The beautiful oak woodlands along the coastal fringes and steep valleys of Exmoor are home to several species of whitebeam (Sorbus spp.) that are found nowhere else in the world. Exmoor is one of a number of regions in the UK well-known for their diversity of endemic Sorbus species. In the recently published Vascular Plant Red List for England, Sorbus as a genus makes up 6% of threatened plants. This is largely due to their small population sizes, sometimes only a handful of trees on one or two sites, which make them particularly susceptible to extinction. The main threats to this group are competition from invasive species such as rhododendron and holm oak, particularly on the coastal sites. Sorbus is also particularly palatable, making them attractive to browsing animals such as deer and wild goats.
They have an attraction for field botanists because of their perilous nature, both in terms of rarity but also the habitats where many of them are found. Small, twisted specimens, often of great age, may be found clinging to cliffs on geographically distant sites such as Torbay in south Devon right up to the Cairngorms and islands off the west coast of Scotland such as Arran and Rum, and west into Ireland. This makes any kind of survey work potentially hazardous – or exciting – depending on your viewpoint. The combination of dramatic location and the potential for discovering a possible new species has ensured that Sorbus has an enthusiastic following.
In addition to cliff habitats, some of the species, often those more closely related to the wild service tree S. torminalis, can be found scattered through woodland where they can struggle to compete with high canopies, preferring the woodland edges or rocky areas where the canopy is sparser. The history of woodland management on these sites may reveal the reasons behind current distributions. Many of the Exmoor woodland sites have been shaped by their history of coppice management and quarrying, so it’s probable that the shade intolerant Sorbus gained a foothold during these cycles of disturbance when intermittent open conditions enabled trees to flower and set seed in a greater capacity than today.
This genus is of particular interest to evolutionary biologists due to its tendency to throw up novel species making the genus Sorbus a taxonomic challenge. Their heritage derives from the three most common members of the genus; rowan Sorbus aucuparia, common whitebeam S. aria and the wild service tree S. torminalis. It’s known that as these three species plus a fourth, the rock whitebeam S. rupicola, colonised after the ice age. As they spread, new species arose via hybridisation events, resulting in a high level of endemism in this group.
Many of these endemic taxa are polyploid and reproduce asexually via seed (although they still need pollinating), resulting in effectively clonal populations. However, this breeding system is somewhat ‘leaky’ with offspring occasionally produced sexually, thereby allowing some variation and capacity for adaptation. The asexual or apomictic nature of reproduction can be advantageous as a short term survival strategy for small populations if those species have a self-compatible pollination system. We are starting to find this is not the case for all apomictic species which means they are reliant on effective pollen flow between species to reproduce!
We are particularly interested in the interactions between these species and how their reproductive ecology may influence their evolutionary potential. For example, we are looking at pollen flow across one of the woodland sites to see how it effects seed production and where the compatible pollination partnerships are. This type of research helps shed light on this particular mechanism of speciation and the ecological factors that influence the process. If we regard these interactions as a network, which may also include common species, we can target management towards conservation of this network rather than solely at the threatened species, see Ennos et al, 2012.
Tracey Hamston was a PhD student based in the Molecular Ecology & Evolution Group at the University of Exeter, studying the endemic Sorbus species of the southwest UK in collaboration with the National Botanic Gardens of Wales. This research was funded by the Whitley Wildlife Conservation Trust
Dr Tracey Hamston