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Contents Introduction Learning outcomes 2 Error! Bookmark not defined. Tree disorders 3 Climatic and atmospheric factors 3 Effects of other organisms 7 Signs of a tree under stress 11 Causes of stress in trees 12 Selection of planting material 13 Planting techniques 15 Damage to the roots of established trees 17 Damage caused by roots 27 Damage to kerbs, footpaths and paving 30 Summary 33 Check your progress answers 37 5862AE: 3 Tree Maintenance OTEN–DE, 2001/049/09/2002 1 Introduction Upon completion of this section the learner will be able to: identify, prevent and/or treat common disorders. You will know you have achieved this when you are able to: 2 identify abiotic and biotic factors which result in tree disorders identify four common symptoms of stress identify common disorders affecting a given list of tree species. 5862AE: 3 Tree Maintenance OTEN–DE, 2001/049/09/2002 Tree disorders Tree disorders are a range of problems caused by a variety of factors and requiring analysis and relevant treatment to alleviate or remove the condition. Apart from sudden physical damage such as that caused by storm, accident or vandals, most of these problems are a result of the tree becoming ‘stressed’. Conditions causing stress to trees result in reduced vigour and reduced ability to respond to those conditions. Shigo defines stress as: ‘a condition where a system or its parts begins to operate near the limits for which it was designed.’ Reductions in plant vigour usually arise from exposure to stress: drought, poor soil aeration, freezing or extreme temperature fluctuation, defoliation, nutrient deficiency, chemical injury, mechanical damage, or transplant shock. This stress may manifest as visible symptoms or it may have a more subtle effect of weakening a plant and increasing its susceptibility to pathogens or attack by boring insects. The greatest threat to survival faced by urban trees is mistreatment by humans, giving rise to stress. Climatic and atmospheric factors Temperature This is possibly the most fundamental plant selection criterion. Plants have evolved under certain temperature regimes so a tropical plant placed in a cool temperate landscape (or vice versa) is going to suffer. The extremes of temperature affect growth and survival. Some plants are frost tolerant, others are not. Consider whether or not you are in a frost-prone area. Take into account the fact that heat may be reflected off buildings or hard surfaces. Light Has the plant evolved in sun or shade? Light conditions may change during the life of a plant; for example, other trees may grow or be removed, buildings may come and go. Photosynthetic efficiency may be reduced if plants have to compete for light. 5862AE: 3 Tree Maintenance OTEN–DE, 2001/049/09/2002 3 If a plant is suddenly exposed to more light and heat it may suffer from sunburn. Over-pruning smooth-barked trees such as Ficus spp and Fraxinus spp can have the same effect. Photoperiod (the relative amount of light and dark) may determine a tree’s seasonal behaviour. Some plants, when grown at lower latitudes than those to which they are native, may experience a reduced growing season and will therefore be smaller than normal. In almost all photoperiod-sensitive woody species, long days and short nights promote vegetative growth. Night lighting such as street lights may affect plants sensitive to day length. One general observation is that vegetative growth may be promoted and dormancy delayed. Continuous lighting suppresses chlorophyll formation and promotes lengthening of shoot internodes and leaf expansion. Thus trees in this situation may be more susceptible to air pollution and water stress. Next time you are out at night have a close look at trees growing close to lights and see if you can see any of these effects. Rainfall The amount of rain that falls and its distribution influence plant selection and requirements for irrigation. Drought is a common stress factor. Some plants have adaptations for drought hardiness (eg reduced leaves, sunken stomates) but others must simply be watered more often or not planted in situations where drought is common. Borer infestations tend to be more common during dry periods. Excess rainfall can also cause problems through reducing soil aeration, while high humidity may encourage diseases. Hail damage may predispose plants to certain foliar diseases; for example, shoot blight (Sphaeropsis sapineae, previously Diplodia pinea) of Pinus radiata (see Hadlington and Johnston, p 96). Bark can also be killed (as well as the cambium) on trees with thin bark. These damaged areas may also be involved by saprophytic or parasitic organisms. The weight of snow on branches may be enough to break them. Wind Knowing the direction of prevailing winds will tell you about the temperature, moisture and salt conditions that may exist in an area during particular seasons. Windbreaks can be grown using hardy species to redirect or lessen the effect of such winds. 4 5862AE: 3 Tree Maintenance OTEN–DE, 2001/049/09/2002 Strong winds and storms are less predictable and may cause extensive damage to trees, especially when the soil is moist. In areas where strong winds are common, early thinning of trees may lessen any damage by allowing the wind to move through the trees more easily. Trees in windy areas tend to be shorter, with thicker trunk bases and a more pronounced trunk taper than trees grown in a more protected environment or close together. What are the prevailing wind directions in your area, and from which directions do storms come? City landscapes can increase wind velocity by creating wind tunnels. Turbulence may be created by the many solid barriers in street scapes. Lightning The nature and extent of lightning damage to a tree can vary greatly. Branches and barkk may be blown completely apart; the crown may be killed or large limbs broken out; trunks may be split open; in rough-barked trees, a continuous groove of bark and/or wood may be stripped out or injured without any external evidence; part or all of the root system may be killed. These variations appear to be related to the intensity of the lightning, the species, the amount of water on or in the bark, and the character of the branch and trunk tissues (see Figures 8.8 and 8.9, Hadlington and Johnston). Lightning is most likely to strike a lone tree, the tallest in the group, a tall tree at the end of a row or on the edge of a grove, a tree growing in moist soil or adjacent to bodies of water or a tree closest to a building. Plants under or near lightning-struck trees are often killed or injured. Protection Trees can be protected against lightning if means are provided by which a lightning discharge can enter or leave the ground without causing damage or loss. Trees close to buildings (within 3m), specimen trees, historic trees, and those under which people may shelter could be equipped with a lightning protection system. In a group of trees, only tall trees need to be protected. Lightning protection systems should be inspected regularly and air terminals may need to be raised every two to three years. A tree protected with a lightning rod will actually be more likely to be struck by lightning, but a properly installed system should protect the tree and people underneath from harm. 5862AE: 3 Tree Maintenance OTEN–DE, 2001/049/09/2002 5 Treatment Depending on the extent of damage, trees may need to be removed, others treated. Since the extent of internal damage cannot be assessed immediately, repairs should be limited to safety pruning and treatment of bark wounds. A period of six months is usually enough to assess whether or not the tree will survive. Root growth should be