The Physiology and Management of Aging Trees

DR GREGORY M MOORE
School of Resource Management,
University of Melbourne, Burnley Campus

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1. INTRODUCTION

The management of urban trees contributes significantly to their health, vigour and longevity. In many cities, the ambiance of the city depends on the quality of the trees in its parks, gardens, avenues and boulevards. Often, these cities have been planned and constructed over a relatively brief period of time and consequently a situation arises where a restricted range of species is planted in large numbers over a short time. Trees belong to a similar age class, and as these trees approach maturity and senescence, a number of important management issues arise.

Proper urban tree management should be based on a sound understanding of tree biology that informs arboricultural and management practices. There are several basis principles of tree biology that are the foundations of modern arboricultural practice, and these are of greater significance when the trees concerned are either stressed or aged (Table 1). Both aged and stressed trees have a lower capacity for coping with stress, pests and disease than healthy, young vigorous trees, often because of lower carbon reserves or stressed root systems.

In modern arboriculture, the model of prevention rather than cure that has worked so well in medicine has been adopted as one of the axioms of good practice. In this model, which is a variation of the old horticultural saying that “healthy trees don’t get sick”, modern management attempts to provide conditions that allow the tree to grow vigorously without stress. Under these conditions, a healthy vigorous tree has sufficient resources to cope with the pests and diseases. These resources include appropriate light, water and nutrients that allow the tree to photosynthesise and respire effectively. Healthy vigorous trees not only have a sound structure, but also have the reserves of carbohydrate to maintain an appropriate chemical protection and response system to many pests and diseases.

The concept that ‘healthy trees do not get sick’ is consistent with basic principles and its significance should not be under-estimated. It recognises that stressed trees have weakened defence mechanisms and so are prone to pest and disease attack, and that in the past symptoms have been treated rather than causes. Maintaining an appropriate environment where the tree can maintain its health and vigour is essential in allowing the tree’s natural defence mechanisms to develop their full potential.

In the urban environment it is not uncommon for significant specimens in parks, gardens and streets, to be highly prized and valued by the local community and the authorities that manage them, but at the same time to be subjected to poor management practices. Such practices may involve poor canopy management, inappropriate pruning, damage to trunks and significant interferences with the root system. Many urban trees can cope with these situations when they are young and when environmental conditions are favourable. However, their capacity to cope diminishes significantly when they are placed under environmental stress and as they age.

2. MANAGING AGED TREES

Many of the trees that form the basic plant infrastructure of major parks, gardens, avenues and boulevards in Australian cities are now in excess of 100 years of age. Many of these trees have experienced significant stress, including road construction, urban development and re-development, major droughts, and fire in their history of urban growth. It is not surprising therefore, that there have been reports of significant infestations of pests, occurrences of rare and unusual diseases and the deaths of significant numbers of older native and exotic trees over the past five years.

Over the past seven to eight years in many parts of Australia there has been a chronic, rather than severe or acute, drought affecting trees that are now aged and sometimes senescing. While these circumstances are often described as ‘unusual’ because it has been the most prolonged drought of its kind since data has been collected, it cannot be assumed that these are rare or occasional events. Such a drought may occur every 100 or 200 or even 300 years, and so be a significant event in the life of trees in urban areas where lifespans in excess of 250 years are not uncommon. The weather conditions may also be indicative of a broader and prolonged climate change. Consequently, steps must be taken to ensure that urban tree management is consistent with tree responses to these circumstances.

Management of mature and senescing trees requires an understanding of their needs. They should be provided with the best growing conditions possible, and care should be taken to ensure that they are not inadvertently interfered with by works that affect their canopies or root systems. These trees may have tolerated poor practices in the past with little or no ill effects, but now aged and stressed, they will not cope with poor management that could seen them decline, fail structurally, or even die.

