Contemporary forest restoration: A review emphasizing function
The forest restoration challenge (globally 2 billion ha) and the prospect of changing climate with increasing frequency of extreme events argues for approaching restoration from a functional and landscape perspective. Because the practice of restoration utilizes many techniques common to silviculture, no clear line separates ordinary forestry practices from restoration. The distinction may be that extra-ordinary activities are required in the face of degraded, damaged, or destroyed ecosystems. Restoration is driven by the desire to increase sustainability of ecosystems and their services and restoration is likely to have multiple goals arising from the motivations of those involved. The process of setting restoration objectives translates vague goals into feasible, measurable targets and ultimately actions on the ground. Our objective for this review is to synthesize the science underpinning contemporary approaches to forest restoration practice. We focus on methods and present them within a coherent terminology of four restoration strategies: rehabilitation, reconstruction, reclamation, and replacement. While not a consensus terminology, these terms have a logical foundation. Rehabilitation restores desired species composition, structure, or processes to a degraded ecosystem. Reconstruction restores native plant communities on land recently in other resource uses, such as agriculture. Reclamation restores severely degraded land generally devoid of vegetation, often the result of resource extraction, such as mining. Replacement of species (or their locally-adapted genotypes) with new species (or new genotypes) is a response to climate change. Restoration methods are presented as available tools; because adding vegetation is an effective restoration technique, the discussion of methods begins with a description of available plant materials. We then discuss altering composition under different initial overstory conditions, including deployment methods depending upon whether or not an overstory is present, how much of the landscape will be restored, and the complexity of the planting design. We present some major approaches for altering structure in degraded forest stands, and describe approaches for restoration of two key ecosystem processes, fire and flooding. Although we consider stand-level designs, what we describe is mostly scalable to the landscape-level. No restoration project is undertaken in a social vacuum; even standlevel restoration occurs within a system of governance that regulates relationships among key agents. Gathering information and understanding the social dimensions of a restoration project is as necessary as understanding the biophysical dimensions. Social considerations can trump biophysical factors.