stimulated by watering and a light fertilising. Struck trees may decline over several years. Pollution Leaves are the main plant parts to show symptoms of air pollution injury. On broad-leaved plants, leaves may develop interveinal necrotic or chlorotic areas, marginal or tip necrosis, stippling of the upper surface or silvering of the lower surface. Conifer needles may show patterns of chlorosis and necrosis. These are symptoms of acute toxicity and usually result from a short exposure to high concentrations of a gaseous pollutant. Pollution injury symptoms can be very variable depending on such factors as: plant species and stage of growth the type of concentration of pollutants the length of exposure moisture content of the leaves humidity light temperature wind nutritional level. Long periods of exposure may result in chronic injury which may be difficult to distinguish from other forms of poor growth. Affected plants will be low in vigour, leaves will be pale green tree, leaves and fruit will colour early and more brightly and will drop earlier. Affected plants are usually more susceptible to other disorders. Air pollution may be from particulates and/or from airborne chemicals. Particulates include various dusts (chemical or natural), soot, oil and lead. Stomates and lenticels may be blocked and photosynthesis may be reduced. 6 5862AE: 3 Tree Maintenance OTEN–DE, 2001/049/09/2002 There are four major chemical air pollutants: 1 sulphur dioxide: a point-source pollutant from coalburning industries, oil and gas refineries 2 fluorides: a point-source pollutant from industries such as steel and aluminium refineries and the production of ceramics, glass and phosphate fertihsers 3 ozone 4 peroxyacetyl nitrates (PAN): oxidants formed in the atmosphere by photochemical reactions between hydrocarbons and nitrogen oxides in the presence of sunlight. Photochemical oxidants can cause plant injury as far as 120-200 km from the origin of the primary pollutants. The hydrocarbons come from the combustion of coal and petroleum-mostly from vehicular exhausts. Some pollutant-tolerant trees and shrubs (From Bernatzky 1978) Buxus sempervirens (sulphur dioxide) Platanus x hybrids (sulphur dioxide) Acer platanoides (fluorine, oxone) Euonymus europaeus (fluorine) Quercus robur (fluorine, oxone, nitrogen trioxide) Chamaecyparis pisifera (fluorine, nitrogen trioxide) Fagus sylvatica (nitrogen dioxide) Ginkgo biloba (nitrogen dioxide) Robina pseudoacacia (nitrogen dioxide, nitrogen trioxide) Acer negundo (nitrogen trioxide) Effects of other organisms Other organisms tend not to be important as selection factors but are commonly involved in tree decline. They may either be the primary cause of damage or they may be secondary organisms taking advantage of a tree weakened by other factors. 5862AE: 3 Tree Maintenance OTEN–DE, 2001/049/09/2002 7 People People can do the right or wrong things by trees. The right things are careful selection and appropriate maintenance. The wrong things include poor selection, lack of maintenance, careless use of machines (particularly lawn mowers and ‘whippersnippers’), inappropriate use of herbicides, and disruptive construction around trees. Unfortunately, deliberate damage to trees is an increasing problem. Usually this is by acts of indiscriminate vandalism. However, sometimes a tree is poisoned or otherwise damaged by someone angered by the loss of a water view or engaged in a dispute with neighbours. Some people don’t like trees because they drop leaves, shade their gardens or may shelter perverts! Animals Large animals such as horses, cattle, sheep and goats can damage trees by grazing and debarking and by compacting the soil underneath. The excreta of dogs and cats may enrich the soil around young trees causing nutritional problems. Soil organisms These may be macro or micro, plant or animal, beneficial or detrimental. Earthworms, mycorrhizal fungi, nitrogen-fixing bacteria and the multitudes of micro-organisms involved in nutrient recycling have beneficial effects on the soil ecosystem and thus on tree health. A change to less favourable conditions especially anaerobic conditions may favour organisms which can have detrimental effects on plant growth. Some tree species are more susceptible than others to certain root rots. For example, Eucalyptus marginata (jarrah) is sensitive to Phytophthora cinnamomi. However, in this case, extra stress is placed on the trees by the effect of burning practices on the nature of the understorey. Pathogens Like all organisms, trees are subject to certain diseases. Susceptibility to disease depends on a number of factors including genetic predisposition, changes in the tree’s environment, and wounding. Most pathogens appear to be secondary problems arising as a result of a primary cause of stress. Wood decay is an example of this process and will be discussed in Section 4. 8 5862AE: 3 Tree Maintenance OTEN–DE, 2001/049/09/2002 Insects Like soil organisms, these can be useful (predators, parasites and pollinators) or detrimental (leaf-chewers, sap-suckers, borers, and rootchewers). Native plants will always be eaten by native insects as everything has its place in the food chain. A borer infestation is a symptom of a tree already under stress. Repeated defoliations of a tree may require control of the insect or may be a factor in future species selection. A knowledge of the type of insect and the extent of damage may influence tree-care practices. Borers, leaf-chewers and sap-suckers will be covered in other Unit 4?. Birds Most birds cause no problems to trees and, in fact, control many insect pests. In some areas the overclearing of low, dense understorey plants has destroyed the habitats of many small, insectivorous birds, which has allowed the balance to tip in favour of the insects. In some instances, however, birds can be harmful to trees. For example, white cockatoos have been known to strip trees and snap branches, possibly in the search for borers or possibly for mischief! Also, the mistletoe bird is closely associated with the mistletoe plant which is a destructive parasite on other plants (see next section on ‘Other plants’). Other plants Other trees and shrubs can compete for water, nutrients and light. In natural systems this leads to the survival of the fittest. Turf and ground covers These present few problems for mature trees but turf in particular can reduce the growth of young trees. Grass roots not only compete for water and nutrients but may also have allelopathic effects (ie growth inhibition as a result of toxic plant exudates). 