7  Scientific approaches

Two complimentary approaches can be used to adequately address the major research questions. In this thesis physical traits and epiphyte communities of aspen will be sampled in:

  1. Natural populations

  2. Experimental populations

Each of these approaches has its advantages and disadvantages.

7.1 Sampling in natural populations

This approach has the advantage of making measurements under realistic ecological conditions, assessing whether genetic effects are sufficiently important, and whether they have a significant influence on dependent community composition in nature (Tack et al. 2010). It is anticipated that sampling natural populations will provide a realistic representation of how the variation in measured traits may influence epiphyte communities in the wild. Significant results obtained from an experimental trial may not be reproducible in a wild system. However it is important to acknowledge that this approach is subject to a number of limitations that restrict the interpretation of the results obtained from the study. A major limitation is sourcing a sufficient quantity of wild clones, within close proximity to one another, and containing enough ramets to be statistically meaningful. A second limitation is that environmental variation within a given site may confound genetic variation between clones; environmental causes of differences in dependent communities may be difficult to disentangle. To some extent this difficulty can be overcome if clones are intermingled, but the problem remains.

A third limitation of utilising observations in natural populations is that the study will necessarily be restricted to a single site. The genotypes will only be observed under a single set of environmental conditions. This means that it is not possible to draw more general conclusions about the wider importance of any effects of genotypic variation within the structural species on the community composition of dependent species. A fourth problem associated with studies of natural populations is that they only allow determination of effects of genetic variation that exist within the single study population of the dependent species. This genetic variation may be insufficient to have an influence on epiphyte community composition. However genetic variation found between populations may be much larger and more important in determining dependent community composition, as has been found for Eucalyptus globulus in south-eastern Australia (Barbour, O’Reilly-Wapstra, et al. 2009; Barbour, Forster, et al. 2009).

A fifth problem with using wild populations is that the different ramets of the same clone may be connected below ground, because of their origin from suckers, and therefore represent non-independent units. It is currently unknown how interconnected the ramets are within their respective clones. However it is believed that independence of ramets from parent trees is likely to occur after 8 years. The aspen clones in the population at Invertromie are 50-80 years old and therefore are likely to have been living on rootstocks independent of the parent ramet for several decades (Section 4). The Highland Aspen Group have also been cutting sections of root away from the Invertromie clones used in this study, further reducing any existing connections between the ramets measured. Taking these facts into consideration the ramets used in chapters 2 and 3 are likely to be largely unconnected, and independent of one another, though the possibility remains that some ramets may be dependent on the same rootstock. For the purposes of the statistical analysis of variation within and among clones, ramets of the same clone will be treated as independent samples of that clone. However it is acknowledged that there is no way to currently establish the degree of connectedness between ramets. If ramets are connected then they will be pseudoreplicates, and the significance of any differences between clones in the statistical analyses will be overestimated.

The final problem with utilising studies from natural populations is that differences in age will exist among the ramets that are compared. Bark characteristics are known to change with tree age. Therefore age may be a confounding factor in the analysis.

7.2 Sampling on experimental populations

In order to overcome the limitations of observations made in natural populations, a more controlled experimental approach may be adopted. Here replicate cuttings of genotypes of the foundation species are taken and planted in trial sites in a randomised block design. The main advantage of this method is that any differences in the epiphyte communities that establish on these different genotypes are due to genetic differences between the clones, not to differences in the environment occupied by the genotypes. A second advantage of this design is that genotypes of the foundation species can be replicated over a number of sites. This allows us to test whether any genetic effects on community composition of dependent species are reproducible or specific to a particular set of environmental conditions. A third advantage of this design is that it allows cuttings to be taken from a range of different populations of the foundation species to ensure that a broad range of genetic differences within the foundation species are tested. The results are therefore not constrained by the limited amount of genetic variation present within the foundation species at a single site. Finally if the experiments are set up at the same time, age of the ramets does not become a confounding factor affecting epiphyte community composition. However there are some drawbacks to employing this method. Clonal trials using multiple genotypes from diverse areas fail to thoroughly deal with genetic effects at varying spatial scales. Increasing the distance at which the effects of genetic variation in foundation species are studied, from common garden, to local, site and landscape level, can show a significant decrease in host genetic effects on associated communities (Maddox and Cappuccino 1986, 1986; Tack et al. 2010). In certain cases, genetic effects on community structure can be reduced by environmental variation. Another problem is that removing a genotype associated with a particular area, and using it in a clonal trial in a location where it would not normally grow may attract a suite of dependent organisms not usually associated with that genotype.