Still, forest ecologists are a curious and plucky bunch. They continue to build on that legacy of effort, by measuring roots of container-grown trees, painstakingly excavating living roots in forests, and, more recently, using modern imaging technology to watch roots grow in place. And most have come to agree on a generalized view of tree roots through the seasons.
These ecologists describe root activity as periodic, with maximum growth in early summer – especially in deciduous species – and pulses of additional growth occurring occasionally in early fall. And complicating things further, they indicate that not all roots grow at the same time. Even within a single tree, some roots may be active while others are not. However, by all accounts, tree roots in our region are thought to spend the winter in a condition of dormancy. This means they are not dead but rather they overwinter in a resting phase with essential life processes continuing at a minimal rate. Full-on root growth resumes in spring, shortly after soils become free of frost, usually sometime before bud break.
But unlike the aboveground parts of most trees that pass the winter in a prolonged dormancy – marked by unbroken inactivity until spring – tree roots seem to maintain a readiness to grow independent of the aboveground parts of the tree. That is, roots remain mostly inactive but can and do function and grow during winter months whenever soil temperatures are favorable, even if the air aboveground is brutally cold. While roots tend to freeze and die at soil temperatures below 20°F, minimum temperatures for root growth are thought to be between 32 and 41°F. So, if soil temperatures warm to or stay above this minimum, winter roots can break dormancy and become active.
This winter quiescence – where roots are resting but ready – is extremely important for the health of individual trees and, by extension, for forests in general. Indeed, it is this trait that allows evergreens to absorb soil water and avoid winter desiccation in their needles, and it is this trait that allows all species, including deciduous hardwoods, the opportunity to expand their root systems in search of water and nutrients in advance of spring bud break.
But there is an important tradeoff. To maintain this quiescence, a tree’s roots necessarily tend to be much less cold hardy than its stems and branches. This is fine, so long as the soil is sufficiently insulated by a covering of snow against extremely low air temperatures. A good early season snowfall – if it persists – can keep soil unfrozen throughout the coldest of winters. In such years, sustained winter root activity may replace previously damaged roots, may ready the tree for spring bud break, and may translate into excellent aboveground growth during the following summer.
Conversely, a deep snowpack coming later in winter, after the soil is already frozen, can also insulate the soil – but in a different way. These late snows actually keep soil frozen for extended periods – even during January thaws and despite the heat of the earth’s core. The surface layers of forest soils do commonly freeze, and when they do, it is not good for roots or the stems and branches dependent on them. Not only do the roots remain inactive under such frozen conditions, but the freezing, heaving, and cracking of winter soils physically damages roots – particularly the fine feeder roots in the uppermost organic layers. This can trigger a cascade of effects on overall tree and forest health. By reducing a tree’s ability to take up water and nutrients, particularly during spring bud break, winter root damage limits subsequent stem and branch growth in summer. In turn, this can contribute to tree mortality and may even explain pockets of dead trees.
Winter injury to feeder roots is an inherent – and natural – part of forests in northern climates. And, through its effects on individual tree health, winter root ecology is an important determinant of overall forest composition, dynamics, and productivity – even though it is difficult to see and measure directly.
Michael Snyder is the Chittenden (Vermont) County Forester.