Ken Baker

Columnist

Picture the sweeping vista of Tanzania’s Serengeti National Park.

Just like a NOVA special, let’s make it a red dawn silhouetting a tower of a half dozen giraffes ambling against a backdrop of flat-topped umbrella thorn acacias.

Their thorns notwithstanding, the acacia’s leaves are a favorite food of giraffes … for a half hour or so. A few minutes after recognizing that its foliage is being grazed upon, the tree pumps quantities of distasteful compounds into its leaves and the giraffes saunter off to continue browsing on another clump of acacias a couple hundred yards away.

Hold on a tic. What’s your take on the concept I just flew past you — that the acacia “recognizes” when its leaves are being eaten and that it quickly responds with an effective defensive tactic? Although we commonly refer to animals’ anti-predator “behaviors,” we wouldn’t refer to the acacia’s actions as a form of behavior, would we?

Even more intriguing, when a giraffe begins grazing on its foliage, the acacia also releases a highly volatile compound, ethylene, to the surrounding air. Neighboring acacias detect the — scent? — and respond as if they, too, had been fed on by the herbivores. Thus, useful information has been communicated to neighboring acacias: “Warning, incoming giraffes! Better initiate defensive action.” No dummies, the giraffes move some distance away before choosing an as yet uninformed acacia to feed upon.

Scene shift: The Douglas fir forested slopes of British Columbia in the mid-1990s with a young graduate student, Suzanne Simard, stooping over a group of three plastic bag-enclosed saplings — a Douglas fir in one bag, a paper birch in the second, and a western red cedar in the third. As we watch, she injects radioactively labeled carbon dioxide gas into each bag with a large syringe.

Plants use CO2 as their source of carbon in assembling sugars in the process of photosynthesis. After waiting an hour, Simard can determine, by waving a Geiger counter over the plants, where the now radioactively labeled sugars go after being manufactured in the leaves. She had the crazy idea (many thought) that some of those sugars might be transferred, underground, between the fir and birch saplings but not between either of these and the cedar.

She was right.

It had long been known that the roots of some 90 percent of all land plants are sheathed in an elaborate network of enormously thin fungal filaments (hyphae). In a classic case of symbiosis, the filaments move nutrients like nitrates and phosphates from the surrounding soils into the plant’s rootlets while the plant shares some of its energy-rich sugars with the fungus. Together the fungal hyphae and plant roots form a complex known as the mycorrhiza.

Simard knew that fungal hyphae extend throughout the forest soil, commonly connecting the mycorrhizae of many different trees. Her radical hypothesis was that perhaps this network of interconnected mycorrhizae could also serve as a conduit for the movement of materials from one plant to another.

She found that Douglas firs and paper birches did indeed share sugars with each other and since their root systems were not in direct contact, the saplings had to be doing so via the mycorrhizal network between them. Why was the red cedar left out of the game? There are two broad categories of fungi that form complexes with plant roots: ecto- and arbuscular mycorrhizal fungi. Firs and birches share the first type of fungi while the cedar forms complexes with the second type.

Over succeeding decades, a lot of work has revealed the workings and importance of these subterranean mycorrhizal networks, which have come to be more popularly known as the “Wood Wide Web.” Scientists have since learned that forest trees not only transfer sugars between each other via the WWW but also a variety of nutrients, hormones and defensive compounds.

A notable discovery was that in a natural, old growth forest, the largest trees tend to serve as hubs in the network, with a multitude of links to other trees. It’s been shown that such trees will preferentially shunt nutrients to their own offspring over those of other individuals.

We’ve come to realize that a healthy forest with a diverse mix of tree species of varying ages is a much more complex entity than initially thought. Individual trees certainly compete for nutrients, water and sunlight, but they also interact — communicate? — with each other via the Wood Wide Web of fungal filaments in ways that can benefit each other.

In later work, Simard’s students discovered that paper birch provide Douglas firs with sugars during the summer when the firs are overshadowed by larger trees, and firs return the favor in early spring before birch trees have a chance to grow out their new leaves.

Ken Baker is a retired professor of biology and environmental studies from Heidelberg University. If you have a natural history topic you would like Dr. Baker to consider for an upcoming column, please email your idea to fre-newsdesk@gannett.com.