Planting Approaches and Considerations
Seed preparation Breaking seed dormancy Planting time Seeding rate
Seeding rates and competitive interference
Planting very low seeding rates
Seed bed requirements
Hay mulch seeding
Planting densities for trees and shrubs Wildings (plants from natural settings) Sod
Bare-root stock Container-grown stock Cuttings Sprigs
Source: Adapted from Whisenant, S. G. 1999. Repairing Damaged Wildlands. Cambridge University Press, Cambridge, U.K.
of this surge in restoration ecology has been the development of commercial nurseries that provide both plants and information on how to do restoration. An excellent example that cuts across several of these areas is Environmental Concern, Inc. of St. Michaels, MD, which is run by Edward Garbisch. Environmental Concern includes a commercial nursery, a consulting firm, and a nonprofit educational component. Garbisch himself is one of the pioneers in wetland restoration and ecological engineering (see Chapter 4 in Berger, 1985), and his company has published a variety of useful materials on wetlands including a planting guide (Thunhorst, 1993), a curriculum plan for teachers (Slattery, 1991), and a scientific journal.
Despite the experience that is accumulating, planting programs often fail when the species that are planted die or do not contribute significantly to the restored ecosystem in the long run. Failures range across the gradient from large to small projects. An example at the large scale was the U.S. Army Corps of Engineers planting project at Kenilworth Marsh in Washington, DC. Here approximately 30
acres (12 ha) of tidal freshwater marsh was planted at a cost on the order of hundreds of thousands of dollars. A well-developed marsh ultimately self-organized on the site but the intentionally planted species made up a relatively minor part of the plant community after 5 years (Hammerschlag, personal communication). On a small scale, for example, Shenot (1993; Shenot and Kangas, 1993) described the results of plantings at three stormwater wetland sites in central Maryland. Eight species were intentionally planted but they made insignificant contributions (less than 12% of the total density and less than 1% of the total diversity at each site) to the plant communities 3 to 5 years after planting. Lockwood and Pimm (1999) reviewed 87 published studies of restoration projects (mostly wetlands or prairies) for success or failure. They found 17 failures, 53 partial successes, and 17 successes. However, their review is biased because it considered only published studies. Many failures probably go unpublished because they would have to report negative results. Of course, failures are important opportunities to learn (see Chapter 9), and the publication of negative results should be especially encouraged in the field of restoration ecology.
One cause of failure in plantings is predation by species of herbivores that are attracted to the restoration sites. Plants in natural ecosystems have a number of defenses against herbivores, such as spines or chemical deterrents, which limit herbivory to on the order of 10% of net primary productivity. Exceptions occur, such as muskrat eat-outs in marshes (see Chapter 2) and insect outbreaks, but these cases are relatively rare. Restored sites represent new ecosystems which must self-organize to conditions different from those experienced by natural ecosystems. One expression of this self-organization is the emergence of new food chains, which may be undesirable to the restoration ecologist. Some of the best examples are herbivory of wetland plants by Canada geese (Branta canadensis) and of terrestrial plantings by white-tail deer (Odocoileus virginianus). The magnitude of herbivore impact was demonstrated by May (in preparation) in his study of freshwater tidal marsh restoration at Kenilworth Marsh mentioned earlier and at other sites along the Anacostia River. He enclosed some plots with fence to keep herbivores away from marsh plants (exclosures) and left other plots with no fencing as controls. In certain areas of the marsh all vegetation was eaten by herbivores (primarily Canada geese), except those plants protected within the exclosures (Figure 5.5). This kind of study demonstrates the power of herbivory to determine success or failure in restoration plantings. Whisenant (1999) describes techniques for protecting plants such as chemical repellents and protective tubes. Extra cost is required to protect plantings, but it is sometimes necessary as a safeguard against project failure.
Failures in planting projects sometimes are due to lack of accountability. Enough projects have been conducted that common causes of failure (such as from herbivory) should be able to be avoided. Some consulting firms who contract for restoration work now guarantee plantings against failure, which is an encouraging indication of the evolution of the field. However, large sums of money are still being wasted in planting programs destined to fail. This money could surely be better invested for conservation purposes, and restoration ecologists must always include this kind of economic perspective in their work.
Often ignored in restoration projects are the free biotic inputs from nearby ecosystems. These are usually seeds that disperse into the site, germinate, and become established. A common problem in restoration ecology is to focus solely on the intentional plantings and to overlook the "volunteer" species that emigrate from the surrounding landscape. These volunteers have also been called spontaneous species (Fraisse et al., 1997; Prach and Pysek, 2001) because they spontaneously appear at a site even though they were not intentionally planted. In many cases these kinds of species come to dominate the site. MacLean (1996; MacLean and Kangas, 1997) was able to split a wetland mitigation site in central Maryland into four experimental cells in which three strategies of plantings were tested: low diversity intentional planting (11 species) of native wetland species typical of local mitigation projects; high diversity intentional planting (132 species) of native wetland species and others; and natural colonization without any intentional planting. The high diversity case emphasized the multiple seeding approach in an attempt to remove seed source as a possible limiting factor to plant community development. The observed plant species richness after two growing seasons is shown in Table 5.3. Some of the intentionally planted species were observed but volunteer species dominated the diversity in all of the experimental cells. This result was even more pronounced in terms of stem density counts from permanent plots at the site (Table 5.4). Facultative (FAC and FACW) and obligate (OBL) wetland species dominated all of the cells in terms of observed species and in three of the four cells in terms of numbers of individuals. Since the presence of these species is an indicator of success for wetland creation, it is interesting to note that the cell which received no intentional plantings had the highest number of wetland species (Table 5.3) and the highest number of wetland individuals (Table 5.4) of all of the experimental cells. In this case, as in many others, the surrounding landscape provided a subsidy to the restoration project through dispersal of a high diversity of species at no cost to the humans conducting the restoration. This kind of result suggests restoration ecologists are either arrogant or naive in thinking that the set of species they have chosen for intentional plantings is the most appropriate for a site. Natural selection often demonstrates that the intentional plantings are incorrect and that volunteer species from seed sources in the surrounding landscape are competitively superior. Unfortunately, knowledge of natural recruitment is not well enough developed to reliably
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