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This article was downloaded by: [University of Tennessee At Martin] On: 04 October 2014, At: 07:30 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Arboricultural Journal: The International Journal of Urban Forestry Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tarb20 WHEN THE ROOTS GO ROUND AND ROUND Gary W Watson a & Sandra Clark b a Morton Arboretum , Lisle , Illinois , 60532-1293 , USA b Mt. Prospect , Illinois , USA Published online: 27 Mar 2012. To cite this article: Gary W Watson & Sandra Clark (1997) WHEN THE ROOTS GO ROUND AND ROUND, Arboricultural Journal: The International Journal of Urban Forestry, 21:4, 347-356, DOI: 10.1080/03071375.1997.9747180 To link to this article: http://dx.doi.org/10.1080/03071375.1997.9747180 PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses,

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This article was downloaded by: [University of Tennessee At Martin]On: 04 October 2014, At: 07:30Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number:1072954 Registered office: Mortimer House, 37-41 Mortimer Street,London W1T 3JH, UK

Arboricultural Journal:The International Journalof Urban ForestryPublication details, including instructions forauthors and subscription information:http://www.tandfonline.com/loi/tarb20

WHEN THE ROOTS GOROUND AND ROUNDGary W Watson a & Sandra Clark ba Morton Arboretum , Lisle , Illinois ,60532-1293 , USAb Mt. Prospect , Illinois , USAPublished online: 27 Mar 2012.

To cite this article: Gary W Watson & Sandra Clark (1997) WHEN THE ROOTSGO ROUND AND ROUND, Arboricultural Journal: The International Journal ofUrban Forestry, 21:4, 347-356, DOI: 10.1080/03071375.1997.9747180

To link to this article: http://dx.doi.org/10.1080/03071375.1997.9747180

PLEASE SCROLL DOWN FOR ARTICLE

Taylor & Francis makes every effort to ensure the accuracy ofall the information (the “Content”) contained in the publicationson our platform. However, Taylor & Francis, our agents, and ourlicensors make no representations or warranties whatsoever as to theaccuracy, completeness, or suitability for any purpose of the Content.Any opinions and views expressed in this publication are the opinionsand views of the authors, and are not the views of or endorsed byTaylor & Francis. The accuracy of the Content should not be reliedupon and should be independently verified with primary sources ofinformation. Taylor and Francis shall not be liable for any losses,

actions, claims, proceedings, demands, costs, expenses, damages,and other liabilities whatsoever or howsoever caused arising directlyor indirectly in connection with, in relation to or arising out of the useof the Content.

This article may be used for research, teaching, and private studypurposes. Any substantial or systematic reproduction, redistribution,reselling, loan, sub-licensing, systematic supply, or distribution in anyform to anyone is expressly forbidden. Terms & Conditions of accessand use can be found at http://www.tandfonline.com/page/terms-and-conditions

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Arboricultural Journal1997, Vol. 21, pp. 347-356 © AB Academic Publishers 1997 Printed in Great Britain

WHEN THE ROOTS GO ROUND AND ROUND:!:

Gary W Watson* and Sandra Clark+

Summary

Investigations into girdling root formation in young landscape trees revealed that Norway and sugar maples had an average of four girdling roots per tree and that red maple had nearly twice as many. While these roots persisted to become a serious problem in older Norway maples, they did not in red and sugar maples. Removing the roots was ineffective since new girdling roots were regenerated from the same location. Of the 60 mature Norway maple examined girdling and potentially girdling roots were completely absent on only two trees. Genetic diversity may allow a rootstock to be selected and propagated so as to reduce or eliminate the character of girdling roots.

Several decades ago, American elms dominated the urban landscape of North America. When large numbers of them succumbed to Dutch elm disease, they were replaced by new plantings that were less homogeneous, but still limited to a handful of species. Maples especially Norway maples (Acer platanoides L.), were commonly included in these replacements in the Midwest and elsewhere as they were considered well suited to urban landscapes because of their attractive form and foliage. Moreover, they were easy to propagate and transplant as well as tolerant of a wide variety of soil conditions.