In managing older trees, it must be remembered that they have real economic value. A number of tree valuation systems have been developed that place a monetary value on urban specimen trees (Moore 2000). A draft Australian Standard was circulated for comment in 1992 but never approved (Anon, 1992), after attracting an enormous response from interest groups and industry sectors. The opposition of the powerful insurance and utility sectors virtually doomed it. However, the draft was quite widely and successfully used in the period from 1992 to 1999. In 1999 a new draft standard of amenity tree evaluation was circulated by Standards Australia for comment, which was a new approach to valuing amenity trees that did not build upon the earlier draft (Anon, 1999). However, it too has not been finalized and Standards Australia is embarking upon another attempt. Australia desperately needs a method of valuation that can be used by professions involved in managing aspects of the urban landscape. Clearly the method must come from Standards Australia, as it is the only agency that could achieve nationwide coverage and find acceptance in a broad range of often competing professions.

While this paper is not the place to canvass the issues surrounding tree valuation, the methods do provide a real value for older trees which can assist in setting priorities and deciding management strategies. However, the methods value the whole tree, which in many instances is appropriate, but there are situations where the components of old trees – root system, trunk or canopy – have value in their own right, and this should be taken into account in managing older trees as assets.

3. CASE STUDIES IN AGED TREE MANAGEMENT IN MELBOURNE

3.1 Southern Conifers

In a survey of over 300 southern conifers, most from the genera Agathis and Araucaria (Andrews, Harris, Skipper 2000), it was found that most were over 70 with many above 100 years of age. Few young specimens were found. Using a modification of the Burnley Method of Tree Valuation (Moore 1992) assessment of form and vigour (Table 2), most trees were found to be in excellent health and vigour with few structural deformities, low incidences of shedding and almost no occurrences of pest and diseases. Given the age of these specimens, the results were a surprising (Table 4).

The suitability of the specimens to their location was also rated (Table 3), and revealed that the trees were well positioned and managed. The very low rating score (Table 4) indicates that the specimens had been planted in almost ideal locations. Almost all of the specimens surveyed were growing in parks and gardens or streetscapes where there was sufficient space for the full development of their canopies and root systems. An investigation at the base of the trees revealed that their root systems had escaped interference, and that soils were largely free from compaction. In short, the trees had been subject to a very low maintenance regime, and so had little damage from poor pruning or the inappropriate management practices of earlier decades.

These trees because of their size had been planted where adequate space had been provided. Consequently, they were in good structural and physical condition, and the incidences of pest and disease attack were minimal.

Table 4: Number of specimens from the genera Agathis and Araucaria surveyed, form and vigour, location and multiple leaders (trunk codominance) and height for the specimens surveyed (Modified from Andrews, Harris, Moore and Skipper, 2004)

The data indicate that these are large trees that exhibit little structural deformity, have had little or no negative impact on the hard landscape, and are almost free of pest and disease. Because these trees have been planted in open space where there has been little interference with their roots and canopies, they have been managed in accordance with the principles outlined earlier – often by default. Consequently they are well placed to cope with stress, and most pests and diseases. Under this management regime although they can be very large trees indeed, they are well suited to urban planting. Future plantings that will allow a broader range of age classes to mature are to be encouraged, provided there is adequate space for their canopies and root systems.

3.2 Phoenix canariensis

In 2003, 147 Phoenix canariensis were surveyed in the Mount Alexander Road plantation (Curnow 2003). The study was initiated as a preliminary investigation of the health of palms in this part of Melbourne and to determine whether any of the specimens were showing initial symptoms of pest or disease attack, and in particular Fusarium wilt (Table 5). Root damage to the palms was rated on a scale of 1 to 5 with one being no, or little damage, and 5 being significant damage. A similar scale was used for suckering with 1 being no suckers and 5 being significant suckers. These were used as general indicators of the quality of management and environmental stress respectively.

Once again, the specimens were found to be in generally good condition, as the plantation provides adequate space for root systems and canopy. There was no evidence of the presence of Fusarium, at the time of survey, but there were practices that raised concerns.

