Thoughts on Urban Forest in Hong Kong

Thoughts on Urban Forest in Hong Kong

First Published on 20th October, 2014 @1825

We all know that Hong Kong is an International City famous for its majestic skyscrapers, robust public transportation system and no lack of places to shop for pleasure. With the city being prosper from concrete and steel, little did we, as Hong Kong people, paid attention to the “greens” in the city – I am referring to all plants, big or small, that are growing around us. Until recent years, with the unfortunate events of tree failure causing casualties and injures, we began to realize the importance of tree care for public safety.

Unfortunately, all these came a little too late and the public already have doubts whether the establishment of the Tree Management Office (TMO) would prevent more tree failing cases to happen again. As recent as August 2014, the fatal accident of a pregnant woman and her 38 weeks baby still fighting for life seems further proving that the catching up game from the government is not accelerating fast enough to avoid even more tragedies to happen.

Few of the forestry concept are rather relevant for urban tree care.

As a forester, I felt that it is an “attitude” problem – the way how we approach trees, as a living life form, rather than the shortage of tree care professionals, are hindering the government departments from being capable of getting whole of the situation. Few of the forestry concept are rather relevant for urban tree care. Here are three major concept worth thinking:

  1. CONCEPT #1 ROTATION: How Old Can A Tree Grow?
  2. CONCEPT #2 THINNING: No Trees Share The Same Faith.
  3. CONCEPT #3 MANAGEMENT: Why We Need Trees?
  4. Public Safety, Public Safety, Public Safety

CONCEPT #1 ROTATION: How Old Can A Tree Grow?

Very often, when asked, people will think trees will grow over hundred, if not thousand of years. While many trees do have this potential, not all – even the most longevity ones, could actually live to their expectancy. Just like human being, how many of us would expect to live more than 100 years old, even the oldest on record was more than 120 years? It will take lots of favorable condition, such as, the environment, climate, tree itself etc. to achieve its maximum longevity. Even with these favorable condition aligned, the health of a tree will weaken over time and at some point, will still die. If we don’t remove a tree in time, gravity will take care for us.

If we don’t remove a tree in time, gravity will take care for us.

In Forestry, we have a term called Rotation, it describe the optimum time to lodge trees for profits. Trees will grow, attain its maximum size and after that, deteriorate and more prominent to irreversible damages, such as weathering or infestation of pests that the trees will die eventually. In urban trees, we have to look for those that are in their ageing stage and determine a time to remove it before being too fridge for the tree to stand by itself. Even if we decided to retain trees for various intrinsic values, such as the Old and Valuable Tree (OVT), safety precaution will need to be thoroughly considered for pubic safety. For forester, our rule of thumb would be 60 years that trees will attain their most favourable condition: maximum wood, minimum damage that are good to be harvested. Anything beyond that point the tree grow slower and more prone to damages that much energy will be used for repairing itself. The mechanical structure of trees will become weaken over time from ageing and weathering.

If public safety is not a major concern, keeping a tree to its natural age is a very sensible management decision.

To talk about Rotation is to borrow the idea behind that we have to identify and make appropriate management decision before the tree fall from its own weight. If public safety is not a major concern, keeping a tree to its natural age is a very sensible management decision. For trees with intrinsic values, we do have to strike a balance in between the site condition and how likely it will cause public safety concerns. Trees that grow on slopes, walls, planter, small pits are never good to be grown oversized and very likely to fail.

CONCEPT #2 THINNING: No Trees Share The Same Faith.

The current tree management strategy resolved around the use of Form 2, a Tree Risk Assessment form developed by the Tree Management Office (TMO) to evaluate trees, (you can download here). All trees are surveyed in government lands – you might already noticed many trees were tagged with a laminated paper or a green strip with strange numbers or bar codes, those are site number and tree number. After a tree is recorded in Form 2, they will pass through different departments to review the condition of each tree and give comments for remedial actions, largely whether to prune, cabled or removed. To some degrees, even dead trees uses Form 2 to administratively declare the tree is dead for removal!

While this method is appropriate to evaluate trees that were planted alone by itself, it is not the best way to evaluate trees that were planted within a patch. Without much information about the spatial distribution of trees and a clear management goal for each site, simply caring each and every tree to its maximum condition would not attain favourable condition for trees to grow and prosper.

When trees planted in the same patch, they compete for the same resources to grow…

When trees planted in the same patch, they compete for the same resources to grow, ie., water, sunlight and nutrients. Different species have their own strategy – some trees grow faster and taller to outpace the others, while some trees grow slower and torrent to shades better. In Biological terms, it is called Succession, referring to the dynamic changes in trees over time. Within the same patch, some trees will grow better than the others because of spatial aspects (e.g. amount of sunlight) and some other biological conditions (e.g. pests) and therefore, simply taking care of all trees with the same standard is not in the best interest, nor being cost effective, for less tree failure cases.

