Trees causing problems for concrete infrastructure – and vice versa – is often viewed as an unavoidable fact of life in the arb world. But a more scientific approach to planting and care can lessen the impact.
LIVING in an urban environment means many of the trees I see on a daily basis are surrounded by concrete, with the impact of one on the other plain to see. Concrete paving could reasonably be regarded as an insurmountable barrier, but actively growing tree roots are an irresistible force that will find their way over and under, around or even through this universal construction material if needs must.
Arboriculture is essentially an urban activity (unlike forestry), carried out in situations where trees will unavoidably interact with infrastructure both above and below ground. Interactions above ground, within the fluid gaseous medium of the earth’s atmosphere, are clearly more flexible, with room for manoeuvre and avoidance, but not so the interactions occurring in the confines below.
UNDERGROUND CONFRONTATIONS
The consequences of these underground, head-to-head confrontations between concrete and tree roots are seen above ground in ailing tree canopies and damaged concrete infrastructure. This is clearly something which the observant urban dweller sees daily in struggling, threadbare trees, raised road and pavement surfaces and fractured brick walls, all suggesting how trees are effectively pushing against what should be an insurmountable barrier to root growth and extension.
I never gave this much thought, assuming it was an unavoidable fact of life when you plant trees in the urban environment, until listening to an expert on the subject from the United States. He showed how, with proper planning and soil preparation, trees and concrete can ‘live in harmony’ without either party suffering undue damage.
The expert was Dr E. Thomas Smiley (Tom Smiley) from Bartlett Tree Experts, speaking at a Barcham Trees Seminar some years ago, who prompted me to study some well-established research conducted in the US (much of it by Tom Smiley himself) detailing the mechanics of root growth, damage to urban infrastructure and the preparations and precautions required to avoid or reduce the risk of damage.
Trees growing in small islands of soil, surrounded on all sides by concrete, are crucially important components of the urban environment, including trees planted along sidewalks (roadside pavements) and inside shopping malls and plazas. The area of ground surface and the volume of soil allocated to each tree are relatively small because of limited space, which in this case is perversely measured in units of area (e.g. square metres), at a premium in availability and monetary cost.
Aerial plant parts such as branches and foliage can be trimmed accordingly by mechanical pruning or the use of chemicals. However, the same close hands-on control cannot be exercised over the growth of the underground root system, which is out of sight and effectively out of reach (but not ‘out of mind’), especially when concrete surfaces start to lift and crack.
TREE ROOTS ARE AN IRRESISTIBLE FORCE
Tom Smiley told an attentive audience, largely composed of tree officers, how tree roots interacting with a concrete mass are an ‘irresistible force up against a solid and rigid object’.
Tree roots are positively geotropic (grow with the force of gravity) and negatively phototropic (grow away from the source of illumination), which means they ‘instinctively’ grow downwards through the soil. But roots are also positively hydrotropic, which means they will also grow forcefully (within a downward direction) towards available water resources.
Roots are dynamic living plant organs and where there is a will, there will be a way for tree roots to achieve their ultimate goal and primary function, to access and absorb water (and minerals) from the soil.
If there is a space to occupy or an avenue to enter, then tree roots will find it and exploit the resource thus presented. This is irrespective of concrete barriers nominally in the way, which tree roots will find a way round, over or under and even through, especially if cracks and fissures develop.
Concrete is the architect of its own demise, due to an intrinsically rigid macro- and microstructure that causes this common construction material to crack under the pressure exerted by roots as they grow through the soil.
STATISTICALLY DESIGNED AND VERIFIED TRIALS
The big difference between what’s said on the subject either side of the Atlantic comes down to scientific method. The consideration of concrete and trees in the UK has traditionally relied heavily on instinct and guesswork, while progress in North America has been achieved through scientifically designed and replicated field trials yielding reputable results which stand up to statistical analysis.