A few decades later, as some of these Norway maples approached maturity, many started to decline inexplicably, raising new fears of devastating tree losses. Typical canopy symptoms included reduced growth, abnormal colour, early autumn coloration and die-back. There was no evidence of diseases, pest infestation or any other above ground problem. However, investigations below the ground began to reveal that a high percentage of these trees had girdling roots (TATE, 1981). A girdling root has been defined as 'a root that grows around another root or stem, thus tending to strangle the plant' (ANON, 1971). Where the girdling root contacts the

tReproduced fromAmoldia Volume 56, No.1, page 15, 1996 by permission of the Editor, The Arnold Arboretum, Jamaica Plain, Massachusetts 02130-3519. USA. *Root System Biologist, Morton Arboretum, Lisle. Illinois 60532-1293, USA. +Superintendent of Forests, Mt. Prospect, Illinois, USA.

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FIGURE 1. The canopy of Norway maple (Acer platanoides L.) at Mt. Prospect, Illinois shows symptoms of girdling roots; reduced growth, abnormal colour and dieback.

FIGURE 2. Excavation of the root crown reveals a typical girdling root formation in an older Norway maple.

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trunk, radial growth of both trunk and root is distorted and reduced. Normal movement of water and nutrients from the roots to the canopy is greatly decreased, leading to stress and then to decline.

How and why these girdling roots form on field grown trees is unknown. When plants are grown in containers, roots often circle round the interior of the pot. If not cut or removed, when the plant is moved for transplanting in the landscape, these circling roots can also strangle the tree as it grows larger. In the case of field grown stock that has never been in a container, however, some other mechanism may be involved.

Girdling roots on Norway maple are recognised all over the country as a minor problem and the Chicago area is no exception. As an example, in the suburb of Mt. Prospect, where they represent 17 per cent of all street trees, some of the Norway maples were reaching mature size by 1987 and some were declining because of girdling roots. At that time a study was initiated to identify the cause of the girdling roots and how they might be corrected or prevented. Because girdling roots had been reported on sugar maple (Acer saccharum Marsh.) and red maple (A. rubrum L.) as well as Norway maple, all three species were included in the study. Three other species, green ash (Fraxinus pennsylvanica Marsh.), honey locust (Gleditsia triacanthos L.), and little leaf linden (Tilia cordata Mill.) were also studied but were found to have less than half as many girdling roots as any of the maples. Girdling roots were especially infrequent in the lindens (WATSON et al., 1990 and WATSON, 1993).

The first phase of the work, involving excavation of the roots of large Norway maples (over 12ins diameter), showed the girdling roots can wrap around the entire trunk circumference before crown symptoms develop. By the time the canopy decline becomes evident, the girdling is often in an advanced stage, with the roots so intertwined that little can be done to correct the problem. In these cases, the advisability of removing girdling roots is uncertain, even where the individual roots are distinct enough to make removal practicable: if a large root constricting the trunk is removed, a substantial portion of the root system may be lost. Root loss can also cause stress, decline and even the death of the tree.

After these disappointing excavations, the Mt. Prospect study focused on younger parkway trees that had been in place for from three to ten years. Trees of this age are well enough established so that stress from root crown excavation is minimal, but young enough so that severe girdling had not yet developed. In this second phase of the study, 120 root crowns were investigated over a two year period. The number of girdling roots were recorded and these girdling roots were removed when this could be done without substantially reducing the total root system. The approximate age of each root removed was determined by smoothing a cross section and counting the number of annual rings.

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FIGURE 3. A schematic drawing showing the probable mechanism involved in the girdling root formation: (a) The major roots of a tree normally radiate out from the trunk. These roots and some of the lateral roots are severed during transplanting. (b) After transplanting, new roots initiated from the cut ends follow the same direction as the original. Growth in existing and new lateral roots is stimulated and these can become girdling roots as indicated by the arrows.

