![]() ![]() It’s when environments change rapidly that trees, like most of us, get into trouble. Over generations, trees have proved adaptable to gradually changing environmental conditions. The second part – messing with temperatures – is a bit more dicey. Most nurseries, for example, pay close attention to frost hardiness and avoid stocking and selling southern source trees for planting in the north. We’ve got the first part reasonably well in hand. Everything’s just fine so long as trees are not planted out of their natural range and nobody messes with the temperature cycles. Presumably, native trees growing on their natural sites have evolved a synchrony with the annual temperature cycles of their site. And whereas a red maple growing in the Upper Valley may require a few months of cold, the same species growing natively in Florida may require no cold at all. Sugar maple tends to be in the middle, needing as many as 2,000 hours of exposure to low temperatures before it will flush. Though their actual dates of bud-break vary, you can count on trembling aspen every year, for example, to be among the first in New England to break bud and white ash to be among the last. Some species need weeks of chilling, some need months. Of course, the exact chilling temperatures and duration of exposure needed to break dormancy vary by species, individual, and geographic location. These environmental cues trigger physiological responses in trees, altering the balance of hormones and enzymes involved in promoting and inhibiting growth. That’s right, trees must first go through prolonged exposure to chilling temperatures (-5 C to 10 C) before subsequent exposure to warmth will force bud break. It turns out the all-important environmental cue for spring budbreak is cold. Sure, some forest trees get nipped by late frosts, but generally they get it right. Somehow, trees seem to “know” when true spring arrives and respond by breaking bud. And clearly it’s not just warmth that stimulates budbreak, as January thaws demonstrate. If that were the case, trees would break bud on the same spring day every year and, as we’ve all seen, the date of bud opening can vary by weeks from year to year. Trees avoid such midwinter mishaps not because they are programmed to simply wait a fixed time period until conditions are right. It’s a perfect way to lose the coming year’s growth, victimized by a cruel winter hoax. But if that sugar maple, impatient for spring, breaks bud and sends out those fleshy young shoots into the unseasonable warmth, its leaves could get zapped and blackened by the returning freeze. It starts with some warm rain followed by spring-like breezes and temperatures above freezing for days. Sugar Maple dormantly waiting out November, December, and a nasty run of cold in early January. It’s a good thing, too, considering the alternative. And in what might well be the height of arboreal prudence, these dormant buds don’t even break during midwinter thaws. They too were made last summer, but they spend the winter dormant and protected under bud scales until favorable growing conditions return in spring. Inside these buds are the miniature beginnings of this coming year’s new shoots. In most northeastern trees, the buds that burst open in spring were formed the previous summer. Dormancy is how trees avoid such unfavorable environmental conditions without moving to Florida. They still burn energy maintaining life, but little goes on in the way of growth. They’re dormant – very much alive but relatively inactive. Thankfully, most northern trees aren’t dead in winter. ![]()
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