Indigenous plant scientists generally have to make do with what is on offer, by way of trial sites.
Dr David Bergin’s wish list, for the open-ground and container-raised plants comparison, was for an area of reasonably consistent contour and soil type, and not so narrow as to expose to trials to too much
edge effect, and in the Mahurangi catchment—in deference to the
project’s initiator.
Mahurangi farmer
Shelley Trotter had just the site. And not only did it meet the ideal trial criteria, it was also accessible, being beside Sandspit Road. There is even ample pull-off width beside the busy road for parking, and a café next door!
But wait, there’s more. The trial area nicely augments a riparian planting, created earlier, and establishes a significant length of indigenous vegetation to a roadside landscape that currently has little such cover.
The second site is quite different.
It is neither in the Mahurangi catchment, nor is it accessible. But those deficiencies it more than makes up for by bringing in two additional terrain types: River flat and steep exposed hillside. It is located in the Silverdale district and accommodates two blocks the size of that at Sandspit Road, similarly minimal edge effect.
The trials were laid out, in early July 2008, by Dr David Bergin, assisted by three of his offspring. The trial layout is a considerable exercise. In addition to the positions of each of the 7000 plants needing to accurately determined, and the appropriate 18 different options of species and nursery type were laid out for each row of 10 plants, for the planters.
The randomised and replicated layout was designed with the input of biometrics expert Mark Kimberley. In each block are sets of three rows of each species, randomly determined. The order of open-ground, PB3-sized container and root trainer, within those rows, is also determined randomised. The randomising ensures that bias, intentional or unconscious, does not influence the positioning of species or raising style.
Planting itself proved to be more time consuming than for normal restoration planting, including because the correct, rather than nearest to hand, plant is used and because the need to plant reasonably accurately to the 1.4-metre grid—ensuring a consistent space between the plants. The space factor becomes critical as the plants approach
canopy closure. Planters were all employees of Scrub Growers, a participant in the open-ground project.
Edge effect Individuals at the edge of a trial are exposed to a somewhat different environment from those in the body of the trial. The impact of wind and browsing, for example, can be greater at the edges of a trial. In addition to avoiding elongated trail plots, additional rows of are established to buffer the trial plants.
Canopy closure The point at which the canopy of plants and/or trees becomes continuous, or closes, is the second of the two most important events in forest establishment. Until canopy closure occurs, there is considerable opportunity for competing weeds to overtop the plants being established. After the canopy becomes continuous, the most potentially troublesome competing species, being shade intolerant, will cease to threaten the planting.
(The first critical event, of course, is the initial establishment of plants, either naturally, by
direct seeding or by transplanting seedlings.)
Direct seeding The obvious first question that should be asked when contemplating, for the first time, the establishment of indigenous trees or other indigenous vegetation is why start with transplants, rather than from seed? Not surprisingly, this approach has been attempted with a number of species, and on a number of occasions.
Laying mänuka brush with ripe or ripening seed is one approach that has been used, with some success. Another approach is allowing mänuka and tötara to regenerate through lightly grazed pasture.
While some of these approaches appear promising, to date consistent establishment on any scale has only been successful with nursery-raised plants.
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The Open-Ground and Container-Raised Indigenous Plants Comparison is a Sustainable Farming Fund project.
Textbook Trial Site: Sufficiently large to accommodate eight replications of the randomised trial layout, and next door to the Parsley Pot Café! Photographer Michael Bergin
Head Start: The open-ground plants, centre, being on average twice the stem size of the potted plants, left, and three times that of the root trainers right. The question as to whether the open-ground plants retain their the initial advantage, after being established at the trial sites at Mahurangi and Silverdale, is about to be answered. Photographer Jonathan Barran