Tree roots can be classified into (i) primary roots which radiate out from the base of the tree like spokes of a wheel; (ii) secondary roots which are lateral branches of primary roots that grow almost perpendicular to them; and (iii) tertiary roots which are lateral branches of secondary roots. (Fig. 3) Almost all the girdling roots found were secondary or tertiary roots. This is not surprising since the nearly perpendicular branches of the spoke like primary roots are more likely to wrap around the base of the trunk or cross other roots. A strong relationship between transplanting and girdling root formation became apparent in all three maple species with the majority of the girdling roots being initiated within one year of transplanting. This fact, together with the finding that most girdling roots are secondary or tertiary roots, enable a hypothesis of the way girdling roots may be formed on the field grown transplanted trees to be constructed. Nor­mally secondary roots grow slowly and remain quite small as long as the primary root is intact. When the tree is removed from the nursery and the large radiating primary roots are severed, the secondary roots often begin to grow more rapidly. In addition new secondary roots may form some distance behind the cut ends of the primary root. All these

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vigorously growing secondary roots are located close to the base of the trunk and are well positioned to become girdling roots as both the roots and the trunk grow larger. If severing the primary roots during transplanting is indeed the stimulus for girdling root formation, it is easy to understand why girdling roots do not occur in forests.

The Mt. Prospect study found that young Norway maples and sugar maples had an average of four girdling roots per tree. Red maple has nearly twice as many. Why is it then, that only the Norway maples show canopy decline when they mature? Excavation of root crowns of sugar and red maples 21-28 years after transplanting revealed very few girdling roots and those that were present were relatively small and all were less than 12 years old. By contrast, girdling roots of Norway maples of similar age were much more numerous and ranged in age up to 24 years. For unknown reasons, it seems that the girdling roots that develop on red and sugar maples as a result of transplanting are short lived, unlike the case of the Norway maples.

How can girdling roots be prevented or corrected? Numerous girdling roots were removed during the first phase of the Mt. Pleasant study in hopes of preventing canopy decline in the future; the process of excavation and removal is time consuming but would be worth the effort if it were effective. To determine whether this was the case, one quarter of the same Norway maples were excavated again in 1992 and the roots compared to the photographs taken at the time of the 1987 excavation. It soon became clear that one or more roots had consistently regenerated from each root removal site. The new roots, just like the old ones, were usually nearly perpendicular to the radially orientated primary roots and well positioned to become girdling roots (Fig 4 a,b and c)

This finding was discouraging. It was, however, encouraging to learn that the root systems of Norway maple have diverse growth habits. Those with many large girdling roots at the time of the first excavation showed many large regenerating girdling roots four years later. Likewise, root systems that initially showed fewer and smaller girdling roots showed fewer and smaller regenerated roots. Of the 60 older Norway maples examined in the first phase of the study, girdling and potentially girdling roots were completely absent on only two trees. Theoretically, this genetic diversity may allow root stock to be selected and propagated so as to reduce or eliminate girdling roots altogether.

Until better rootstocks are developed, overplanting of Norway maple (or any other single species) should be avoided, otherwise substantial losses from girdling roots as the trees reach maturity can be anticipated. What these losses can amount to can be estimated only imprecisely at between ten and forty per cent (TATE, 1981 )*. It is not known how long a Norway maple

*Percentages based on data and experience from the author's own research.

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FIGURE 4. The girdling roots of Norway maple (a) before and (b) after corrective treatment and again (c) four years later. The arrows indicate the same roots.

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FIGURE 5. Examples of the diverse character of Norway maple root development. Selection of rootstock like that in the lower photograph on the right may be the best way to eliminate girdling root problems in the future.

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would survive if girdling roots did not develop. The average life span of all urban trees is only 37 years, so it is possible that on average girdling roots shorten the life by only a few years (SKIERA and MOLL, 1992).

