Gebiz Spiral Watch

Quotation No.: HDB000ETQ18300071

QUOTATION FOR ISA CERTIFIED ARBORIST CONSULTANCY SERVICES FOR THE PROPOSED PUBLIC HOUSING DEVELOPMENT AT KALLANG WHAMPOA KWC57

Company A - $7,713
Company B - $8,800
Company C - $3,008 (Awarded)
Company D - $4,000

Company C seems like a relatively new-comer to the local arboriculture and landscaping scene and  claimed to be doing business since 2005. Winner had bidded 25% lower than the second lowest bidder.

Quotation No.: HDB000ETQ18300057

Quotation for ISA certified arborist consultancy services at demolition site at Bedok South Avenue 3

Company A - $12,000
Company B - $3,897 (Awarded)
Company C - $8,800
Company D - $4,500
Company E -  $12,500
Company F -  $8,155

Winner's price was 13% lower than the second lowest bidder, who in turn is about half that of the median price. More newcomers are bidding than the established ones. Surprisingly, one of the company who was under-pricing everyone not long ago is now at the median.

Quotation No.: HDB000ETQ18300004

Quotation for ISA certified arborist consultancy services for demolition of industrial blocks at Sin Ming Industrial Estate

Company A - $4,998 (Awarded)
Company B - $13,500

Winner's price was 63% lower than the other company. This job seemed as good as any, but strangely there was only two bidders.

Quotation No.: HDB000ETQ18300097

QUOTATION FOR ISA CERTIFIED ARBORIST CONSULTANCY SERVICES FOR THE PROPOSED TOA PAYOH NEIGHBOURHOOD 1 CONTRACT 25/25A AND PARK


Company A - $4,200
Company B - $7,430
Company C - $4,200 (Awarded)
Company D - $4,650
Company E -  $8,205

Winner's price was  10% lower than the second lowest bidder. Company C was the same company in Quotation No.: HDB000ETQ18300071, and was also the awarded company. As a relative new-comer to the scene and/or Gebiz bidding circle (for arborist), they managed to win over Company A, which was bidding at the same price and seemed to be more established in the industry.

LTA Car Crashed into OG Boxes Before Being Stopped by Tree

On 26 April 2018 (Thursday) around 2.30pm, an Land Transport Authority (LTA) car crashed into overground (OG) boxes before smashing into a tree along Kranji Way. Swietenia macrophylla (Broad-leafed Mahogany; Honduran Mahogany) no less. The incident also involved a Kranji Countryside Association minibus, which goes along a set route and brings visitors to farms in Kranji. A spokesperson from the LTA said that no injuries were reported - either to the driver of the LTA car, and driver and passenger in the minibus. A Mobike (sharing economy bicycle) appeared to be toppled in the path of destruction. The tree with its apparent buttress roots remained standing and a  large wound appeared to have been created at the point of impact.

Photos credited to a "Taxi Poh", with exception to the first photo.

 Screenshot captured on Google Streetview dated Sep 2015.

 Minibus involved in the incident with the damaged OG boxes and car in the background.

LTA car smashed into Swietenia macrophylla tree.

 Along Kranji Way near the Sungei Buloh Wetland Reserve Visitor Centre (60 Kranji Way) and Kranji Reservoir Carpark B.

 Damaged OG boxes and toppled Mobike in the path of destruction.

Another view of the point of impact. No obvious soil movement and uprooting despite the impact from the car.

One online commenter asked - "If you hit a tree..... Does nparks come after you?"

NParks said that when roadside trees are hit and damaged by a vehicle and if established that it was due to recklessness, negligence or intoxication of the motorist, the motorist may be fined, starting from $2,000 a tree. The motorist may also have to pay for the damage caused. And that I suppose refers to "making good" based on the value of the tree, which depends of the age, size and type of the tree.

Can tree management reduce trees overturning during storm events?

Some trees fall during storm events in strong wind and rain (these trees were termed as windthrow or wind snap in the paper below) but some trees managed to stay upright. So what factors contributed to these trees to stand firm while the others to fall? For this entry, the following paper would be referred to heavily.

