Mangroves: A Carbon Source and Sink

Study Number:

14

Author:

J.E. Ong

Abstract:

The mangrove ecosystem in many wet tropical areas represents one of the most, if not the most productive of natural ecosystems. The question that has occupied the minds of many mangrove scientists is “What is the fate of this high productivity”? More recently this question has gained added relevance as a result of the increase in global carbon dioxide concentration. (Climate Change). Are mangroves sinks of atmospheric carbon? We try to answer these questions using 15 years of data from the Matang Mangrove Forest Reserve and the Sungai Merbok Forest Reserve, in Peninsular Malaysia. We take a quick look at the palaeo-geological evidence on sea level changes (climate change) in the Straits of Malacca during the recent past (Holocene) to give us a better perspective of the Matang and Merbok mangroves and emphasise the dynamics and ephemeral characteristics of the mangrove ecosystem. The pristine forest of Matang has a mean nett annual above-ground productivity of 18 t dry organic matter ha-1 yr-1 whereas the same forest managed on a sustained yield basis is a good 20% more productive. If harvested timber is used as fuel wood then much of what is fixed is released back into the atmosphere. (Deforestation). On the other hand, if harvested timber is used as pilings then significant amounts of mangrove carbon are locked away. We estimate that for the mangroves of Matang some 1.5 tC ha-1 yr-1 is buried each year over the past 8,000 years or so. The impact of man (since the beginning of this century) has resulted in an initial increased release of carbon into the atmosphere (in the first half of this century) as a result of the use of mangrove timber as fuel-wood but sustained yield management has ensured a carbon balance between what is fixed as timber and what is burned. The present management system (which produces significant amounts of slash and stumps) may result in increased amounts of burial (i.e. more than the 1.5 tC ha-1 yr -1). To demonstrate that the terms “source” and “sink” are relative terms, we show that mangroves may (at the same time as being a sink for atmospheric carbon) also be a source of carbon in that they may out-well significant amounts of carbon to adjacent coastal ecosystems and thus play a vital role in coastal fisheries production. Conversion of mangrove to aquaculture ponds could result in the release (from about 1,000 years accumulated mangrove sediments) of some 75 t C ha-1 yr-1 to the atmosphere over a 10-year period. This is 50 times the sequestering rate.

Main Results and Conclusions:

• The following conclusion about mangrove trees acting as both carbon sinks and sources is as follows: “From what is said above, mangroves act as a source (carbon out-welling) as well as a sink (burial of mangrove assimilated carbon in sediments) – an apparent contradiction! The fact of the matter is that the terms source and sink are relative terms:
• “The Matang mangroves acts as a sink for atmospheric carbon (climate change), fixing an estimated 75-150 t C ha-1yr -1 (as gross primary productivity). Of this 80% – 90% is returned to the atmosphere as respired carbon dioxide, leaving an estimated 7-18 t C ha-1 yr -1as net productivity. Some 5 t C ha -1 yr -1 of this net productivity is shed as litter. In a mature stand (30 year-old) there is a standing biomass of about 150 t C ha -1. An estimated 1.5 t C ha -1 yr -1 is sequestered in the sediments. It is estimated that under sustained yield management the system is about 20% more efficient at sequestering carbon than if left undisturbed.
• “Carbon that is not buried in the sediments either remains as standing biomass or is exported from the ecosystem. In this way mangroves are a source of carbon to its adjacent coastal system. The quantity of export has not been determined but there is an ongoing project to determine this” (1104).
• The following quote is about the release of carbon of from mangroves when these forests are converted to aquaculture ponds: “Still, irrespective of how the mangrove timber is used, the Matang mangroves acts as a carbon sink in the accumulation of carbon (some 1.5 t C ha -1 yr-1) in its sediment. The amount of organic carbon sequestered in non-wetland forests is comparatively insignificant. It is only when mangroves are converted to aquaculture ponds that there will be a release of carbon back to the atmosphere not only from the removal of the forest (deforestation) (loss of 150 t C ha -1 of standing biomass) but even more so from the perturbation (and oxidation) of about 2 metres of mangrove soil during pond construction. This will return to the atmosphere what has been accumulating in the mangrove sediments for about a thousand years (about 750 t C ha -1, even if only about half the carbon in the mangrove sediment is oxidised; even if this process takes 10 years we are looking at a very significant 75 t C ha -1 yr-1). This is a hidden cost of pond aquaculture in mangrove forests and yet another good reason why pond aquaculture in mangroves should be discouraged. Thus, if the increasing concentration of atmospheric carbon dioxide is of concern (climate change), then the mangrove ecosystem is an important one to carefully manage or conserve”(1105).