Some of the palms had been damaged around the lower trunk and the skirt of aerial roots by mowers or whipper snippers. These palms showed some production of plantlets or suckers and some showed signs of stress, which would suggest vulnerability to fungal attack. Such occurrences were most evident in areas where the bases of palms had been damaged during the construction and extension of car parks. There is a correlation between root damage and the height that the root skirt extends above the ground, with the higher the root skirt the greater the likelihood of root damage. Damage from mowers was the major cause of damage, and given the nature of palm stems is permanent. Priority must be given to the base of the palm in managing landscapes rather that the surrounding turf.

An attempt was made to investigate the foliage and the ratio of new to old fronds as an indicator of plant health. The hypothesis being that as palms do not undergo secondary growth, stem taper and the ratio of new to shed leaves should depend on plant growth and vigor. If the plant is healthy the stem caliper should be constant and the ratio of new to old leaves should be greater or equal to one. Twenty palms were measured but while it was relatively easy to identify new season’s leaves, it was almost impossible to identify the leaves being shed in that season from those of earlier seasons. This problem was compounded by the pruning of older leaves, and the difficulty of access to tall palms on a busy roadway. There is promise in this form of monitoring, but it would require accurate tagging of leaves as they are produced each season over a number of years.

The pruning of leaves from the palms is also a regular practice. There was a preliminary survey done of these practices to see if they were contributing to damage or poor canopy health. While there is little evidence that this has been done poorly, there was some evidence of minor damage. It is to be hoped that the basic practices of hygiene are being followed and that chainsaws and other equipment are being disinfected as a matter of routine with disinfectants such as hypochlorite. Once again this would be in accordance with the principles of modern arboricultural practice.

3.3 Survey of Elms

In a study of Elms in Melbourne (Graham and Ferrier, 1997), five criteria were chosen as the basis for assessing the condition of mature elms growing along roadsides or in city parks. These criteria (Table 6) included some obvious symptoms of condition such as the presence of decay and the amount of deadwood present in the canopy. Other criteria including the measurement of bud burst and leaf shedding of leaves were used to indicate tree health and the length of the growing season, A fifth factor used in assessing trees was their capacity to produce callus after pruning or wounding

The general results of the survey undertaken in the study are shown in Table 7; they reveal that the trees in Princes Park were generally in better condition than those growing along any of the rows along Royal Parade, a busy major road. The results also show that the elms growing in Yarra Park, which surrounds the MCG and is subject to both vehicular and pedestrian traffic, regardless of their location were in generally poor condition, and that there were no significant differences between the elms growing at these sites.

Interestingly, the use of deadwood in the canopy cannot be used as an indicator of condition in these situations. The street trees are so intensively managed compared to parkland trees, for traffic and pedestrian safety that there is little or no deadwood in the canopy due to regular pruning. The lack of deadwood contrasts with modest levels of deadwood in parkland trees, and perversely could be used as an indicator of the intensity of management, which in turn reflects concerns about the trees’ poor condition and risk status.

 

 

3.4 Survey of Tree Failure

In a survey of 80 mature trees that exhibited major trunk failure or were wind thrown, it was found that many were mature and senescent. The work extended earlier work (Moore 1998, 2004). The study was intended to determine whether there were any indicators of root system failure that might be useful in identifying the likelihood of future tree failure. A series of criteria were used to assess trees after failure (Table 8), and the data collected were based on rapid qualitative assessments.

There was evidence (Table 9) in almost all cases of major interference with the root systems. This interference may have been from trenching and other construction works, or from mowing practices, which repeatedly damaged the roots that had come to the soil surface through secondary growth. Such wounds provide access for pest and disease entry. While root damage was a common factor associated with the failure of these specimens, there was also a strong correlation of failure with changed soil/water conditions.

These changes to soil/water conditions are of two major types: the first was the existence of waterlogged soils which have a significant impact on descending roots, which often die back under such conditions. Under these circumstances, the root plate often fails and the whole tree tilts in the ground as it falls. The second condition is where subterranean water flows are altered and the specimen faces a sudden imposition of a water deficit. Here trees show the effects of wilting and are often significantly stressed. Some of these trees would appear to be ideal candidates for sudden limb.

