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3.1

Introduction

Work carried out by Mitchley and G rubb (1986) on chalk grasslands (details of which are presented in section 1.3) suggested that the ability of a plant to interfere with its neighbours was a major factor in controlling its abundance in the field. This experiment was designed to test how effectively each of the species investigated competed as an established plant with the other species of the turf as a whole. T he performance of each species in the experiment could then be compared with its firequency in the field (as presented in table 2.1) to test whether these were linked. A strong positive correlation would be taken as an indication that competitive ability might be a controlling factor in the field abundance of that species.

Interference between established plants

3.2

M ethod

3.2.1 Mitchley and Grubb's (1986) experiment

Experimental work on competition between species in hay meadows does not appear to be detailed in the literature, hence there was no obvious protocol to follow in designing this experiment. However, the work done by Mitchley and Grubb (1986) on the control of relative abundance of perennials in chalk grassland (described in section 1.3) was a useful starting point from which to design the procedure for this experiment.

Whilst their study provided convincing evidence to justify their results, Mitchley and G rubb drew attention to some of its probable shortcomings. They state that the artificiality of such an evenly-aged and evenly-spaced experiment makes its

interpretation difficult, although the long time-scale of the experiment negates these objections to a certain extent. They criticize the field experiment, since the soil on which it was conducted was cleared of vegetation and would have had a modified nutrient status. Also, grazing by mammals was absent, so the experimental plants were not subjected to the normal environmental conditions of chalk grassland.

3.2.2 Design of the adult competition experiment

This present study on neutral grasslands was intended to tackle some of the deficiencies that Mitchley and Grubb identified in their work. As a result of this, and of the

structural and ecological differences between chalk grasslands and neutral grasslands, the design of the experiments in this study differs significantly from that of Mitchley and Grubb.

In the first instance, it was intended that the environmental conditions during the interference experiment should be as close as possible to those in the rest of the grasslands studied. So, rather than grow the "test" species against one or two "competitor" species in a plot cleared of vegetation, the existing turf was used as a competitor. Thus, the test species would be exposed to the normal management regime of the grassland habitats, including grazing and, in the meadow, mowing.

At the time of setting up the experiment seed was not available, so plants were dug up, split into similar-sized pieces and grown on in pots, then transplanted back into small holes in the turf. T he following year their performance was assessed and related to their abundance in the field.

T he total density of plants in the turf of the experimental plots was essentially unknown. Consequently each test species could not be grown in monoculture at the same total

density as the turf, and could not act as a standard against which to base the

performance of each test species in the experiment. Instead, the dry mass of the test species in each of the different habitats would be compared to gain a ranking of "favoured" habitats according to yield for each species. However, it is possible that any difference in fertility between the different habitats may have a significant effect on the yields of the test species. T o control for this, the dry mass of the plots in which the test species grew was measured, and this allowed the dry mass of the test plants to be expressed as a percentage of the plot mass. If it were found that there was a significant difference in yield between habitats, this percentage would be used as an indicator of how well each species interfered with the other constituent species of the turf, and would allow a comparison of performance between the different habitats. The correction factor relies on the assumption that the productivity of the various plots is controlled by their fertility. This may not be the case in all instances (see section 3.4.3). T he comparison of the interference ability between different species in the same habitat requires no such correction factor. It does, however, assume that all test plants,

irrespective of species, were approximately the same size at the time of transplanting back into the field. This did appear to be the case, at least from a casual inspection, and was probably because their size was limited by the resources available in the small plant pots. Furtherm ore, it takes no account of the species having different potential

maximum sizes and growth rate. This should be borne in mind when interpreting the results.

3.2.3 Collection and preparation of experimental plants

T he plant species chosen to be tested were collected in May 1990 as follows; From M artin's M eadow Alopecurus pratetisis

Anthoxanthum odoratum Holcus lanatus Cerastium holosteoides Plantago lanceolata Ranunculus acris Rumex acetosa Primula veris From Bradfield Woods Agrostis capillaris

Deschampsia cespitosa Centaurea nigra Mentha x verticillata Prunella vulgaris

Interference between established plants

A large num ber of plants of each test species was dug up and the turves containing them taken to U C L where the test species were removed from the turves underwater, to prevent the roots from drying out. For each species, 60 roughly even-sized small plants were selected and potted-up with John Innes No. 1 compost into 5cm pots. Large plants were split into several smaller ones where necessary. The pots were kept on the roof of the Darwin Building at U C L and were watered regularly, but no additional fertilizer was applied.

