How can we get our urban forest more contributing to carbon sequestration?

(the original article was written for Friend of the Earth (Hong Kong)

The COP26 (the 26^th United Nations Climate Change Conference) just finished in November has eye-caught us again on the urgency in trying to reverse the global warming situation that you and I are all feeling worse in recent years.  Other than the several major deals made according to the agenda of the conference, there was a separate critical pledge agreed amongst 110 countries (with about 85% of the world forests) that deforestation is to be stopped or even reversed by 2030.

 

This agreement was seen as a vital action to limit temperature raise because trees absorb enormous amounts of CO₂.  Without trees, it appears that other efforts trying to achieve the goal of limiting temperature rise within 1.5^oC without avail.  It is trees that can be utilized by human being to effectively fix carbon in a relatively easy manner as an offset of our emission, as long as we start this earlier.

 

The terms deforestation and reforestation are not far away from city dwellers.  While that COP26 agreement focused on rainforest, yet its spirit is inspirative to our attitude towards the trees nearby – our urban forest.  Urban forest refers to the trees within built-up areas and those that are closely related to city life.  These trees contribute to the process of carbon sequestration – like the trees in rainforest do.  We all acknowledge this process to be our fate’s salvation, but some may question: how far our urban trees can contribute?

 

Like many green plants, tree capture CO₂ from the air almost continuously in their whole life. The carbon so captured is used or converted via many routes, such as a fuel for energy (carbon will then return back to the air), for building blocks (stored and fixed), or lost finally in litterfall/death (returned to the land – then decay will release the carbon back to CO₂).  The most efficient C-fixers should be those that are comparatively fast growing, allocating more carbon in proportion for building blocks (in tissues such as wood) and long-life (so that the fixed-C won’t get back to the land very soon).  The above in fact differentiates trees from many other plants (that are annual or shorter-life) in term of their capacity in carbon fixing.

 

With this understanding, trees in our urban forest can undoubtedly contribute to the global carbon sequestration campaign, if we can provide the environment optimal for their services.  Various factors affect urban trees’ ability in fixing atmospheric carbon.  These include:

1.      The quantity of trees that we have – suggesting the potential scale of their services;

2.      The size of trees and wood quality that can be achieved – reflecting the actual amount of wood (C-sink) that can be produced;

3.      Health and longevity of trees – determining the rate of returning wood onto the land and then its carbon back to the atmosphere;

4.      How we deal with the dead wood – also determining the rate of returning wood back to carbon in the atmosphere.

 

Tree quantity

For sure more trees will uptake more carbon from the air in principle.  However, quantity itself is not always equivalent to efficiency, especially in the built-up area with various site constraints to tree growth.  In the past people had focused too much on the number of trees that we planted (or replanted), transplanted or even retained during our urban development.  This sometimes ended up with trees congested in very small planting areas, having their roots severed as they are too close to structures/facilities, resulting poor health and even risk to the public.  Ultimately these trees were considered problematic instead of beneficial and could not escape the fate of removal prematurely.

 

Obviously, if the trees so established do not have sufficient space for development, they cannot produce sufficient building blocks (C-sink) within shorter time.  Under this scenario, strategic management such as pruning and thinning of trees can help making room for the remaining to continue the C-fixing services.  A single tree provided with larger planting space will likely produce more C-sink (e.g. wood) than several poorly-grown trees will do.

 

Having said that, it is always recommended to explore space at the beginning to increase the number of trees in a reasonable manner.  This will fundamentally increase the carbon-fixing capacity of the city.

 

Size and wood quality

Comparatively trees take up less ground surface area - when compared with other plants (i.e. high plots ratios in building term) - upon which they build up carbon storage above.  To further maximize the aboveground C-sink (i.e. wood volume), the way of city planning needs to be refined to allow space for tree root development underground.  This should be done holistically with the planning and design of other grey infrastructures such as cables, pipes and drainages.  All utilities should be tactfully aligned so that the space usage efficiency underground can be maximized in order to make room for root development.  When there is space for roots, the ultimate wood volume can be increased.

 

With proper space demarcation, damage to utilities due to tree roots could be minimized.  This does require a change in the conventional way of utilities alignment, which has been weak in coordination.  Such old practice not only results high maintenance cost, but tree roots are also prone to damage during repair and replacement.  When we expect larger trees for bigger C-sink, potential damage on trees that may jeopardize safety must also be minimized.

 

Different tree species vary in growth rate and wood density, and these factors amongst others affect the amount of carbon that can be sequestered in woody stems.  Our arborists, city planners and landscape architects should also take this kind of scientific data into consideration when species are selected for “installation”.  As planting space in congested urban environment is valuable, we cannot just do the species selection by only their amenity value.  Planting pits will be wasted if only a single attribute of tree is considered during the design process, when there is other option(s) that can provide better, multiple services (e.g. stronger C-fixer).  This brand new perspective on trees – trees as a “green infrastructure” (FAO, UN) - should be widely shared in the society.

 

Health and Longevity

Trees as a C-sink will hold carbon longer when their health is well maintained and their life-span is extended.  However, growing condition in urban area usually impacts negatively to tree health and limits their life-expectancy.  Improving site condition as aforementioned will prolong the “shelf-life” of the C-sink.  Maintenance quality does also place a critical role in keeping our trees (and their parts) healthier and longer.  Excessive pruning (or damage) of trees without justified reason would shorten the shelf-life of the tree parts, not to say impacting the longevity of the whole tree.  The HKSAR government has improved on the requirements of qualified tree professionals in the recent years.  This approach in requiring trained personnel to deliver tree care should also be extended to the private sector because all trees – no matter public or private – serve us similarly in term of carbon sequestration and many other functions.  All tree owners have the responsibility to take care of this infrastructure component.

 

Handling dead wood

No matter how the trees and their parts end their life (damage/pruning/felling/natural death), the dead wood remains are still holding carbon.  The approach that we treat this C-sink (so termed “yard-waste”) have direct impact on the fate of carbon so locked in it.  In the past the felled trees (parts) were used to be dumped in landfill.  This is a practice that actually speeds up the process of releasing the fixed carbon back to the atmosphere as decaying organism breakdown wood, releasing CO₂.  In fact, there are ways of preserving wood in good condition for a prolonged period of time so that the carbon can be remaining locked.  An example is many historical buildings or structures that were built from trees felled several hundred years ago.  Wise use of such yard-waste would extend the life of wood and contribute to the environment.  Strategically, wood can also be turned into other forms such as biochar, which can lock carbon for even over thousands of year.

 

In fact, how our urban forest can contribute the process of offsetting carbon is a function of planning, design and management.  Just like what we do for other components of grey infrastructure, when the services of green infrastructure are well defined and realistically quantified there is a way to achieve it.  City trees come from the nature originally and have the same capacity in carbon sequestration in principle.  Although in the urban environment they are not likely to achieve the size as if they were growing in a rainforest, how far city trees could perform in carbon sequestration actually depends on how we value them and care of them.