1
The Context of Ecological Silviculture

Brian J. Palik1 and Anthony W. D'Amato2

1 USDA Forest Service, Northern Research Station, Grand Rapids, MN, USA

2 Rubenstein School of Environment and Natural Resources, University of Vermont, Burlington, VT, USA

1.1 What Is Ecological Silviculture?

In forestry, the tools to carry out on-the-ground management are found in the toolbox of silviculture, that is, the part of forestry that deals with the development and care of forests. This development and care can be all encompassing, including establishment, growth, composition, health, and quality of trees and stands to meet the diverse needs of society. The planned program of all these activities, spanning the entire life of a forest stand, is termed a silvicultural system.

Ostensibly, such a definition for silviculture would seem to recognize most forests, outside of plantations, as natural ecosystems and that the tools to manage these ecosystems would be based on ecological principles. In reality, classical silviculture is largely based on principles of production agriculture, more so than the ecology of natural forests [1]. The outcome of the long-term application of an agricultural model to managed forest is near global-wide simplification of structure and composition [2], as well as a reduced ability to adapt to unexpected and novel threats [3].

This is where ecological silviculture comes in, which we define as an approach for managing forests, including trees, associated organisms, and ecological functions, based on emulation of natural models of development [2]. There are other closely aligned approaches that also are ecological that have similar aims, such as close-to-nature forestry (e.g. [4]). An explicit goal of ecological silviculture and allied approaches is to manage forests in ways that reduce the disparity between natural forests and their managed counterparts in terms of composition, structure, and function. Recognizing that this disparity cannot be eliminated in managed forests, particularly those where timber resources are highly valued, ecological silviculture still rests on the premise that managers can get closer to the natural model than classical silvicultural has allowed. Let us look at why this might be, by contrasting some of the main differences between the two approaches.

1.2 How Does Ecological Silviculture Differ from Classical Commodity-Focused Silviculture?

The central emphasis of ecological silviculture is on maintaining the full array of structures, functions, and species found in a healthy natural forest ecosystem [2]. To achieve this goal, ecological silviculture builds from an understanding of natural disturbances and forest development to craft silvicultural systems that generate and maintain complexity and heterogeneity in ecosystem attributes. This often includes the application of regeneration harvests patterned after the prevailing natural disturbance regime for an ecosystem, including their scale, severity, and frequency. In addition, the legacies of these disturbances, namely surviving trees, and coarse woody material, are accounted for by placing an equal emphasis on what is left behind relative to what is removed at each silvicultural intervention over the course of the silvicultural system. Although economic objectives are still a priority with ecological silviculture, those associated with ecosystem diversity and resilience are given high priority in the design and implementation of ecological silvicultural systems.

In contrast, commodity-focused silviculture emphasizes the "forest crop" and is correspondingly concerned with silvicultural regimes that generate forest conditions to optimize growth and development of economically desirable species and tree forms. Silviculture treatments under this approach are largely based on agronomic models with a corresponding emphasis on simplifying and homogenizing forest-wide structure (e.g. spacing) and composition (i.e. primarily commercial tree species) to maximize the area occupied by economically desirable trees and to enhance the efficiency of harvest operations. Not surprisingly, the timing, frequency, and severity of silvicultural treatments are largely governed by economic criteria, including maximizing net present value, and avoiding risk of crop-tree loss to damaging agents [5]. Forests managed under this approach can satisfy noneconomic objectives to a degree [6]; however, achievement of economic objectives is the central priority under timber-focused regimes.

1.3 Why Is Ecological Silviculture Needed?

To appreciate the need for ecological silviculture, consider how the forces shaping forest management have changed over the last 70?years. First, globally, it is estimated that there are 4.0 bha of forest, including 0.3 bha of plantations and 3.7 bha of natural-origin forests; moreover, of the latter, 0.65 bha are legally protected reserves [7]. Generally, plantations are managed intensively for wood, such that ecological benefits are marginalized, while reserves usually exclude harvesting. This leaves about 3 bha of natural-origin forests where multiple environmental, economic, and social objectives are pursued [8] and, for the portion of this forest that is managed, ecological silviculture may be the appropriate tool for meeting diverse objectives.

In fact, the drivers of policy and practice across the global forest have changed fundamentally to be more ecological [2]. For example, conservation stewards, such as The Nature Conservancy, have emerged as key players in the forest management world, with their goals being overtly ecological. Also, the objectives of traditional forest stewards, e.g. United States National Forests, have evolved to focus on sustaining whole ecosystems. Additionally, third-party certification, as well as best management practices (BMP's), have led forest stewards to adopt ecological approaches for silviculture. Many largely unprecedented health threats facing forests, including climate change, catastrophic wildfires, and invasive species, require silviculture that restores the structural and functional integrity of forests to better position them to adapt to uncertainty [9]. Finally, the vast increase in our understanding of forests - how they are structured, how they function, and how they are connected across the landscape - argues for a different kind of silviculture, one that focuses on forests as ecosystems, not agricultural ecosystems.

1.4 What Are the Foundational Concepts of Ecological Silviculture?

While the definition of ecological silviculture may vary a bit among stakeholders, all are based on an understanding and modeling of natural disturbance and forest development. For silvicultural application, such a model begins with canopy disturbance and proceeds through stages that reflect structural maturation. The model may include a preforest stage, followed by young, mature, and old forest stages (Box 1.1). The model may apply to whole-stands or patches within stands. As you will learn in the chapters that follow, ecological silvicultural systems often emulate this developmental model and are catered to the ecological and socioeconomic contexts of the ecosystem being managed.

Box 1.1 Some Concepts and Terms Associated with Ecological Silviculture

• Biological legacies : biological legacies are healthy trees, decadent trees, large deadwood, and derivatives of dead trees, e.g. tip-up mounds, and pre-disturbance vegetation (advance regeneration, understory plants) that survive from the previous forest into the new forest after disturbance.
• Deadwood: dead trees, including snags and downed wood, and tree components (branches, roots).
• Forest developmental stages
  • Disturbance and legacy creation: a disturbance that kills trees and leaves a legacy of organisms and structures from the pre-disturbance forest.
  • Preforest: a period when herbs and shrubs dominate growing space and tree abundance is not enough to dominate the site.
  • Young forest: establishment of new cohort trees of relatively uniform size, along with large legacy trees; density-dependent mortality is a dominant process in this stage.
  • Mature forest: new cohort trees dominate; mortality shifts to density-independent agents; advance regeneration of shade-tolerant trees are released and species of lesser tolerance may establish in canopy gaps; spatial heterogeneity of structure increases; new deadwood accumulates.
  • Old forest: a diverse mix of live and dead trees of varying size and condition; high levels of spatial heterogeneity; increasing decline and mortality of trees from exogenous agents; significant inputs of large deadwood.
• Recovery period : the time between regeneration harvests of sufficient duration to allow for development of mature forest structures.
• Retention tree: a tree retained during a regeneration harvest to live out its natural lifespan so as to provide mature forest structural elements and associated functions.
• Structural complexity : abundance, size, conditions, and degree of heterogeneity in living and dead components in a forest.
• Variable retention...