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Arboricultural JournalThe International Journal of Urban Forestry

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DEVELOPMENT OF PRUNUS ROOT SYSTEMS INA CITY STREET: PAVEMENT DAMAGE AND ROOTARCHITECTURE

Bruce C. Nicoll & Alan Armstrong

To cite this article: Bruce C. Nicoll & Alan Armstrong (1998) DEVELOPMENT OF PRUNUSROOT SYSTEMS IN A CITY STREET: PAVEMENT DAMAGE AND ROOT ARCHITECTURE,Arboricultural Journal, 22:3, 259-270, DOI: 10.1080/03071375.1998.9747209

To link to this article: https://doi.org/10.1080/03071375.1998.9747209

Published online: 27 Mar 2012.

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Arboricultural Journal 1998, Vol. 22, pp. 259-270 © AB Academic Publishers 1998 Printed in Great Britain

DEVELOPMENT OF PRUNUS ROOT SYSTEMS IN A CITY STREET: PAVEMENT DAMAGE AND ROOT ARCHITECTUREt

Bruce C. Nicoll* and Alan Armstrong**

Summary

We surveyed the pavement (sidewalk) damage around five 30-year-old cherry trees in a street in Sheffield and examined the architecture oftheir root systems. The tarmac and soil were removed, and the positions of the woody roots mapped. Most roots had been constrained by the road on one side and a wall on the other. Pavement damage had been caused not just by roots directly below the surface, but also by fast growing roots as deep as 0.4 m below the pavement. Almost all damage was caused by roots over I 00 mm diameter. Large surface roots that had been chiselled down during previous pavement repairs had callused on both sides of the damaged area, increasing the area of contact and the risk of damage to the new pavement.

Keywords: pavement damage o roots o Sheffield o trees and the built environment.

Introduction

Damage to roadside pavements occurs as tree roots growing close below the surface undergo secondary thickening. In a survey of street trees in Manchester, UK, WONG et al. (1988) reported that 30% had caused damage to pavements and 13% had damaged kerbs. The resulting repairs are expensive (McPHERSON and PEPER, 1996), and also result in damage to the trees themselves (NICOLL and COUTTS, 1997). When the pavement is replaced, damaging roots are often removed or chiselled down, risking introduction of disease and threatening the stability of the tree (DOBSON, 1995). Sometimes street trees that

tThis paper was presented at the 31st National Arboricultural Conference, Exeter, 8-10 September 1997. *Northern Research Station, Forestry Commission Research Agency, Roslin, Scotland, UK, EH25 9SY **Alice Holt Research Station, Forestry Commission Research Agency, Wrecclesham, Farnham, Surrey, UK, GUIO 4LH

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260 ARBORICULTURAL JOURNAL

FIGURE 1. Example of pavement damage caused by Prunus trees.

cause damage are removed completely and concern about recurring problems makes road engineers reluctant to allow replanting (PATCH, 1994).

COUTTS and NICOLL (1991) demonstrated that the roots that develop into surface roots often have an innate upward growth pattern that takes them to close below the soil surface where they encounter conditions that stimulate a downward deflection. Upward directed roots commonly emerge from the upper part of conifer root systems, and increasingly downward roots develop with increasing depth (COUTTS and NICOLL, 1991). However, in root systems of broad-leaved trees the roots are more randomly directed, with upward roots also emerging from greater depths (NICOLL and COUTTS, 1997). The combination of upward growth and downward response enables roots to track close below the soil surface even when it is undulating or inclined. However if the soil is covered with a solid layer such as concrete or tarmac, roots may not experience normal surface conditions and often grow upwards until they make contact with the hard surface (NICOLL and COUTTS, 1997). Tree roots

DEVELOPMENT OF PRUNUS ROOT SYSTEMS IN A CITY STREET 261

directly below a paved or tarmac surface can experience conditions that are much more favourable for growth than the conditions encountered by deeper roots. For example, temperatures can be higher, and water condenses on the under side of the hard surface, making the soil particularly suitable for root growth (WAGAR and FRANKLIN, 1994). As a result, roots develop faster in these optimum conditions, resulting in accelerated secondary thickening, which can cause damage to the surface covering as they develop (Figure 1). Roots thicken most close to the tree, and growth rings are commonly thicker on their upper side (WILSON, 1975; NICOLL and RAY, 1996), resulting in the worst damage to structures within aIm radius of the stem (WONG et al., 1988).