Source: Moore GM 2014, Wind-thrown tree: Storms or management? Arboriculture & Urban Forestry 40(2):53-69

There is a Chinese saying that "big trees attract more wind", and that is true but there is also the assumption (not just by the Chinese) that tree crowns act like sails on a boat/ship, and the total surface area of the foliage were taken in the past. But doing that overestimated the wind force exerted on the tree. Rather, tree crowns behave more like a perforated sieve.

I do not know if calculation were made to estimate wind forces exerted on "sails" or "sieves" for the trees locally, but crowns were reduced or tree were removed I suppose to show that something is being done. It is not clear that crown reduction to reduce the foliage surface area would necessarily reduce the wind load, because depending on which branches are removed, the capacity for mass damping (see previous blog post) may also be diminished.

Most tree roots are near the surface and spreads 3-4 times the spread of the tree crown (also known as the drip line). There are different types of roots in the root system - descending (or vertical, sinker, or oblique) roots, spreading roots, tap roots, and fine roots. Fine roots bind closely to the soil and consolidate the root plate as they are large in number and surface area.

Structural roots are important to tree stability and have to be protected within a critital root zone (Matheny and Clark 1998, Anomymous 2009), or structural root zone (SRZ) - same meaning.
Anonymous, 2009. Protection of trees on development sites, Australian Standard # 4970, Standards Australia, Sydney.

Roots that contribute to the anchorage are the resistance of leeward roots to bending (25%) and the resistance of tap roots and decending roots to uprooting (75%). Therefore the windward roots, which is pulled upwards during toppling, is the most important component in resisting windthrow. When they do get pulled up, the descending roots, if present, often follow intact. If there are no descending roots in the exposed windward side, it is often an indication that they have not been present, which may be important in diagnosing the causes of failure. Failure usually occurs closer to the trunk in wet soils.

Other professions (geotechnical, civil engineering, etc.) have noted the important of descending roots in stabilising slopes, with deeper rooted trees stabilising slopes only to the depth to which descending roots can penetrate (Gray and Sotir 1996).
Gray DH and RB Sotir. 1996. Biotechnical and soil bioengineering slope stabilisation. Wiley and Son, New York City, New York, U.S.

Root plates are often depicted as being circular but that is a simplified approximation. A study suggested that an elliptical root plate may be a better approximation to reflect root systems (based on two Pinus species).

Figure 5 below was flipped horizontally to match the directions in Figure 4. It was the other way around in the original paper, which is strange. The root plate is more likely to extend to the windward side of the trunk, and damage to anchoring roots at that side, by trenching or construction, is more likely to lead to the root plate tilting (Figure 4). Especially if the roots are damaged or severed close to the trunk. Mowing could have also repeatedly damaged the lateral root that had come to the soil surface. While the wounds created may not be structurally imparing, they may provide access for pests and diseases into the tree. A number of them would start to decay and desending roots would also die along, causing the tree more prone to toppling by wind.

Construction work such as adding backfill soil or re-contouring surfaces could divert water flows away from the root systems, and pose sudden water deficits to the trees, which would show signs of wilting and dieback. However, many trees with significant dieback could remain standing while trees with healthy crown could be wind-thrown. Hence caution should be applied when observing the crown in visual assessments. Signs of dieback and significant deadwood in the crown may also indicate that the trees may have been stressed for some time and that there had been a loss of root mass.

A study have shown that depth of root plate is not significant in tree failure. In addition, urban soil profiles are altered and compacted such that penetration by tree roots is limited to no more than 1000mm depth. In the case where soils are compacted near the surface - at heavily trafficked areas of a park, sports activities, people grouping under trees for shade - reduces aeration and water penetration. This in turn inhibit descending and lateral roots, which can affect tree stabililty. However, compaction increases soil strength.

Heavy rains would saturate soil and reduce the strength of the connection between soil and tree roots, resulting in trees overturning and the descending rotos of the root system of the windward side would be intact. Soil may also be waterlogged due to poor drainage or altered subterranean water flows. Due to "high water table", root developement could be restricted to depth of 200mm and above.  In waterlogged soils, desending roots would die back over time. After the tree had been overturned by wind in such a case, the exposed soil would have a pungent odour and there would be a large number of small blackened roots. 

