The Perilous Existence of Endangered Whitebeam
Genetic research has expanded the conservation aims of these endangered Devonshire trees and shrubs. It turns out the sorbus genus is a lot more complicated than first thought.
Sorbus Hybrid - Avonensis Seeds by Wild Planet TrustWild Planet Trust
Devon: Home to some rare trees
The beautiful oak woodlands on the coastal fringes and steep valleys in Devon are home to several species of whitebeam (Sorbus spp.). Some are found nowhere else in the world.
Sorbus Sites in Devon by Dr. Tracey Hamston, Wild Planet TrustWild Planet Trust
Sorbus makes up 6% of endangered UK species
Devon is one of a number of regions in the UK that is well-known for it’s 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.
Walls Hill Sorbus Specimen Leaves by Oliver Newton - Browne, Wild Planet TrustWild Planet Trust
Small Population Sizes
Sometimes there are 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.
Goat in Ranch at Paignton Zoo by Oliver Newton - Browne, Wild Planet TrustWild Planet Trust
Particularly Palatable
Deers and goats love the leaves.
Tasty
Sorbus Walls Hill Torquay by Oliver Newton - Browne, Wild Planet TrustWild Planet Trust
If you know where to look
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.
Walls Hill Site Torquay by Oliver Newton - Browne, Wild Planet TrustWild Planet Trust
Survey work is potentially hazardous...
...or exciting depending on your viewpoint. The combination of dramatic location and the potential for discovering a possible new species ensures that Sorbus has an enthusiastic following.
These trees 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.
Woodland Canopy Opening - Clennon Gorge by Oliver Newton - Browne, Wild Planet TrustWild Planet Trust
Not all Sorbus trees live on cliff edges
In addition to cliff habitats, often trees 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.
Many woodland sites have been shaped by their history...
...of coppice management and quarrying. 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.
Illustraion Tryptic by Oliver Newton - Browne, Wild Planet TrustWild Planet Trust
Evolutionary biologists love this genus...
...for its tendency to throw up novel species. This makes the genus a taxonomic challenge. Their heritage derives from the three most common members of the genus; rowan S. aucuparia, common whitebeam S. aria and the wild service tree S. torminalis.
Walls Hill Sorbus Flowers by Oliver Newton - Browne, Wild Planet TrustWild Planet Trust
They really flourished after the ice age
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.
Sorbus Saplings by Wild Planet TrustWild Planet Trust
Endemism...
...is the state of a species being native to a single defined geographic location, such as an island, state, nation, country or other defined zone.
Sorbus Saplings Growing by Wild Planet TrustWild Planet Trust
Polyploids
Many of these endemic taxa are polyploid. This is the heritable condition of possessing more than two complete sets of chromosomes. Polyploids are common among plants, as well as among certain groups of fish and amphibians.
Sorbus Seed Prep by Wild Planet TrustWild Planet Trust
Polyploid organisms adapt well to their environments
They reproduce asexually via seed (still via pollination), resulting in effectively clonal populations. However, this breeding system is somewhat ‘leaky’ with offspring produced sexually, through pollination, allowing some variation and capacity for adaptation.
Sorbus Saplings at Paignton Zoo by Wild Planet TrustWild Planet Trust
The asexual or apomictic nature of reproduction...
...is due to the lack of fertilisation - pollination does happen, but the genetic makeup of the plant cells are mirrors of the mother plant. This can be good for short term survival if those species have a self-compatible pollination system.
Sorbus Saplings at Paigtnon Zoo by Oliver Newton - Browne, Wild Planet TrustWild Planet Trust
This is not the case for all apomictic species...
...which means they are still 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.
Sorbus Saplings Young by Wild Planet TrustWild Planet Trust
Pollen flow studies help show pollination relationships...
...shedding light on this particular mechanism of speciation and the ecological factors that influence the process. If we regard these interactions as a network, we can target management towards conservation of the network rather than solely at the threatened species.
Sorbus Seed Prep by Wild Planet TrustWild Planet Trust
Our role
Wild Planet Trust has been working to study the genetic relationships between these trees in an effort to understand the evolutionary processes that gave rise to Devon’s whitebeam diversity.
Sorbus Growing by Wild Planet TrustWild Planet Trust
Our findings have been used to guide the management plans for these trees in order to conserve them for the future.
Interested in Natural history?
Get updates with your personalized Culture Weekly
You are all set!
Your first Culture Weekly will arrive this week.