When I first started looking at my forest with an eye toward managing it for increased health and faster growth—for MORE BIG TREES, basically—I was still in the habit of assessing a tree primarily by looking at its trunk. “Hey, that’s a big one!” or some variant of “This tree is only 8 (or 10 or 12 or 14) inches in diameter, so it really doesn’t merit my attention right now”………(insert phallic joke here). It took a walk of my woodlot with a State Forester for me to begin to see where the action is really going on—in the upper canopy.
It is estimated that in the forests of the northeastern U.S., of the 5000 or so tree seedlings that germinate per acre, only 100 will live to full maturity. Put another way, if you were to lay out a square on the forest floor with 25 feet per side, that area would ultimately be claimed by one mature tree. This makes for some pretty merciless competition, as ultimately only the trees best adapted to their specific circumstances are going to be able to outlast their fellow competitors to claim a portion of the sky—to gain direct access to sunlight. This process creates a feedback loop, as the tree that overtops its neighbors maximizes its photosynthesis and capacity for growth, while the neighboring underlings struggling in varying degrees of shade are simultaneously crippled in their abilities to make food at a time when they most need it to be able to compete. So a multi-tiered canopy structure usually develops, consisting of four crown classes: dominant (the tallest, broadest crowned), co-dominants (receiving full light from above but little from the sides), intermediate (shorter than co-dominants, receiving still less light), and suppressed (shorter still, receiving no direct sunlight).
It is a common (and forgivable) misconception that tree size correlates directly with age: that a forest consisting of trees of differing heights and diameters is by definition a forest made up of trees of all different ages. While uneven-aged forests exist (and once were the norm), they are generally a product of many centuries of undisturbed tree growth—old-growth character forests. In the northeast most of the land was either cropfield or pasture 100 years ago, so the forests around us are relatively young, and are overwhelmingly even-aged. What I’m getting to here is the fact that the majority of the ‘small’ trees in a forest are in fact the same age as the biggest ones. There are a few major lessons to be taken from this: 1) if you want to assess the true health and future potential of a tree, don’t be fixated on the size of the trunk, look up at the crown; 2) in most cases, the suppressed trees have already lost the race, and will never become dominants themselves, so cutting all the biggest and best trees will not lead to another stand of ‘best’ trees (this is the fallacy leading to the patently unsustainable practice of “high-grading”, a topic to be explored further in another post); 3) that managing a forest by selectively thinning to maximize sunlight to the crowns of the most desired trees can bring about exponential increases in growth. For now, I often find myself with a stiff neck after walking in my woodlot, as I’m constantly fixated on looking upward (I trip a lot too). One last note: assessing and comparing tree crowns is a lot easier without leaves on the trees, so get out sometime soon and see for yourself, as springtime is right around the corner.