It had been intended to return the potted test plants to the field as soon as it could be seen that they had recovered from the trauma of transplantation and had started to grow again. This would have allowed the field experiment to run for a longer period and to have produced more meaningful results. However, the summer of 1990 was

exceptionally hot and dry and the test plants would not have survived in the field without frequent watering, which was not possible logistically, and which may have produced artificial results. Consequently, the test plants were returned to the field in mid-September, after the drought had broken. Whilst this delay reduced the length of time in which the test plants could come to equilibrium with the vegetation into which they were transplanted, it did serve to ensure that all the test plants were very closely m atched in size, since they had become effectively pot-bound. Consequently, any observed differences in the mass of the plants at the end of the experiment could be attributed to the experimental treatments with greater certainty.

3.2.4 Arrangement of experimental plants in the field

T he test species were re-introduced into the field to form a series of grids, or

experimental blocks. Each block was sub-divided into 13 plots. The test species were each randomly allocated to one of these plots and nine individual plants of each test species were planted in their respective plots. Within the plots, the test plants were planted in a rectangular grid at 30cm intervals so that each plot measured 90cm x 90cm (see figure 3.1). The spacing was chosen so that each test plant could be considered to be independent of its neighbours for the purposes of replication. The test plants were planted directly into the otherwise unmodified turf. This was accomplished with the aid of a metal quadrat with cross-wires at 30cm intervals. The positions for planting were marked with pencils and then were drilled to a depth of about 7.5cm with a 5cm diameter auger. The holes were thoroughly soaked with water and then the plug of soil containing the test plant was removed from its pot and pressed gently into the hole. The comers of the blocks were marked with wooden stakes driven into the ground so that 15cm of the stake remained above the ground.

Six blocks were planted this way. Three were planted in Martin's Meadow in rather similar habitat (and were referred to as M M l, MM 2 and MM3) and 3 blocks were

planted into differing grassy habitats in Bradfield Woods (see section 1.5). The arrangement of species within each block is shown in figure 3.2.

3.2.5 Harvesting of experimental material

After the initial watering that each test plant received at the time of planting, no further watering was given and the plants could be considered to be influenced by natural environmental conditions. The test plants (with the exception o i Primula veris) were harvested in mid-July 1991, immediately before the meadow was mown, and had been growing as part of the meadow or woodland turf for 10 months. As the position of each plant, relative to the wooden stakes, was known, each test plant could be relocated with the quadrat. In practice, it was usually easier to find them by feeling for the slight depression formed where the soil had settled into the holes in which they were planted. Each test plant was cut at ground level and put in a separate paper envelope. The 90cm X 90cm plots in which the test plants had been growing were also mown at as close to ground level as was possible and collected. The test plants and mown plots were dried in ovens at 100 °C overnight, were allowed to cool, and then their dry mass was found using a top-pan balance accurate to 0.01 g.

Interference between established plants 90cm 30 cm

o

Nom inal area occupied by given species (90cm x 90cm ) = One experimental plot

H ole occupied by plant of given species (diameter 5cm)

H ole occupied by plant of different species Quadrat corner marker post

Des

Pla Pri Pru Ran Hoi Ala A n t Agr Ceti Cer Men Rum M artin’s M eadow block 1

Pri

Des Hoi A nt Pru Pla Rum Cer Rati Ala Cen Men Agr M artin’s Meadow block 3

Men A nt Des Pri Cen Ran Agr Rum

Pru

Cen Alo Rum Men Pla Des Ran Agr Hoi Cer Pri A n t M artin’s Meadow block 2

Pla

Des Ran Rum Hoi Agr Men Alo Cen Cer Phru Pri A nt Bradfield W oods Triangle

Hoi Cer Pru Alo Pla Bradfield Woods Coppice

Pru Des Hoi Cen A nt Rum Alo Men Cer Agr Pla Pn Ran Bradfield Woods Ride

Figure 3.2 The arrangement of the experimental species plots within the experimental blocks.

Interference between established plants

3.2.6 The special case of Primula veris

The performance oi Primula veris was assessed differently from the other test species. At the time of harvesting, there existed the possibility that the experiment might be

continued for another year. As Primula vens is a low-growing rosette plant, it would be at most minimally damaged by the mowing process, so harvesting its leaves would not mimic the normal meadow management regime. Consequently, the lamina length of each leaf of each plant was measured to the nearest 1 cm and was then squared to give a relative measure of leaf area. Thus, the relative performance of Primula veris among the different blocks could be assessed, but its performance relative to the other test species in a given block could not be assessed. Regrettably, the experiment was not carried on for a further year, so the value of the Primula vens data is somewhat reduced.