5862AE: 3 Tree Maintenance OTEN–DE, 2001/049/09/2002 9 Climbing plants These may affect trees in several ways: constriction of young trees, causing deformity competition for light-trees may be smothered constriction of vascular tissues competition for water and nutrients increase in crown weight and wind resistance concealment of structural weaknesses, decay or insect damage. If you want a specimen tree, don’t grow a climber up it. If you do choose to have a climber on a tree, then understand the consequences. Once climbers such as Ivy, Wisteria and Bougainvillea are established they are very difficult to remove. Just one more point: the adventitious roots of Ivy are not parasitic; that is, they do not penetrate and feed off the host tree. Mistletoe There are over 70 species of mistletoe in Australia. Mstletoe is a native to Australia and is adapted to drought and low-nutrient soils by being a parasite (ie it feeds off a living host—it may eventually kill the host). Some mistletoes are parasitic on a wide range of hosts, and others are very host specific. It may be possible to find a mistletoe growing on a mistletoe growing on another mistletoe! Mistletoes are found in all parts of Australia except Tasmania and alpine areas. They are found in all plant communities including rainforests, mangroves, and areas, woodlands and coastal areas Mistletoe flowers twice a year and the fleshy green pseudoberries it produces are eaten by a range of birds and animals. One of the main agents of spread is the mistletoe bird Dicaeum hirundinaceum. This small bird feeds on insects, fruits and particularly the pseudoberries of mistletoe. (By the way, the bird is not found in Tasmania or alpine areas.) It has a modified digestive system which allows the seed to pass undigested through the gut. The sticky seed is then excreted onto (rather than off) a branch as the bird has to wipe it on the branch to get rid of it. A sticky coating of viscin glues the seed to the host’s branch. When conditions are right, the seed germinates and produces an haustorium (the point of attachment between mistletoe and host). Some mistletoes produce one point of attachment whilst others produce a long creeping limb that repeatedly penetrates the host. Mistletoes are usually well camouflaged in the host. 10 5862AE: 3 Tree Maintenance OTEN–DE, 2001/049/09/2002 Control This should be considered for specimen trees and those of high value for shelter or amenity. The best method is to remove the mistletoe from the branch if the mistletoe is small enough or, if the infestation is heavy, to remove the entire branch. This is labour intensive but is the most effective method. A weak solution of herbicide can also be used but this is hazardous. There is some biological control by fungi, insects, possums and birds (see Hadlington and Johnston, pp 129). Dodder This is a leafless, rootless, twining plant that is parasitic on other plants. It is a native to Australia but is also found in all other temperate and tropical areas of the world. The vine, sometimes known as devil’s twine, develops an extensive branching habit and contacts the host at many points, producing haustoria. Dodder is propagated by seeds and by broken-off segments that accidentally come in contact with a fresh host. Manual removal is recommended as a control but this can be difficult and timeconsuming. Signs of a tree under stress The signs of stress in a tree vary from being quite obvious to very subtle. It is an advantage, in recognising stress, to know what a healthy specimen of the species in question looks like. The signs of trees under stress can include: wilting leaf fall and a thin crown ‘dieback’- dead twigs and branches epicormic shoots basal suckers small leaves and buds pale or discoloured leaves reduced or compressed extension growth borer damage early autumn colouring slow callus growth spring growth starting late fruit and bud drop 5862AE: 3 Tree Maintenance OTEN–DE, 2001/049/09/2002 11 heavy flowering or fruiting. Once the symptoms are observed it is important to diagnose the cause of the stress and correct or treat it if possible. Causes of stress in trees Symptoms of stress in trees may be caused by any of the following factors: Abiotic factors—those relating to the physical and chemical properties of the soil or environment. These include the suitability of: soil type soil pH levels soil aeration and compaction soil water availability and drainage climatic factors such as light, temperature, wind exposure cultural techniques employed (eg mowing, mulching, pruning) sudden or gradual changes to any of the above storm, accident or vandalisation Biotic factors—those relating to other living organisms. Competition for space, light, water, nutrients Pathogens (pests and diseases) and parasites 12 5862AE: 3 Tree Maintenance OTEN–DE, 2001/049/09/2002 Selection of planting material With regard to the climatic factors, it is important to choose plants which are suitable to the local region. Plants struggling to survive outside their ‘comfort range’ will always be stressed and susceptible to problems. Similarly plants (particularly large, long lasting trees) need to have adequate space to grow, and not be subjected to ‘cutting back’ unnecessarily. When people go to a nursery to buy plants, most spend some time looking at the size and condition of the stem and leaves. Few stop to consider the root system concealed in the container. Root characteristics are important when selecting stock for planting. Root and shoot quality can determine not only performance but survival. A well developed, healthy root system is essential to a vigorous plant. If you intend to buy a large number of trees from a nursery, it may be wise to randomly select several plants for a close inspection of the roots. Containergrown stock should have fibrous roots sufficiently developed so that the root mass will retain its shape and hold together when removed from the container or when handled during planting. Plants should not be ‘potbound’: that is, have too many overdeveloped roots which may circle the pot or which may be kinked. If you can’t lift the container from the ground because the roots have grown out into the ground, leave it and go elsewhere. Circling of roots within the container can take place several times if plants are not potted-on at the right stage. Even if peripheral roots are satisfactory (ie fibrous, brown roots with white tips), roots within the centre of the root ball may be girdled from previous potting. Be suspicious of this if, when removing the plant from the container, loose potting mix falls away from the outer edges of the root ball. Check also that the tree has grown into its container before you buy it, particularly as in NSW trees and shrubs are priced and sold according to container size. The tree may appear to be the right size for the container but may not have grown into it. If you are unsure of the condition of the inside of a root ball, then select a random sample from a large order and wash the potting mix from the roots and check for disorders. Some problem roots may be corrected by pruning and ‘teasing’ at planting. However, if the roots are very woody and circled it may be cheaper in the long run to throw it away and plant a better specimen. Specifications for the purchase/supply of plants could include aspects of root condition and/or inspection for root defects as a condition of acceptance on delivery. It is not uncommon to find that trees in the landscape are not thriving because they were planted in a pot-bound condition. Signs of this can include poor stability in the ground (hold the trunk and push the tree back 5862AE: 3 Tree Maintenance OTEN–DE, 2001/049/09/2002 13 and forth-if the tree moves in the ground, suspect a problem with the roots). Other symptoms include undersized and paler leaves and susceptibility to borers. Figure 1: Pot bound roots Container size, colour and shape may influence root development. Taller, deeper pots provide good drainage characteristics; tapered and ribbed pots tend to produce less root circling than flat-based pots; light-coloured pots are cooler than black pots but are more expensive, are often translucent and are short-lived. A recent development in the in-ground production of super-advanced trees has been the use of special fabric root-control bags that stimulate secondary root branching within the bag. Trees are planted into the bags, which have been ‘planted’ into the soil. They are filled with the same soil as surrounds the bag. The roots grow outwards and when they contact the bag, they penetrate it. However, as the roots that penetrate the bag increase in diameter, the fabric restricts root expansion and causes a swelling or nodule to develop just inside the bag. It is from this nodule that fibrous, secondary roots develop. Following removal of the bag at planting time in its new site, many root tips develop rapidly into the surrounding soil. This method has advantages over traditional in-ground or open-ground production of trees in that a maximum number of roots are retained after digging. Likewise, it compares favourably to above-ground containerising of plants in that it avoids high temperatures within the container and the problem of containers being blown over. The system was developed by Carl Whitcomb from the United States and is being used successfully in several large nurseries and tree farms in Australia. 