Mangroves - a carbon source and sink

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. 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 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. 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 signi-ficant 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.