It would be easy to advise not to plant Norway maple because of the girdling root problem. On the same basis it could be argued that redbuds (Cercis canadensis) should not be planted for fear of verticillium wilt, or green ash (Fraxinus pennsylvanica) because they are attacked by borers and red oak (Quercus rubra) because it is susceptible to oak wilt. There is an appropriate place for every tree. For reasons of historical significance American elms are still planted on the National Mall in Washington DC, even though they will eventually die from Dutch elm disease. They survive the harsh site conditions as well or better than any other species. A Norway maple may not be ideal for a location where survival for many decades is important, but if a life span of 30 years is acceptable there is no reason why it should not be planted. Mt. Prospect village continues to plant Norway maples, but tries to use them in mixture with other species so that no single problem can wipe out all the trees in the area.

Frequency of girdling roots in relation to planting depth for all maple species

It is often contended that trees planted too deeply have more girdling roots, but the matter has never been formally studied. The Mt. Prospect study found no relationship between planting depth and girdling roots. However the data do show how often tree trees are planted too deeply (Table 1 and

TABLE 1. Frequency of girdling roots in relation to planting depth for all maple species

Root flare No. of trees Average number of relative to grade girdling roots per tree Inches (em)*

+1(2.5) 4 at grade 21 5 -1(2.5) 2 3 -2(5.0) 13 5 -3(7.5) 2 9 -4(10.0) 7 5 -5(12.5) 4 -6(15.0) 4 5 -7(17.5) 1 -8(20.0) 2 2

*The first root of trees planted 'at grade' was within one inch of the soil surface

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2.0

1.8 -s- Norway maple

Q) 1.6 ~

-e- Red maple -- Sugar maple

1-- 1.4 ..... Q) a. 1.2 t1J -0 1.0 0

0:: 0.8 0) c:

"E 0.6

(5 0.4

0.2

0.0 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7

Age Relative to Transplanting Date (years)

FIGURE 6. The age of the girdling roots of Norway, red and sugar maples in relation to the number of years since transplanting.

Fig. 6). Over half the trees studied had been planted two to eight inches too deep, even though all had been planted by reputable commercial landscape contractors. Planting too deep is one of the major causes of death of trees of all species. No soil should cover the top of the root ball when the planting operations have been completed.

References

ANoN (1971). A Technical Glossary of Horticultural and Landscape Terminology. Horticultural Research Institute, Washington DC. SKIERA, R. and MoLL, G., (1992). Trees in the Red. Urban Forests, 12, 1, 9. WATSON, G.W., CLARK, S. and JoHNSON, K. (1990). Formation of girdling roots. Journal of Arboriculture, 16, 197-202. WATSON, G.W. & CLARK, S. (1993). Regeneration of girdling roots after removal. Journal of Arboriculture, 19, 278-280.

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Resume

Des recherches portant sur la formation de racines d'etranglement parmi les arbres du pay sage ont revele que I' erable plane presentait en moyenne quatre racines d'etranglement par arbre et que l'erable rouge, 21-28 annees apres transplantation, montrait un nombre reduit de petites racines d'etranglement !igees de mains de 12 ans. Chez l'erab1e plane du mt!me !ige, les racines d'etranglement etaient beaucoup plus nombreuses et d' !ige varie, certaines all ant jusqu' a 24 ans. Parmi les 60 erables planes adultes examines, seulement deux specimens etaient completement exempts de racines d'etranglement existantes ou potentielles. II se peut que la diversite genetique permette de selectionner et propager un porte-greffe pour reduire ou eliminer le caractere responsable de la formation des racines d'etranglement.

Sumario

Las investigaciones sabre la formaci6n de raiz en cintur6n para arboles paisajfsticos han revelado que el arce de Noruega tenfa aproximadamente cuatro raices de cintur6n por arbol y que el arce rojo de 21 a 28 afios despues del transplante mostraba unas pocas menos raices de cintur6n que uno de 12 afios. Las raices de cintur6n en arces de Noruega de igual edad eran mucho mas numerosas y basta la edad de 24 afios. De los 60 arces de Noruega desarrollados que se examinaron, el cintur6n y las raices potenciales de cintur6n faltaban s6lo en dos arboles. La diversidad genetica puede permitir que un rizoma sea seleccionado y propagado de modo que sirva para reducir 6 eliminar el caracter de raices de cintur6n.

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