14 5862AE: 3 Tree Maintenance OTEN–DE, 2001/049/09/2002 Planting techniques Many people feel that they have an innate ability to plant a tree. However, like most things horticultural, planting is a skill! Planting has been covered in module 5862MD Planting Skills but it is important to re-consider some aspects of planting now, as the roots may be damaged at this stage. The planting hole needs to be wider and deeper than the root ball. It is most important in fine-textured soils (ie clays) that the sides and bottom of the hole are opened up and are not smooth-sided—otherwise it is similar to putting the plant in a larger container. The excavated material should be broken up to a fine tilth. Do not dig into the subsoil—especially in clays—as it may create a well and therefore create anaerobic conditions. With large plants or transplants, the soil may need to be mounded. It is not recommended that organic matter be placed at the bottom of planting holes as anerobic breakdown may either produce toxic gases or displace soil oxygen. Similarly, do not place a layer of gravel or coarse material in the bottom of the planting hole. (Contrary to popular belief, this does not improve drainage but actually impedes drainage as it usually creates a perched water table. The soil and ensuing maintenance procedures should supply the roots with adequate water, oxygen, nutrients and support.) When planting, carefully remove the container. Matted and circling roots should be cut or teased. It may be enough to just roughen the outside of the root ball. This will break small roots and encourage branching of the root system. When placing the root ball in the hole, it is most important that the top of the root ball is level with the soil surface, not below it. (One common cause of poor establishment is planting trees too deeply. It is particularly important to avoid this in fine-textured soils. Due to the difference in texture between the soil and the potting mix (usually very coarse), any water applied tends to stay in the soil and will not move into the root ball until saturation of the soil occurs. Apart from aeration problems and possible trunk damage, trees planted too deeply tend to suffer water stress. Another certain way to damage tree roots is to dig too small a hole and then push the root ball in with a size 10 boot!) When backfilling, ensure good contact between soil and root ball. The plants should be watered in and mulched. 5862AE: 3 Tree Maintenance OTEN–DE, 2001/049/09/2002 15 If staking is required, do not drive the stake or stakes through the root ball— this also damages roots. (Staking will be covered in Unit 6.) When planting bare-rooted stock, remove dead, diseased and twisted roots. It is most important that bare-rooted trees are planted at the correct depth (it is usually possible to see a soil level mark on the plant) and that the soil is in good tilth for backfilling. There must be good contact between the roots and soil, with no large air pockets otherwise the roots will dry out. Figure 2: Correct techniques for planting 16 5862AE: 3 Tree Maintenance OTEN–DE, 2001/049/09/2002 Damage to the roots of established trees Stress factors can affect the roots of existing trees directly by killing them or by preventing or reducing branching, elongation, radical growth and mycorrhizal formation. Indirectly, stress can affect roots by reducing photosynthesis and other physiological processes, which may lead to less sugar for use by, or storage within, the roots, or disrupt transport of materials between roots and leaves. Stress factors which damage roots can include: excavation and construction work soil compaction changes in the water table soil buildup or surface sealing contamination and gas leaks pH and salinity changes competition with turf and other trees insects and diseases drought nutrient problems girdling roots damage to the crown or trunk. Each of these causes will be discussed and some possible solutions suggested. Excavation and construction damage Visit a construction site where trees have been retained and observe the amount of room required for building work to proceed. Apart from direct loss to roots by trenching, excavations and foundations, construction around trees often leads to chemical and structural degradation of the soil and changes in the water table. Direct root loss will make a tree more susceptible to drought, and the reduction in water uptake will lead to a reduction in the crown and hence photosynthesis. Trees may also be destabilised and be more subject to windthrow. 5862AE: 3 Tree Maintenance OTEN–DE, 2001/049/09/2002 17 Recommendations If trees are to be retained on building sites where excavation is to occur, as much of the root system as possible (at minimum, to the drip zone) should be protected by a root curtain or by sheet-piling. Sheet-piling involves contiguous sheet-piles being driven vertically down through the roots before excavation commences. (See Figure 3.) Figure 3: Sheet-piling This will cut the roots cleanly, retain the root ball intact and allow excavation to occur up to a solid barrier. The sheet-piling should be in short lengths to minimise crown damage. Once excavation and construction work is completed, the piling is removed. A root curtain involves manually digging a trench at least 500 millimetres wide and 1.5 metres deep between the tree and the future excavation site. All roots are cleanly cut. (This technique is used also when root pruning for transplanting.) On the development side of the trench, a ‘fence’ is constructed of stakes (steel or timber), wire mesh and hessian. (See Figure 4.) 18 5862AE: 3 Tree Maintenance OTEN–DE, 2001/049/09/2002 Figure 4: Root curtain The trench is backfilled taking care to replace soil horizons in the correct sequence. New roots can grow from the cut ends. Application of a rooting hormone may assist in the initiation of new roots. Excavation of the site can occur up to the root curtain. If root treatment can be done only during the excavation, the roots should be protected from drying out. An alternative could be to spray concrete onto the exposed root ball. With all techniques it is important to maintain adequate moisture and drainage within the retained root ball. When trenching is necessary for laying of pipes or cables, it should ideally be carried out beyond the drip zone even though roots will still be destroyed. If it is impossible to trench so far away, then it is best to take the trench below the centre of the tree where there are fewer roots. As far as possible, hand digging should occur in sensitive areas and large roots tunnelled under rather than severed. If water pipes are to be installed they should be of PVC with solvent-weld joints and with enough space between root and pipe to allow root expansion (see Figures 2.6 to 2.9, Hadlington and Johnston 1988 or later). If developers are serious about retaining trees on construction sites, and governments are serious about imposing tree preservation orders, then good specifications must be written and followed in order to provide the tree with means of survival and growth. It is inevitable that some damage will occur 5862AE: 3 Tree Maintenance OTEN–DE, 2001/049/09/2002 19 