It can be seen that in many situations there are multiple factors that contribute to root system failure that leads to the tree falling. The study is limited in that it only focused on trees that fell and so almost by definition had poor root systems. There is no capacity for comparison with trees that did not fail.

3.5 The Value of Tree Components – Roots and Trunks
In the past decade, removals of whole trees because of canopy defects have been very common, especially as concerns about risk and insurance liability increased. Often, the removal of trees with co-dominant stems, columns of decay or dangerous branches is warranted. However, the decision with established trees should not be rushed for all species as there has been considerable investment in the development of their roots systems and trunks, and these may be assets in their own right.

For species with lignotuberous or epicormic buds, there is a capacity by proper management to re-establish a sound canopy that can restore a valuable amenity tree in a fraction of the time taken to grow the plant from a seedling. Most species of eucalypts possess both types of buds, while many exotic species are known to have a “basal burl” - the equivalent of the lignotuber - as well as epicormic or other dormant buds. The epicormic and other dormant buds can allow the rapid reconstitution of a canopy and branching structure after fire or defoliation due to stress, drought or grazing. They can also allow the establishment of a sound canopy after pruning as they take full advantage of an established root and trunk system. Initially shoots that develop from these buds are weakly attached, but with proper management and arboricultural skill the tree can rapidly return to high amenity value.

Similarly, lignotuberous shoots can provide a very sound attachment if the canopy and trunk have been removed. Under ideal growing conditions, such shoots can grow up to 27cm in a week and in excess of 6m in a year (Moore 1982), while growth of 4m in a year is more usual. Even young seedlings show a capacity for rapid canopy restoration (Figure 1). These prodigious rates of growth are possible because the shoots exploit the full nutrient and water absorbing capacities, as well as the carbohydrate reserve, of a mature root system. It can be seen that at these rates a significant tree can be developed in 3-5 years compared to 15-20 for the same species grown from a seedling.

For many species of eucalypts the value of a mature root system should not be underestimated as the root system is often the key to successful tree establishment. To often trees are removed because of structural weaknesses in the canopy and the stumps ground. But the trunk and root system have value as real assets if properly managed. Recently Summers (2005) reported the management of 2000 E. cladocalyx that were once a farm shelter belt, but are now part of an urban development. The trees had been pollarded and were shedding large limbs on newly constructed homes during storms.

In managing the situation Summers (2005) noted that the trees had been lopped at a height 1.0m from the ground to limit shedding and that the coppice growth from epicormic buds would be monitored and pruned every 10 years. Already the trees have re-sprouted and formed bushy shrubs 2-3 m high – some amenity value has been returned in months rather than years. The risk is that if these are not properly managed then there will be future shedding. However, there has been recognition of the value of the root systems. If the trees had been cut at ground level re-growth would be from lignotuberous buds and if appropriately thinned after 2-3 years, a sound tree could be developed. This would seem an appropriate strategy to derive full value from an established root system, but again requires appropriate arboricultural and management skills if future risks are to be mimimised.

 

4. CONCLUSION

The studies of aged trees reported in this paper indicate that management practices have a profound influence on the health and vigour of aged trees, especially when they are growing under significant environmental stress. Many trees that are growing in ideal locations, where they are not subjected to invasive management practices, remain healthy and vigorous as they age and are capable of dealing with many of the pests and diseases that might otherwise affect them.