Management may have contributed to forest tree failure after forest thinning operations, which has been well-researched because of the economic losses that results. Due to heavy rains that reduce soil strength and accompanying strong winds. I would argue that this applies to urban trees and shrubs thinning as well.

Standard protection systems cannot deal with the nuances of every tree and root system that develop in response to particular environments.

Arborists should include as part of their inspection protocols, trees that show damaged or decayed lateral roots and the loss of descending roots, evidence of site or trenching work close to the trunk, and whether trees are growing in compacted soil, waterlogged soil and backfilled soil. However in my opinion, I do not think an arborist could observe loss of descending root and waterlogged soil.

The question is raised as to whether the storm was the final trigger in a lengthy chain of events relating to tree management, leading to wind-thrown and whole-tree failure.

Dead branch on the tree! Cut it down right now before it drops on someone!

A slightly over-dramatic exclamation whenever an enthusiatic manager/member-of-public/person-with-nervous-energy spots a dead branch on a tree. This post would be based on Arborist News, the February 2018 edition.

Removal of dead and diseased branches that are attached and have broken off from tree is called deadwooding. Crown cleaning is not widely used in arboriculture outside of North America. It is done for safety, aesthetic, and biological: for compartmentalisation and wound closure to occur, and reduce incidence by pest and disease attack. Clearing of deadwood after storm events could be more expensive than scheduled deadwooding due to reduced economies of scale and crown thinning due to repairing branch attachment points. However when left uncleared, deadwood could provide a habitat for wildlife, and nutrient resource for insects, fungi and other organisms.

This branch was topped and resulted in a dead stub that is blocking wound closure. Note the wound wood surrounding the base of the dead stub.

 Acacia and Eucalypts usually have a lot of relatively small deadwood. As the deadwood would be constantly present in such species and give a messy look too the surroundings and the tree crown, it would not be practicable and economic to remove them all the time. Parks and estate management should be aware of this characteristic instead of yelling for the arboriculture team to turn up every other week.

Two Eucalyptus deglupta with quadpod staking at the entrance of a condo estate at Pasir Ris. At first I thought they were Hopea odorata.

A row of Eucalyptus deglupta along the periphery of another condo estate at Pasir Ris. The crowns were overhanging a cycling/jogging path and it does not seem to be littered with deadwood. Perhaps due to the trees' relatively young age. Wait till they get to 50 years old and things may be different.

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Branches on trees act as coupled masses and in winds develop a mass damping effect which helps distribute, reduce and dissipate the wind energy. (James and Haritos 2014)

Source: James KR and Haritos K 2014 Branches and Damping on Trees in Winds 

23rd Australasian Conference on the Mechanics of Structures and Materials (ACMSM23),  9-12 December 2014, S.T. Smith (Ed.)

Mass damping is part of the tree's response to strong winds (Moore 2014), reducing the impact of dynamic forces on the branches and minimising the occurence of harmonic waves in branches on windy days.

Source: Moore GM 2014, Wind-thrown tree: Storms or management? Arboriculture & Urban Forestry 40(2):53-69

Dynamic loads can be defined simply as time-varying (Clough and Penzien 1993) and may vary with magnitude, direction and/or position with time. The responses of tree structures also vary with time (Coutts and Grace 1995).

Source: James KR 2010, A dynamic structural analysis of trees subject to wind loading. Doctoral thesis, School of Land and Environment, University of Melbourne.

 Tree with most of its large branches, smaller branches and twigs intact.

 Crown raising, crown thinning, crown reduction, deadwooding... for "good measure".

Soruce: James K 2003. Dynamic loading of trees. Journal of Arboriculture 29:163-171

Air Flown Trees Landing on a Rooftop

In the Arborist News magazine, there is a series called Detective Dendro - The Diagnostic Sleuth. In the April 2018 edition, The Case of the Rooftop Restaurant written by James Komen (a consulting arborist himself) is a (fictional?) narrative whereby two arborists were invited by a building manager to examine the decline of some olive trees (Olea europaea). The restaurant was built on top of a 40 storey building, and the restaurant's floor, I believe, was suspended above the building roof.

