Forest pathogens limit species’ distributions, boost biodiversity
STRI/DICYT The forests on the Caribbean and Pacific sides of the Panamanian isthmus are separated by a mere 60 kilometers but are strikingly different in plant species composition. This has much to do with rainfall — precipitation on the Cari-bbean slope averages 4 meters per year, more than twice what falls on the Pacific side. Tree species have evolved or adapted accordingly. While it’s intuitively clear why wet-side species cannot survive punishing dry seasons, it’s a bit of a mystery why most drought-tolerant trees don’t appear in wetter forests. New research by Smithsonian scientists suggests that plant pathogens play a role.
Pathogens have come into the spotlight as a key driver of biodiversity in tropical forests. The study, published in Journal of Ecology, shows pathogens may restrict tree species’ ranges, a finding that has implications for forest conservation in the context of climate change. The research may also recast one of the long-standing hypotheses seeking to explain tropical tree biodiversity.
During a STRI fellowship, lead author Erin Spear collected seeds from Panama’s forests using every method at her disposal — long forest hikes, kayak expeditions around the Panama Canal and ascents to treetops in STRI’s canopy cranes. She then planted the seeds in common gardens, planting dry- and wet-side species on both sides of the isthmus. For 21 weeks, she monitored seedlings for damage and death caused by pathogens.
All seedlings in wet forest gardens fared considerably worse. Compared to dry forest gardens, seedlings were 74 percent more likely to suffer pathogen-caused damage and 65 percent more likely to die from pathogen attack. Dry-forest seedlings were five times more likely to die when damaged by pathogens than their wet-forest counterparts.
The elevated risk of pathogen attack in wetter forests combined with decreased survival for dry-forest species, suggests that pathogens help promote regional forest diversity. But the circumstances surrounding why dry-forest species suffer as much or more pathogen attack in a forest where they do not naturally occur is perhaps the most intriguing outcome of the study.
Pathogens are generally believed to promote or maintain diversity because they keep tree species from becoming overly common. Known as the Janzen-Connell hypothesis, pathogens theoretically do this by being highly adapted to specific species. Under this scenario, a seed needs to travel far from its parent tree to escape host-specific pathogens.
In theory, dry-forest trees should have been freed from their specific pathogens in wet forests and suffered less attack than in the dry forests. That this did not happen suggests that some, or many, pathogens are widespread and/or host-generalized. “If so, this will challenge the conventional thinking about the roles of pathogens in tropical forests,” said Spear, a Ph.D. candidate at the University of Utah. “That lack of escape presents a number of interesting follow-up questions.”
Dry species’ greater susceptibility to pathogens may be key to understanding why trees grow where they do today, and it may provide insight for the future of Panama’s drier forests.
“Greater pathogen sensitivity of dry-forest seedlings should be considered when forest management decisions are made,” said Spear. “We should conserve existing dry forests as dry-forest tree species are adapted to those specific conditions and may not be able to persist elsewhere.”