Biomass estimates
Total aboveground biomass estimates for Carex lyngbyei from the Nunn’s Island
low-marsh transect were greater (P < 0.004) than those of other treatments and other sites in both 1988 and 1994 (Fig. 13). Of interest, however, is the lack of significant difference in Carex lyngbyei biomass between the planted and unplanted blocks as early as 1988.
Total aboveground biomass on the Nunn’s Island low marsh was different from other sites and other treatments in 1988 (P < 0.001) (Fig. 14). However, by 1994, there was no difference in total aboveground biomass, either between sites or between treatments (P > 0.30).
By 1988, there was no difference between the 0.5-m and 1.0-m planted treatments and unplanted treatments for either the Carex lyngbyei biomass or the total biomass estimates (P > 0.30).
Inundating water salinities
Although the small sample size of salinities precluded statistical comparison
(Table 1), it appeared that the lower elevation transects 14 and 15 on island 3 experienced higher salinities at the substrate level than did other transects at higher elevations (24-25 ‰ vs. 3-14 ‰), which, coupled with the longer inundation periods, could have been a contributing factor to the plug failures there.
DISCUSSION
The marsh creation project in the Campbell River estuary has resulted in the establishment of plant communities with typical intertidal marsh plant species at this latitude. Over the 13-yr study period, the constructed islands remained stable, with little apparent erosion. As a result, most of the planting blocks on the islands established Carex lyngbyei- or Juncus balticus-dominated communities with total cover similar to that of the natural low-marsh or mid-to-high marsh communities of adjacent Nunn’s Island. A striking result was the vegetation growth on the unplanted blocks that, between the 7th and 13th years, had reached
species composition and aboveground biomass levels comparable to those of both the planted and the natural marshes. Thus, the project has achieved Zedler's (1988) marsh creation goals of establishing plant cover and species richness near natural levels.
Despite this establishment, however, the marsh communities on the constructed islands are not as stable as those on Nunn’s Island, as suggested by the movements of transects in the PCA ordination (Fig. 5). These changes are due more to year-to-year shifts in relative abundance and cover than to actual changes in species composition. Moy and Levin (1991) and Zedler (1988) note that, in general, coastal salt marshes are subject to high temporal variability and are inherently unstable. Our data from this system, however, suggest the opposite. The Nunn’s Island low- and mid-to-high marshes were, in contrast to the planted
and unplanted blocks, relatively stable over the study period, as suggested by only minor relative movements within the PCA ordination.
Climatic variation undoubtedly affects year-to-year variation in marsh vegetation growth. Allison (1992) found that rainfall during both the dormant period and early growing season was correlated with vegetation changes in a California salt marsh. Sampling error, despite attempts to minimize the problem, could also contribute to the minor shifts in the ordination of the Nunn’s Island transects.
Noticeable marsh establishment on the constructed islands in the Campbell River estuary began to appear about four years after the initial planting in 1982. By 1988, most of the planted transects were similar in species composition to the Nunn’s low or mid-to-high marshes, but it was not until some time between 1988 and 1994 that species cover and frequency values attained values similar to those of the natural marshes. Vegetation in the unplanted blocks was initially
slower to establish, but by 1988, cover and species richness values approached those of the planted blocks.
Broome et al. (1986) found that primary productivity levels of transplanted Spartina alterniflora marshes resembled natural marshes after four years, and stabilized thereafter. This was not the case in our study, however. Although the cover and species composition of the constructed marshes were similar to those of the Nunn’s low marsh six years after planting, total aboveground biomass estimates of the created marshes were only about 40% that of the Nunn’s low marsh; Carex lyngbyei biomass was even less, at about 25%. Thirteen years after planting, a significant difference in the total aboveground biomass estimates of the planted marshes vs. the Nunn’s Island marshes could not be detected. Carex lyngbyei biomass of the planted and unplanted sites, however, reached only about 40% that of the Nunn’s Island low-marsh Carex biomass.
