Elsevier

Forest Ecology and Management

Volume 265, 1 February 2012, Pages 37-46
Forest Ecology and Management

Applied nucleation as a forest restoration strategy

https://doi.org/10.1016/j.foreco.2011.10.013Get rights and content

Abstract

The pace of deforestation worldwide has necessitated the development of strategies that restore forest cover quickly and efficiently. We review one potential strategy, applied nucleation, which involves planting small patches of trees as focal areas for recovery. Once planted, these patches, or nuclei, attract dispersers and facilitate establishment of new woody recruits, expanding the forested area over time. Applied nucleation is an attractive option in that it mimics natural successional processes to aid woody plant recolonization. To date, results of experimental tests of applied nucleation are consistent with theoretical predictions and indicate that the density and diversity of colonists is higher in planted nuclei than in areas where no planting takes place (e.g. passive restoration). These studies suggest that the applied nucleation strategy has the potential to restore deforested habitats into heterogeneous canopies with a diverse community composition, while being cheaper than projects that rely on plantation designs. We recommend several areas where research would aid in refining the methodology. We also call for further comparisons as nuclei age beyond the 2–13 years that have been studied, thus far, in order to confirm that practical applications continue to match theoretical predictions. Finally, we suggest that applied nucleation could be effective in the restoration of a variety of habitat types or species guilds beyond the ones to which it has been applied thus far.

Highlights

► We review applied nucleation as a reforestation method. ► It may be more successful and/or cheaper than other reforestation methods. ► To date, its influence on composition beyond 2-13 years is not well-understood. ► Applied nucleation may also be effective in other, non-forested, habitats.

Introduction

Deforestation is recognized as a prime element of human-caused global change affecting biodiversity, carbon storage, soil erosion, habitat connectivity, and soil nutrient dynamics (Foley et al., 2005). Whereas preventing conversion of forest into pastures, urban habitat, or other non-forested land must be a key element of efforts to reduce future impacts, restoration of formerly forested land is increasingly recognized as a parallel strategy that can have significant benefits (Chazdon, 2008). However, the scale of the challenge facing efforts to restore cleared habitats is daunting. Purely passive restoration relying on natural succession results in highly variable recovery rates which may take several decades (Holl, 2007, Jones and Schmitz, 2009) or degraded ecosystems may remain in an alternative stable state (Suding and Hobbs, 2009). Meanwhile, the area of land in need of restoration is so large that intensive replanting programs are only feasible in specifically-targeted circumstances. For these reasons, cost-effective methods to facilitate forest recovery are greatly needed.

Recent discussions highlighting the links between succession and restoration have emphasized the potential to utilize natural processes governing dispersal, establishment, resource availability and community assembly to achieve specific management strategies (e.g. Luken, 1990, Walker et al., 2007). Often the most efficient restoration strategies, in terms of cost and effectiveness, facilitate natural successional processes (Chazdon, 2008, Lamb et al., 2005). Applied nucleation (i.e. establishing small patches of shrubs and/or trees to serve as focal areas for recovery) is a strategy that uses principles of colonization of non-forested landscapes by woody vegetation to restore forest cover. It borrows elements of both natural pathways of succession and active restoration to influence the direction and rate of natural succession. We propose that it has the potential to succeed to a greater extent, from a management perspective, than if either passive restoration or intense active management was undertaken by itself.

We review nucleation as a natural process of succession and discuss how it can be applied in a restoration context. We also review the ways in which applied nucleation may be more or less effective than common management strategies designed to restore deforested landscapes, primarily natural recovery and large-scale tree plantings. Finally, we offer suggestions as to how further research can help refine applied nucleation as a strategy to restore deforested areas and develop applications for a wider range of habitat or species types.

Section snippets

Nucleation as a natural process

Natural forest recovery, including colonization of open habitats created by natural and anthropogenic disturbances, frequently has been observed to take place in a discrete pattern whereby initial colonization by pioneer species creates “clumps” or clusters of vegetation around which other species establish (e.g. Archer et al., 1988, Castellanos and Figueroa, 1994, Del Moral and Bliss, 1993, Franks, 2003, Yarranton and Morrison, 1974). Yarranton and Morrison (1974) referred to the initial

Applied nucleation as a restoration tool

The application of nucleation in a restoration context has been suggested as a way to influence the trajectory and pace of restoration (Hooper et al., 2005, Reis et al., 2010, Rey Benayas et al., 2008, Robinson and Handel, 2000, Toh et al., 1999). In this review, we define restoration as assisting the recovery of a degraded, damaged, or destroyed ecosystem (SER, 2004), and aim in particular for a diverse ecosystem that resembles relatively undisturbed nearby forest. Restoration via applied

Influence of disturbance intensity and the surrounding landscape

The intensity and spatial extent of the disturbances that led to forest clearing influences the rate and direction of forest recovery (Chazdon, 2008, Holl, 2007, Parrotta, 1992). For example, areas that have been used for mining, pasture, or industrial scale agriculture for many years may require extensive intervention, such as planting trees throughout to ameliorate stressful soil conditions or to shade out pasture grasses. By contrast, sites that were used less intensively or for shorter

Rate of nuclei spread and establishment

As noted earlier, most studies of applied nucleation have spanned less than a decade following planting. Therefore, long-term monitoring of recovery is needed to answer a host of questions about the efficacy of this restoration method (Table 1B). An important question is whether and at what rate the nuclei spread over time. Experiments suggest that, over the short-term, there is relatively little establishment of seedlings at nuclei edges compared to areas under planted trees (Holl et al., 2011

Generalization beyond forest restoration

Thus far, applied nucleation has been tested in a relatively few habitat types, specifically forests dominated by animal-dispersed tree species. This restoration approach has the potential to be useful in a wider range of ecosystem types, although this remains largely untested (Table 1C). Given the role of nuclei in attracting birds (e.g. McDonnell and Stiles, 1983, Robinson and Handel, 2000), applied nucleation may be most effective in situations where birds are important dispersers. Seed

Conclusion

The scale of deforestation worldwide necessitates the development of techniques to re-establish forest cover on cleared land at a faster pace than strategies that rely exclusively on natural colonization processes, but at a cheaper cost than planting the entire disturbed area. Thus far, experimental tests suggest that applied nucleation matches theoretical predictions and patterns seen in natural succession; continued monitoring and testing will be useful in determining whether favorable

Acknowledgements

The authors would like to thank R.A. Zahawi, J.L. Reid, H. Briggs, and the rest of the Holl Lab Group at UC Santa Cruz for discussions and suggestions that improved this paper, and A. Meyghani for creating Fig. 1. Two anonymous reviewers made valuable suggestions. JDC would also like to thank the Eastern New York Chapter of The Nature Conservancy for providing research space during manuscript preparation.

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