Ancient Tree Forum: Areas of Research Identified As Potentially Useful 1

Ancient Tree Forum: areas of research identified as potentially useful[1]

Note: pages 3 to 6 of this document contain reviews of the main areas A to L below. For quick access on-screen, please hover the cursor over each shaded letter A to L. This should reveal a hypertext field, which will take you to the relevant review by the use of Ctrl+Click

A.Tree population surveys

1. Further field testing of the Specialist Survey Method (SSM) to establish how well it is working.

1. Evaluation of which levels of the SSM are best for use in different circumstances.

1. Research on methods to allow analysis of data about populations of veteran trees (as distinct from data gathering like the SSM).

1. More information on veteran tree survival rates.

1. More information on tree survival in relation to management techniques.

1. Further analysis of historical records of tree management (e.g. pollarding).

1. Research on tree physiology in relation to maintaining a functional envelope of sapwood and bark.

B.Soil micro-organisms

1. Research on the impacts of pesticides and veterinary drugs on mycorrhizal fungi and tree root function.

C.Mulching

1. Research (or at least a review of practical experience) on the impact of mulching on veteran trees, especially comparing mosaic mulching and conventional mulching.

D.Management of trees with decay caused by particular fungi

1. Further research on the impacts of specific fungal species such as Perenniporia fraxinea on the development of decay in trees especially in relation to impact on roots and interaction with tree pruning (similar studies to those carried out on Ganoderma australe (= G. adspersum), G. applanatum, Meripilus giganteus, Inonotus hispidus and Ustulina (Kretzschmaria) deusta).

1. Research on organisms that can have either beneficial or deleterious effects on the health or mechanical integrity of trees such as mycorrhizal fungi, nematodes and hyperparasitic fungi.

E.Tree work for management of veterans

1. Evaluation of management practices and their long-tem impact on various tree species through replicated experiments. This could include examination of before and after records where work has been carried out on tree populations such as Burnham Beeches, Epping Forest, Hatfield Forest, Hainault Forest and Windsor Great Park.

F.Tolerance oftrees to pruning

1. Evaluation of the responses of trees to pruning in relation to their reserves of stored energy, predisposition to produce epicormic shoots and genetic composition.

G.Pruning in relation to light and shade

1. Further research on tree responses to pruning looking at relative exposure to sunlight/shading and tree species differences such as shade tolerance and bark thickness.

H.Retrenchment pruning

1. Assessment of the efficacy of retrenchment pruning through condition monitoring of cut trees.

I.Accelerated crown reduction

1. Comparison of the responses of trees to relatively severe crown reduction in relation to tree species, crown structure and branch foliar cover. Is a good response due to location of pre-existing carbohydrate reserves?

J.Pole thinning of lapsed pollards

1. Evaluation of the responses of trees following pole thinning.

K.A sustainable density of trees

1. Further research on the average optimum number of trees of various ages per hectare to maintain a sustainable population.

1. Further research on the availability of habitat for specific saproxylic species and the numbers of trees needed to provide sufficient for a sustainable population.

1. Further information on dispersal range for specific saproxylic species.

L.Invertebrate-fungus associations

1. Research on interactions between specific invertebrate and fungal species.

1. Are any invertebrates dependent on fungi only found in ancient trees?

Last revised 25-Feb-2016

Reviews of the above research areas

A.Tree population surveys

The general methods currently available for the survey of tree populations are probably sufficient in order to improve knowledge about the size and condition of the resource of veteran trees. The application of the Specialist Survey Method (SSM) has been reviewed (Fay & de Berker, 2003) but further field testing is needed in order to determine how well it is working in practice. Continued field testing could help to indicate whether resources would be best devoted to operating the SSM at Level 1, 2 and/or 3. There is also a need for further research in order to develop methods for evaluating data, as distinct from the gathering and recording of data, as in the SSM.