The two arborists were supposed to investigate the eight mature olive trees that the building manager was concerned with. There was a scaffold over the restaurant that supported sun shades that could retract in segments, and duct system for climate control. The building manager shared that 10 years ago, the olive trees were lifted up to the roof with a helicopter in 1.5 m boxes. Since then, the trees have been creating a garden-like environment. However over the last few years, the branches have been dying back and they were pruned off. The arborists noticed that about half the remaining branches had leaves with necrotic tips. Furthermore, each tree had one drip irrgation dripper and below the mulch the soil appeared to have enough moisture. There was a possibility that the duct system, which was used to transport hot air could have dried the leaves but the patten of dieback does not correspond with the location of the duct system. In addition, the problem only started to appear in the last four years. If the hot air was the issue, the symptoms would have appeared almost immediately 10 years ago.

The arborists that suspected that the leaves could have been infected by bacteria, which has been found recently in urban olive trees. The particular bacteria species, Xylella, can cause a systemic infection that restricts the flow of water and result in drought stress symptoms in the leaves. They took leaf samples and then took photos of the drainage system (not obvious in the photo below) and tree containers in the space underneath the restaurant floor. Much later after the site visit, the lab test did not detect Xylella in the leaves or stem tissue. The tree containers were shown to be not much larger than the 1.5m boxes that they were transplanted in, and the arborists deduced that the trees were likely to be root-bound.

One of the arborists dug around a tree container with a trowel and found some adventitious roots, which are produced by trees with issued below ground like girdling roots or poor soil conditions. Girdling roots are a sign that the trees were root-bound, which means that they could have out-grown their containers.

The other arborist calculated the amount of soil volume recommended based on the crown projection (three cubic feet of soil per square foot of crown projection) and found that the soil volume in the container was far less than the recommended amount. He proffered his view that the soil volume for each tree should be larger and there was no way to correct the problem without "demolishing and rebuilding the entire restaurant and steel casings [that suspends the entire floor above the roof]". The arborist counselled the building manager that he could try to keep the trees alive for as long as he could but plan for their eventual replacement, as the tree containers would limit their lifespan.

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The (fictional?) consultants were observant and knowledgeable enough to consider the duct system across the scaffolding and the possibility of a recent bacteria infection occurence, which turned out to be negative from those olive trees. However, they may not have considered that olive trees does not tolerate too much water and are unhappy in soggy soils. The top few inches of soil should be allowed to dry between waterings, and the irrigation system should be adjusted towards this result. In addition, the drainage system of the tree containers should be checked that it is free from blockage, which would allow excess water to drain freely. The necrotic tips of the leaves and adventitious roots could be symptoms that the trees are receiving too much water from the irrigation system.

While it is likely that the trees have girdling roots and do not have the recommended amount of soil volume in the slightly larger than 1.5m tree containers, the arborists could have offered favourable solutions other than to rebuild the entire restaurant from ground up (or in this case, roof up) or to wait for the trees to die. Perhaps, in order to do improvement work within that crawlspace meant breaking a dozen occupational safety and health rules, and hence the other solutions were not suggested.

Tree roots generally spread three to four times wider than the crown spread, and when trees are grown in containers, the roots tend to spiral around within the container creating a root-bound condition. Roots dessicate when exposed to air, and this principle is how air-pruning pots/containers work - by pruning roots that try to extend outward using air - and solving the root-bound issue.

Image Air-Pot.com

The tree containers could be enlarged by first removing the floor of the restaurant. But that does not mean giving up valuable real estate for the trees' well-being. Next, structural cells (trade name: Silva Cells) could be installed in the enlarged soil space, which would hold up decking or slabs and prevent compaction of the soil. The soil added should be sandy and suitable for the mediterranean climate plants. The opportunity should be taken to upgrade the drainage system. And the sides of the tree containers should be air-pruning panels instead of steel panels.

Image Air-Pot.com