Experimental units are often too large to be measured completely. A sample of part of the experimental unit may be used as the basis for measurement. Care must be taken to ensure that the sample is representative of the whole experimental unit.

Details of how to sample, what to sample and when to sample are very much specific to each weed-control agent combination. For development of sampling techniques for control agents, see Southwood (1978) or other general textbooks on ecological techniques. Preliminary sampling of the weed population and control agent population should be carried out for at least a year and at several sites to determine seasonal and spatial variation in the populations. The preliminary sampling should be carried out as part of pre-release studies of the weed and the study of the establishment and spread of the agent. Information gained from this work can be used to develop the sampling program and contribute to the timing of samples and location of sample sites. Elaborate or overly complicated sampling designs should be avoided as the time and labour required may conflict with the need to sample large numbers of sites over a wide area.

Quadrats of varying sizes are used for counting plants or insects per unit of ground area, or for measuring biomass. The choice of quadrat size depends on the vegetation to be studied and the question to be asked, but a good rule of thumb is that, for square quadrats, the length of a side should not be less than the height of the plant. See Krebs (1989, pp. 64-72) for a discussion of quadrat size.

Litter trays (Farrell et al. 1992) provide an estimate of production per unit area of seed output, leaf fall or biomass. They can be emptied as often as required; more frequent emptying gives a more detailed time course (e.g. Lonsdale 1988).

Surface samples using vacuum cleaners and soil cores using soil augers allow one to estimate the soil seed population. Monitoring soil seed banks can be very time consuming, particularly if the soil is heavy, and it is important to ask whether it is actually necessary to study seeds in the soil at all before embarking on a major project. It may be that a sufficient understanding of the plant's demography can be obtained by simply monitoring input (seed fall) and output (seedling emergence), both of which involve rather less earth-moving than do soil cores. Such information can be made more complete by using seed bags for burying known numbers of seeds in the soil to investigate their longevity. Bags are retrieved at intervals and the seeds taken out for germination tests. A rate of decline can then be calculated by regression techniques (See Lonsdale et al. 1988).

[ Back ] [ Next ]