Introduction
A significant length of the Ohinemuri River has been fenced off from stock as a result of the Hauraki Rail Trail development. Discussions about such an eventuality had occurred a number of years ago (Waikato RC, DOC, Waihi Walkways and HELP met a number of times to look at progressing a walkway from Waikino to Waihi) and both authorities had then proclaimed a desire to remove stock from the river bank. The fencing will mean that pasture grasses will grow relatively undisturbed and in itself this greater sward could provide a better protection against flood flows. However the pasture grasses do not represent a native ecosystem and in some areas have been invaded by weeds which will proliferate without the grazing competition. The project aim is to introduce native sedges into the environment to enhance both the bank stability and the ecology of the riparian margin. The total length of the river with pasture margins is considerable and planting its entirety in dense sedges would be a very costly project. Therefore the aim of the project is to trial various planting strategies to see if the cost of achieving a suitable riparian strip could be achieved. The aim of the trial is not to form a complete riparian margin including shrubs and canopy trees. In part this is limited by the establishment of the cycle trail and fencing and consequent set width and height above high flood levels. Also it is felt that if a shrub zone was established alongside large sections of the rail trail the present appeal of the rail trail (with its river views and openness) would be adversely affected. Rather it is to see if sedges can be introduced into the river margin with minimal disruption of the existing pasture sward.
The Plan
The intention is to trial two carex - geminata and secta - at various planting densities with varying pre- and post- planting strategies. Areas of the riverbank between Waikino and Waihi in the McKinney Road area have been selected - based on keeping some uniformity of sites and planted according to the strategies. Survival and growth rates would be assessed over a two year period and the results should provide enough information to make decisions about optimal planting programmes.
McKinney Road area during the construction of the Rail Trail (August 2013). The area was unfenced to the river but now the new fence (left) excludes stock. The cropped grass has now become rank, with increasing numbers of weeds.
Details
Establish two control areas - each say 100 metres length of stream bank. Control One - untouched pasture sward left to grow in its own manner with no management by man Control Two - pasture left to grow in its own manner but with management by man involved weed control (eg sprays to remove gorse, blackberry, honeysuckle etc) Establish a number of experimental planting areas
Experiment One - carex geminata planted at two metre intervals into the existing sward
Experiment Two - carex geminata planted at one metre intervals in spot sprayed sward
Experiment Three - carex geminata planted at 0.75 metre intervals in spray cleared areas
Experiment Four - carex secta planted at two metre intervals into the existing sward
Experiment Five - carex secta planted at one metre intervals in spot sprayed sward
Experiment Six - carex secta planted at 0.75 metre intervals in spray cleared areas
Each of these six experimental areas would be sub-divided into areas which are left unsprayed and areas which are managed by spraying for weed growth (as in Control Two).
Each area would be plotted by GPS, photo surveyed before the trial starts so that any potential weaknesses (or strengths) can be noted, and then growth reviewed. At the end of Year One a formal survey would be undertaken to assess survival and expansion of plants in each of the areas, as well as noting other relevant features such as erosion features and the presence of weeds. This survey would be repeated at the end of Year Two. The relative survival and expansion of plants in each area could then be compared and conclusions reached about the effectiveness and financial implications of different planting strategies.
Benefits
How will the project benefit the Waikato region's environment or promote environmental education?
Benefits will occur in a number of ways. Local representatives of Waikato Regional Council, Hauraki District Council, DOC and HELP Waihi have already met to discuss strategies to deal with weeds in the area. This will add to knowledge for this group. The results of the trial would be applicable not just to the Ohinemuri River and Waihi basin but to other areas of the Waikato and wider afield. On each area of the river bank under experiment there should be some improvement of the riparian strip.
HELP Waihi would also look to use the trials to raise cyclists' awareness of the need for environmental management and seek their help in weed control and riparian planting along the river and the rail trail. A first step in this will be to erect some signage on site explaining the purpose of the trials.