3.3

Results

3.3.1 The yield of experimental material

The dry masses of all the test plants and their plots are given in appendix 2 and appendix 3, respectively. The leaf length data for Primula veris are presented in appendices 4 and 5.

The summed dry masses (yield) of the surviving test plants from each plot are given in table 3.1.

Experimental site

Species M M l MM2 MM3 BWCop BWTri BWRid

Agrostis capillaris 8.554 10.36 8.327 15.44 16.12 15.45 Alopecurus pratensis 9.550 5.970 7.186 3.750 11.66 10.27 Anthoxanthum odoratum 13.42 9.853 6.543 14.99 8.038 17.20 Deschampsia cespitosa 2.432 4.833 2.158 16.68 19.84 13.73 Holcus lanatus 5.628 11.02 4.988 13.09 5.575 10.48 Centaurea nigra 1.406 2.172 5.449 0.210 6.338 6.412 Cerastium holosteoides 5.119 6.943 5.394 11.52 1.688 1.337 Mentha x verticillata 0.784 0.551 0.963 0.410 3.12 1.718 Plantago lanceolata 4.292 13.43 9.896 1.724 1.458 8.292 Prunella vulgaris 0.355 0.989 0.044 0.557 2.478 0.796 Ranunculus acris 5.106 6.671 5.230 0.019 1.509 2.670 Rumex acetosa 4.132 6.881 4.377 0.491 0.183 0.316

Table 3.1 The yield (the summed masses, in grams) of the test plants in each plot

The summed dry masses of the test plants, expressed as a percentage of the mass of the plots in which they grew (percentage yield), are presented in table 3.2.

Interference between established plants

Experimental site

Species M M l M M 2 MM3 BWCop BWTri BWRid

Agrostis capillaris 4.05 2.96 3.04 11.70 4.53 4.98 Alopecurus pratensis 3 6 3 1.67 2.39 3.15 3.67 3.84 Anthoxanthum odoratum 4.83 2.99 2.35 13.75 2.06 5.73 Deschampsia cespitosa 0.79 1.43 1.00 19.84 5.11 5.22 Holcus lanatus 3.04 3.87 2.15 10.55 1.35 3.72 Centaurea nigra 0.73 0.62 1.62 0.26 1.97 2.56 Cerastium holosteoides 1.85 2.16 2.32 30.32 0.55 0.64 Mentha x verticillata 0.30 0.23 0.50 0.34 0.77 0.70 Plantago lanceolata 1.74 5.48 2.77 2.05 0.44 2.72 Prunella vulgaris 0.13 0.41 0.01 1.86 0.68 0.34 Ranunculus acris 1.51 2.20 1.55 0.02 0.35 1.19 Rumex acetosa 1.41 2.22 1.38 0.53 0.07 0.13

Table 3.2 The percentage yield (the summed masses expressed as a percentage of the mass of the plot in which they grew) of the test species for each plot

T he relative leaf areas (the summed squares of lamina length) of the Primula veris plants are presented in table 3.3. The yield measure (total relative leaf area for each plot) and the percentage yield measure (yield measure divided by dry mass of the plot in which they grew) for the Primula veris plots are presented in table 3.4.

Block Summed squares of lamina lengths for each plant ( c m 2 ) .

M M l 181 172 121 182 134 9 297 149 - MM2 32 61 301 263 36 122 59 129 149 MM 3 29 187 102 41 50 90 199 50 - BWCop 109 62 16 97 - - - - - BWTri 180 75 77 85 129 - - - - BWRid 167 168 34 16 61 108 61 162 ■

Table 3.3 The relative leaf areas of the Primula veris plants for each plot. A dash (-) denotes a dead plant.

Block M M l MM2 MM3 BWC BWT BWR

Yield measure (cm-) 1245 1152 748 284 546 777

Percentage yield measure (cm^/g)

3.62 4.41 2.68 5.26 1.57 2.82

Table 3.4 The yield measure (total relative leaf area per plot) and percentage yield measure (yield measure divided by mass of plot) of the Primula veris plots.