Other marsh creation projects have had mixed results in establishing successful plant communities. In a marsh transplant experiment in the Fraser River estuary, Pomeroy et al. (1981) found that one of three sites had negligible plug survival, while at the other two sites, Carex lyngbyei plug survival was 65% and 84%, which is somewhat lower than our plug survival rate of 90%. In both of their successful transplant sites, however, transplant shoot heights were not as great as those in the donor sites. Moy and Levin (1991), in a study of Spartina salt marsh creation in North Carolina, found that some constructed marshes achieved rates of primary productivity comparable to those of a natural marsh, but noted that salt marshes should not be considered a replaceable resource. Mason and Slocum (1987) found that nine of 19 constructed wetlands successfully established salt marsh vegetation in Virginia's coastal zone, while six more were apparently on the way to success.
Substrate elevation appears to be a major factor in the establishment of marsh communities on the Campbell River estuary. There, islands 1 and 2 had a higher elevational range than island 4 (Table 1) and, correspondingly, had a higher cover and richer plant communities. Island 4 vegetation was more closely aligned with Nunn’s low marsh in cover and richness, and was roughly at the same mean elevation as the Nunn’s low marsh. The least successful transects, with little or atypical plant establishment, were found on island 3. The low elevations on island 3 probably contributed to the poor plant establishment on
transects 13-15, because of the relatively longer inundation periods and the higher salinities of the inundating waters. Transect 12, within the elevational range of the other island transects, was successful. Pomeroy et al. (1981) also found that elevation and the associated salinity effect were strong determinants of plug survival in Fraser marsh transplants. Dawe and White (1982) note that substrate elevation is probably the most agreed-upon factor in determining vegetation zonation in brackish and salt marshes.
The diversity of species in the plugs planted in 1982 appeared to have little effect on the species composition of the resulting plant communities. Most planted and unplanted areas became increasingly dominated by Carex lyngbyei or Juncus balticus, regardless of the species composition of the plug or the species proximate to the unplanted areas. The fact that the unplanted transects eventually reached higher mean species richness than the planted transects
and showed less of a decline in cover between 1988 and 1994 may have had more to do with our moving the donor vegetation from its mid-marsh location to lower elevations on the islands, with their longer periods of inundation, than to any other factor. Seeds or propagules arriving at the unplanted areas, no matter what the elevation, would not be likely to establish unless they found site conditions suitable for colonization (van der Valk and Davis 1976).
These results suggest the importance of ensuring that donor vegetation is moved to a location where elevation and inundating water salinities are similar to those of the donor site. Although seeds or propagules may not establish if conditions are unsuitable, the vegetation in the plugs could appear to be doing well initially, even in less-than-suitable conditions. It has been our experience that marsh vegetation is quite resilient. Even when changes are thrust upon the plants
that make conditions unsuitable, they often tend to “hang on” for years. For example, Carex growth at the low elevations of island 3, much reduced in cover, but still extant six years after planting, had disappeared by 1994. Dawe and McIntosh (1993), found that Carex lyngbyei stands were relatively stable in terms of cover and frequency for the first five years of their marsh restoration project, but by year 10, the Carex stands had disappeared because of increased interstitial soil salinities. Simenstad and Thom (1996) also
noted a precipitous decline in the shoot density of Carex lyngbyei on their study area seven years after transplantation; the decline occurred after a gradual, linear increase over the first six years. The importance of such slowly changing variables can only be appreciated if monitoring continues for the long term.
Other factors reported to influence the rate of plant establishment in marsh creation are substrate characteristics and sedimentation rates (Seneca et al. 1976), grading and erosion control (Mason and Slocum 1987), and proximity to a natural marsh for seeding (Mason and Slocum 1987, Zedler 1988). On the Fraser River estuary, the main factors leading to the failure of marsh transplant projects included unstable substrate, incorrect substrate elevations, saturated soils, driftwood accumulations, and grazing by Canada Geese (Adams and Williams, in press).