but carefully prepared specifications and liaison between engineers and horticulturists and arborists may keep it to a minimum. Soil compaction Take a screwdriver and, using the palm of your hand, try pushing it into an area of lawn or garden which receives little traffic. Try again in an area of heavy traffic. Feel the difference? Compaction is common in parks, streets and public areas and is a greater problem in fine-textured soils than in coarse soils. Soil aeration is reduced and eventually root functions are impaired. This leads to a deterioration of the crown and a reduction in overall growth, although the effects on growth may not be noticeable for several years. Various researchers have found that when compaction reduced air space to less then 15%, root growth was inhibited; when dropped to 2%, root growth was negligible. Air space should be at least 20% for adequate gas exchange between soil and atmosphere, and oxygen concentration should not drop below 10%. Recommendations All problems have a cause. What is the cause of the compaction? Can the cause be treated? The most common cause is foot and vehicle traffic. In some cases traffic can be redirected, but in many cases (eg parks, golf courses) it is unavoidable and hence symptoms must be treated. Soil can be aerated by coring, slicing or fracturing with compressed air. Changes in the water table and soil water levels Whether the water rises from below or excess water is applied to the surface, flooding, like compaction, leads to poor soil aeration. However, in the case of flooding, the oxygen deficiency is immediate, whereas the effects of compaction occur gradually. Flooding immediately affects most aspects of tree growth—height, leaf growth, cambial activity, reproduction, leaf initiation and it may also induce leaf senescence and abscission. High soil moisture levels may also encourage pathogens such as Phytophthora fungi, and predispose otherwise resistant species to attack. Waterlogged soils encourage anaerobic breakdown of organic material, which can lead to the production of phytotoxic compounds such as sulphides. The roots of some trees under waterlogged conditions manufacture products which can also be toxic to the tree; for example, cherries produce cyanide. Plants can be indirectly affected by flooding through the effect on micro-oganisms involved in nutrient recycling. For example, the bacteria involved in the nitrogen cycle stop functioning in saturated conditions. 20 5862AE: 3 Tree Maintenance OTEN–DE, 2001/049/09/2002 Rising water tables can have even more devastating effects on tree growth if the water is saline. Lowering water tables by drainage can also damage trees which have adapted to wet conditions. These trees will suffer from a water shortage. Some species of trees (eg Nyssa aquatica, Taxodium distichum) found in swamps and floodplains, have evolved special anatomical features that allow conduction of oxygen to at least 300 millimetres below the surface of the water and special metabolism that eliminates toxic byproducts of anaerobic respiration. Other species which grow in wet sites (eg Willows Salix spp) have shallow and multi branched root systems. Recommendations Sometimes flooding occurs because of accidents such as burst water mains or sometimes as a result of excessive rainfall. In these situations it is important to recognise that the trees will be stressed and that some dieback is likely. Removal of excess water as quickly as possible is the only remedial action that can be taken at the time. In areas prone to flooding, species adapted to such conditions should be planted. Water tables may be changed as a result of construction and site development. This is best anticipated at the planning stage and decisions made to either provide drainage or to supply water to trees retained on the site. Soil build-up and surface sealing In your travels around streets, car parks and other built-up areas, assess the health and vigour of trees that have been retained whilst the area around them has changed. When an area is sealed or the soil level is built up, soil aeration and water penetration are reduced. This not only directly affects root functions and thus overall plant growth but it also affects soil organisms. The most useful soil organisms are aerobic. Under anaerobic conditions, aerobic organisms are replaced by anaerobic species which may produce undesirable products such as toxic gases. Mycorrhizal fungi will also be affected, thus further reducing root functioning. A reduction in water and nutrient uptake eventually affects the leaves and their ability to photosynthesise. Hence sugar production and, the products of metabolism are reduced and the tree may become susceptible to secondary problems such as borers. 5862AE: 3 Tree Maintenance OTEN–DE, 2001/049/09/2002 21 Figure 5 shows the effect that build-up or surface sealing may have on a tree. Figure 5: The effect of soil buildup One of the signs of soil level build-up is an absence of flaring at the base of trees. The tree comes out of the ground like a pole. Recommendations In some cases, increasing the soil level around trees can be avoided. For example, on construction sites materials can be stored away from trees. If level changes are unavoidable, maintain as large an area of the drip zone at original level and then backfill with very coarse material (eg gravel). However, this may not guarantee survival. Care should be taken to avoid excess run-off into these situations as this will cause drainage problems. Drainage systems may have to be installed as well as the gravel. If surface sealing around existing trees is necessary, use a material or materials which will allow some aeration, such as, in decreasing order of permeability, gravel, unit pavers on coarse sand (no mortar), asphalt and concrete. Other materials include decomposed granite, no-fines concrete, ‘B.G.’ blocks or grass-crete blocks, Ritter-rings® and similar products. 22 5862AE: 3 Tree Maintenance OTEN–DE, 2001/049/09/2002 Figure 6: Alternative to impervious paving. In many cases of build-up or surface sealing, trees may take up to ten years to die. Trees planted either in the new soil levels or after paving survive better than those older trees retained on the site. Note that brick or dry-stone collars are not effective in preserving trees after soil buildup (see Figures 2.4 and 2.5, Hadlington and Johnston 1988 or later). This is because they collect water and litter, can be a danger to small animals and children, and lead to poor aeration, drainage and water infiltration. Drought and nutrient problems Drought can affect roots both directly and indirectly. Root tips can die from lack of water; drying of soil prevents root penetration; photosynthesis is reduced and hence food for root growth and storage is reduced. Therefore, drought can reduce root growth during the current season and also at the beginning of the next season through its effect on food reserves. The deficiency or excess of certain nutrients may directly damage roots or may indirectly affect plant growth and storage of reserves. Plants require 16 essential elements in the correct proportions, for normal growth and reproduction, so fertiliser application must be carefully considered. Excessive fertiliser causes problems with salinity and toxicities. Some plants have adapted to drought stress by morphological modifications (eg sunken stomates, pubescent new growth, waxy leaves). Likewise, many plants (particularly some Australian natives) have ways of coping with low nutrients: nodulation in legumes and Casuarina spp for nitrogen fixation; for example: nitrogen-fixing associations between blue-green algae and cycads: mycorrhizal associations: proteoid roots; lignotubers and epicormic buds. These are explained more in Section 4. 