(the original article was written for Friend of the Earth (Hong Kong)
The COP26 (the 26th 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 1.5oC 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.