Several authors (e.g. WAGAR and BARKER, 1983; WONG eta[., 1988; BIDDLE,

1992) have observed variation between tree species in root development and damage to pavements. However, there have been no studies investigating pavement damage in relation to root architecture. Only by conducting detailed surveys of pavement damage and relating them to measurements of excavated tree root systems, can we start to explain the variation that is reported between trees and sites. The aim of this study was therefore to relate the pavement damage caused by trees growing in a city street to root system development and architecture.

Methods

Selection of street trees for investigation

We selected five cherry trees were that had caused different amounts of pavement damage in a street in the Stannington district of Sheffield, UK. The trees were 30-year-old Prunus serrulata var. purpurescens 'Kanzan' grafted onto wild cherry (Prunus avium L.) rootstock, and were situated on the road side of a narrow (up to 2.0 m wide) tarmac pavement that had a property boundary wall on the other side. Trees had been planted in 0.7 x 0. 7 m planting pits that had been prepared by replacing the original soil with loam. The pits were edged with slabs that were 780 mm long with 110 mm below the surface. The pavement sloped at 6° (mean) from horizontal. There was pavement damage up to 7.0 metres from the study trees. There were also cracks in some boundary walls and suckers from cherry roots in adjacent gardens.

Investigation of Tree Root Architecture

A standard technique for measuring tree root architecture developed by the UK Forestry Commission (QUINE et al., 1991; NICOLLet al., 1995) was adapted for this investigation. This technique examines the directional distribution of

262 ARBORICULTURAL JOURNAL

FIGURE 2. Excavation of a street tree root system using a jet of air from a 'supersonic soil pick'. Soil is removed from around tree roots and services without causing damage.

roots around the tree, number and sizes of structural roots, root branching, and root angles relative to the vertical.

After the trees had been felled, we hammered a 1 m long, 5 mm diameter, steel spike into the biological centre of the stump. One end of a 3 m length of steel frame was secured to the spike 0.5 m above the stump and levelled using a spirit-level. We measured the depth from the frame to the pavement surface at 0.25 m intervals from the tree centre to the end of the frame, using a plumb­bob attached to a measuring tape. We repeated these measurements on each 15° separated radius, omitting radii in the direction of the road, and recorded the positions of the road, wall, and all pavement cracks around each tree on a map.

After lifting the pavement using hand tools, we recorded the thickness of tarmac and the gravel layer underneath it at five randomly selected positions within 2 m of each tree. We removed the soil using hand picks and a 'Supersonic Soil Pick' (MBW, Bolton, UK). This air tool, that works by directing air from a compressor through a small nozzle (Figure 2), removed soil from around tree roots and utility cables without damaging either. To fully expose the roots close to the tree, we excavated and cleaned roots back to their origin at the stump, and removed soil to the depth of the lowest part of the root system.

DEVELOPMENT OF PRUNUS ROOT SYSTEMS IN A CITY STREET 263

Root architecture measurement

Using the frame and plumb-bob system that was used for the pavement surface survey we measured the root positions. Firstly, we painted concentric circles onto the root system, incrementing by 250 mm relative to the tree centre, then we numbered each root over 10 mm diameter. We measured and mapped the position of each of these roots at each point that it crossed one of the circles. At each point, we recorded azimuth angle (compass direction from the tree centre, relative to north), depth below the soil surface, and diameter of the root (measured vertically and horizontally).

Results

Soil description

The tarmac was on average 58 mm thick over a 75 mm layer of gravel and clinker. Apart from these layers there were no distinct soil horizons. None of the soils found during the excavation was thought to be natural to the site. The soil around all trees (excluding the good quality top-soil in the planting areas) consisted of a mixture of generally very moist pockets of compacted yellow silty soil, red sand, and dark loam.

Root system development

Root systems on three trees had developed predominantly downslope, and on two trees they were predominantly upslope (Table 1). The road completely constrained the roots on one side, i.e. no roots had grown under the road. The property boundary wall on the other side of the trees also constrained the roots to a large extent, although small roots had grown through cracks in the walls and some roots had grown under the walls. Major changes in the direction of roots were common where growing root tips had been deflected by stones and bricks.