5862AE: 3 Tree Maintenance OTEN–DE, 2001/049/09/2002 23 Recommendations When selecting plants for sites, consider available moisture and nutrients and either select accordingly or provide adequate moisture and nutrients. Avoid applying excessive fertilisers and avoid changing the pH, to avoid upsetting nutrient availability. If nutritional problems are suspected, check the pH and, if necessary, have the soil analysed. If plants have been overfertilised, leaching may be required. If pH is a problem, adjust it accordingly. Contamination, pH changes and salinity A significant proportion of the root system of trees is concentrated in the top few millimetres of soil and extends well beyond the drip zone. The roots intermingle with leaf litter, grass roots and are even found in the cracks and crevices in concrete and asphalt paving. Due to this proximity to the surface they are vulnerable to damage by chemical contamination caused by herbicides used to kill broad-leaved weeds. Dicamba is an example of a herbicide which is known to damage trees. Some organisations have found problems in trees where Roundup(has been used repeatedly around their roots. This is a widely used herbicide but more research on its action in soil and tree roots is required. Applications of Velpar , bromacil and other sterilants and residual herbicides can kill trees over 20 metres distant from where they have been applied. Other chemicals that can damage roots include fuels and oils (including diesel), solvents, detergents and chemicals used for root control in pipes. Gas from gas leaks or from anaerobic respiration (eg old tip sites) can also damage roots directly and indirectly by displacing soil oxygen. Rapid changes in pH can occur around building sites due to contamination by lime and cement. This can upset nutrient availability, leading to deficiencies and/or toxicities of various elements. Salinity from rising salt or from over-fertilising damages roots and interferes with water uptake. Recommendations If residual herbicide damage is suspected, several diagnostic tests can be used. A bio-assay using annual plants can be carried out to see if the soil is still toxic enough to damage new plantings. Toxic levels of herbicide will twist, stunt or kill the test plants. The damaged tree can be analysed for herbicide residues. 24 5862AE: 3 Tree Maintenance OTEN–DE, 2001/049/09/2002 The surrounding soil can be analysed for residues. Chemical analysis can be quite expensive. If you suspect a particular herbicide, check with the manufacturer about symptoms of phytotoxicity and necessary treatment. Soil containing a toxic level of herbicide can be removed, mixed with organic matter, leached or planted with tolerant plants, depending on the herbicide, soil texture, drainage, area involved, desired planting and the available budget. Activated charcoal, at 40 grams for each 150 millimetres depth of soil in a hole one metre in diameter, should be thoroughly mixed into the soil. This approximates the recommended rate of 150 parts (weight) of activated charcoal to one part of active herbicide in the soil. Other researchers have reported that nitrogen-enriched sawdust could be as effective as charcoal. Gas leaks can be detected by smell and by gas detection meters. The source of the leak should be found and repaired, the soil aerated; and the plant replaced. Plants tolerant of poorly aerated soils (such as swamps) may be more tolerant of gas-contaminated soils. pH and salinity problems should be treated at the cause. In some cases the pH and salinity are site constraints and therefore tolerant plants should be selected. Competition Turf and ground covers are not usually a great problem for established trees. However turf can greatly reduce the growth of young trees. The roots of grass intermingle with tree roots and directly compete with them for water and nutrients. Turf may also increase run-off. These factors tend to exacerbate the effects of other stresses such as drought and nutrient deficiences. Some turf species have been found to have allelopathic effects on young trees. Activities associated with lawn care, such as applications of lime, herbicides and fertilisers, may also damage tree roots. Competition can also come from other trees and shrubs in the landscape. Recommendations Adequate irrigation and appropriate fertilising may help to overcome some of these competition problems. If turf can be kept at least 300 millimetres from the trunks of young trees and shrubs, most retarding effects will be eliminated. The area should be mulched. Trials using applications of organic mulch versus grass and bare soil as surface treatments have shown that mulching produces several beneficial results. Under organic mulch, rooting density and root surface area were increased and soil moisture content was significantly higher. 5862AE: 3 Tree Maintenance OTEN–DE, 2001/049/09/2002 25 The mulched circle also reduces the exposure to damage from whippersnipper type trimmers. Insects and diseases When you’ve been digging in the garden, have you ever noticed curled white larvae with quite prominent legs? (See Figure 3.6, Hadlington and Johnston 1988 or later.) They are probably Christmas beetle or scarab beetle larvae. These larvae live underground and feed on plant roots. In favourable seasons, these larvae can seriously debilitate and even kill young trees. They can also be a serious pest of containerised plants. There are several species of root rot fungi that can cause death or instability including Armillaria spp and Phytophthora cinnamomi. Information on these organisms is provided in Hadlington and Johnston pp 93-94. In the production of trees in nurseries, seedlings may be susceptible to damping-off diseases. The organisms concerned are varied and include Pythium spp, Phytophthora spp, Rhizoctonia spp, and Fusarium spp. Damping-off is favoured by low soil temperatures, surface compaction, soils of poor structure and excessive watering. Recommendations If Christmas beetle larvae are numerous and causing major problems, an insecticide such as ‘Lawn Beetle Killer’ & can be applied as directed on the label. For Amillaria and Phytophthora refer to Hadlington and Johnston or your Pest and Diseases modules. Damping-off diseases are best controlled by avoiding the conditions that cause them, by pasturising potting mixes at 60’C for 30 minutes or by using specific fungicides. These problems and others will be dealt with in more detail in Section 5. Girdling roots Trees can be weakened or killed by roots that girdle the trunk or main roots. Circling roots usually start in the nursery as a result of inadequate pottingon. Sometimes surface roots are deflected by poor soil or rocks and may grow back towards the plant. 26 5862AE: 3 Tree Maintenance OTEN–DE, 2001/049/09/2002 Recommendations Girdling roots are best avoided by potting-on at the right stage, by checking roots before planting and by providing adequate room for root growth. Sometimes surface girdling roots can be removed with a chisel and mallet. If one root is girdled, others lower down may also be girdled. This may require careful excavation with a hand trowel close to the trunk to determine the extent of girdling. If girdling is severe, the plant will not only be inefficient but may be unstable. In severe cases the tree should be removed. Figure 7: Girdling roots Damage caused by roots Most people tend to ignore the root systems of trees—that is, until the roots cause problems. A healthy, vigorous root system is essential for a healthy and vigorous tree but it is true that the root systems of trees can themselves cause problems. Fore-knowledge about which trees are likely to have too vigorous root systems for a particular site can lead to more careful plant selection and avoid serious problems later. Roots can crack and plug sewerage and drainage lines, lift and break kerbs, footpaths, paving and foundations, and may exacerbate the problems of swelling and shrinking clays which may cause further structural damage. Tree-caused damage makes up a large component of the insurance and legal costs of local councils. This includes damage caused by street trees to private property and to pavements, in the latter case making them uneven and causing people to have accidents. This section will look briefly at the main types of problems caused by roots and at some possible solutions. 