Trees in urban areas that have had their canopies and root systems interfered with are more likely to be stressed and undergo a premature senescence. They are also more likely to suffer structural defects, sudden limb failure or catastrophic trunk failure. In many instances, older urban trees have been subjected to the lopping practices that were common for many decades in the last century, and which lead to major canopy structural defects, and the tendency to shed large epicormic shoots. Flush cutting was practiced widely for most of last century and still occurs in parts of Australia. It not only results in large columns of decay, and structural weaknesses, but can also facilitate insect and disease attack as it undermines the trees’ capacity to compartmentalize and grow over

Major root damage from construction, road and infrastructure work is common. Works are often implemented without thought of the effects of open trenching on tree root systems, or considering the option of tunneling under trees that modern boring technologies provide. Boring options are not only less likely to damage trees root systems, but are often cheaper than trenching. However, the misconception that trees have taproots or a large root mass under their trunks persists, and so alternatives to trenching are not even considered. These practices add significantly to the stress levels that older trees endure and the consequences can take many years to emerge. The trees can be left prone to infection and decay, and the prospect of complete failure.

Modern arboricultural management requires that the biological principles that are the foundations of good practice be adhered to. While the knowledge and technology exists to prolong the useful lives of older and stressed trees, good management involves prevention rather than cure. Trees must be given appropriate environments in which to grow – environments that provide for the development of canopy and root systems with a minimum of interference. Formative pruning of young specimens should then allow trees to grow and develop largely unmanaged and at low cost. Any pruning practice must be undertaken within the principles of good pruning and in accordance with proper plant hygiene.

The studies undertaken reveal that trees usually do best when left to their own devices. This is an indictment of tree management practices. Trees such as the southern conifers, which have been left alone and free from interference are doing well compared with those trees that have been subjected to intense management because of their locations, particularly in streetscapes. It is disappointing that so many of the basic rules of proper tree design, planning and management that we take for granted, such as the right tree in the right place at the right time, providing an appropriate environment for the tree to reach maturity and ensuring that management practices accord with the proper principles of arboriculture and good pruning practice are given low priority. This low priority results in trees that are prematurely aged, structurally unsound, considerably stressed and prone to pest and disease attack.

5. REFERENCES

Andrews L, Harris B and Skipper R (2000) The Horticulture and Management of Melbourne’s Southern Conifers. Horticultural Project, Burnley College, University of Melbourne Victoria

Andrews L, Harris B, Moore G M and Skipper R (2004) The Horticulture and Management of Melbourne’s Araucaria and Agathis Species, International Dendrological Society, New Zealand (in Press)

Anon, 1992, Draft Australian/New Zealand Standard: Amenity Trees – Guide to Valuation, Standards Australia, Sydney.

Anon, 1999, Draft Australian/New Zealand Standard: Amenity Trees – Guide to Valuation, Standards Australia, Sydney.

Curnow A J (2003) Characterising the Arboriculture of Phoenix canariensis of the Mount Alexander Road Planting. Horticultural Project, Burnley College, University of Melbourne Victoria

Graham A and Ferrier H (1997) An Investigation into the condition of Elms in Melbourne. Horticultural Project, Burnley College, The University of Melbourne Victoria

Moore G M (1982) The Effects of High Temperatures on the Growth and Physiology of E.obliqua Seedlings" Ph D Thesis, University of Melbourne.

Moore G M (1992) Amenity Tree Evaluation: A Revised Method, in The Scientific Management of Plants in the Urban Environment, p166-171. Proceedings of the Burnley Centenary Conference, Centre for Urban Horticulture, Melbourne.

Moore G M (1998a) Melbourne’s Elms: Their Condition and Management, in Proceedings of Elm Pests and Diseases Symposium, Melbourne

Moore G M (1998b) Managing Ageing Urban Trees, in Smith K D and Moore G M (Eds) Managing tree Hazard Conference Proceedings, 4-11, International Society of Arboriculture Australian Chapter, Australia

Moore G M (2000) Amenity Trees: Setting the Standards of Value, in Proceedings of the 3rd National Conference, 1-9, International Society of Arboriculture Australia, Brisbane.

Summers J (2005) Retention of Non indigenous trees in development: A Case study and lessons to be learn't, in Managing The Urban Forest Seminar, Basalt Plains Urban Forest Group, City of Hume.

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