Vegetation structure, species diversity, and ecosystem processes as measures of restoration success

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Abstract

Most restoration projects have focused on recovery of vegetation to assess restoration success. Nevertheless if the goal of a restoration project is to create an ecosystem that is self-supporting and resilient to perturbation, we also need information on the recovery of other trophic levels and ecosystem processes. To provide an example on how to assess restoration success, we compared four measures of vegetation structure, four measures of species diversity, and six measures of ecosystem processes among pre-reforested, reforested, and reference sites. In addition, we described how Bray Curtis Ordination could be used to evaluate restoration success. Vegetation structure recovered rapidly due to the increase in vegetation height and the decrease in herbaceous cover. Other measures such as litter cover, number of litter layers, and DBH size class values are recovering at slower rates, but they also have increased vegetation heterogeneity in the reforested site. Species diversity recovered rapidly. The increase in vegetation structure changed the local conditions in the reforested site facilitating the colonization of woody seedlings, ants, reptiles, and amphibians. Ecosystem processes, particularly litter production and turnover, have enhanced the incorporation of nutrients and organic matter in the soil. By including vegetation structure, species diversity, and ecosystem processes measures we have better information to determine the success of a restoration project. Moreover, the Subjective Bray Curtis Ordination is a useful approach for evaluating different restoration techniques or identifying measures that are recovering slowly and would benefit from additional management.

Introduction

Most restoration projects have focused on the recovery of vegetation (Young, 2000) to assess restoration success. Nevertheless if the goal of a restoration project is to create an ecosystem that is self-supporting and resilient to perturbation (SER, 2004), we need to measure more than just vegetation. What measures need to be assessed to determine if a restored site is self-supporting? Vegetation structure, species diversity, and ecosystem processes have been identified as essential components for a long-term persistence of an ecosystem (Elmqvist et al., 2003, Dorren et al., 2004). Measures of vegetation structure provide information on habitat suitability, ecosystem productivity, and help predict successional pathways (Jones et al., 2004, Silver et al., 2004, Wang et al., 2004). Measures of species diversity provide information on susceptibility to invasions (e.g., proportion of native and exotic species), and trophic structure necessary for ecosystem resilience (Parmenter and MacMahon, 1992, Peterson et al., 1998, Nichols and Nichols, 2003). Measures of ecosystem processes provide information on biogeochemical cycles and nutrient cycling necessary for the long-term stability of the ecosystems (Herrick, 2000). Most restoration projects measure some aspects of vegetation structure or diversity, arthropod diversity or nutrient pools (Ruiz-Jaén and Aide, 2005), but studies rarely assess more than one measure of each component.

Along with assessing many measures in a restored site, it is necessary to compare this information with similar data from pre-restored and reference sites (Hobbs and Norton, 1996). The pre-restored and reference sites should occur in the same life zone, close to the restoration project, and should be exposed to similar natural disturbances (Hobbs and Harris, 2001, SER, 2004). If chosen correctly, these sites can provide endpoints to evaluate the success of a project (Passell, 2000, Purcell et al., 2002). The use of reference points can help to identify whether the response of the restored site is caused by the restoration activity or by unassisted recovery (White and Walker, 1997).

The goal of this study is to provide an example of how to evaluate restoration success in an integrative way using measures of vegetation structure, species diversity, and ecosystems process. We evaluated restoration success by comparing four measures of vegetation structure, four measures of species diversity, and six measures of ecosystem processes among pre-reforested, reforested, and reference sites in Puerto Rico. We addressed the following questions: (1) how does the vegetation structure of the reforested area changed in comparison to both the pre-reforested and reference sites? (2) how does the change in vegetation structure enhance species diversity in the reforested site? and (3) how does the ecosystem processes of the reforested site changed in comparison to both pre-reforested and reference sites?

Section snippets

Study area

The study was conducted in Sabana Seca, Puerto Rico (18°27′N, 66°12′W). This site is located in the northern limestone region, and is classified as subtropical moist forest (Holdridge, 1967). Mean annual rainfall is 1693 mm with a rainy season between April and December and a dry season between January and May (Eusse and Aide, 1999). The pre-reforested site was a park in a Karst valley where the grass was cut on a regular basis. This site was abandoned (i.e. no longer mowed) in May 2000. The

Vegetation structure

Three years after planting, the growth of woody stems in the reforested site created a diverse vegetation structure (Fig. 1). The number of stems in the 1 to <5 cm DBH size classes was higher in the reforested site (171 stems) than in the pre-reforested site, which had only one stem in this category. The reforested site had fewer stems in the 5–10 cm DBH classes (n = 8) in comparison with the reference site (n = 156; Fig. 1a). Nevertheless the reforested site has stems >10 cm DBH (n = 3). Vegetation

Vegetation structure

Woody vegetation height and herbaceous cover were the measures of vegetation structure that changed most rapidly due to the planting and early establishment of pioneer species in the reforested site. These pioneer species (e.g., Cecropia shreberiana, Roystonea borinquena, and Thespesia grandiflora, planted; Delonix regia and Senna siammea, colonizers) quickly accumulate biomass (Guariguata and Ostertag, 2002), and provide a diverse vertical structure and canopy cover necessary for arboreal

Conclusion

This restoration project has initiated rapid changes in vegetation structure, species diversity, and ecosystem processes, but has restoration been successful? The rapid increase of vertical stratification, low herbaceous cover, rapid colonization of species from different trophic levels, high litter production, and rapid C3 incorporation in SOM suggest that the reforested site could be left without further management assistance. These results indicate that the restoration project has been

Acknowledgments

This project was funded by the Biology Department Graduate Program of the University of Puerto Rico, NASA-IRA, DEGI, and the Laboratory of Plant Ecophysiology of the Venezuelan Institute of Scientific Research. Neftali Rios-Lopez and Miguel A. Acevedo provided the herpetofauna and bird census data, respectively. The δ13C values were analysed in The Stable Isotope Ratio Facility for Environmental Research (SIRFER) at the University of Utah. The comments of Jim Ebersole and Carolina Monmany

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