When we think of a forest, most of us have a mental image of a stand of more-or-less dense trees that covers a sizable piece of the landscape. Plantations don’t count: too manicured, too uniform. So our mental images assume a diversity of tree ages and some understory shrubs, ferns, and mosses. For some of us, the image is not good enough until we add some birds and mammals and other critters that call the forest home.
Yet even that image is not sufficiently deep. Much of what makes a forest goes on underground — call it the unseen forest.
Consider first the roots. Trees and many other plants have roots that penetrate the soil, rather shallowly in most of our area, but cottonwoods can send roots down dozens of feet. The main roots branch out into smaller and smaller roots, then into fine rootlets. On the rootlets are thousands of root hairs: tiny, thin-walled structures that do the job of collecting water and water-borne nutrients from the soil. From there, the conducting tissues of the plant transport water to the twigs and leaves. The canopy of a tree can only be as big as the capacity of the roots to deliver water and nutrients.
The roots of some trees are known to graft to each other, even to trees of other species. Via these connections, they can exchange nutrients, hormones and defensive compounds. In such cases, each tree may be something more than the individual trunk that we can see. However, root-grafting by the trees of our local forests has apparently not been studied.
The roots of many (probably most) plants have associations with fungi. The closest associations are referred to as mycorrhizae (fungus-root) of several types. Some actually penetrate the roots, while others are external. Mycorrhizal fungi take up additional nutrients from the soil, and so plants grow better when they have their particular kinds of root fungi. These fungi also can connect neighboring plants, conducting nutrients from one to another — making possible both sharing (especially if neighbors are genetically related) and thievery (if one tree can somehow commandeer a greater proportion of the transported nutrients).
Some plants have both mycorrhizal fungi and symbiotic nitrogen-fixing bacteria. Associated with the roots of many plants are bacteria (and other, unrelated microorganisms) that capture molecular nitrogen from the air (which is about 80 percent nitrogen) and “fix” it into a usable form (ammonia, which is soluble in water). Some of these bacteria colonize the root surface or the inside of the roots; others interact with the roots (of alders, sweetgale, and lupine, for example) to form nodules: compartments in which nitrogen-fixation is supported by energy from the plant and a low oxygen concentration.
Through a series of chemical interactions, these symbiotic bacteria create ammonia, which the plant can use to increase growth, build better chemical defenses or deal with environmental stresses. Ammonia is toxic, however, and so it is rapidly incorporated into small organic compounds (amino acids) that are transported via the water-conducting system to all parts of the plant, where it is used in further syntheses. Still, other bacteria and microorganisms that live near the roots take nitrates from the soil and create ammonia, which is taken up with water by the roots and transformed into non-toxic amino acids and circulated to the plant.
Plant growth and health is often limited by the availability of usable nitrogen, a component of proteins, enzymes, DNA, defensive chemicals and other essential items. I have read that about 90 percent of the nitrogen used by plants comes from the activity of bacteria and other microorganisms, both free-living and symbiotic (the rest comes mostly from effects of lightning). Consider, then, that all plant life — and therefore all animal life — depends on these microorganisms.
Some of the nitrogenous products of symbiotic bacterial activity leach out into the soil, but most of it eventually returns to the soil when parts of plants drop off and decay. Here the actions of lots of invertebrates, fungi, a few saprophytic plants (which feed on decaying material) and still more bacteria break down the plant parts into simple forms that are recycled through the system.
We are creatures of the ground surface, and the complex interactions I have just mentioned — and many others — take place beyond our vision. However, what we do see ultimately depends on those interactions, to which the microbial world is essential.
A forest is much, much more than the trees.
• Mary F. Willson is a retired professor of ecology.
