Erin Spear, University of Utah. Diseases that kill individual trees enhance rainforest diversity, and therefore health

It's the ultimate case of seeing the forest by not focusing on the trees: Pathogens that kill individual plants make for a healthier ecosystem.

While always tropical, the Isthmus of Panama has starkly different climates, with rainfall dramatically higher on the Caribbean side than the Pacific. Panama City averages 1900mm of rain a year, but it is approximately double that at the other end of the canal.

As a result, forests on the Pacific side only have trees that can handle dry periods. However, ecologists have been puzzled with why the Caribbean side lacks the same trees. After all, both locations are only 30 miles apart.

In the Journal of Ecology Professor Phyllis Coley of the University of Utah solves this mystery, and demonstrates that the ideas involved have wider application. She found that the species that inhabit the dryer Pacific areas are more resistant to diseases, be they fungal, bacterial or viral. Some of these thrive in the wet conditions, and the presence of water increases opportunity to spread from tree to tree. When a variety of trees were planted in two trial plots, diseases were 74% more common in all species in wetter areas, but the drought tolerant were five times as likely to die.

"Because seedlings of disease-sensitive tree species can't survive in the wetter forests and drought-sensitive tree species cannot survive in the drier forests, different tree species inhabit the wetter and drier forests even though they are only 30 miles apart,” says Coley.

The differences in forest type are the reason Panama has such extraordinary diversity, with 800 known tree species in the area around the canal. This has made the country, and neighboring Costa Rica, a focus for efforts to save tropical rainforests, and the enormous ecosystem services they provide.

The variation provides the ecosystems with considerable resilience because if the climate changes, one type will most likely be suited to fitting the new conditions. The observation that what is bad for the individual may benefit the community as a whole is not new, but Coley and Spear have provided a new example, which may open the door to other examples.

First author, Ph.D. student Erin Spear says that for endangered systems, "conservation planning and predictions about how tree species distributions may shift with climate change require an understanding of the factors currently influencing where species can and cannot survive."

While forests on both sides of Central America are under threat, the sunlight on the Pacific side is more attractive to farmers, making the arboreal inhabitants a particular priority for conservation programs.

Erin Spear, University of Utah.A healthy seedling, left, and one killed by a pathogen, right. By different vulnerabilities helps maintain forest diversity.

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