|There's quite a bit of controversy about which are "native" British trees and which aren't. The purest brigade have shouted the loudest and bullied most people into accepting a cut off point at the formation of the English channel. Anything growing in Britain before the channel formed is considered native and anything appearing after is not. Which is fine up to a point, but it seems that when the channel was born most of Britain was covered in Tundra or melting glaciers which makes the growth and spread of most trees virtually impossible. |
Firstly tree seeds need raw earth or competition free soil/humus to grow in, and secondly the seeds need to travel if the species is to spread. Birch seeds are airborne seeds as are Alder, Poplar, Willow, Aspen and Elm, all trees that flourish in wet soils which composed much of early Britain, so it's no surprise that they would establish themselves amongst the soil spills, mud flats and sand banks besides the many waterways in early Britain. Scots Pine was also re-establishing itself along with Juniper on the higher rocky grounds and they seem to be the only true native species as they were self establishing or pioneer species.
There are exceptions to this, some of the big seeds that are produced by trees ie. oak have the reserves of energy to compete with grass and heather by been capable of driving their roots below the level of soil competition and tall enough to escape the shading before their energy reserve's are exhausted, but the size of oak acorns prevent them from spreading too quickly, it has been established that some species of birds (Jays) will collect acorns for food and on occasion lose them whereby the acorn will colonise new territories. As oak seedlings can only survive and grow in good light in the 'wild', this means that they only colonise open spaces.
However, pollen counts taken from peat bogs laid down in these early days (9000 bc) indicate that other species of tree dominated and I believe that all other species were deliberately introduced by advancing humanity. Just like the nomadic hunter gatherers of the rainforest plant beneficial trees near their temporary camps, the early inhabitants of Britain after the last ice age also introduced their favourite trees as they moved about. Mainland Britain had Hazel widely introduced alongside Lime trees, both very useful trees as they both provide food, tools, timber and in the case of Lime trees rope made from the inner bark. These trees although broadleaf and consequently called hard woods are nevertheless soft enough to be cut with stone axes. The pioneer trees also had their uses, but were supplemented with more beneficial trees. Different tribes of nomads would bring their own favourite tree species to Britain. There is coincidental evidence of the Beaker people bringing the Strawberry tree (Arbutus unedo) and the Yew to Britain and Ireland from their homelands in Iberia. Oak was introduced along with Hawthorn, Blackthorn, Pear and Apple all around the time that farming appeared. The English Oak was introduced/expanded approx. 4000 BC along with the Holly and Spindle. The Beech tree was initially introduced by some Belgian tribes, they were also reintroduced by some tribes from Germany, (Oak acorns were used as part of the diet, and the trees were widely spread replacing the lime tree as the main sacred tree.) Celts introduced the Sycamore tree from Gaul where they used the ground down seeds to make a flour or gruel from. There seems evidence that without special breeding programs most introduced species of trees take approx 1000 years before they have adapted themselves to compete and grow in the wild with, and amongst other native species. (specialist pollinators, mychorriza and weather hardiness are just some of the requisites to aid survival) The Sycamore tree was re-introduced by the Normans due to it's hardiness in salt winds near the coastlines, although there's no actual proof of this.
The next influx of foreign trees was in the 16th Century when trees from the far flung corners of the British empire were introduced, none that I know of have yet "naturalised" themselves, but it will happen given time. I have seen self seeding Conker (Hippocastenum) trees, but the young trees seem to be dying at about 15 years old and these were in a derelict garden with no real tree or shrub competition. There are exceptions, like Rhododendron ponticum, Mahonia and Amelanchier all of them reaching large shrub or small tree status, growing in shade and woodland.
So most of our useful trees have been deliberately introduced and probably most of the pioneer species have been helped either by humans or animals, for example birds are well known to pass small seeds from eaten berries. Most of the pioneer species are amongst our rarer trees in the wild, although Birch seems to keep popping up whenever the Forestry commission clear fell some of their forest but outside parks and gardens you won't often find Aspen, Willow, Poplar and Elm.