Step 3. Which strategy for selecting provenances will increase the likelihood of the local population surviving in the future?
The strategy to select the provenance/s best suited to a particular revegetation project is context-specific and driven by variables such as the species’ phenotypic plasticity (the ability to cope with change through physiological and/or morphological means rather than by genetic alteration), levels of in-situ genetic diversity18, and the location of the revegetation site. Therefore, in this Guide, the selection of the appropriate provenance strategy and subsequent recommendations can only be general and not site-specific.
Even though a species distribution may indicate that the species is able to tolerate a broader range of climate conditions than those currently being experienced at the proposed revegetation site, the survival of local populations is not guaranteed. For example, local populations may not have sufficient genetic diversity to have the capacity to adapt to a rapidly changing climate. The traditional approach of using only locally collected germplasm is based on an assumption that local genotypes are best adapted to local conditions. Given the rapidity of observed and future climate change, this ‘local provenance is best’ approach is unlikely to provide the most effective basis for long-term sustainability of revegetation projects.
Provenance selection strategies
Including propagation material from locations currently experiencing climatic conditions similar to that projected for the revegetation site will reduce the risk that the revegetation project will fail as the climate changes. Several approaches to selecting germplasm for revegetation are shown in Figure 7 and are briefly explained here: (but see18 and references therein for more detailed information http://journal.frontiersin.org/article/10.3389/fevo.2015.00065/full).
- The ‘Climate-adjusted’ approach18 (Figure 7a) builds on previous research that recommends including non-local provenance with local provenance material to increase genetic diversity and adaptive potential19, 20. Germplasm is sourced from locations in the direction of predicted climate change.
- A ‘Local provenance’ approach (Figure 7b) uses germplasm that is only found within a certain geographic distance to the revegetation site.
- A ‘Composite’ provenance approach (Figure 7c) recommends mixing a small proportion of genotypes from non-local high quality and genetically diverse populations with local sources to reinstate historical gene flow and address inbreeding and adaptive potential issues19.
- Where changes to the local environment are expected but uncertainty about the scale and rate of this change is high, the ‘Admixture’ approach may be appropriate (Figure 7d). This approach incorporates a wide variety of provenances with no spatial relationship to the revegetation site and is predicted to build evolutionary resilience20.
- A ‘Predictive’ provenance approach (Figure 7e) uses genotypes solely from a source population that has been experimentally determined to be the best match for the revegetation site21.
The following points should be considered when selecting non-local provenances:
3.1 Where should non-local provenance material be sourced?
Use the process in Step 2 to select potential provenances. In the E. melliodora example, to use the ‘Climate-adjusted’ provenance method, select points on the scatterplot that represent a gradient towards and/or those that represent climatic conditions that are projected for the region where the revegetation site is located. These selections will then appear as red circles on the distribution/occurrences map. Click on the red circle on the map to get location details. Figure 7 illustrates how to determine which locations are chosen, depending on the provenance strategy. Always strive to collect genetic material from large populations, with consideration of other factors such as soil type.
- Go to the main ALA menu http://www.ala.org.au and select the ‘Mapping and analysis’ section (click on the orange ‘Browse mapping’ tab).
- Select ‘Add to map | Species’ and enter the species name e.g. ‘Eucalyptus melliodora’
- Using the example of E. melliodora, the species occurrence map (Figure 3) shows that the species has a wide distribution from Victoria to the Gladstone region in Queensland, and also occurs in New Zealand. To check that any ‘outliers’ are credible records, click on that occurrence dot and then select ‘Full record | View details’ to display detailed information about the record. If you are not confident that the record is credible, the record can be removed (see Box 4). There is some value in leaving in cultivated locations, and locations where the species has naturalized or has been planted overseas (especially eucalypts16) because this gives some indication of the thermal tolerance of the species. However, these records should not be used to indicate rainfall requirements because the plants may be artificially watered.
3.2 What should the proportion of non-local provenance to local provenance be?
Figure 7 illustrates how to determine the proportions of different provenances to satisfy the different strategies. The star represents the proposed revegetation site and the green circles represent different populations of the target species. The size of the circles indicates the relative quantities of genetic material included from each population for use at the revegetation site18. For example, to adopt a Climate-adjusted provenance strategy where the revegetation site is projected to become warmer and drier, provenances should be selected along a gradient of warmer and drier locations18. The proportion of local germplasm that is combined with proportions of non-local germplasm along the gradient reflects the confidence in climate change projections and the likely genetic compatibility of the different provenances18.
This Guide cannot be prescriptive about the best provenance strategy for individual sites. However, the provenance strategies shown in Figure 7 (a), (c) and (d) will increase genetic diversity and adaptive potential, compared to the sole use of local provenance. Further information regarding the different provenance strategies is available from18 http://journal.frontiersin.org/article/10.3389/fevo.2015.00065/full.
3.3 Is there a risk of outbreeding depression when mixing local and non-local provenances?
Historically, there has been a reluctance to incorporate non-local provenance material into revegetation programs because of concerns that outbreeding depression (OD) will occur if two or more populations are mixed. Whilst OD has been experimentally shown to occur at some sites, (e.g. in old, naturally fragmented, landscapes such as those in WA), a recent review of the evidence for this concern concluded that the risk of OD is overstated and can be minimized22. In summary, OD is predicted to be low for crossing populations where22:
- The populations are the same species (the species is not currently under taxonomic review and appears to be taxonomically stable);
- The karyotype is the same (there are no differences in the number and structure of chromosomes between breeding individuals (see http://www.tropicos.org/Project/IPCN as a guide);
- There has been gene flow between the populations during the last 500 years (this is generally the case in Australia, where populations have not been fragmented for more than 500 years); and
- There are no marked differences between the habitats of the different populations e.g. wet areas vs dry areas, different soil types, different day lengths.
Information may not be available to assess all these points, especially with regard to chromosomal differences. However, to balance the urgency of planning for climatereadiness against waiting for perfect knowledge, Climate-adjusted, Composite and Admixture provenance approaches alleviate some of the uncertainties (Figure 7).
3.4 What other factors should be considered?
This Guide cannot provide answers for site-specific questions because there are many factors other than provenance and species choices that need to be considered during revegetation projects. Which of these factors are relevant to a particular project will depend on the site and the species involved. We urge users of this Guide to read the papers and reports referenced here, relevant planning documents (e.g. the SERA Standards for the Practice of Ecological Restoration in Australia) and to fully explore the capabilities of the on-line tools and their associated caveats.
The ANPC is proud to be hosting ‘Climate-ready revegetation – a guide for natural resource managers’ on behalf of the authors. However its content has not yet been formally endorsed as ANPC policy. The ANPC suggests that any user of the guide should carefully consider its appropriateness to their particular circumstances. The field of climate-anticipatory revegetation is still developing, is locally contingent, and should be considered in conjunction with established best practice