3.4.1 Methods: Establishment of 12 ground cover plant species
A site within farmland maintained by Massey University at Poultry Road near Palmerston North (40o22’57.5’’S, 175o36’19.9’’E) was used for establishing these species. The site measuring 12m by 12m was on Manawatu fine sandy loam.
Manawatu fine sandy loam is categorised as a soil composed of alluvium material found along river levees. The top layer is about 25 cm deep and described as “dark greyish brown fine sandy loam; friable; with a moderate nutty texture”. It is also considered to be a well-draining soil type which occurs on “infrequently flooding river flats”. It may also dry out in summer, and topsoil structure could deteriorate if frequently cultivated (Cowie & Rijkse 1977). Prior to use for the trial, the site was used for grazing by sheep and dairy cows. Pre-existing vegetation was removed by rotary hoeing.
The ground cover species were planted using a randomised complete block design with three blocks. Each block contained all 12 species in separate plots. Within each 2m by 2m plot, three plants of the same species were planted in an equidistant triangle pattern 1.0 m apart from each other. The apical position of the triangle alternated between neighbouring plots to reduce the risk of overcrowding when the plants expanded (Fig 3.14).
Each plot received 200 kg/ha N in the form of ammonium sulphate by surface application following planting, followed by sawdust laid 7 cm deep to control weeds during establishment. The sawdust had aged for about a year prior to purchase. Care was taken not to have the sawdust too deep at the base of each plant to avoid disease problems, and all stems or branches were laid above the sawdust. All planting, fertilising and mulching was completed on 24 Nov 2008.
Bloc k 1 Ophiopogon planiscapus Juniperus procumbens 'Nana' Pimelia prostrata 'Anatoki' Ajuga reptans 'Caitlin's Giant' Lithodora diffusa 'Grace Ward' Veronica peduncularis 'Oxford Blue' Grevillea lanigera 'Little Drummer Boy' Persicaria capitata Acaena inermis 'Purpurea' Sedum mexicanum 'Acapulco Gold' Coprosma acerosa 'Taiko' Muehlenbeckia axillaris Bloc k 2 Acaena inermis 'Purpurea' Muehlenbeckia axillaris Ajuga reptans 'Caitlin's Giant' Ophiopogon planiscapus Grevillea lanigera 'Little Drummer Boy' Persicaria capitata Pimelia prostrata 'Anatoki' Veronica peduncularis 'Oxford Blue' Juniperus procumbens 'Nana' Lithodora diffusa 'Grace Ward' Sedum mexicanum 'Acapulco Gold' Coprosma acerosa 'Taiko' Bloc k 3 Juniperus procumbens 'Nana' Muehlenbeckia axillaris Persicaria capitata Ajuga reptans 'Caitlin's Giant' Pimelia prostrata 'Anatoki' Acaena inermis 'Purpurea' Grevillea lanigera 'Little Drummer Boy' Lithodora diffusa 'Grace Ward' Veronica peduncularis 'Oxford Blue' Ophiopogon planiscapus Coprosma acerosa 'Taiko' Sedum mexicanum 'Acapulco Gold'
Figure 3.14: Planting layout of the 12 species in a randomised complete block design. Triangles at the top indicating planting position for the column
3.4.2 Methods: Rate of establishment -- Measuring diameter and height
The rates of establishment were determined by making regular measurements of plant diameter and height. Each plant diameter was measured three times in planes approximately 45 degrees to each other to get a mean estimate, as some of the plants were irregularly shaped. Height was determined from the base of the plant to an approximate level of the ends of the majority of the shoots. This avoided taking the heights of singular “rogue” twigs. In addition to these physical dimensions, a visual estimate was also made of the percentage of each plot covered by the three plants. These measurements were taken every 2 months in 2009 (February, April, June, August, October, December); and then in May and November of 2010.
3.4.3 Methods: Visual estimate of plot coverage of ground cover plants
This measure was an estimate of the proportion of the 2 m by 2 m plot which was covered by the ground cover plants so that the underlying sawdust was not visible. In 2009, hand-weeding was carried out prior to recording observations so that the visual estimate was not confounded by the presence of other plants.
