Silvics: The Scientific Foundation of Silviculture

By Bob Seymour

When foresters use the term silvics, we are referring to characteristics of tree species that define their life history, growth, response to its environment, and ecology. When forestry first began in the United States in the early 20th century, job one for the newly trained foresters, lead by the U.S. Forest Service chief Gifford Pinchot, was to study the biology of American tree species and how they respond to human interventions. Zon’s monograph on balsam fir and Frothingham’s treatise on eastern white pine, both published in 1914, remain valuable classics. Ultimately these efforts led in 1990 to the encyclopedic compilation of Silvics of North America in two volumes: 675 pages about conifers and 877 about hardwoods.

The Forest Trees of Maine is a wonderful reference for describing and identifying Maine’s trees, but gives little insight into how, why, and where they grow and reproduce. I could find no convenient summary of silvics just for Maine species, so I plan to use this column to fill this void, emphasizing silvicultural applications. Given that a typical Maine woodlot may have 10 or even 20 species, each with its own unique suite of characteristics (its ecological niche), mastery of this subject can be a lifetime endeavor linking published literature honed by in-woods experience.

Although the study of silvics is valuable in its own right, the real value of this knowledge is for devising silvicultural systems. Silvics is akin to materials science in structural engineering, or human anatomy in medicine – the fundamental building blocks without which the discipline would have no underpinnings. The silvical properties of a tree species define both its potential and limitations, and thus comprise essential working knowledge for the practicing forester and engaged landowner.

Silviculture is fundamentally about managing two distinct biological processes: interactions among trees that are well established and growing in stands, and timely regeneration of new trees when older trees mature. Silvics informs both aspects, and must be coupled with operational realities to create useful silvicultural solutions. Two very important silvical properties that influence nearly all aspects of silviculture are discussed below.

Shade tolerance. Trees vary greatly in their ability to carry out photosynthesis over a gradient ranging from full sunlight to dense shade. Very intolerant species such as tamarack, aspen and black locust must have full sunlight at all life stages or they will perish, whereas very tolerant species such as beech, balsam fir and eastern hemlock can persist at light levels of under 1-5% of full sun. Intermediates such as white pine and red oak lie between. All species grow better as light increases up to their light saturation point, which varies between about 40% of full sunlight for hemlock up to full sun for all intolerants. Intolerant species tend to have very efficient leaves (measured by the amount of CO2 captured per unit of leaf), but do so at the expense of having less total leaf area per acre. Such intolerant forest canopies thus “leak” light that could be captured by more tolerant species.

This light-capture phenomenon of forest canopy structure allows us to grow many valuable intermediate or tolerant trees in stratified (layered) stands where the upper stratum races ahead while the lower strata grow happily beneath it. Common Maine examples include white pine beneath aspen, hemlock beneath red oak or white pine, and spruce-fir beneath paper birch. We are fortunate in New England to have abundant precipitation evenly distributed during the growing season. Without summer rains, the upper canopy captures all the water and lower canopy strata cannot develop.

Managing light available to the understory is a key aspect of the shelterwood regeneration method, in which new seedlings are recruited in the understory of the parent stand. The photo above depicts a shelterwood establishment cutting made about 15 years ago in a mature white pine stand. The small pine sapling in the foreground originated 11 years ago but is being outcompeted by more shade-tolerant, somewhat older red spruce, hemlock, and fir that were present as small advance regeneration when the cut was made. The pine overstory has expanded, creating too much shade for the pine sapling but not the other tolerants. The understory pines could be released by cutting nearby competitors in the understory, but a better solution would have been to harvest some of the overstory 5 years ago, brightening the understory to favor the pine. A complete overstory removal would benefit pine growth even more, but possibly subject pine saplings to weevil attack.

Response to release from competition. Silvicultural treatments in established stands are about reallocating growing space (mostly obviously light, but also water and nutrients) to trees we want to favor. We do this by cutting or killing other trees whose crowns (foliage) are taking up space that could be used by the crop trees. Trees must grow upward to expand their crowns outward into the vacated space. Hence, short trees that are far from their maximum height are more likely to respond well than taller ones, regardless of species. If we want to favor intolerant species such as paper birch, we must release them when they are still young (under 30-40 years old) and favor vigorous trees with healthy crowns. In contrast, tolerant species of the same age but only half as tall as the main canopy (the midstory) will usually respond well, because they can quickly rebuild their crowns upwards.

Contrary to some popular belief, even shade-tolerant trees that are severely suppressed in the understory, such as old flat-topped “umbrella” red spruces, will respond dependably if given time. The photo below depicts such a spruce (right) just released from overstory competition along with a mature spruce after 80 years of growth. The flat-topped crown with no distinct leader is the most efficient needle arrangement to capture light and survive in a very shaded understory, a characteristic unique to some tolerant species. The yellow arrow shows base of live crown when released. On the resulting mature tree at left, note the distinct transition between branch-free main stem below live crown base and large dead branches above which supported vigorous living foliage during the decades after release. Although the suppressed spruce may be 100 years old, its “effective age” is little different than a much younger sapling of the same height.

Bob Seymour recently retired from the University of Maine School of Forestry Resources faculty after over 30 years as Curtis Hutchins Professor of Silviculture. rseymour@maine.edu