Trees had between two and ten roots over 10 mm diameter, originating from the stump, and of these, up to five were major 'structural' roots over 100 mm diameter (see Table 1). No roots had grown deeper than 0.57 m below the pavement surface (max. depth range = 0.36-0.57 m). The root system of one tree is shown as a line diagram in Figure 3. For individual trees, the mean diameters of measured roots within the excavated area were between 40 and 130 mm, with maximum root diameters between 150 and 310 mm (Table 1 ).

TABL

E I.

Cha

ract

eris

tics

of

five

30-y

ear-

old

stre

et t

rees

exc

avat

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n th

is s

tudy

. R

oot

diam

eter

s ar

e m

ean

and

max

imum

dia

met

ers

of

all

mea

sure

men

ts

from

the

exc

avat

ed p

art

of

the

root

sys

tem

. T

he '

Num

ber

of

mai

n ro

ots'

ind

icat

es t

he t

otal

num

ber

of

root

s on

eac

h sy

stem

tha

t ar

e ov

er 1

0 m

m

diam

eter

at

larg

est

poin

t, th

e nu

mbe

r o

f th

ese

that

are

ove

r 50

mm

dia

met

er,

and

the

num

ber

that

are

ove

r IO

O m

m.

Tre

e T

ree

Dbh

P

redo

min

ant

No.

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r m

ain

root

s M

axim

um

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t (d

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ight

(d

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dire

ctio

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vem

ent

root

de

pth

at 1

.3 m

) cr

acks

>

IO

>5

0

> 1

00

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ax

m

mm

m

m

mm

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m

m

mm

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m

8.2

37

0

upsl

ope

4 6

5 4

0.47

II

0

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> " "' 0 " ?5 c::: ,... .., c::: " > ,... 0 c::: " z > ,...

DEVELOPMENT OF PRUNUS ROOT SYSTEMS IN A CITY STREET

FIGURE 3. Line dia­gram of the woody roots on one example tree in plan view. Superimposed (thick grey line) is the position of pavement cracks, from the map drawn before root excavation. Contours (dotted lines) show the profile of the pavement surface, sloping downward from left to right. From left to right, each contour is 5 mm lower than the previous one. The scale line on the left hand side represents 0.5m.

FIGURE 4. A root that had damaged the pave­ment 0.4 m above it. This root had been thickening at a rate of up to 10 mm per year.

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266 ARBORICULTURAL JOURNAL

Damage to the pavement by roots

Roots larger than 100 mm in diameter caused almost all of the pavement cracks (Figure 3); only one crack was caused by a root smaller than 100 mm diameter (Table 1). Most damage was caused by roots growing in contact with the underside of the pavement, however, deeper roots also caused damage. Large, fast growing roots as deep as 0.4 m had caused pavement cracks, for example, one root (Figure 4) that was between 0.3 and 0.4 m below the surface had caused a pavement crack extending 4.9 m from Tree 1. This root was between 190 and 230 mm diameter, and measurements of its growth rings revealed that it had been thickening at a rate of up to 10 mm per year. Pavement cracks approximately followed the directions of root growth (Figure 3), although this was less precise for deeper roots.

Damage to roots during pavement relaying

The root systems of two of the trees had been severely damaged during previous pavement repairs. Both had major roots that had been chiselled down with others removed. Where surface roots had been chiselled down, there was strong thickening of 'callus' on both sides of the chiselled area that had again lifted the pavement (Figure 5). These roots had regions of decay that

FIGURE 5. Partially excavated root system. Roots immediately below the pavement that have been chiselled down during previous repairs are indicated by arrows. These roots have callused on both sides of the wound, causing the new pavement to lift over a larger area. Damaged roots were infected by soil pathogens, and decay had extended into the stem.

DEVELOPMENT OF PRUNUS ROOT SYSTEMS IN A CITY STREET 267

extended from the damaged area back into the stem. Some large roots had been completely removed, also introducing disease that had spread into the stem.

Discussion

The cherry trees in this study had between two and five major roots. This is comparable to the number of major 'structural' roots on trees growing in forest conditions. For example, ROWE (1964), EIS (1974), FAYLE (1975), COUTTS (1983) and NICOLLet al. ( 1995) found between three and 11 major roots on a variety of conifer species. However, the spread of the cherry root systems had been largely constrained by the road on one side and a wall on the other, and root systems had grown mostly under the pavement with no predominance of either up or downslope development. Downward growth was also limited: none of the root systems excavated in this study had grown deeper than 0.57 m, possibly because the soil was too compact (NICOLL and ARMSTRONG 1997).

