Panama's rainforest canopy borders the Chagres River and the Caribbean Sea. (Erin Spear/University of Utah)

In Panama's forest, death is a beautiful thing.

It kills ultimately for a good reason, so that trees in a wetter part of the forest can grow big and strong. according to a new study published in the Journal of Ecology.

"That’s exactly right," said Erin R. Spear, lead author of the research and a doctoral candidate in biology at the University of Utah. "We have this negative perspective on pathogens and disease. We think of pathogens as being destroyers, and in this case they are constructors."

The killing part, Spear admits, is still ugly. Death sends a gang of pathogens in a wetter part of the forest to dispatch seeds that wind up there from a drier part of the forest. They meet their end in a slow, agonizing demise that looks perfectly awful on time lapse video.

But the cold truth is that seeds from the drier side carried about 40 miles by wind and birds had no business being in a place where they can hardly survive. Seedlings native to the wetter side have developed better defenses against attacking viruses, bacteria, disease and fungus that thrive there, but for drier seedlings it might as well be the set of a horror film.

Essentially, killing drier seedlings in their cribs creates space, and more space allows more diverse species of trees that can not only survive but grow into leviathans on the wetter side. "They’re allowing more tree species to exist in a given area," Spear said. The wood and canopy creates a greater habitat for birds and other fauna. They also provide "better water quality, [regulate] carbon in the atmosphere, raw material for medicine and food."

How seedlings live and die in Panama provides a lesson to conservationists trying to determine how tree canopies will shift as the climate warms. It will inform their understanding of whether trees in some temperate areas can survive in others, before they embark on massive and expensive replanting efforts that are doomed to fail.

It had long been suspected that the pathogens slither up to seeds that wander too far from home. But Spears and her fellow researchers -- Thomas A. Kursar and Phyllis D. Coley, professors at the university and researchers at the Smithsonian Tropical Research Institute in Panama -- set out to investigate if it actually happened and how.

Panama, as everyone knows, is a narrow strip of land connecting Central and South America, sandwiched by the Atlantic and Pacific oceans, and slashed by the famous canal.

The volume of rain on the Atlantic side basically doubles the amount on the Pacific side, giving life to more types of pathogens than anyone has bothered to count. By comparison, the drier side is virtually disease free.

Doctoral student Erin Spear stands in front of a 150-foot-tall giant rainforest tree in Panama as she hunts for tree seeds. (Joseph Sertich/Denver Museum of Nature & Science) Doctoral student Erin Spear stands in front of a 150-foot-tall giant rainforest tree in Panama as she hunts for tree seeds. (Joseph Sertich/Denver Museum of Nature & Science)

Spear, Coley and Kursar established 30 gardens in each part of the forests and seeded them with the progeny of 12 species of trees. Spear hiked for miles to collect some seeds, and kayaked in the canal to grab overhanging tree fruit that contained others. She even rode "a crane-carried gondola more than 100 feet upward into the forest canopy" for seeds, a statement for the study said.

When the seeds were planted for the experiment, it didn't take long to spot damage. One species was dead within a week. "In some cases, we could actually see the pathogen growing on the seedling," Spear said.

Within three weeks, nearly 40 percent of more than 700 seedlings were damaged, and most of it by far was on the pathogen-rich wetter side. Seventy-percent were more likely to be damaged there.

Every seed planted was at a much greater risk of an attack in the wetter forest, but drier seeds didn't put up much of a fight. "On the whole, seedlings of the dry-forest species suffered relatively high levels of pathogen-caused damage and mortality in both forests. In fact, seedlings of dry-forest species were at a greater risk of pathogen-caused mortality in the drier forest than seedlings of wet-forest species," the study said.

Death's defense of the wetter part of the forest could actually save it from greater harm. Because it gets less rain, the dry forest is favored area for agriculture, housing developments and resorts that help denude it. In a sample of a 40 by 40 meter plot, the researchers counted 57 species of trees on the drier side, compared to 174 species on the wetter side.

Overall, about 495 tree species in the wetter forests compared to 101 restricted to the drier forests with several intermediate species of trees between them.

"I think a cool aspect to our study is it's broadly applicable to other plant communities," including forests in the United States, Spear said. "We can control where species can and can’t grow, and better predict how distributions might change as the climate warms, especially where we’re at risk of losing species."


Animals, like this sloth in the canopy of rainforest on Panama’s Atlantic coast, capture our attention but trees are essential for feeding and sheltering those animals. (Erin R. Spear/University of Utah)