The Elements

Earth, fire, wind and water all affect trees, woods and forests.

Climate and soils are two major factors influencing what the dominant vegetation types are on a global, regional and local scale.

This section introduces how the elements influence trees.

EARTH

To grow trees successfully, species choice needs to be carefully matched with soil types.

The type and depth of soils have a major effect on which trees grow where and how well.

Soils need to be deep enough to allow tree roots to penetrate and create strong anchorage and to provide adequate water and nutrients. The root system or root architecture of trees varies - some conifers such as spruce have shallow or superficial rooting systems and can survive and prosper on shallow soils. Most broadleaf trees need deeper soils to grow well.

Soil fertility is a vital factor. Many lowland woods enjoy fertile soils - although many woods only survived clearance because they were on the worst of the soils locally. Challenging sites for tree growth include upland moorland, sandy soils and thin soils over chalk and limestone along with very wet soils. Hard or compacted soils make tree root development difficult if they have been impacted by man by ploughing or impaction and some upland soils contain an ironpan that roots cannot penetrate through.

Creating new woodlands on former agricultural land presents its own challenges - it can be too nutrient rich with luxuriant weed and grass growth swamping young planted trees.

On calcium rich chalk and limestone soils, lime-induced chlorosis can occur, especially in conifers.
The ground flora in native woodlands often reflects the type of soil and its moisture regime.

Trees can be essential to fix or stabilise sandy soils. One reason for planting conifers in coastal zones like Pembrey in South Wales or Culbin near Inverness was to stabilise the shifting dunes.

FIRE

Fire is a constant source of worry for folk who manage woodlands and forests.

Prevention is better than cure. Prudent management aims to stop fires starting or spreading into the forests from neighbouring land.

Firebreaks should be incorporated into good forest plans. These reduce the risk of ground vegetation fires spreading and double up as access roads for emergency vehicles.

When the risk of fire is high - for example after a period of prolonged dry weather - fire lookouts may be posted. Using notices and the media are vital to alert the public to the high fire risk conditions.

Close liaison with the local fire brigade is vital as is creating and maintaining good vehicle access. Artificial ponds or reservoirs can also be important water storage facilities in case of fire as well as wildlife sites.

Many fires actually start outside of the treed-area and precautions must be taken during muir burning. The prohibition of stubble burning after harvesting cereal fields has helped cut fire risk.

The ways of fighting a fire depend on factors such as accessibility, the scale of the incident and what type of tree crop it is in and the ground vegetation.

Small spot fires can be extinguished with fire beaters or knapsack sprayers. Increasingly foam is used to combat forest fires rather than water.

Specially adapted helicopters or aircraft can be vital in fighting larger fires in remote areas.

In other countries where fire is a regular part of life, certain trees have become adapted to survive and benefit from periodic burning of the surrounding vegetation. For instance, they may have thick bark to protect the tree trunks from the heat. Others have cones which only open when the heat from the fire triggers that so the seeds are released into the new, clear, ash-rich seedbed the burn leaves behind.

In much of upland Britain, the practice of burning heather moorland or muir burn suppresses re-colonisation by trees. With no burning and fewer grazing livestock, those areas would revert to what they were formerly - which is upland woodland with species such as birch and oak predominating.

WIND

Britain has a severe wind climate compared to most of the world, including the rest of Europe. Strong winds typically result from the passage of Atlantic depressions and very rarely from thunderstorms, tornadoes or squalls. About 150 depressions affect Britain annually, forming out in the Atlantic and travelling in from west to east. Air circulates anti-clockwise round a depression so the strongest winds are often found to the west and south of the centre of the low pressure. Winds are generally stronger in northern and western Britain.

Wind damage is a serious threat to managed forests in the UK. It can take many forms and can be made worse by other conditions such as wet snowfall or soil conditions. Wind can have many effects on trees:

• leaves may become abraded, causing desiccation;
• young trees may socket as the wind sways them and loosens the soil around the root collar; in extreme cases it may topple young trees with a poor root system;
• leading shoots, branches and crowns may break or snap;
• older trees may be windthrown where the stem and root plate overturn but do not break;
• wind snap is where the stem breaks above the ground.
• trees in very exposed areas may be wind pruned where persistent winds from one direction snap the twigs on the windward face and the branches and foliage become lop-sided.

Much of the expansion of UK forestry cover during the 20th century was on uplands, marginal for agriculture, and sometimes buffeted by the winds. Minimising risk of losses from the wind by planting the right species in the right site and by the design of the forest itself are all part of the forester's art. Prediction and prevention of wind damage are basic elements of forest management and a windthrow hazard classification is used to try to predict and minimise this problem.

Records of wind damage to trees in Britain date from as early as the 13th century. Storms in the late 19th century caused serious windthrow and resulted in gluts of timber on the market.

Notable storms affecting Britain's forest within living memory are those of 31st January 1953, 15th January 1968, 2nd January 1976, 16th October 1987 and 25th January 1990.

On the night of 16th October 1987, south-east England was hit by an unexpected and violent wind storm. No one knew exactly how many trees were lost but the total has been reliably estimated at 15,000,000. Some were uprooted; others had their trunks snapped like a twig. Countless trees lost major branches from their crowns. Yet in terms of the lifespan of many of our broadleaved trees, such violent storms are almost a normal event in the lives of woodlands.

WATER

Water can affect trees in the short and long term.

Too much or too little soil water or at the wrong time can have major effects on trees, their growth and their distribution.

Some species like alders and willows thrive in wetter soils - others need a well drained substrate to survive and prosper.

Some trees can use vast amounts of water - photosynthesis in the leaves needs some but transpiration to keep the leaves cool demands massive volumes.

But the presence of trees can also affect water flow.

Forests can reduce peak stream flows and hence flooding risk, and there is a major opportunity in the headwaters of rivers with high risks of flooding, e.g. the Severn & Wye.

Floodplain forests are a robust land use and could have a role in buffering river flows upstream of settlements.

Forests can increase infiltration and hence aquifer recharge in pervious geology.

Tree canopies and roots intercept rainfall in towns too, slowing run-off in downpours and reducing the risk of flash floods.

More: Why and how storms damage trees and the measures that can be adopted to mitigate such damage are described in FC Bulletin 114, Forests and Wind: Management to Minimise Damage. By C.P. Quine, M.P. Coutts, B.A. Gardiner & D.G. Pyatt. (1995)

Recent FC Publications:

• FC Tech. Note 002. Planning Controlled Burning Operations in Forestry. By Bill Jones (2002)
• FC Tech. Note 003. Forest and Moorland Fire Suppression. By Ian Murgatroyd (2002)
• FC Bulletin 125. Climate Change : Impacts on UK Forests. By Mark Broadmeadow (2002)
• FC Guide Book 01. The Identification of Soils for Forest Management. Fiona Kennedy (2002)
• FC Info. Note 065. Water Use by Trees. By T. R. Nisbet. (2005)
• FC Info. Note 69. Climate Change & British Woodland. By Mark Broadmeadow (2005)