Empirical Evidence


Results from research and field tests conducted over a five year period from 1990 to the present, involving over 100 test seedlings, demonstrated the encased method to be the most successful alternative in satisfying goals established for the project. The field study proved it to be the only alternative method that effectively achieved all goals, particularly that of replanting in areas with high tide depths of several feet.

An interesting planting application is along bulkheaded properties where mangroves were removed when the sea walls were installed. Although the removal of mangroves when installing sea walls is prohibited today, it was a common practice up until the early 1990's. This application represents one of the most challenging environments since water depth, tidal movement, and wave activity from boats combine to present a number of factors with which the replanting method must contend. In the field project, which is described in detail in the following, the Encased method demonstrated its capability in replanting along canal front bulkheading.

Seedlings were planted in a canal located in south Melbourne Beach where they were intentionally located in waters with significant tidal variances. The seasonal tidal depths at the planting site have varied from 30.5 centimeters (12 inches) during the summer months at low tide, to a maximum 132 centimeters (52 inches) at seasonal high tide. In addition, boat traffic on the canal is a daily occurrence creating periodic and substantial wave activity throughout the year. The mangrove seedlings planted at this location, using the encased method, have matured and shown significant growth since their planting. They have generated the prop roots which are a critical factor in long term survival and in the employment of successful Encased Replanting. One of the test plants has reached a stem height of 155 centimeters (61 inches) and prop roots now measure over 76 centimeters (30 inches) in length.

In contrast, seedlings planted using conventional methods and located in shallower water but in close proximity to the encased plants were unsuccessful in establishing themselves. All seedlings planted according to conventional methods were dislodged due to wave and/or tidal variances and were subsequently unsalvageable.

The techniques developed on applying the Encased method are relatively simple and the materials required are inexpensive. The essential component is the encasement itself which effectively accomplishes the isolation and protection previously discussed. Standard PVC pipe was tested as one encasement alternative. PVC proved to be sufficiently rigid to perform the tasks of supporting the seedling through its development of aerial roots. Other materials could also be used but the semi-rigid, light weight characteristics of PVC made it an ideal candidate. Results of the study confirmed that PVC pipe offered suitable properties for the successful planting of mangrove seedlings.

Selecting a diameter that would hold sufficient soil in order to provide adequate nutrition for the seedling to root was a subjective judgement. Since no previous experience could be used for guidance two pipe sizes, 3.8 centimeters (1.5 inch) and 10 centimeters (4 inch) diameters, were tested. The vertical dimension or length of pipe was also somewhat of a subjective decision, therefore the major factor used in determining the lengths for experimentation was the depth of the water during the peak of the seasonal high tide. From evaluation of tidal variances at the site it was decided that two extremes in planting height be tested.

The PVC was cut to length in order to accommodate the desired planting elevations. The pipe was then driven into the canal's bottom until it felt solid, approximately 30 centimeters (12 inches). The encasement was filled with soil from the site to 5 centimeters (2 inches) from the top of the pipe. By keeping the soil 5 centimeters below the rim it provided added protection in instances when water would rise to cover the top of the encasement. The seedlings were then planted vertically with the thicker end down and soil covering about 1/3 the seedling's total length.

One seedling was set at an elevation of 76 centimeters (30 inches) above the bottom. The average water depth at the site typically does not exceed 76 centimeters; however, water depth during seasonal high tide has been measured at 132 centimeters (52 inches). This setting ensured that the seedling would not be under water during normal seasonal conditions. A specific concern with this elevation was that during the summer when tide levels are seasonally low, the seedling would not receive adequate moisture for nourishment. However, the soil remained sufficiently moist for the seedling to develop without any unusual curtailment in growth. The seedling has developed normally and the stem has reached 152 centimeters (60 inches) in length and has spawned multiple prop roots.

Another seedling was set at an elevation placing it at 50 centimeters (20 inches) above the bottom. This setting ensured that the seedling would be under water periodically for varying periods of time throughout the year, although the frequency of being submerged would decrease during the summer and increase during the fall through the spring months.

The experimentation with extremes in elevation was somewhat subjective but was an effort to determine which extreme might reveal a deficiency in the Encased method. Obviously the first extreme or highest planting elevation provided the greatest isolation and protection from environmental factors. The second extreme or lowest planting elevation subjected the seedling to the greatest influence of wave and tidal activity, and therefore a higher probability of physical damage or displacement.

