Is local planting material best? This is the question David Boshier from the University of Oxford is currently trying to answer through a systematic review – digging through decades of literature on provenance trials from the archives of FAO, IUFRO and various other agencies and research institutes. In this interview he explains about how choice of seed sources matters for adaptation and how a systematic review approach helps to assess the evidence for or against local seed sourcing in trees.
Q: Why is it important to understand local adaptation in trees and what practical implications does it have for forest conservation or management?
DB: Planting of native tree species for forest or habitat restoration and other purposes on farms depends on a ready supply of germplasm (seeds or vegetative material), which needs consideration of the most appropriate source. Choice of seed source, both in terms of location and genetic composition, can have important consequences for the immediate success and long-term viability of plantings and associated risks (e.g. loss of diversity – genetic bottlenecks, future adaptability or past adaptation, contamination of native gene pools prioritised for conservation).
Local genotypes are often assumedto be better adapted to local conditions, with natural selection over time increasing the frequency of genes that improve fitness. However, the importance of local provenance in sourcing planting stock for forest production, habitat conservation and restoration remains contentious. Concerns exist over the scale of adaptation in trees and the relative dangers of incorrect seed source or restricted seed collection which can lead to establishment of trees with limited genetic diversity and adaptive potential. Discussion of suitable seed sources often emphasises “local” in a very narrow sense or on political boundaries, rather than hard evidence for the scale of adaptation. Undue emphasis on local seed sources or poor practice in seed collection may drive sourcing from small populations or a limited number of trees, producing bottlenecks, with populations that are unlikely to be both adapted and adaptable.
Most evidence of local adaptation comes from herbaceous plants with low gene flow and strong selection pressures (e.g. tolerance to heavy metals in soils). Long life cycles, wide distributions and gene flow (pollen and seed dispersal) in trees, compared to herbaceous plants, suggest that scales and patterns of adaptation maybe more extensive in trees. So local adaptation over a small geographic scale maybe unlikely unless selection forces are very strong e.g. frost resistance. Given the long life of trees, the environment is also likely to have altered such that a site no longer experiences the same conditions under which the trees originally developed. Such temporal environmental variation may be particularly important for trees, not only in terms of past adaptation but also in the context of climate change, requiring more adaptive sourcing strategies.
Q: Can you share with us some preliminary findings from the review?
DB: No, the review is on-going and we are not at the data analysis phase yet. However, what is evident is the importance of the 'grey literature' in reviewing this question. Many systematic reviews on environmental conservation topics can focus on recent literature, most of which comes from journals with papers available on-line. However, much of our review is based on the reporting of provenance trials, some of which were established as far back as the end of the 19th century. The results were published in technical reports of research institutes (some no longer in existence), IUFRO working party conferences, FAO congresses and other such meetings. Just because the papers are old, or not written in English or not in an academic journal does not mean the research was not rigorous and done well – such studies can still be of great relevance and the decades from 1950s to 1980s provide rich pickings. This stresses the importance of library collections with physical holdings of such literature for being able to complete such a review.
Q: How does a systematic review differ from a conventional literature review? Do you have some advice from your experience for researchers who are interested in the systematic review approach?
DB: The systematic review process started in the medical field, with the desire to evaluate the scientific evidence base that supports application of a particular treatment. The process starts by precisely defining a question and then deciding what research addresses that question. The approach makes no preconceived decisions on what sources will be used, but defines criteria for inclusion vs. exclusion of studies prior to exhaustive literature searches, to find research that fits the defined criteria. All this is usually laid out in a review protocol (our protocol is freely available at -http://environmentalevidencejournal.biomedcentral.com/articles/10.1186/s13750-015-0046-3). In this way it is also possible for a review to be repeated in, say, another 10 years time, by a different group of people, using exactly the same process to see if and how the evidence base and possible conclusions have changed. Systematic reviews often, but not always, use statistical techniques (e.g. meta-analysis) to combine results of the eligible studies.
My advice for researchers interested in possibly using this approach is don't bother unless you are prepared to leave your prejudices and preconceived ideas at the door before you start the process. This is both in terms of the eventual outcome of the review and what or whose publications will make the cut that forms the evidence base. Absolute objectivity is a prerequisite of the process, rather than what often happens in a conventional literature review of looking for evidence that supports views you already hold. Systematic reviews aim to make objective use of scientific evidence to inform policy or practice, with rigour, objectivity, transparency and restricting bias in reaching conclusions.
David Boshier is Senior Research Associate and James Martin Fellow in the Department of Plant Sciences at the University of Oxford, and Honorary Research Fellow with Bioversity International. His research focuses on the genetics of tree populations, human impacts (e.g. fragmentation, logging) on such populations and applications to issues of use and conservation in natural and agro-ecosystems.
