Applied nucleation as a forest restoration strategy
Graphical abstract
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.
References (145)
- et al.
Potential contribution of the seed bank in coniferous plantations to the restoration of native deciduous forest vegetation
Acta Oecologica-International Journal of Ecology
(2001) - et al.
Influence of tree species on understory vegetation diversity and mechanisms involved - A critical review for temperate and boreal forests
Forest Ecology and Management
(2008) - et al.
Exotic tree plantations and the regeneration of natural forests in Kibale National Park, Uganda
Biological Conservation
(1996) - et al.
Forest fragmentation differentially affects seed dispersal of large and small-seeded tropical trees
Biological Conservation
(2007) - et al.
The role of native species plantations in recovery of understory woody diversity in degraded pasturelands of Costa Rica
Forest Ecology and Management
(2004) - et al.
Biodiversity conservation in human-modified landscapes of Mesoamerica: Past, present, and future
Biological Conservation
(2010) - et al.
Mechanisms of primary succession: Insights from the eruption of Mount St Helens
- et al.
Selecting framework tree species for restoring seasonally dry tropical forests in northern Thailand based on field performance
Forest Ecology and Management
(2003) - et al.
Globalization, forest resurgence, and environmental politics in El Salvador
World Development
(2006) - et al.
When and where to actively restore ecosystems?
Forest Ecology and Management
(2011)