Not just spades in the ground - growing trees the natural (colonisation) way

When it comes to creating woodland, sometimes you can just let nature take the lead. For National Tree Week, Anna Holliday, our Farming and Conservation Officer, puts away her spade to talk about natural colonisation.

Woodland and scrub creation can be achieved in several ways; along riparian corridors, we favour a method called natural colonisation.  This is where existing trees set seed to colonise new ground and grow new trees.

Used in the right place, natural colonisation creates new woodland of local provenance, with native genetic diversity that will help create a resilient woodland better able to cope with the challenges of climate change, pests and diseases.

This approach also reduces the:

• cost and drain on resources to source and plant new trees,
• use of plastic to protect trees – particularly important when creating woodland on flood plains of watercourses, and
• biosecurity risk reduces the risk of pests/diseases being transferred.

Natural colonisation lends itself to places where there is a strong, diverse and native seed source – maintaining local provenance is ideal as the trees have evolved and adapted to that environment. Natural colonisation can occur almost anywhere in the right conditions, but traditional planting can be hard, especially on rocky outcrops.

In flood-prone areas, particularly along main rivers in spate, planting trees in tubes can be risky as they may be ripped out and the plastic is washed away down the river.

Natural colonisation can be better suited to some woodland management objectives more than others. If the priority is to increase biodiversity, then establishing woodland of local provenance using natural colonisation is key, however this process may be less effective if the priority is carbon sequestration.

Natural colonisation typically takes longer to create woodland as scrub cover with localised tree development forms over a longer period, creating a range of structures and habitats for many of our native declining woodland species of birds and invertebrates.

There are limits to where natural colonisation can be applied. The two main limiting factors are browsing and out-competition by vegetation.

Browsers, such as deer and hares, can impede natural colonisation by eating the new growth on emerging seedlings, thus stunting the young trees and hindering their development.

Out-competition of young trees can occur by other vegetation, such as more dominant grass species and nettles, particularly if the soil is nutrient-rich.

Use of this technique for timber production is possible – if browsers are limited and if the desired upon tree species are dominant on site – but the intended tree density might not be reliably achieved.

Combining woodland creation approaches is another useful technique to ensure the successful establishment of trees. Some supplementary planting with tree seedlings can be used to enrich the species composition available and speed up the development of dense habitat.

Funded by the Woodlands for Water project, over the last two years we have assisted landowners in designing and submitting grant applications for woodland creation through the Forestry Commission’s England Woodland Creation Offer

One of these projects is at Redbeck; a  3.7 ha site adjacent to ancient semi-natural woodland and a priority watercourse – the River Irthing.

The site has been approved to go into natural colonisation with supplementary planting that will predominantly safeguard the local provenance of the trees for the site.

First, grazing livestock will be introduced to lower the sward height and perhaps create patches of bare ground to allow seed establishment. Ways to limit the opportunity for browsing by wildlife through fencing or culling were then considered.
Rabbits aren’t a problem at Redbeck, although deer can be found here. Techniques employed to reduce their impact include culling, and installing small fenced-off areas to establish dense tree growth (Miyawaki establishment method) with supplementary planting.

Anna explains why natural colonisation was chosen to create woodland here:

Although it is by the riverbank, this site doesn’t flood, and there are a good variety of mature, native broad-leaf trees on site – a good seed source. We were aiming for low density of tree cover to maintain the light needed so that other existing vegetation could also thrive.

Redbeck and sites like it, are vital for maintaining wooded corridors through the landscape and creating a substantial buffer to the river, to intercept and trap polluted highway and agricultural runoff before it enters the watercourse.

Natural colonisation is just one of the nature-based solutions used by Eden Rivers Trust to create and improve habitat that ensures Eden’s rivers have a brighter future for all. If you’re interested in finding out more about woodland creation, contact Anna on 01768 866788.