forest Ecology Liza Comita reported that, within tree species, seedlings sourced from both wet and dry areas of central Panama performed equally well. That’s good news for restoration experts searching for seeds.
“Tropical forest restoration is a major tool for biodiversity conservation and climate change mitigation, and you want to make sure the seeds you collect are going to thrive under future conditions,” said Comita, who is also co-director of the Yale Center for Natural Carbon Capture. “We found in our study region that it’s possible to collect seeds across a wide range of each species, which increases genetic diversity and resistance to pests, without compromising drought resistance.”
Comita and the research team tested the drought resistance within 16 different tree species in tropical Panamanian forest sites where the dry season typically lasts between three and four months. Their study was conducted in an area between the Pacific and Caribbean coasts where rainfall and drought severity range greatly.
Tropical forest restoration is a major tool for biodiversity conservation and climate change mitigation, and you want to make sure the seeds you collect are going to thrive under future conditions.”
The team harvested seeds from tree species growing across the studied area and compared seedling survival for trees grown under simulated drought conditions or within irrigated plots. They also transplanted seeds from across the rainfall gradient to determine how they performed at forest sites that differed in dry season severity.
Surprisingly, they found the opposite of what they had expected: seedlings sourced from wet sites performed just as well under drought conditions as seedlings of the same species sourced from dry sites.
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“We were initially confused by the results,” Comita said. “Across this gradient you see very different species growing in the wettest sites compared to the driest sites. We expected that, similarly, the severe dry season would select for more drought-resistant individuals within species at the drier sites.”
The researchers found that pollen and seeds are able to traverse across the range of each tree species. Instead of being genetically isolated populations, trees across the site are genetically similar, resulting in similar responses to drought for seedlings of the same species. Nonetheless, in related work led by Andrew Muehleisen '20 PhD, the research team did find evidence of local adaptation to insect herbivore pressure in this study, indicating that within-species variation in this region is more strongly shaped by biological interactions than by the direct effects of climate.
“At sites with larger differences in rainfall across the area, you might see local adaptation to drought at very dry sites,” Comita said. “However, these results are positive in terms of the ability of tropical tree populations to persist under increasing drought stress. We know more about how these tree species will respond to climate change, and restoration specialists will have a wider range of seeds to choose from when preserving Panama’s rainforests.”