As the sample size is not large, the data could be skewed. Skewness measures the tendency of the deviations to be larger in one direction than in the other. The skewness value can be positive or negative; it measures the asymmetry of the data distribution. Observations which are normally distributed should have skewness near zero. Table 3.1 illustrates the improvements to minimise skewness after appropriate data transformation. If data transformation improved skewness, then the transformed data set was used in analysis.
Table3.1 Assessment of data set skewness to determine best transformation, if necessary, on plot coverage by ground cover plants prior to ANOVA.
Data period Untransformed data skew value
Optimal transformation
Type Improved skew value
Feb 09 1.77 Natural log 0.22
Apr 09 1.15 Natural log -0.10
Jun 09 0.89 Natural log -0.04
Aug 09 0.73 Square root 0.20
Oct 09 0.98 Natural log -0.35
Dec 09 0.07 n.a.; none better n.a.; raw data used
May 10 -0.25 n.a.; none better n.a.; raw data used
Nov 10 -0.76 n.a.; none better n.a.; raw data used
3.4.4 Methods: Light quality under the ground cover foliage
The light quality within the environment under the ground cover foliage canopy was of interest from the weed control perspective as it determined whether the conditions would be conducive for weed seed germination under the ground cover plants. This was carried out twice, in June and November 2010. Light measurements were recorded using a LI-1000 data logger, manufactured by the Li-Cor company with input from a pair of Skye Instruments SKP2155 light sensors. The data recorder and light sensor bases were mounted on a wooden platform, and placed
in an open-top box measuring 40 cm by 60 cm. The light sensors were mounted on flexible metal arms extending 55 cm out of the box, with the actual light sensitive area being a circular pad 3 mm in diameter at the end of the flexible metal arms. These sensors were able to detect the quantity of light at the wavelengths of 660 nm (red light), 730 nm (far-red light), as well as compute the ratio of red light against far-red light at any given instant.
At each plot, ambient light conditions were measured by placing the light sensors on the ground facing up and not in any shade. The sensors were then placed on the ground at the edge of the plant base and gently pushed inward under the plant as far as the sensors allowed without resistance from plant stems, sometimes lifting the metal arms slightly to avoid stem disturbance. This was generally a distance of 5-15 cm from the edge of the plant base. As there was minimal disturbance to the plant stems, foliage cover was maintained at normal density. As the light sensors were mounted on paired metal arms taped together near the ends, accidentally flipping the sensors upside-down could be avoided by ensuring that the paired arms did not rotate when held. Once the sensors were in place, the light readings were recorded in quick succession. For each plant, this process was repeated three times.
3.4.5 Methods: Weed germination in plots with established ground cover plants
The ground cover plants were deemed to be established 18 months after they were planted, and hand weeding was performed for the last time on 17 May 2010. The intent was to clear the plot of weeds, and then observe subsequent weed invasion in each plot. This period lasted 24 weeks from 18 May to 2 Nov 2010.
At the end of this period, all the weeds were harvested from each plot. Within each plot, the weeds were cut at ground level and bagged according to whether they were found growing in the uncovered spaces between ground cover plants, or had germinated through the ground cover plant foliage. The weeds were then dried in an 80oC oven for 5 days, and the dry weed mass weighed and collated according to whether or not the weeds were growing within or between ground cover plants.
To ensure that the dry weed mass collected across the different plots are easily comparable despite differences in plant diameter and ground coverage, the dry weed mass results are expressed below in grams per square metre of open area and of area covered in ground cover. The total area within the plot under ground cover foliage was estimated assuming that the plants approximate an elliptical shape, and the minimum and maximum plant diameter measurements used accordingly. The difference between these elliptical area estimates and the plot size of 4 m2 was assumed to be
the estimate of open space within the plot not under ground cover foliage. For plots where the ground cover plant reached the plot boundaries, the area of open space within the plot, if any, were visually estimated with approximate measurements, and ground cover foliage area was the difference between 4 m2 plot size and estimated open space.
3.4.6 Methods: Data Analysis
Statistical analysis on the data was performed using SAS 9.2. Analysis of variance was used to determine which plant species were significantly different to each other for the characters measured. To improve the fit of data to assumptions for ANOVA, data transformation was necessary at times (see Table 3.1).