Most pavement cracks had been caused by roots over 100 mm diameter. Although large 'surface' roots had caused the most severe pavement damage, roots as deep as 0.4 m below the surface that were undergoing fast secondary thickening had also caused damage. Pavement cracks mostly followed root direction, but this was less apparent with deeper roots.

The remedial operation of chiselling down damaging roots before relaying a pavement appeared to have only short-term benefits. Damaged roots 'callus' around the chiselled area, subsequently lifting the new pavement over a larger surface area. The force exerted by tree roots, applied over a wider area increases the risk of damage to structures (MACLEOD and CRAM, 1996). The damage to roots during pavement relaying also renders the roots susceptible to soil-borne pathogens (POPOOLA and Fox 1996) and threatens the health of the tree. Complete removal of large damaging roots during pavement repairs avoids recurrent damage, but may impair stability if the tree has few major roots and if root spread is already restricted. Again, it also risks the introduction of disease into the tree. If large roots have to be removed, a strategy of removing and replacing the offending trees with less vigorous ones may be more cost effective. Further research is required to examine other options such as species selection and the use of alternative paving materials to reduce or prevent damage.

Where possible, the use of large uncovered planting areas would avoid the pavement damage that is common close to the tree. Improving soil preparation at depth encourages growth of deeper roots (NICOLL and COUTTS, 1997). HODGE (1992) recommended that the planting pit should be 2.25 m x 2.25 m, with soil prepared to 1 m depth, for species that will grow into medium sized trees, and that a greater volume should be prepared on particularly

268 ARBORICUL TURAL JOURNAL

inhospitable sites. A medium that has a matrix of small stones with the gaps filled with loose soil (see GRABOSKY and BASSUK, 1995) will permit healthy root growth at the same time as supporting a pavement. However, such a substrate will also efficiently transmit the loads exerted by expanding roots. A firm substrate will lift as roots thicken and, as in this study, even deep roots will cause damage. Where space is too limited to use a large unpaved area, possibly the most effective strategy would be to use species that grow less vigorously (WAGAR and BARKER, 1983, NICOLL and COUTTS, 1997). For example, in this study Prunus trees had caused severe damage, while adjacent slow growing Malus trees in the same street had caused no damage. Unfortunately there have been too few studies of variation in root development between species to permit reliable selection of less damaging street trees.

This study was a limited, preliminary investigation of the ways that roots damage pavements, concentrating on only five trees of one species on one site. However, the excavation and root mapping techniques used in this investigation would also be effective in future studies on other sites. Applying the same method in future studies would permit analysis of site and species differences. It is essential that more information is gathered on pavement damage in relation to tree species, tree age, soil type, planting pit size, pavement characteristics and root development. Such information could be used to improve species selection, planting methods, paving types, and management regimes, to prevent or reduce the considerable damage caused by street trees. Developing strategies to reduce damage caused by street trees would benefit the urban tree stock, as the health and survival of trees are put at risk whenever root systems are cut back or damaged during pavement repairs.

Acknowledgements

Thanks to Ian Tubby and Alan Dowell from Forest Research-Alice Holt for their considerable assistance with this work. The street tree excavation was carried out in Sheffield with the cooperation of Sheffield City Council. We are particularly grateful to the residents of Marchwood Avenue for their interest in this study and their tolerance during the work. Thanks also to the Arboricultural Advisory and Information Service for their help in finding the site, to Richard Gill from City of Sheffield Leisure Services for his help in co­ordinating the work, and to Helen McKay and Bill Mason for reviewing the draft. This research was funded as part of the 'Arboriculture VI' contract from the Department of the Environment, Transport and the Regions.

References

BIDDLE, P.G. (1992). Tree roots and foundations. Arboriculture Research Note 108192/EXT. Arboriculture Advisory and Information Service, Farnham.