Despite the need for more empirical assessment of survival potential and of management techniques, there are other sources of information that have contributed to the guidance presented in this book. These include historical records of traditional practices, such as pollarding. Also, there is some understanding of the processes by which trees maintain a functional envelope of sapwood and bark. Such knowledge has been very useful but there is a need to develop it further through research.

B.Soil micro-organisms

Pesticides and veterinary drugs are toxic to a range of micro-organisms, which in some cases include those that are important for the maintenance of good root function in trees (e.g. mycorrhizal fungi, which might be affected not only by fungicides but also by drugs that have activity against certain bacteria that are involved in the establishment of the mycorrhizal association). The extent to which such organisms are harmed in practice is uncertain, and so research is required. According to the precautionary principle, however, it is desirable to eliminate or at least reduce the use of these chemicals in the vicinity of veteran trees. Also, there is no doubt that certain drugs, especially the avermectins, are very toxic to dung-inhabiting invertebrates, some of which are endangered species (see Chapter 5 of AoVM).

C.Mulching

Thesuggestionthat mulching helps veteran trees is not currently based on experimental evidence specifically involving such trees. Research, or at least practical experience, is therefore needed, especially regarding the relative benefits of mosaic mulching and conventional mulching (see 3.7.1 of AoVM). Mulching is, however, good for trees in general and is, in principle, likely to provide better conditions than occur under grass

D.Management of trees with decay caused by particular fungi

Since the early 1990s, much has been learned about the colonisation strategies of various decay fungi and the associated effects on the mechanical integrity of trees. This information is helpful in prognosis of the development of decay in trees and thus in long-term management. For example, trees affected by certain root decay fungi can eventually die owing to loss of conductive roots. Physiological dysfunction, induced by pruning such trees, could accelerate fungal development. Fungi which have been studied in some detail include Ganoderma australe (= G. adspersum), G. applanatum, Meripilus giganteus, Inonotus hispidus and Ustulina (Kretzschmaria) deusta. There are, however, other fungi (e.g. Perenniporia fraxinea) that occur in veteran trees and that would be useful candidates for similar research.

There is also a need to learn more about other organisms that can have either beneficial or deleterious effects on the health or mechanical integrity of trees. These include mycorrhizal fungi, nematodes and hyperparasitic fungi.

E.Tree work for management of veterans

Continuing research is needed in order to evaluate management practices with regard to their long-term effects on trees of various species and of various degrees of mechanical integrity and vitality. Such research should ideally involve properly replicated experiments but these are often limited by practical or financial restrictions. There is, however, an increasing bank of well-documented information, based on “before and after” records at places in the UK, where numerous veteran trees are present, including Burnham Beeches, Epping Forest, Hatfield Forest, Hainault Forest and Windsor Great Park. Also, various methods of pruning beech pollards have been compared in replicated trials in the Basque Country of northern Spain (Read et al., 2013).

F.Tolerance of trees to pruning

If pruning is required in order to reduce mechanical stress, there can be a difficult balance between removing enough material for this purpose and removing no more than the tree can reasonably tolerate. Even in the initially less severe practice of retrenchment pruning, the tolerance of the tree depends on its subsequent capacity to produce and maintain new foliage, which can then sustain physiological function in the sapwood and inner bark.

Maintenance of a good, well-foliated canopy is favoured if the retained part of the crown already has a good structure of twigs and small branches before pruning. If not, adequate leaf cover can be re-established after cutting only if the tree can produce new shoots on old branches or stems. Its capacity to do so will depend on its reserves of stored energy and its predisposition (partly genetic) to produce epicormic shoots.

G.Pruning in relation to light and shade

An important factor in the ability of the tree to respond to pruning is the subsequent exposure of its foliage to sunlight. It is difficult to get a good balance between light and shade when pruning veteran trees. New shoots often fail to form or die back after forming if a tree of a light-demanding species (see Table 4.2 of AoVM) becomes heavily shaded when reduced below the height of surrounding trees. The latter could be pruned so as to alleviate shading. On the other hand, in a more open-grown situation, pruning can expose the retained stem and branches of the same tree to sudden, excessive sunlight, which tends to induce severe dieback in almost any tree species. Thin-barked species are, however, especially liable to such damage.