5862AE: 3 Tree Maintenance OTEN–DE, 2001/049/09/2002 27 Damage to sewerage and drainage lines Roots can damage pipes in two ways: by entering through cracks in the pipes and then growing and expanding, or by cracking them by external pressure as a result of root expansion. Roots are not the only cause of damage to pipes. Other factors to consider are: soil movement caused by site disturbances or natural causes movements due to shrinking clays the age and type of pipes the quality of the installation work. Damage to sewers can be prevented by laying PVC plastic pipes with solvent-weld joints, rather than earthenware pipes. Other preventive or treatment measures include the following: Avoid planting large trees with vigorous root systems within two metres of sewers and drains. (Several local councils, the Water Board and other authorities provide lists of plants considered unsuitable for planting near drains and sewers). Have a plumber remove root blockages—this is temporary and may have to be carried out on an annual basis. Root-prune near pipes on a regular basis. Excavate a trench along the line of the sewer or drain and encase the pipeline with about 150 millimetres of concrete all around the pipe. Excavate a trench between the tree and the pipe and fill the trench with concrete. Treat drains and sewers with commercially available chemicals (but carefully investigate the effects of these chemicals on trees, sewers and drain outfalls before you use them). Trees commonly associated with drain problems include Ficus spp, Populus spp, Salix spp, Cinnamomum camphora, Erythrina spp, Robinia pseudoacacia and Ulmus spp. (Refer to the authorities mentioned above for more complete lists.) Structural damage to foundations Roots can cause damage to foundations of buildings by expanding and physically disrupting foundations, and by removing water from soils near foundations causing the soil to shrink and thus the foundations to settle. The 28 5862AE: 3 Tree Maintenance OTEN–DE, 2001/049/09/2002 latter problem is particularly serious if the soil is a shrinking and swelling clay and is made worse by dry seasons and droughts. As a tree’s demand for water levels off after it reaches maturity, it is less likely that the roots of mature trees will cause damage to new foundations nearby, unless the roots are cut during excavation work. Trees planted after construction will cause most damage. Older houses with shallow footings are most susceptible. Tree removal is one solution to the problem, but if the tree is to be retained, the following actions could be considered: Soak the soil near the foundations to alleviate shrinkage damage. Underpin the footings to a depth which avoids the drying effects of the tree (see Figure 8). Figure 8: Underpinning of foundations In shrinking clays, position foundations carefully by carrying them through the clay layer to firm sand or bedrock. Construct a root barrier between the tree(s) and footings by trenching and then filling with lightly reinforced concrete (see Figure 9). 5862AE: 3 Tree Maintenance OTEN–DE, 2001/049/09/2002 29 Figure 9: Root barrier between tree and footings Trees near houses should be selected for shallow and compact root systems if foundations problems are likely. Not only can the growing of trees near foundations be a problems, but so too can the removal of trees, especially on the shrinking clays. Due to the fact that a tree uses water in the ground, the removal of a tree may mean extra water in the soil, expansion of clay particles and thus heaving of the ground under wet conditions. Damage to kerbs, footpaths and paving Footpath and kerb damage is a common occurrence and major expense for local government. Roots cause problems to these structures by simply attempting to grow and survive in a fairly hostile environment. Due to soil compaction around street plantings and the aerobic nature of roots, many problems are caused by roots forced to grow near the soil surface, and subsequently expanding. The root/trunk interface is prone to maximum growth and commonly causes buckling and cracking of paving when too small a hole has been provided for the plant. Most footpath problems are associated with the roots of young and vigorous trees. To avoid future problems with pavement distortion and kerb damage, the following techniques could be considered: At planting, provide planting holes that are as large as possible with deep soil and good drainage. Adequate soil depth discourages an excess of surface rooting. The soil should be as uniform as possible to avoid interface problems. Select trees that have deep roots; are small to medium at maturity, and are suited to harsh environments. In some situations it may be suitable to plant shrubs such as Photinia fraseri, Nerium oleander or other species which can be trained as trees. 30 5862AE: 3 Tree Maintenance OTEN–DE, 2001/049/09/2002 As roots follow the path of least resistance, create aerated and friable conditions away from problem areas. This may be done by ‘water-jetting’ (forcing water deeper into the soil to create spaces) or by deep ripping before pavement construction. Avoid heavy pruning and lopping of trees as this can upset the root/crown ratio and lead to increased root growth. Prune the roots periodically. As with pruning the above-ground parts of trees, root pruning should start early in a tree’s life to avoid major damage to the plant. Modified concrete cutters can be used for root pruning near paving. The use of root-control boxes or deep-root planters has been tried in some areas. These planters are designed to force root growth downwards. However, because of the tree’s need for aeration, the roots tend to grow back up towards the surface after being temporarily deflected. Therefore these products generally delay root problems rather than solve them. In Canberra, polythene sheeting and the root-retarding herbicide dichlobenil have been trialled in an attempt to limit root invasion. A trench was dug and dichlobenil (as Gasoron ) was applied at the rate of one kilogram per cubic metre of backfill. This herbicide kills treated root tissue without affecting untreated parts. For the trees used in the trial (Eucalyptus bicostata— Tasmanian Blue Gum), the barriers were still working after eight years (see Figure 10). Figure 10: Root control Barrier The digging of trenches has been indicated as a possible means of controlling problem roots, in combination with root pruning. However, trenching has some problems of its own: 5862AE: 3 Tree Maintenance OTEN–DE, 2001/049/09/2002 31 it may destabilise trees; it may lead to crown dieback and susceptibility to other stresses and secondary problems; suckering from severed roots may occur in some species, (eg Elms— Ulmus spp); it may stimulate new root growth; it needs to be an ongoing treatment unless permanent barriers are constructed within the trench. As with most tree problems, the possible problems caused by roots should be anticipated before planting and either suitable species selected or appropriate maintenance procedures initiated early. In the following Units we will continue to learn about tree disorders and stress, in relation to tree wounding, pruning techniques and pests and diseases. 