Grazing by Canada Geese may yet become a factor on the Campbell River marshes, and is an example of the “moving target” aspect of ecosystems discussed by Walters and Holling (1990). During the early years of this project, there was a small amount of grazing on the plug vegetation, but the birds did not seem to affect the growth of vegetation on the islands. Few Canada Geese used the estuary at that time. For example, during weekly bird surveys over the period 31 October 1982 through 18 March 1984, a total of only 31 Canada Geese was observed on or near the islands (Dawe et al. 1995). However, on the east coast of Vancouver Island, these introduced, resident geese have increased their populations significantly over the years, and roughly 250 or more geese can now regularly be found on the Campbell River estuary. Heavy browsing of the vegetation was noted on some of the islands in the estuary in 1994. On the Little Qualicum River estuary, some 95 km south of Campbell River, natural, monospecific stands of Carex lyngbyei have been eliminated through the browsing effects of large numbers of Canada Geese now using that system (N. K. Dawe, unpublished data).
The establishment of high-cover plant communities on the unplanted blocks suggests that transplanting may not be required, provided that there is a nearby seed or propagule source with suitable species, and provided that the substrate is stable and at suitable elevation. The proximity of Nunn’s Island and the Nunn’s Creek marshes probably had some influence on the success of marsh plant establishment on the unplanted blocks. Some of the colonization may also have resulted from seeds in the plugs of adjacent planted blocks or a seed bank in the dredge
spoils. The results here suggest only a slight delay in success between planted and unplanted blocks. Frenkel and Morlan (1991) also note that planting may be unnecessary in some cases.
The concept of ecosystems as moving targets (Walters and Holling 1990) seems particularly well suited to the developing island marshes at Campbell River. Had our monitoring survey ended in 1988, we might have concluded that the project was a success, and that the planted marsh communities had achieved a species composition and cover similar to those of the natural marshes. Our 1994 results show, however, that the community composition of the islands is still changing.
Also, the large areas devoid of vegetation on some of the planted blocks did not appear until some time after year 6, another example of slowly changing variables. A combination of island design and substrate settling that resulted in areas of standing water and waterlogging of the plants probably contributed to the dieback. This problem may have been avoided initially, had the islands been constructed with a rounded cross-section or been sloped like island 3 and Nunn’s Island.
Broome et al. (1986) found that a 10-yr sampling period was sufficient to compare the primary productivity of a transplant site with that of adjacent natural marshes, and to determine that the transplanted marsh was persistent and self-sustaining. Results of our study are not so clear, even after 13 years. Mitsch and Wilson (1996) advise a period of 15–20 years before
judging the success of creating freshwater marshes, and they suggest that coastal wetlands may require even more time. Our study and others highlight the need for long-term monitoring of coastal marsh creation and rehabilitation projects (Race and Christie 1982, Zedler 1984, 1988, Frenkel and Morlan 1991, Dawe and McIntosh 1993). The time required will differ depending, in part, on the dynamic nature of the system, but should continue at least until annual variations
in species composition, cover, and productivity of the planted vegetation are similar to those of a nearby natural system.
CONCLUSIONS
Because adaptive management is a process of “learning by doing,” the following is a brief discussion of what we have learned from this project to date.
First, substrate stability is a prerequisite for any intertidal marsh creation project. There is little likelihood of success if the transplanted vegetation must deal with a shifting substrate.
Second, the created substrate should be of a type, and built to an elevation, as close to those of nearby intertidal marshes having the vegetation species composition and biomass levels that the project is trying to recreate. Because the substrate elevation affects the inundation periods and inundating water salinities, errors here could result in ultimate shifts in the species composition to a type not planned for, or could result in the species being eliminated altogether, as was the vegetation at the lower elevations of island 3 in this study.
Third, it seems likely that, if a stable substrate has been created at the right elevation and there is a proximate seed or propagule source, such as a nearby marsh, transplanting of plugs would not be necessary at all. This could be a significant factor for projects with few financial resources and lots of time.
