- Top of page
- Supporting Information
Most plant species exhibit spatial structuring of genetic variation throughout their range (Hamrick 1990), which may arise through local adaptation or genetic drift. Genetic variation provides evolutionary flexibility and enables a response to environmental change (Lande & Shannon 1996; Booth & Grime 2003). It has therefore been suggested that preserving genetic variation within, as well as among, species should be a priority in restoration programmes (Crozier 1992; Moritz 1994). While translocation of individuals is sometimes proposed as a strategy to increase genetic variation within populations, individuals transferred across different environments may be poorly adapted to the new conditions. Furthermore, there is a risk of outbreeding depression or genetic swamping when divergent populations interbreed (Hufford & Mazer 2003; Potts et al. 2003; McKay et al. 2005). These consequences of seed transfer must therefore be weighed against the potential benefits of increased genetic variation within founding populations.
To conserve locally adaptive genotypes and prevent outbreeding depression, collection of source seed of the local provenance is frequently advocated for the restoration of plant populations (Keller, Kollmann & Edwards 2000; Jones, Hayes & Sackville Hamilton 2001; Sackville Hamilton 2001). However, such a strategy requires knowledge of the level and distribution of genetic diversity within the species of interest, and assessment of this can vary considerably, depending upon the technique used to detect it (Storfer 1999; Hedrick 2004). While molecular markers such as allozymes, AFLP (amplified fragment length polymorphism), RFLP (restriction fragment length polymorphism)and microsatellites provide powerful means of assessing genetic structuring within and among populations, such markers have repeatedly been shown to be poor predictors of variation of adaptive traits (Karhu et al. 1996; McKay et al. 2001; Reed & Frankham 2001). This disparity among molecular and quantitative measures of genetic variation may be attributed to the influences of different processes. While the effect of neutral genetic drift is consistent across all loci, natural selection affects loci controlling traits that confer adaptation, which may represent only a small proportion of the genome (Eriksson 1995). In the absence of barriers to gene flow (a necessary precursor to divergence by genetic drift), the likelihood of detecting genetic differentiation among populations using techniques that do not specifically target loci controlling adaptive traits is low, even if there is strong, divergent selection. In a restoration context, success of the restored population is contingent upon the survival, growth and reproduction of the founding individuals, hence consideration of adaptive variation is important.
The opportunity for divergence through drift vs. natural selection varies across landscapes and species, in response to factors such as life history, habitat and dispersal (Loveless & Hamrick 1984). In species that are highly outcrossing and continuously distributed, such as many forest trees, extensive gene flow is likely to prevent genetic divergence of neutral variation, although adaptive divergence may still arise through localized selection (Loveless & Hamrick 1984; Hamrick 2004). In such cases, reliance on molecular markers alone to guide decisions about where to source material for restoration may result in the introduction of poorly adapted genotypes.
Assessment of adaptive variation within a species’ range requires the measurement of ecologically important, heritable traits of plants from different source populations, grown at a common site to control for environment as a cause of phenotypic variation (Crandall et al. 2000; McKay et al. 2005). Provenance trials have been used in forestry since the early 19th century for detecting populations with economically desirable characteristics to be targeted for tree-breeding programmes (Guries 1990). Because the source of material included in such trials is known, existing trials may be utilized to study variation of ecologically important traits and identify appropriate source populations for restoration. For long-lived, slow-growing species, such as many forest trees, long-established trials provide a unique opportunity to examine later life-history traits, which may reveal important variation not evident in younger plants.
We assessed survival, growth and reproductive traits of jarrah Eucalyptus marginata Donn ex Smith, grown in a provenance trial in south-western Australia. Eucalyptus marginata is a dominant forest tree species, endemic to the south-west of western Australia. Restoration of E. marginata is needed primarily to replace populations following bauxite mining, which clears approximately 700 ha of E. marginata forest each year (Gardner 2001). In an effort to conserve locally adapted genotypes and maintain intraspecific genetic variation, bauxite mine restoration is undertaken using local provenance seed and plant material (Gardner 2001). However, delineation of appropriate seed collection boundaries would benefit greatly from improved understanding of the geographical distribution of ecologically important genetic variation within key species.
Eucalyptus marginata is long-lived, typically surviving for 300–400 years or more (Dell & Havel 1989). It exhibits substantial morphological variation throughout its distribution, ranging from a tall, straight tree to a shrubby plant with a mallee growth form (Dell & Havel 1989). In natural populations, the height and volume of E. marginata tend to be greatest in the southern jarrah forest and the high rainfall zone on the western edge of the northern jarrah forest, decreasing clinally to the north and east, where mean annual rainfall is lower (Dell & Havel 1989). Despite this apparent high intraspecific variability, analyses of nuclear and chloroplast RFLP variation revealed minimal genetic structuring within the jarrah forest (Wheeler, Byrne & McComb 2003; Wheeler & Byrne 2006) and it has been suggested that seed transfer could be undertaken at a regional scale (Wheeler, Byrne & McComb 2003). However, as much of the genetic variation detected by molecular markers is likely to be neutral, the absence of divergence at marker loci does not necessarily mean that seed collected from throughout the region will be equally well adapted to conditions at a particular site.
If local adaptation has driven genetic divergence of ecologically important traits of this species, it is expected that trait variation among seed sources will increase with environmental divergence. Given an understanding of this relationship, it may be possible to use characteristics of the site of origin to predict the performance of seed at restoration sites. Therefore, the objectives of the study were to: (i) assess whether seed source is associated with variation of survival, growth and reproductive traits of this species and (ii) determine how these trait types vary in association with environmental variation over different geographical scales.