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Forests for the future: planting resilient woodlands

Today, we are faced with a changing environment.

Over the last 350 years – less than the lifespan of a mature oak – humans have left an indelible footprint on our environment. Climate change, predicted decades ago, is now a reality with global temperatures on the rise and the scientific consensus predicting greater increases before the end of the century. This warming will cause significant changes in our weather patterns with more winter storms and summer droughts.

The UK is also facing an unprecedented challenge from introduced plant pests and diseases – some of which can damage or kill healthy trees, as we have all too often seen.  The Forestry Commission’s timeline shows just how many outbreaks we have had since the 1970s. The rate has risen in recent decades as international trade has increased, bringing new opportunities for pests and pathogens to be transported quickly across the world from one ecosystem to another.  Climate change can exacerbate this problem, because a warmer climate can make it easier for new pests and diseases to get established, and for existing ones to become a bigger problem, for example by breeding more frequently.

In addition, browsing pressure from significantly increased deer populations is limiting woodland regeneration. Without adequate control, bark stripping by grey squirrels can destroy thin-barked species’ financial value.

These challenges have highlighted our dependence on very few tree species, and the high proportion of trees which are in even-aged monocultures (forests where all the trees are the same species and age). This lack of diversity makes our woods more vulnerable to threats.

But all is not lost. The good news is that we have enough scientific knowledge to mitigate these challenges by allowing us to proactively manage our woodlands. If we use this expertise to act boldly, and with urgency, we have the opportunity to leave a legacy of resilient forests for generations to come.

But, what does this really mean for the future of our trees and woodlands? What are the adaptive strategies we need to adopt? In this article, I’d like to consider how we have found ourselves here and, crucially, how we can increase the resilience of our woodlands in the face of a changing environment.

Increasing resilience to a changing climate

Scientists are suggesting that our climate is warming at such a rate that it is roughly equivalent to the temperature warming ten metres a day from north to south. So, it’s clear why our trees need a helping hand. But, can our woodlands adapt fast enough?                             

In short, yes – provided we can make interventions to our forest environments. After all, forests thrive all over the world, in hotter, drier climates than ours – but the techniques and planting stock needed to be adapted to suit. The challenges we face will not disappear but increasing resilience is a way to combat them.

Woodland adaptation is key to stronger resilience – with diversity at its core. This can be achieved through:

  • planting a wider range of tree species, using seed from a wider range of origins and provenances (regions in which we know they grow well);
  • staggering harvesting and replanting to achieve a diversity of forest structure and tree ages;
  • creating opportunities, such as open spaces, for trees to regenerate (self-seed) naturally; and
  • protecting them from damaging mammals.

Nearly 60% of England’s broadleaf woodlands are not actively managed – this needs to change if we are to protect them from our rapidly changing climate. Active management of woodlands helps to maintain productivity – and, as the saying goes, the ‘wood that pays is the wood that stays’.

Central to this active management of woodlands is the opportunity to plant a more diverse selection of tree species. For the origins and provenances of our planting stock, ‘look south’ is the mantra. That’s because saplings descended from trees which have thrived for hundreds of years in warmer areas further south than our woodland are likely to do better in the climate we expect to have in 50 to 100 years’ time than plants descended from local trees. This is a big decision, because while we can change arable crops at a year’s notice, there are few opportunities to switch tree species, whose lives span decades. So we must plan carefully.

It has long been recognised by UK scientists that trees grown from seed sourced from stands located 2° of latitude south of the planting site are more productive in timber terms than seed sourced from the same latitude, in part because they flush earlier and have longer growing seasons.

If game or wildlife habitat is a prime objective, we at the Forestry Commission recommend planting trees from a mix of provenances. This can involve:

  •  making up about one third of the planting stock from plants grown from local seed;
  • sourcing the remainder of our plants from seed gathered from regions with slightly warmer climates: current guidance recommends between 2° and 5° latitude (about 140– 350 miles) further south. However, avoid eastern European sources, which are unsuitable for England;
  • sourcing a significant proportion of the plants from genetically improved stock from provenances slightly further south (i.e. plants which have been selectively bred to maximise desirable timber qualities such as straightness and strength), if timber production is also an objective. Using a forestry consultant can help get this decision right; and
  • encouraging natural regeneration (self-seeding), where it is likely to be successful, to encourage evolutionary adaptation as the climate changes.

In general, seed sourced from about 2º south of the growing site usually outperforms material sourced from local provenances, and this is considered a safe distance to transfer planting material. Although seed sourced from provenances up to 5º south matches current climate change predictions up to 2050, it does carry risks from frost and maladaptation, but might offer the greatest productivity gain.

The makeup of England’s woodlands today

77% of England’s broadleaf woodlands today are made up of just five tree species – oak, ash, beech, sycamore and birch. It’s a similar story with England’s coniferous woodlands - 89% of which are made up of just six tree species (Sitka spruce, Norway spruce Scots pine, Corsican pine, Larch, Douglas fir) mainly grown in monocultures.

This is leaving us vulnerable to serious damage from tree pests and diseases like Chalara dieback of ash or acute oak decline, and illustrates the urgent need for diversification in the native tree species which we are planting.

There are many productive broadleaf trees such as alder, small leaved lime, hornbeam, poplar which have fallen from popularity in recent years but now must be brought back into our planting mix. We must also accept that tree species brought to us from mainland Europe during the last couple of centuries should be considered as native except where genetic conservation is the highest priority or where priority/protected species present on a site are dependent on native tree species.

 A close examination of the facts shows that we have been planting a lot of trees that had their origins in Eastern Europe. This is clearly demonstrated by the fact that Chalara dieback of ash seems to have arrived here on young planting stock imported directly from continental Europe at the start of this century. Much of this is likely to have been driven by lower costs and a focus simply on species and not the genetic origins. We must get smarter and use the best available tree seed of the right genetics grown on our own island.

This also presents an opportunity to highlight the importance of practicing good biosecurity when sourcing tree stock – foresters should always ask where plants have been grown: purchasing UK-grown plants can help avoid accidentally introducing pests or diseases on imported stock.

It is also worth remembering that human activity is a key factor in the spread of tree pests and diseases. By undertaking basic biosecurity practice day to day, and minimising the amount of soil, water and plant material moved between sites, we can also reduce the spread of pests and diseases.

People say that trees ‘have seen it all before’ and therefore must be resilient to the changing environment around them. However scientists say they believe the rate of climate change to be around 30 times quicker now, than it has been in the past. This time it’s man-made and whilst humans may be agile enough to adapt to the environment, it is probable that trees will need our help to do the same.

We know that to help our woodlands adapt and become resilient to the changing environment we need to plant more diverse forests, observe good biosecurity to limit the introduction and spread of pests and diseases, and protect our trees from damaging mammals.

So, we know what we need to do – and we need to act now!

 

John Weir is a Fellow of the Institute of Chartered Foresters.

 

Last updated: 11th April 2017