Plants are not often thought of as predators. They’re the nice guys. With more than 300,000 species known to exist, only a small fraction are known to be meat-eaters. In our northern bogs, for example, insects are trapped on the sticky hairs of sundew or drowned in the pitcher plant’s water.
Research now suggests that at least one tree may owe its size to more than just sun, water and good soils.
The eastern white pine is one of the tallest native tree species in our region. Give them a few hundred years in ideal floodplain habitat, with roots sunk deep into sandy and silty soils and protected from winds and lightning by hillsides, and they’ll grow to more than 200 feet tall, with nearly eight-foot diameter trunks.
It takes a lot of energy and nutrients for a tree to grow to such grandeur. One thing that might help the eastern white pine is its surprising relationship with a meat-eating fungus.
The bicolored deceiver (Laccaria bicolor) appears above ground as a small, tan mushroom with lilac-colored gills. It is found in most coniferous woodlands throughout temperate regions around the globe. The fungus has a symbiotic relationship with many trees, including the eastern white pine. It forms a mycorrhizal sheath, like roots of the fungus, around the small root tips of the tree. The fungus receives sugars from the tree’s photosynthesis that takes place above ground, while it supplies the plant with essential nutrients and helps to increase water uptake by the tree roots from below ground.
Such symbiotic relationships between trees and fungi are common. About ninety-five percent of plants get some nutrients from fungi, and fungi play a critical role in the food web. In particular, fungi (along with lightning strikes and soil bacteria) are critical for converting atmospheric nitrogen into reactive forms, such as nitrate and ammonia, which other living things can use for growth.
What makes the eastern white pine’s relationship with the bicolored deceiver surprising is the way the tree benefits from the fungus’ meat-eating habits. This discovery occurred by accident, during a study of tiny soil arthropods called springtails.
Many observers know springtails as snow fleas, the wingless insects often seen by the thousands jumping across the snow in late winter. They are incredibly small, but they can occur in huge numbers. Soil ecologists John Klironomos, now at the University of British Columbia, and his colleague Miranda Hart, wondered if springtails had an adverse effect on trees since they ate fungi that helped secure nutrients for many plants. They set up a simple experiment to feed the springtails a diet of fungi, including bicolored deceiver.
That’s when their experiment took a strange turn. All of the springtails died when they were with bicolor deceiver. "It was as shocking as putting a pizza in front of a person and having the pizza eat the person instead of vice versa," Klironomos told Science News.
To confirm their findings, Klironomos and Hart fed a few hundred springtails a diet of bicolor deceiver while others were fed a diet either devoid of the fungus altogether or with another fungi species. After two weeks, only five percent of the springtails that were with bicolor deceiver remained alive. In contrast, nearly all the springtails that ate other species of fungi or whose diet was devoid of fungi survived.
The fungus was killing the springtails and breaking them down with a special enzyme. The researchers believe that the fungus first paralyzes the springtails with a toxin and then extends fine filaments into them to absorb nutrients.
So how does this make the eastern white pine tree a meat-eater? As a follow up experiment, Klironomos and Hart fed a batch of springtails a diet with nitrogen that was tagged radioactively so they could follow it through the food web. The insects were added to containers of bicolor deceiver growing with white pine seedlings. After a few months they tested the seedlings and found that 25 percent of the nitrogen in the trees was radioactive, and thus had come directly from the springtails. It’s as if white pine were fishermen using the fungus like a giant net to capture their prey.
Now, new research from scientists at Brock University in Ontario suggests that this adaptation may be shared by many plants. Green muscardine fungus, a soil-dwelling fungus found in many ecosystems, has long been known to infect insects. It has now been shown to associate with plant roots and transfer nitrogen from its insect prey to grass and even beans.
With webs of mycelia hunting tiny prey underground to help giants grow and capture the sun above, understanding who is eating whom just got a lot more complicated.
Kent McFarland is a biologist with the Vermont Center for Ecostudies. The illustration for this column was drawn by Adelaide Tyrol. The Outside Story is assigned and edited by Northern Woodlands magazine and sponsored by the Wellborn Ecology Fund of New Hampshire Charitable Foundation: firstname.lastname@example.org