There is no substitute for scientifically sourced data allowing conclusions to be made with confidence simply because the field trial was properly designed. The ‘economy option’, whereby measurements are taken from a handful of trees, and which is too often a feature of ‘research’ in the UK, is essentially a waste of time.
Dr Smiley and his colleagues working for private arb companies and universities in the US state of North Carolina (NC) carried out the definitive work to determine the most appropriate soil structure and composition for trees forced to grow and develop in a restricted volume of medium overlain by concrete and other paving materials.
Work was started and initial definitive conclusions made almost 20 years ago by a group of scientists representing Bartlett Tree Research laboratory in Charlotte, NC, Clemson University, South Carolina, and Queens University, also of Charlotte, NC.
SOIL OPTIONS AND TREATMENTS FOR TREES SURROUNDED BY PAVEMENT
Trees in areas surrounded by pavement often experience inhospitable root environments, which shortens their life expectancy, according to the authors in the introduction to their work. This involved a comparison of five different soil treatment options under pavement for two different silvicultural subjects, namely snowgoose cherry (Prunus serrulata) and Bosque lacebark elm (Ulmus parvifolia).
Trees were planted in 5.4 m3 of soil medium and of 5 different types as follows:
1. Gravel/soil* mixture
2. Stalite/soil* mixture
3. Stalite
4. Compacted soil*
5. Non-compacted soil* with suspended pavement
*Existing (native) sandy clay loam soil
**All treatments were covered with concrete
Fourteen months later, the team used a range of tree-growth and tree-health parameters to assess and compare how the trees had performed under the five different soil medium regimes. The parameters used were:
1. Radial stem growth measured as trunk diameter in cm
2. Extension growth measured as increments in twig length in cm
3. Root growth assessed via a rhizotron (an underground laboratory with viewing window) installed for each tree species/soil treatment
4. Foliar colour and crown dieback visually assessed using scoring systems of 0 to 5
5. Chlorophyll content measured with a SPAD meter on five leaves per tree
6. Bark borer infestation measured as total borer holes for each treatment
7. Scale insect infestation rated visually on a scale of 1 to 5; 1 = tree free of scale insects; 5 = twigs completely covered with scale insects
Results were statistically analysed using analysis of variance to a probability level of 0.05 (5 per cent).
COMPARATIVE TREE GROWTH AND CONDITION
This type of experimental design, which typically features a large number of soil treatments applied across more than one tree species, utilising different growth and health parameters to gauge tree response, is notoriously difficult to interpret due to the interactive nature of the results obtained.
Be that as it may, the overall picture showed that trees in the non-compacted/suspended pavement treatment were larger, faster growing, of superior colour and showing more root growth than the other treatments.
Following on in quality were trees in the gravel/soil mixture with those in the stalite/soil mixture and compacted soil treatments slightly inferior. The stalite treatment was the easiest to install but did not provide an environment conducive to acceptable tree growth and condition.
These observations were based on the actual measured growth/assessed condition of trees grown in the various soil media treatments. However, when the results were viewed statistically, with respect to statistical significance at the five per cent probability level (P = 0.05), more subtle differences appeared.
Most striking and perhaps most important from the tree growth, condition and health points of view is that for both Prunus (cherry) and Ulmus (elm) the chlorophyll content readings and twig extension measurements made on trees in the non-compacted soil/suspended pavement treatment were significantly superior (P = 0.05) to trees in all other treatments.
For other growth/health parameters, including radial stem growth (trunk diameter) and visual assessment of foliar colour, trees in the non-compacted soil/suspended pavement treatment invariably came out on top, although the differences between these and trees in the other soil treatments were not necessarily significant at the 5 per cent (P = 0.05) probability level (Tables 1 and 2).
Nine years later, in 2015, differences in performance between trees planted in the non-compacted soil/ suspended pavement treatment and trees planted in the other treatments were still very apparent.
This pioneering work essentially paved the way for the commercial installation of suspended pavement.
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