Tilia background

The term ‘ancient woodland’ has been frequently used to describe tiny fragments of isolated UK forest. Of course, rarely are the individual organisms found on such sites truly ancient. However, Tilia (Lime trees) may be the exception.

 Tilia (Malvaceae) are a broad-leaved temperate forest tree with an almost circumpolar northern hemisphere distribution. Two species naturally occur in the UK, T. cordata (Small-leaved lime) and T. platyphyllos (Large-leaved lime). These populations are at the edge of their ecological and geographical range and many, particularly northern populations, rarely regenerate from seed. Southern populations have more success in seed development and reproduction but successful recruitment is limited.

 Once dominant in the UK landscape, Lime woods are now highly fragmented resulting in isolated populations. Isolation of long living taxa with little or no gene flow can lead to changes in demographic structure, and reproduction strategies. This in turn can affect genetic diversity. Populations at the edge of their natural range often show less genetic diversity due to genetic drift (the random change in allele frequencies), limited gene flow, inbreeding, and clonal reproduction.

 Tilia cordata and T. platyphyllos propagate easily and both species were once an important resource throughout the Medieval period. Regular cycles of manual coppicing for timber use and fodder etc., as well as layering for hedgerows and field boundaries ensured that trees were maintained. Self-propagation from epicormic shoots, low hanging or fallen branches, or indeed fallen trees, and long-term maintenance through self-coppicing may have provided a pathway for the continued existence and establishment of genotypes from ancient Tilia populations. In the absence of sexual reproduction, this early management regime and self-preservation strategy may have played a significant role in not only conserving individual trees (some are dated several hundreds of years old), but more importantly in preserving the genetic material (potentially many thousands of years old) of past individuals/populations.

 These small pockets of ancient Lime woods have been awarded some level of protection under various statutory regulations. However, the management approaches of some sites are being questioned. The very measures that were put in place to protect these ancient relics may actually lead to their demise, in the sense that the genetic integrity of these ancient remnants may become at risk through dilution with foreign cultivars. And so the careful management of past generations (that is planted trees would most likely have come from cuttings of individuals within close proximity, thereby maintaining the genetic integrity of the populations), are now being replaced by modern management which occasionally sources trees from commercial growers, thus introducing genetic material that is not specific to that particular region.

 The two UK species naturally hybridise, producing the more familiar Lime, T x europaea (Common lime). Although little is currently known to what extent this occurs in the UK and whether or not backcrosses are also present (currently being investigated), the assumption is that any unsolicited mix of genetic material will only reduce the ancient nature of our UK Lime woods, and thus will jeopardise their unique status.

 This study will use current molecular methods to address the genetic structure and diversity, gene flow, and reproduction methods of our ancient lime woods and assess how these differ to their European counterparts. Additionally Next Generation Sequencing techniques will be developed to address important evolutionary questions such as adaptation and selection.