32 5862AE: 3 Tree Maintenance OTEN–DE, 2001/049/09/2002 Summary ‘Out of sight, out of mind’—this attitude is a major reason why tree roots are so easily damaged. When you look around the typical urban environment it is easy to see why so many trees are declining. Apart from being damaged, roots can also cause damage to roads, buildings and other structures. Both problems of damage to roots and damage caused by roots should be addressed in the planning and implementation stages of projects so that the problems are miminised or avoided. The next unit looks at damage to the trunks and branches of trees. 1 Read the following and circle the letter of the best answer. (a) When selecting container-grown stock, the root systems of the plants should: (i) consist of large numbers of woody roots circling the pot (ii) have no or few roots visible near the outer edges of the root ball (iii) be sufficiently developed to retain the shape of the root ball but not be overgrown (iv) always be reduced by half to produce hardy plants. (b) When planting container-grown stock: (i) avoid digging into the subsoil, thus creating a well (ii) always plant the tree slightly deeper to encourage stability (iii) leave smooth sides on holes in clay to encourage the roots to grow down (iv) place a layer of gravel at the base of the hole to provide drainage. 2 Label the following statement True (T) or False (F). (a) Direct loss of roots may predispose a tree to windthrow. (b) Damage to mycorrhizal fungi will have no effect on the health and vigour of a tree (c) Sheet-piling is a technique for protecting trees on construction sites. (d) If trenching is to take place close to trees, it is best to take the trench below the centre of the tree. (e) Compaction occurs readily in sandy soils. (f) The acceptable minimum for soil oxygen concentration is 5%. 5862AE: 3 Tree Maintenance OTEN–DE, 2001/049/09/2002 33 (g) The nitrogen cycle is unaffected by soil moisture levels (h) If the soil level around a tree must be increased then it is best done gradually with coarse material (i) A bio-assay using annual plants is a useful tool for detecting the presence of residual herbicides (j) Turf growing to the base of young trees has no effect on their growth rate. 3 Why do oxygen concentrations decrease and carbon dioxide concentrations increase in the soil in the growing seasons? 4 Why may trees be damaged by broad-leaf herbicides applied to lawns? 5 You have been called to inspect two trees, both ten years old Corymbia maculata (Spotted Gum). Tree number one is reported to be dying (and has borer damage) and your clients are worried about the second tree. On questioning the owners, the following sequence of events emerges: Two years ago, an in-ground pool was installed and the area of soil between the house and the pool was built up by 200mm, concreted and paved. The trunk of tree number one was protected by sacking. Tree number two was unaffected by pool construction as there was no access for large machines. One month ago a new drain was constructed to carry overflow from the pool and stormwater from the rool. A -small trenching machine dug a trench 800 mm deep, 250 mm wide and 1 m inside the side fence. The stormwater pipe was installed but as yet the trench has not been refilled. (a) In point form explain why tree number one is exhibiting the symptoms of die-back and borer damage. ___________________________________________________________________ ___________________________________________________________________ ___________________________________________________________________ ___________________________________________________________________ ___________________________________________________________________ (b) List two steps that could be taken to compensate for the trenching injuries to tree number two. ___________________________________________________________________ ___________________________________________________________________ ___________________________________________________________________ 34 5862AE: 3 Tree Maintenance OTEN–DE, 2001/049/09/2002 6 Answer the following questions, imagining that you have been called to a site where trees are suspected of damaging sewer pipes. (a) How do roots damage pipes? ___________________________________________________________________ ___________________________________________________________________ ___________________________________________________________________ (b) Apart from roots, what other factors may be involved in damage to pipes? ___________________________________________________________________ ___________________________________________________________________ ___________________________________________________________________ (c) How would you confirm the cause of the problem? ___________________________________________________________________ ___________________________________________________________________ ___________________________________________________________________ (d) If tree roots were the cause, how might you treat the problem: (i) in the short term? ___________________________________________________________________ ___________________________________________________________________ ___________________________________________________________________ (ii) in the long term? ___________________________________________________________________ ___________________________________________________________________ ___________________________________________________________________ 5862AE: 3 Tree Maintenance OTEN–DE, 2001/049/09/2002 35 7 You are in charge of planning a new street planting in a shopping centre that is soon to be paved. Your employer is keen to minimise the risk of litigation as a result of root problems. List five possible methods of avoiding pavement distortion and kerb damage. _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ Check your answers with those given at the end of this section. 36 5862AE: 3 Tree Maintenance OTEN–DE, 2001/049/09/2002 Check your progress answers Check your progress 1 1 (a) When selecting container-grown stock, the root systems of the plants should: (iii) be sufficiently developed to retain the shape of the root ball but not be overgrown (b)When planting container-grown stock: (i) avoid digging into the subsoil, thus creating a well 2 a, c, d, h, i all True b, e, f, g, j all False Respiration by roots and soil organisms and decomposition of organic matter all use oxygen and produce carbon dioxide 4 Surface roots in the top levels of the soil can absorb them. 5 (a) possibly: physical root damage compaction and/or soil level build up cause poor aeration and restricted water infiltration (b) (i) (ii) Cut roots cleanly, possibly add a rooting hormone refill, keep roots moist to reduce stress during recovery 6 (a) Enter through cracks and expand crush externally by growth (b) Soil movement Clay shrinkage Age and type of pipe Quality of installation (c) Excavate or use an electric eel (d) (i) annual use of the electric eel, or root-prune near the pipe regularly, or use chemicals in the drains to kill the roots 5862AE: 3 Tree Maintenance OTEN–DE, 2001/049/09/2002 37 (ii) excavate pipe and enclose in concrete and/or fill trench with concrete 7 38 Use large planting holes Choose species with deep root systems Provide friable and well aerated conditions away from the paths Avoid heavy pruning and lopping as this increases root growth Install root control barriers or deep root planters to force growth downwards. 5862AE: 3 Tree Maintenance OTEN–DE, 2001/049/09/2002