H.Retrenchment pruning

Retrenchment pruning seems to make good sense in principle because it is designed to mimic natural retrenchment. Similar work has been done successfully in various undocumented cases over many years. Retrenchment pruning of veteran trees is, however, a relatively new technique and so there is a need to assess its efficacy and perhaps revise the guidance, if necessary, by monitoring the condition of trees that have been managed in this way.

I.Accelerated crown reduction

Improved guidance is needed regarding trees that appear more amenable to relatively severe crown reduction in the short term than to retrenchment pruning. The former appears to be most successful in trees that already have a good lower crown structure. Apart from the obvious value of reducing mechanical stress sooner rather than later, there could in theory be a physiological advantage in removing or shortening sparsely foliated branches. The thinking behind this idea is that there is some benefit in removing parts of the woody structure that bear too little foliage to “pay their way” in the form of carbohydrates that are essential for maintaining the growth and vitality of the sapwood. The abundance of growth that often follows cutting seems to support this idea to some extent, but could simply be the result of pre-existing reserves being less sparsely distributed and by water being transported along a shortened distance from the roots. Further research would therefore be valuable.

J.Pole thinning of lapsed pollards

Further study is required to find whether pole thinning might be helpful in the management of lapsed pollards that have not yet been cut again. In a small proportion of such trees, the natural failure of individual branches appears to have stimulated the development of epicormic shoots, which have formed a secondary lower crown, thus making the tree more amenable to subsequent reduction of the primary crown. It seems likely that pole thinning could emulate this beneficial process, but further study is required before this can be recommended. In any case, there would often be a need for an accompanying first stage of retrenchment pruning, in order to lessen the probability of failure of the retained pollard branches.

K.A sustainable density of trees

It is currently not feasible to give a firm recommendation for the average optimum numbers of trees of various ages per hectare. Suggestions can, however, be made. For example, on the basis that the crown footprint of a single Pedunculate oak can occupy about 700 square metres, an average of one ancient oak per hectare has been suggested as a sustainable density in southern Britain, provided that there are enough younger trees to succeed them without shading them or each other. If, on this basis, the younger trees are too many or too few, a programme of planting or felling should be planned.

In Sweden, an average density of 2.8 ancient oaks per hectare has been proposed as desirable (Bergman, 2006), taking account of the need to sustain a population of ancient trees, while also allowing space for regeneration and for the retention of open areas.

Survey data in Sweden indicate that there is one tree of 1.25 m or more in stem diameter for every 10 trees up to 0.7 m in diameter (V. Bengtsson, pers. comm.). The same survey also indicated, however, that certain types of habitat might be provided only by a relatively small proportion of ancient trees within the dispersal range of the species concerned. Also, there is uncertainty about factors affecting the long-term survival of successor trees. Continued research might help to provide better guidance about maintaining a suitable density of trees in various age groups.

L.Invertebrate-fungus associations

There is still much to be learnt about the associations between particular invertebrate species and the species of fungi in which they develop, and it is therefore not known whether any invertebrates depend specifically on fungi that are found only in ancient trees.

Literature references

Bergman, K.O. (2006). Living coastal woodlands – Conservation of biodiversity in Swedish archipelagos. Report to EU LIFE – Coastal Woodlands project, 46 pp. Available online only via:

Fay, N. & de Berker, N. (2003). Evaluation of the Specialist Survey Method for Veteran Tree Recording. Research ReportNo. 529, English Nature, Peterborough, UK.

Read, H.J. Dagley, J., Elosegui, J-M., Sicilia, A. & Wheater, C.P. (2013). Restoration of lapsed beech pollards: Evaluation of techniques and guidance for future work.Arboricultural Journal 35, 74-90.

Last revised 25-Feb-2016

[1] extracts from “the need for further knowledge” In: Lonsdale, D.(ed). Ancient and other Veteran Trees: Further Guidance on Management (2013), abbreviated in this document as AoVM.