what good is a tree?
All agree that tree-lined stream banks are essential for the life of the freestone river (Figure 38). The trees that grow there and the many roles they play in helping to maintain the integrity of those banks cannot be emphasized enough. To begin with, a tree’s root system acts like an anchor for the soil and rock, two essential ingredients needed for maintaining the physical course of the river. Siltation is an unusual event for rivers that run through intact hardwood or boreal forests, even after heavy rains. In contrast, it is common-place for rivers that have had their stream-side plants removed, or greatly reduced. The consequences of soil from the bank repeatedly washing into the river are devastating for the macroinvertebrates. Silt gathers around in-stream rocks and other bottom structure, and limits the amount of space that is available for clinging larval forms of macroinvertebrates. Oxygen levels are depleted whenever soil washes into the water. It is hard to sit on the bank and watch as a river with extensively defoliated banks changes color to that of a good cup of strong coffee with cream during a summer thunder storm, as rivulets of soil-laden water rush out of the surrounding hillsides into the main stem. Such is the case for many northeastern American rivers.
Trees shade the river in summer months and function as nature’s air conditioners. Why this ecosystem function for the river is over-looked by most of us is a mystery, since we never fail to replace a faulty air conditioner in our own homes, and we always seek the cool refuge of shade when we go on a picnic or park our cars. Thermal pollution events are common among rivers that are missing significant numbers of trees along their banks.
The leaves of trees harbor an astonishing variety of terrestrial insects and arachnids (spiders, and their kin) that are constantly falling into the water, providing a much needed supplemental protein source for fish at times when aquatic insect hatches are reduced. This is particularly so in the height of summer during daylight hours.
When a tree eventually dies and falls into the river, or an unusual storm rips them from the bank, the amount of in-stream organic material they supply for the larvae and nymphs of aquatic insects can be significant (Figure 39). Trees begin to decay the moment they enter the water, and nutrients are mobilized by leaching which directly aid in the growth of macroinvertebrate communities. Dead trees and branches act like a slow time-release capsule of energy that may take ten or twenty years to be fully utilized. Dead trees also provide protective cover for trout, allowing them to avoid predators.
When a tree dies leaving an opening in the forest canopy, pioneer plant species that are shade intolerant germinate and grow, filling up the gap. Typically, a new tree seed germinates and a sapling rises under the protection of the pioneer plant canopy, since most tree species are shade tolerant. If for some reason this does not happen (unusual flooding, killing frosts, over-browsing, etc.) within the first several seasons after the death of the tree, erosion re-shapes that part of the river, as it has for eons. In addition, with a barren stretch of riverbank, each rain removes essential nutrients (e.g., calcium, potassium, nitrates and other organic compounds), and siltation increases.
Lastly, and most critical to the bio-productivity of the river, the leaves that fall into the water provide up to 60% of the annual required energy in-put for macroinvertebrates (Figure 40). No leaves, no insects. No insects, no trout. So a good motto for remediation where trees are needed is: “Plant a tree and save a trout”. This is one long-term project in which we can all participate, just in case we’re looking for something to do on Arbor Day each Spring. What good is a tree, then? They are so good that we cannot afford to under-value them as the keepers of the riverbank and the river, itself. Without streamside trees lining the entire bank on both sides, a river cannot operate at maximum efficiency. To insure that each and every river bank is maintained as intact as is humanly possible is good stewardship, and at the same time, increases our chances of living out our lives in greater harmony with the processes of nature that got us here to begin with.