The expression "right tree, right place" is a familiar one in the tree-related professions. But what does this mean at a time of changing climate, when the conditions under which woodland trees mature decades hence may be very different from now? Does the idea of "local" sourcing need to be rethought?
With woodland creation high on the political agenda, a new research report from the Forestry Commission, Genetic considerations for provenance choice of native trees under climate change in England, aims to provide timely guidance on these points. Prepared by commission specialists with input from Natural England and the Woodland Trust, it assesses whether trees of more southerly provenance will cope better with projected climate scenarios than those of local provenance, though it does not look at what impacts such provenance choices might have on wider ecosystems.
Such "assisted migration" goes against much standard advice, including some from the commission itself. It has even recommended taking seed or cuttings from trees in the same site to enable regeneration of rare species in ancient woodlands.
Even definitions of what counts as "local", based on geographic zones, can be problematic for a number of reasons, because "plants from a geographically proximal location may be adapted to very different temperature, moisture and exposure regimes", it points out.
England could be 4°C hotter on average by the end of the century — that is, within the lifetime of many being born now. Rainfall patterns are also forecast to change, with drier summers likely to stress drought-sensitive species, though otherwise tree growth is likely to benefit.
Given this, the closest template for the climate projected for southern England is in western France. But even here, "other aspects of the environment, including soils, pathogens, herbivores and mutualists, photoperiod and solar radiation are very different", it points out.
It adds: "Drought-adapted trees sought from regions with a history of more frequent drought episodes would also have to be able to tolerate flooded conditions in winter, and earlier-flushing trees from further south will be more exposed to late frosts."
Nor should the resilience of local tree populations be dismissed. "Levels of within-population genetic variation in trees are very high, which provides the raw material for evolutionary adaptation," the report notes. "Trees have also evolved to exhibit high levels of phenotypic plasticity [to] enable individuals and populations to tolerate the climatic changes projected over the coming century", though trees’ response to this "will lag behind a directionally moving optimum".
The report even ponders whether the whole issue is ultimately "trans-scientific", touching on emotions, belief systems and environmental ethics, and therefore "unlikely to be resolved by scientific evidence alone".
But to make such decisions as evidence-based as possible, it draws on results of 44 "provenance tests" of English native or naturalised trees, in which seed collected from many different locations is grown in a common environment to establish their underlying genetic differences and hence suitability for particular sites.
These show some overall commonalities, such as that material transferred from warmer to colder environments tends to show greater early vigour than local provenances, though is less likely to tolerate adverse weather.
For this reason, seed sourced from further south "may be desirable for production-oriented planting schemes, provided the risk of frost damage is considered acceptable", the report concludes, in which case: "Improved seed sources ought to be favoured as they should have shown some evidence of genetic worth."
However, not all such differences are predictable. A study from the Pyrenees found that beech populations from a higher altitude were earlier to flush, whereas the opposite was found in ash and oak, and no difference was seen in sycamore, holly and silver fir.
Some traits, such as Dothistroma tolerance in Scots pine, have also been found to have an east-west rather than north-south correlation. Others show little correlation at all with provenance. A UK-wide study found growth of rowan of four different provenances was very similar at each of four different UK sites, even while the site-to-site differences in growth were considerable.
In all, it concludes: "The motivation for assisted migration will very much depend on the management objectives and so it is impossible to choose a single, measurable indicator of suitability."
When to assist?
The case for employing "assisted migration" is strongest if:
- The main motivation is wood production on a single rotation.
- Selected or improved material is being considered.
- The site is not exposed, easy to access and/or will have regular management.