Oak has a special place in the hearts of many people and there is strong support for the conservation of oak and oak woodland. However, what exactly is it we should be conserving? The standard definition of biodiversity refers to the conservation of variety at genetic level as well as that at the species and ecosystem (landscape) scale. Is that the genetics of the past, or of the future?
Modern genetic analysis techniques have confirmed that the oaks in Britain represent only a subset of the variation that exists across Europe. As trees spread back naturally (or were carried by people) with the warming climate about 10,000 years ago we received mainly oak colonists from the Iberian Penninsula glacial refuge, and fewer from other refuges such as those in Italy or the Balkans. Our oaks would since have diverged somewhat from their Iberian forebears under the natural selection imposed by our distinctive cool, wet climate, but this divergence would have been countered by other forces. Wind-blown oak pollen could bring in genes from other trees, possibly from across the Channel; and throughout the historic period (at least) acorns and oaklings have been moved about both within Britain and from the Continent to Britain.
Pollen from these catkins could blow for miles; New Forest acorns, some of which are now growing in my Oxfordshire garden
So how do we identify and conserve important genetic variability in our oaks today? One consideration might be that we should try to maintain the mixture of genes that characterised the natural populations of oak from Britain after the ice age. Ancient oaks should be closer in their genetic make-up to those original colonists, because they are derived from populations less influenced by more recent human movement of acorns and trees than (say) oaks arising in the 20th century. However, the ancient trees that have survived through to today are only a tiny proportion of the oak trees that regenerated five hundred or a thousand year ago. They are very unlikely to be a random genetic sample of the past oak population, because their genetic make-up will be biased towards characteristics, e.g. growth and survival in the Little Ice Age, that have been important in their lifetime on that particular site.
We could also select as genetic reserves remote isolated woods that appear not to have been managed – for example Wistman’s Wood on Dartmoor – to try to reduce the likelihood of recent mixing with genes from elsewhere. But the same problem arises: the distinctive conditions that have led to these woods remaining genetically ‘uncontaminated’ may make them atypical of what the genetic variability was like over the rest of the country, particularly as most such sites are small.
Another way of looking at conserving genetic variability is to consider what will be promote oak survival under future conditions in terms of the climate the trees will face, the sites on which they will grow, and the competition they will face from other tree species. The genes that allow oaks to thrive at Wistman’s Wood on Dartmoor, may not be as useful for survival in a new mixed woodland on former farmland in Essex. Since we cannot be certain exactly what genes or combination will be most useful at any one site over the next 50-500 years, this is an argument for trying to include a variety of genotypes particularly in new woodland and also in new non-woodland situations.
Would seedlings from Devon or Hereford parentage have done well over in the east of Britain? The genetic origin of the planted trees on the left is unknown, but so is that of the regenerating oak in a field in a landscape with many planted trees around, but both seem to be doing well.
Encouragement of natural regeneration is one way of promoting future genetic variability, but in small, isolated sites the variety in the seedlings may be limited by the variety in the parents unless there is a lot of long-distance pollen input as well. The variety that can be expressed in the regeneration is also limited by the number of seedlings produced. Oak regeneration in both woodland and open conditions can be quite sparse – one to a few hundred individuals per hectare rather than the thousands of seedlings commonly produced by ash, beech and sycamore – so less of the potential variety may make it past the acorn stage. If planting trees, there is a tension between using transplants grown from selected seed which may have better than average growth and survival (or disease resistance if that has been selected for); or getting more variable stock from a wider range of parent trees.
In the end, though, it is something of a lottery as to which oaks survive to maturity, let alone to become the veterans of tomorrow: good genes can help, but may be trumped by whether or not a hungry deer is passing by.
“They may be from selected seed or natural regeneration, but I don’t really care: they are just food to me!”