For planting of the seedlings, PVC pipe was cut longitudinally for its entire length . This cut allowed for growth in the cross sectional area of the tree without restriction. As the plant's girth increased beyond the diameter of the pipe, the pipe opened and expanded, allowing the trunk and the root system to enlarge An additional benefit of the longitudinal cut is it will facilitate the removal of the pipe. Once the tree matures sufficiently so that its foundation of prop roots are adequate to provide for the long-term nourishment and support of the tree independent of the encasement, the encasement then can be removed. The initial roots that were sprouted by the seedling followed the pipe, reaching downward toward the lagoon's bottom. The root bundles that developed inside the encasement eventually traveled the vertical length expanding the pipe as they grew. The continuing expansion of the longitudinal split increased to the point where the soil that was originally set inside for planting was being discharged, leaving only the root system. It was evident that as prop roots began to develop, about the third year, the mangrove was well on its way to becoming self-reliant, gradually attaining independence from the encasement .

Actual removal of the encasement will be heavily dependent upon the specific environmental conditions at the replanting site. Obviously the deeper the water, the stronger the wave activity, the more extreme the tidal variances, and the greater the effects of upland run-off, the longer the mangrove will need to rely on the encasement for support. Since the encasement is a passive form of support it can remain in place for as long as necessary, but typically its removal would take place between the second and fifth year of growth.

In my original project design, the intent was to remove all PVC as the tree matured; however, based on the most recent tests using a modified two piece configuration my concept has changed such that the lower portion driven into the bottom will not be removed. Only the top portion would ultimately be removed after the first couple of growing seasons and prior to the formation of aerial roots . The new two section system has several advantages over the original single tube design. First, the top section can be removed early in the plants development. This removal conceals the most visiable portion of the encasement, thereby offering improved aesthetics over the single piece configuration. Second, it avoids any potential damage to the young tree that might occur with removal of the tube section containing the root system. Third, the foundation portion that anchors the seedling and prevents its dislocation remains in place. Finally, since the lower section will not be removed, holes can be located in the bottom segment to facilitate the migration of the roots into the surrounding soil. Based on experience with the single piece configuration, the lower portion of the encasement will become covered with barnacles and mosses blending with its surroundings.

Results from experimentation indicated that the seedlings are not highly sensitive to the planting elevation relative to tidal activity. Even when the seedling initial growth has been physically damaged and it is partially submerged for a considerable period of time, as evidenced by the formation of barnacles on the seedling itself, the mangrove will sprout new growth and continue to develop. A characteristic of new growth following physical damage to an immature seedling is a two stem formation. Empirical evidence supports the author's opinion that planting elevation is only critical at the point where the seedling becomes adequately protected from the harsh environmental factors that would result in dislodging the plant prior to it achieving self-reliance. Self reliance being when the tree can stand on its own, supported by its prop roots without assistance from the encasement.

Differences in diameter of the encasement had no significant effect on the test plants, whether 3.8 centimeters (1.5 inch) or 10 centimeters (4 inch diameter) was used, the plants developed at about the same rate. The longitudinal cutting of the pipe minimized the effect of initial pipe diameter by allowing the plants to grow without restriction. Based on the study results, 3.8 centimeters (1.5 inch) diameter, 160 psi pipe can be effectively utilized for encasement without constraining normal growth.

Additional replenishment sites have been planted and are under evaluation. Each site, like the Melbourne Beach bulkhead demonstration, has been chosen to analyze the methodology relative to the specific environmental factors that make conventional methods ineffective. A demonstration project which has been underway since fall of 1994, is located at Sebastian Inlet State Park. The park is located on the Florida east coast and is the northern boundary line for Indian River County and the southern boundary of Brevard County. Two previous attempts by St. Johns River, Water Management District to replenish mangroves at this site have failed and hence the reason for our site selection. This locale is situated on the lagoon side of the inlet with a southern exposure that is regularly subjected to boat wakes and periodically subjected to extreme wave activity when storms and winter fronts pass through the area. In the summer of 1995, the eye of Hurricane Erwin passed directly over Sebastian Inlet resulting in considerable damage to the local foliage. Some of the encased plants were damaged but all survived the wind and waves generated by the storm. Seasonally this location is subjected to significant tidal variances that result in considerable shoreline erosion.




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Abstract
Ecological
Importance
Framework
for
Replenishment
Conventional
Planting
Methods
Limitations
Conventional
Planting
Project
Goals
Encased
Replanting
Empirical
Evidence
Summary
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