DEVELOPMENT OF PRUNUS ROOT SYSTEMS IN A CITY STREET 269

CouTTs, M.P. (1983). Development of the structural root system of Sitka spruce. Forestry 56, 1-16. CouTTs, M.P. and NICOLL, B.C. (1991). Orientation of the lateral roots of trees I. Upward growth of surface roots and deflection near the soil surface. New Phytol. 119, 227-234. DoBSON, M. (1995). Tree root systems. Arboriculture Research and Information Note I 30/95/Arb. Arboriculture Advisory and Information Service, Farnham. Eis, S. (1974). Root system morphology of western hemlock, western red cedar, and Douglas fir. Can. J. For. Res. 4, 28-38. FAYLE, D.C.F. (1975). Distribution of radial growth during the development of red pine root systems. Can. J For. Res. 5, 608-625. GRABOSKY, J. and BASSUK, N. (1995). A new urban tree soil to safely increase rooting volumes under sidewalks. J. Arboric. 21, 187-201. HoDGE, S.J. (1992). Good roots will put you streets ahead. Hort. Week, November 20 1992. 24-27. MACLEOD, R.D. and CRAM, W.J. (1996). Forces exerted by tree roots. Arboriculture Research and Information Note 134/196/EXT. AAIS, Farnham. MACPHERSON, E.G. and PEPER, P.P. (1996). Costs of street tree damage to infrastructure. Arboric. Jour. 20, 143-160. NICOLL, B.C. and ARMSTRONG, A. (1997). Street tree root architecture and pavement damage. Arboriculture Research and Information Note 138/971 S/LN. AAIS, Farnham. NICOLL, B.C. and CouTTs, M.P. (1997). Direct damage by urban tree roots: Paving the way for less damaging street trees. Arboriculture Practice­Present and Future. Research for amenity trees No. 6: 77-84. Department of the Environment, Transport and the Regions, London. NICOLL, B.C. and RAY, D. (1996). Adaptive growth of tree root systems in response to wind action and site conditions. Tree Physiol. 16, 891-898. NICOLL, B.C., EASTON, E.P., MILNER, A.D., WALKER, C. and Courrs, M.P. (1995). Wind stability factors in tree selection: distribution of biomass within root systems of Sitka spruce clones. Wind and Trees. Eds. M.P. Coutts and J. Grace: 276-292. Cambridge University Press, Cambridge. PATCH, D. (1994). Management of trees and the environs in which they grow. Scott. For. 48, 96-101. PoPOOLA, T.O.S. and Fox, R.T.V. (1996). Effect of root damage on Honey Fungus. Arboric. Jour. 20, 329-337. QuiNE, C.P., BURNAND, A.C., CouTTs, M.P., REYNARD, B.R. (1991). Effects of mounds and stumps on the root architecture of Sitka spruce on a peaty gley restocking site. Forestry 64, 85-401. RowE, J.S. (1964). Studies in the rooting of white spruce. Project H-131, 2nd prog. rept. Mimeo Dep. For. Can. No 64-H-13. WAGAR, J.A. and BARKER, P.A. ( 1983). Tree root damage to sidewalks and curbs. J. Arboric. 11, 165-171.

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WAGAR, J.A. and FRANKLIN, A.L. (1994 ). Sidewalk effects on soil moisture and temperature. J. Arboric. 20, 237-238. WILSON, B.F. ( 1975). Distribution of secondary thickening in tree root systems. In The development and function of roots. Eds. J.G. Torrey and D.T. Clarkson. Academic Press, London. pp. 197-219. WoNG, T.W., Gooo, J.E.G., and DENNE, M.P. (1988). Tree root damage to pavements and kerbs in the city of Manchester. Arboric. Jour. 12, 17-34.

Resume

Nous avons inspecte les deg~ts subis par le trottoir au tour de cinq prunus de trente ans poussant dans une rue de Sheffield, et avons examine I' architecture de leurs systemes de racines. A pres avoir retire le macadam et le sol, Ia position des racines ligneuses a ete relevee. La plupart des racines avaient ete genees par Ia route d'un elite et par un mur de I' autre. Loin d' a voir ete uniquement causes par les racines se trouvant immediatement so us Ia surface, les de gats avaient ete aussi causes par des racines ala croissance rapide, pouvant se trouver jusqu' a 0,4m de profondeur sous le trottoir. Presque tousles deg~ts etaient causes par des racines ayant un diametre superieur a 1 OOmm. Les racines a large surface qui avaient ete entamees au burin au cours de reparations anterieures du trottoir presentaient des cals de chaque elite de l' endroit endommage, ce qui accroissait la surface de contact et le risque de voir le nouveau trottoir endommage.