In Forestry, we have a term called Thinning. It refers to the periodical removal of trees in order to create favourable condition for other trees to grow stronger. Forester usually plant a site with higher density of trees during establishment and as time goes by, trees that are being less favourable to attain maximum size and form will be removed, leaving those with more resources (soil nutrients, space, rain and sunlight) to attain its maximum size.

…trees that grow within the same patch should be managed as a single unit…

Borrowing the concept of Thinning, trees that grow within the same patch should be managed as a single unit to develop management goals for what structure we would like to attain in a few years time, say a 3, 5 or 10 years plan. Trees which are in favourable condition should be retained and trees that are weak, doing too much shading to others or not promising enough to become healthy and strong should be removed. An onsite decision for tree management and comparison of dynamic change over time would be a much better way than servicing each and every tree without a clear management goal in mind.


This is a question I always enjoyed to challenge my students when they take my class. At the beginning, I will ask why we need trees in the city? Students will usually right away blast out tons of reasons – Privacy, Temperature Regulations, Wind & Sound Shields etc. to name a few. Then what comes next will simply bring the discussion to a pin drop silence. Simply, why we need trees when all these qualities can be done by man made structures? Privacy, we can build walls. Temperature, we can install air conditioning. Wind & Sound, there are plenty of steel structure placed along the highway for this purpose already. Franky, it is not necessary to have trees in the city!

…trees are living organisms…

Don’t get me wrong, I like trees and a big believer that trees have a significant role in cities for the number of same reasons mentioned above (collectively referred as ecosystem services). Point is, if we can’t understand and be mindful that trees are living organisms, we won’t be able to develop any appropriate plans for trees to fit better in our city. The function of a urban forest has to be thoroughly understood in order to make it relevant to the city. Otherwise, it will be a waste of public space and depleting our social resources for maintenance. It is simply an appropriate management strategy has to be developed ahead of planning and providing clear goals for people to follow.

Take roadside trees for example, when they are too close to the roads, it would be a disturbance to the drivers and maintenance cost for regular pruning seems not being cost effective (e.g. Bus roof ripped off by trees in the UK). Trees at large bus station terminus is also another example that it is purely planted for planting sake, regular maintenance is needed not to block the drivers’ sights and this in turn, constantly stressing the tree from cutting. Roadside planter seems function more like a dustbin and the regular change of potted plants seems to be a drain of money for lollipop landscaping. Stonewall trees are simply anti-physics that either the tree fall by its weight or the walls will crumble over time.

What goals we want to attain from planting trees in the city?

What goals we want to attain from planting trees in the city? If trees are not regularly managed, it will lost its aesthetic beauty and its services that was originally designed for – densely shaded areas by trees will be a welcoming gesture for crimes, trees that are planted on the road side with inappropriate tree form will be a visual blockage to drivers and pedestrians and trees growing on retaining walls on slope without safety measurements will become a falling log in just a matter of time. I believe these are pure management decision in light of public safety.

Public Safety, Public Safety, Public Safety

In Hong Kong and cities around the world, urban trees are planted very close to human activities and therefore, public safety should alway be the top priority in tree management. Urban trees are grown on rather challenging condition: compact soil, heavy shading, pollutants, poor irrigation/drainage system to name a few and quite naturally, they suffer from various stress that affects them become strong and healthy. It is a management challenge to keep urban trees in good condition so they won’t be ticking time bombs for public safety.

A clear management goal would be important to plan ahead for better site condition, selecting favourable trees and gradually tending the site in growing strong and healthy trees. Trees with substantial damage, pest infestation and being geriatric are more prone to failure that are common reasons for tree failures in Hong Kong these days. While there are no silver bullets in solving the problem of tree failure, decision makers should always be reminded that protecting citizen safety is always more important than saving trees (e.g. what’s the point of over cabling a tree?). Planners should always be reminded that replanting is always an option that also provide a chance to grow trees in better condition.

The concepts discussed above hopefully would raise awareness that trees are living organisms and therefore the management system should resolve around their live cycle: Don’t let the tree fall by itself before you can do something (concept #1) , they compete resources to outgrow each other (concept #2) and we need to ask what stand structure (the appearance of the site) we would like to achieve in the years to come if not decades (concept #3). It is an industry that takes more than a generation of us to take care of them and benefiting the generations to come.