Fourth, the system itself is a “moving target” evolving through natural and human impacts, and it has many potential outcomes (Walters and Holling 1990). The challenge here is to distinguish ongoing changes in the created system resulting from the project’s management activities from the natural and human-influenced changes that would affect both the project and the natural system.
Fifth, many of the system variables change slowly on a temporal scale and can only be observed through long-term monitoring.
Sixth, this study emphasizes the importance of an adaptive approach to environmental projects, with long-term monitoring as a key component. With learning as an inherent objective (Johnson 1999), monitoring, evaluation, and response must take place (Walters 1986, Hilborn 1992). Monitoring, and then evaluating the results of the monitoring effort, allows managers to instigate new management actions or to modify the original management actions where necessary.
For example, as a result of our monitoring efforts in 1994, we were quickly able to react to the unforseen problems of the vegetation dieback due to ponding of the water on three of the islands. In an attempt to reverse the dieback, we created a number of channels to drain these areas, mimicking the dendritic channels of natural systems. The channels were dug on two of the islands, leaving one island with areas of dieback as a control. Another complete sampling session planned for 2002 will allow us to evaluate the effect of these new management actions. As Hilborn (1992) observes, “If you cannot respond to what you have learned, you really have not learned at all.”
Finally, although this project does appear to be successful, at least based on the few factors of the system we have been monitoring, we repeat the cautions of Race and Christie (1982) and others regarding the use of marsh creation as mitigation for coastal development projects. The result may be the trading of natural coastal wetlands or mudflats for human-made marshes that may ultimately fail to become productive systems.
Fahselt (1988) notes that meeting conservation objectives through transplantation could suggest that habitat conservation of important natural areas is not urgent, and, as a result, development of these sites may be unwisely allowed, much as a “no net loss” policy may tend to do. Gibson et al. (1994) also “remain skeptical” that wetland creation can mitigate losses of natural wetlands; they emphasize the importance of preserving existing marshes.
Holling and Meffe (1996) argue against a reduction of the range of natural variation of any system. When such a reduction occurs, the system loses its resilience, becoming more prone to a change in structure and thus increasing its vulnerability to perturbations that normally could be buffered. Simenstad and Thom (1996) report that, after seven years of monitoring a total of 16 ecosystem attributes at their marsh creation site, only a few showed functional trajectory
thresholds indicative of natural, mature systems. This suggests that created wetlands are much simpler systems, with less range in variation than natural wetlands, even after a number of years of apparently successful growth, and are likely to be more vulnerable to perturbations than are the natural systems.
As a management concept, Holling and Meffe (1996) suggest that system processes and variables should be maintained as a default condition rather than changing them, unless there is no other option. We, too, urge caution in trading existing, productive estuarine habitat for the promise of mitigating the impacts of developments through marsh creation techniques until we have a better, long-term understanding of the status of the existing marsh creation projects on the eastern Pacific coast.
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Acknowledgments
British Columbia Forest Products and, later, Fletcher Challenge Canada, allowed access to the site and provided technical assistance in obtaining some of the elevation measurements. Michael J. Brownlee (deceased), Don Gordon, and Colin D. Levings, Fisheries and Oceans, Canada, provided input to the initial research design. The following people assisted with the fieldwork over the years of the study: Heather Jones, Terri Martin, John McIntosh, Edward Nygren, Roy Ostling, Jacques Sirois, Louise Waterhouse, and Michaela Waterhouse. Students from Human Resources Development Canada (then Canada Manpower and Immigration), Summer Employment Program, also helped with the fieldwork over the first 5 years of the study. Lance Gunderson, Pam Krannitz, C. S. Holling, and two anonymous reviewers provided helpful comments on the manuscript.
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Address of Correspondent:
Neil K. Dawe
Canadian Wildlife Service, 3567 Island Highway West, Qualicum
Beach, British Columbia, Canada V9K 2B7
Phone: (250) 752-9611
Neil.Dawe@ec.gc.ca