Revegetation Plan

Competitive native plants as a deterrent to Hydrilla invasion. Establishment of competitive native plants as an ecological approach to exotic plant control offers many advantages to more traditional control options. A-thriving-native plant community is the best deterrent to invasion by exotic weeds. Contrary to popular opinion, exotic weedy species are not highly competitive, but they do have several adaptations for exploiting disturbed environments. The widespread prevalence of exotic aquatic plants is due, in large part, to the creation and maintenance of large, artificial, aquatic systems. These man-made reservoirs are disturbed systems – they have broad areas of shallow water that can support large submersed aquatic plants, yet they contain no native plant populations to provide propagules for vegetating these areas. Like a freshly plowed field, they offer an abundance of sites open to colonization by species that are adapted to exploit such opportunities. However, if existing vegetation is present, weedy species are afforded little opportunity for invasion. By occupying the space and capturing the available sediment nutrients, competitive native species preempt the resources of the environment – effectively blocking establishment of later-arriving exotic, weedy species.

Another advantage is that most native aquatic plant species do not grow as abundantly as do the problem exotic species such as hydriila, water hyacinth, and Eurasian water milfoil. Most native species also do not form extensive surface canopies that prevent utilization of the water resource and obstruct the entry of atmospheric oxygen. Most native plants are incapable of rapidly spreading throughout an entire water body and, in Lake Conroe, their development will likely be restricted to water depths less than IO feet rather than the greater than 20 foot depths that hydrilla colonized in the past.

Native plants as fish habitat. In addition to their importance in protecting reservoirs from invasions of exotic weedy species, established communities comprised of diverse native species offer a diversity of habitats for various fish and invertebrate species especially littoral fish species such as largemouth bass, sunfishes, and crappies that rely, as juveniles, directly on aquatic plants for shelter from predation. These fishes also feed heavily on invertebrates that colonize aquatic vegetation.

Native aquatic plants also provide food for variety of waterfowl and other wildlife as well as water quality and sediment stabilization benefits and reduce shoreline erosion.

Recommended approach

Restoring aquatic plant communities is relatively new. As a result, the costs and best methods of establishing native plants are not well known. Costs of establishment will depend, in large part, on the rapidity of desired results. It is currently not practical to conduct large-scale plantings of aquatic plants such as is done with terrestrial plants (e.g. tree plantings for reforestation). Given time, however, we can establish founder populations of desired species and allow these to expand naturally to fill the available niches. The trade-off here is the cost of establishment versus the time required to achieve complete cover. The more founder colonies, the faster the revegetation project will be completed. In a race against hydrilla (as in Lake Conroe), the cost of establishment of natives should be weighed against both the cost of chemical control and the damage that would result if hydrilla wins the initial sprint.

Initial plantings should consist of small-scale, caged (to prevent grass carp herbivory) test plots of several species in various parts of the reservoir. These plots would be evaluated both for successful establishment of plants within the protective cages and also for growth outside the cage as an indicator of the potential magnitude of the remaining grass carp population for affecting the revegetation effort. Results of the test plots would help in the selection of plant species and sites for larger-scale revegetation efforts to follow.

A hydrilla early warning/monitoring system should be initiated to detect hydrilla as soon as it occurs so that it could be controlled immediately. A mechanism for prompt chemical control would have to be in place and there should be no question that hydrilla control would be carried out, how this would be accomplished, and by whom.

Objective: To promote the development of diverse native aquatic plant communities.

Establishment of competitive native aquatic plants will form the basis of an aquatic plant management plan for Lake Conroe. Diverse communities comprised of competitive native plant species:

Approach

Establish protected founder populations of desirable native plant species in selected areas of the lake and allow these to expand naturally to other areas of the lake.

Tasks

1. Establish small-scale test plots within grass-carp exclosures in several areas of the lake. These small-scale test plots would be evaluated during the first year to maximize the cost-effectiveness of later, larger-scale plantings. The objectives of the small-scale plantings will be to:

a) evaluate the suitability of several native plant species for establishment in different areas

of the lake, and

b) determine the potential effects of grass carp and other herbivores on revegetation efforts.

2. Expand test plantings to include additional plots of the most desirable and successful species. Particular methodologies employed for the expanded plantings will be determined from the results of the initial tests.

3. Initiate on-site propagation and production of native plant species.

4. Production of plant propagules for large-scale plantings.

5, Conduct large-scale plantings.

6. Monitor lake for the presence of exotic species.

7. Provide immediate treatment of any detected occurrences of the following exotic aquatic plant species:

hydrilla

Eurasian watermilfoil waterhyacinth

The following exotic species may require treatment in local areas if their growth interferes with the establishment of native species:

8. Monitor changes in plant distribution and species composition.

Year 1 (1995)

Task 1 - Small-scale test plantings:

species (Note that all species are native to Texas. All species have been grown in ponds at the LAERF in Lewisville, Texas for several years. Source material for all of the plants was collected within the state of Texas.)

submersed forms

muskgrass (Chara)

water star grass (Heteranthera dubia)

American pondweed (Potamogeton nodose)

wild celery (Vallisneria americana)

lotus (Nelumbo lutea)

emergent forms

arrowhead (Sagittaria)

spikerush (Eleocharis quadrangulata)

Exposures Two types of protective exclosures (fences) will be used:

1) a 5 ft by 5 ft by 4 ft high cage constructed of rebar and orange plastic construction fencing

(see attached photo)

2) a 7 ft by 14 ft by 3 ft high rectangular plot constructed of silt fencing

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