Interference between established plants

3.4

Analysis

3.4.1 Analysis of Variance (ANOVA)

D ata from designed experiments are usually analysed by an analysis of variance

technique. This calculates how m uch of the variation in the data is accounted for by the different parameters, or effects, in the experiment. ANOVA tends to slightly suppress the significance of differences between data, so any significant differences detected by the analysis are likely to be real. In this experiment the analysis of variance is performed upon the summed dry masses of the test species (the yield, table 3.1) and the

experiment is treated as being of a randomised block design. The analysis cannot be performed upon the dry mass data for each individual experimental plant (which would have been the case had the experiment been of a fully randomised design) because the experimental plants were grouped into plots of 9 plants of the same species rather than being randomly distributed within each block. A fully randomised design would have increased the sensitivity of the analysis and would have allowed the effect of a

species/block interaction to be investigated more fully, but also it would have introduced too many logistical difficulties in setting up the experiment and relocating the

experimental plants for harvesting.

Consequently, the yield of the 72 plots is analysed with respect to the species of plant in the plot and the block in which it grew. The ANOVA table is presented in table 3.5.

Source Degrees of Sum of M ean F Probability

freedom squares square

Species 11 1019.8 92.7 6.3 0.000

Block 5 53.5 10.7 0.7 0.607

Error 55 810.4 14.7

Total 71 1883.6

Coefficient of Variation = 61.9%

Table 3.5 The ANOVA table for the yield of the test species

T he ANOVA table indicates that the yields of the species are very highly significantly different (sig. at 0.1%) but that no block effect has been detected at a significance of 5% or greater. This means that the yield of the experimental plants in a plot varies very

species grew does not have a detectably significant effect upon yield. Consequently, the raw masses of the experimental plants should be used in the analysis, rather than the masses corrected for variations between blocks.

It should be pointed out, however, that the coefficient of variation is very large and this indicates that the analysis is not very sensitive. Hence, some marginally significant differences in yield may be masked by the overall level of variation. The large coefficient of variation is a reflection of the heterogeneity of the environment in which the

experimental plants grew, and whilst it would be unacceptably high for an experiment evaluating crop varieties, for example, it is probably to be expected for a field situation such as this.

T he analysis of variance fits the observed experimental data into a model of how the mean plot yield is varied by the effects of the particular species grown and the block in which it grew. Table 3.5 shows that the basic model is of a significant species effect but no significant block effect upon the yield of a given plot. However, the details of the model are presented in table 3 .6.

Interference between established plants

Overall mean yield for plots = 6.202 g

Species M ean yield (g) Effect upon overall mean (g)

Agrostis capillaris 12.38 + 6.18 Alopecurus pratensis 8.064 + 1.86 Anthoxanthum odoratum 11.67 + 5.47 Deschampsia cespitosa 9.943 + 3.74 Holcus lanatus 8.463 + 2.26 Centaurea nigra 3.664 -2 .5 4 Cerastium holosteoides 5.334 -0 .8 7 Mentha x verticillata 1.258 - 4.94 Plantago lanceolata 6.515 + 0.31 Prunella vulgaris 0.870 - 5.33 Ranunculus acris 3.534 - 2.67 Rumex acetosa 2.730 - 3.47

Std Error of mean = 1.567 g Std Error of diff. = 2.216 g

Block M ean yield (g) Effect upon overall mean (g)

M M l 5.065 - 1.14 MM2 6.639 + 0.44 MM3 5.046 - 1.16 BWCop 6.572 + 0.37 BWTri 6.501 + 0.30 BWRid 7.389 + 1.19

Std Error of mean = 1.108 g Std Error of diff. = 1.567 g

Table 3.6 The main effects of the ANOVA model

The table shows that in the model to which the data have been fitted, the mean dry mass yield for any plot is 6.202 g and this is raised or lowered by the amount indicated for each of the species and blocks shown.

At the species effect level, the five grass species all raise the overall mean yield whereas all the dicotyledons except Plantago lanceolata lower the overall mean yield.

The block effect is less marked, as would be expected from the model, but the three woodland blocks slightly raise the overall mean and of the meadow blocks, M M l and MM3 lower the mean and M M 2 raises it. However, the model states that there is no significant block effect, so we m ust conclude that the block effects from table 3.6 do not represent real differences in yield but are possibly the result of random variation.

The Standard Error of the mean allows the calculation of the range of values (in the yield of a species or a block) that we would expect for a given confidence interval. This

is done by multiplying the Standard Error of the mean by the value of "t" in the