Mary Van Vleck | Contributor

Often when driving along the highways or walking in the woods, I am struck by the species of trees that seem to dominate in one area or another. In the fall when the hillsides are aflame with fall colors, streaks of dark green evergreens stand out on the upper regions, and one might wonder why certain trees grow in some areas and not so well in others. There is a tremendous variety of both evergreen and deciduous trees, and where they grow best is something of a mystery. We can see quickly that evergreens cover the mountain tops with fewer on lower slopes where deciduous trees (maples, birches, beeches) predominate.

We can guess that differences are due to temperature variations and varying amounts of water in the soil. These in turn are influenced by elevation and exposure: south- facing slopes are warmer than north-facing slopes.

Consider too that glaciers once covered the entire northeast with ice nearly a mile thick. This compressed the land beneath the ice, and with rocks and boulders embedded in and carried along by the ice, the ice and rocks together scoured the land, gouging and smoothing over great boulders on the mountain tops, scraping away stones and soils accumulated there; it ground up the bedrock underneath the soils much as sandpaper grinds away at pieces of wood. The southbound glaciers carried the sand and gravel scraped from the bedrock to places miles away, depositing these either along the sides, forming long ridges of moraine, or at the southern end of the glacier, resulting in hills or mountains. Our soils today are the result of this glacial action. Some soils in the Champlain Valley are primarily sand and gravel, but most soils here are composed of clay, which was carried farther than the sand or gravel; it settled out in the still waters of the ancient lake that covered most of the area as the glacier melted.

The rocks, gravel, silt and sand contained minerals that were in the original boulders, influencing the soil quality. Soils with more minerals such as calcium and phosphorus are considered nutrient-rich, such as the soils of the Champlain and Hudson valleys; by contrast the bedrock of the mountain peaks of the Adirondacks, the Whites and the Greens and the soils that came from them are nutrient-poor, as these rocks lack these minerals or are in short supply.

Soil at these high elevations is so shallow that very few plants can survive there. At the top, only stunted spruce and fir grow low to the ground, in the thin, nutrient-poor soils with colder temperatures, strong winds and plenty of moisture. Where there is less wind, these evergreens are taller, their roots spreading sideways since they cannot penetrate the bedrock. At lower elevations and on rocky slopes with little soil depth, the familiar white pine predominates, its roots anchored in whatever soil or cracks it can cling to.

With more minerals, good soil depth and ample water, deciduous trees flourish. Both evergreens and deciduous trees can grow at the lower elevations, but here the deciduous trees out-compete the evergreens. Higher up the mountain, spruce and fir are more successful. On lower areas, where the land slopes more gently or has leveled off, the glaciers and moving waters would have deposited more of the clay, sand or gravel, so the soils accumulated there. Coming down a mountain, one notices that evergreens give way to pine, birches, red maple and beech, and farther down will be the familiar sugar maple and fewer evergreens.

There are other important factors to consider, especially acidity of the soils and solar exposure. Evergreens generally do well and out-compete the deciduous trees in more acidic soils; in more neutral soils, although all types of trees grow, deciduous trees are the most successful and the most numerous.

So what about those dark green lines of evergreens seen high on a hillside? With more information, one can hazard a few guesses: water flowing there washes away the small particles, leaving mostly gravel and rock and less soil, and in a ravine-like area there will be more water, all things that enable the spruce or pine to thrive where maples and other deciduous trees cannot.

Mary Van Vleck is a Charlotte resident and a member of the Charlotte Conservation Commission. This group meets monthly on the third Tuesday at the Town Hall; meetings are open to the public.