Abiotic & Biotic Factors
Abiotic problems come in four major groups:-
- Planting failures - this is when seedlings fail to take because something went wrong when they were planted out such as poor plant quality to start with, bad handling or storage, poor site preparation, sub-standard planting technique, insufficient weed control or a combination of these.
- Site-related problems often involve water supply - too little or too much or at the wrong time - or nutrition problems due to the soil type such as Chlorosis, common on chalk or limestone soils. Particularly in plantations, nutrient deficiencies of nitrogen, phosphorous or potassium can cause problems, particularly with the foliage yellowing. Widespread use of fertilisers is uncommon in plantations in the UK.
- Damage due to chemicals can be from windborne pollution or acid rain, drift of herbicides from surrounding areas, poor use of agrochemicals within the forest itself, salt spray from the sea or from salt used on roads in winter.
- The climatic elements can also damage trees. Wind can snap or blow them over; heavy wet snow fall can cause branches to snap; cold spells and particularly late frosts can kill young buds and leaves; lightning can score large trees from top to bottom; too much or too little water can have dire consequences; sun scorch may occur; and both ground and air pollution can take their toll.
Biotic factors include -
- damage caused by vertebrates (specifically mammals and birds);
- and by invertebrates (mainly insects and some other arthropods);
- a number of tree diseases are caused by some fungi, bacteria and viruses.
Some trees will always die in a wood or forest. Losses may reach an economically or ecologically unacceptable level to either the trees or the ground flora.
The fungi and micro-organisms that infect trees are often invisible to us until we see the visible symptoms - and by then it may be too late to cure. Sometimes more than one disease may be at work and the causes of problems are often complex and cumulative.
Not all fungi damage or destroy trees. Many types of fungi and trees enjoy a mutually beneficial or symbiotic relationship. These are known as mycorrhizal fungi.
Deer can damage trees by browsing them, stripping the bark or fraying the trees with their antlers. Too many deer eating the succulent native ground flora can cause unwanted environmental degradation to the native ground flora and the numbers may need keeping in check. The effect of deer and other grazing animals may prevent any natural regeneration taking place as well.
Other mammals such as voles may also damage trees - new trees in long grass can have their bark gnawed off right round the stem, causing the tree to die, particularly where they are planted in long grass which was previously field.
The common grey squirrel, introduced from North America and now found throughout much of Britain, often strips the bark off of particularly broadleaved trees about June and July. At its worst, that can kill even mature trees if a complete ring of bark is removed right around the trunk or branches. For that reason, numbers of grey squirrels are humanely controlled in many woods and forests to keep damage to an acceptable level.
Wherever you have large numbers of the same plant species - in your garden, in agriculture or in forestry - severe damage from insects is on the cards.
Many species of insects live on trees but most of those are completely innocuous and live in harmony with their hosts. Many of them also have natural enemies which keep a check on their numbers.
Insects can affect trees in a number of ways - some live in the developing seeds and cones. When infestations are heavy, the seed crop can be a write-off. The knopper gall wasp can destroy large numbers of acorns by transforming them into galls.
Some insect pests are a real problem in tree nurseries, others out in the woods themselves and still others in the final timber products.
Steps may need to be taken to reduce the possibility of insect damage. Some species such as the large pine weevil (Hylobius abietis) gnaw the bark of new conifers just above soil level and kill them if protective measures are not taken.
Other insects have larvae called defoliators - which eat particularly the new leaves or needles. Some insect species only attack particular species of trees, others are less fussy or selective.
More: Many diseases and disorders can affect forest trees in Britain. Identifying these is not always as easy as it may seem and specialist input is often advisable. However, there are three good publications which will give you some useful guidelines and help determine the agents causing diseases and disorders in both forest trees and amenity ones. They are:-
- S.C. Gregory & D.B. Redfern. FC Field Book 16. "Diseases and Disorders of Forest Trees - A Guide to Identifying Causes of Ill-health in Woods and Plantations" (1998).
- D.H. Phillips & D.A. Burdekin. "Diseases of Forest and Ornamental Trees" (1992) Macmillan.
- R.G. Strouts & T.G. Winter. "Diagnosis of Ill-health in Trees" (1994) HMSO.
- B.A. Mayle. "Controlling Grey Squirrel Damage to Woodlands (2004) FC Practice Note 004.
- B.A. Mayle. "Managing Deer in the Countryside" (1999) FC Practice Note 006.
- H.W. Pepper. The Prevention of Mammal Damage to Trees in Woodland (1998) FC Practice Note 003.
- C.P. Quire. Managing woodlands and their mammals (2004) FC Research Report 006.
- English Squirrel Initiative: www.europeansquirrelinitiative.org
TREE DOCTOR is a CD-Rom, an interactive programme for tree owners, woodland managers and educational purposes. Caring for trees is not always easy and Tree Doctor takes you through a series of on-screen options to help those who care for forestry and amenity trees identify the problem and get clues to prevention and treatment. It can be purchased from the Forestry Commission Research at Alice Holt.
The Tree Advice Trust offers a Tree Helpline (calls charged at premium rates - see their website for details) on 09065 161147, and issues "Tree Damage Alert" leaflets with advice on topical aspects of tree care.
For more information about the effects of non-living factors on tree health, click on The Elements.