挖掘紅樹林固碳的真相

文 ‧ 圖 / Kate Evans
 http://blog.cifor.org/14197/dirty-science-excavating-the-truth-about-mangroves-and-carbon/#.Ua8hstJTBqL
摘要 / 人禾環境倫理發展基金會

挖掘紅樹林固碳的真相
Dirty Science’: Excavating the truth about mangroves and carbon

摘要

        關於潮間帶紅樹林生態系的研究一直是一個未知的領域，潮濕跟一腳踏入會彷彿拔不出來的泥濘一直讓人們對它不怎麼感興趣。然而，在印尼一群來自國際林業研究中心CIFOR (Centre for International Forestry Research) 跟印尼政府研究發展部門 FORDA (Indonesian Government’s Forestry Research and Development Agency) 的科學家卻在這樣艱困的環境下，在西加里曼丹 (Kalimantan) 一片高達25公尺的紅樹林投入進行研究，希望能了解對於紅樹林固碳的效果跟氣候變遷的關係。

        國際林業研究中心(CIFOR)去年公佈的研究結果顯示，紅樹林對於減緩氣候變化極為重要。紅樹林可以從大氣中吸收大量的二氧化碳，並將將碳儲存在樹木和土壤中。研究發現，平均每公頃紅樹林可儲存1000噸的碳，是陸地上熱帶森林的三倍以上。儘管紅樹林只佔世界的0.7％。但砍伐紅樹林卻佔了全球毀林所致的碳排放量的10％左右。科學家還發現，紅樹林當中的碳有49％-98％的碳儲存在地面以下。但在國際氣候變遷委員會IPCC的計算中，僅以總生物量的10％，來計算樹木地下部的碳儲存量。這類知識的落差常常造成決策的錯誤。

    此外，紅樹林不但可以固碳，在這個生態系中也存在著很多特有的生物。同時也有重要防災制洪的功能。僅管如此，紅樹林卻一直沒有真正受到重視。在印尼，大部分的紅樹林都被砍伐改成養蝦場以及種植油棕，造成紅樹林的覆蓋面積快速且大量的消失。更因此，這群科學家表示了解紅樹林生態系是一個很緊急的事情，而且必須快點找出它會如何影響氣候變遷，否則的話就來不及了。

原文

Dirty Science’: Excavating the truth about mangroves and carbon

Chest-deep in brackish, tea-coloured water, a team of scientists wades into a towering mangrove forest in West Kalimantan, Indonesia.

It’s not the easiest place to do research. The trees are up to 25 metres tall and densely packed, with webs of entangled prop roots extending like skirts from each trunk. High tide floods the forest, while low tide means negotiating sucking mud and clouds of biting insects.

Nevertheless, the team from the Centre for International Forestry Research (CIFOR) and the Indonesian Government’s Forestry Research and Development Agency (FORDA) is committed to finding out more about this unique ecosystem.

“We are doing science in a dirty way,” says project leader Joko Purbopuspito, a CIFOR post-doctoral research fellow.

“You get wet, you get mud everywhere, it is a challenge.”

“But this work is crucial – we are filling the gap in our knowledge about the total carbon stored in these mangrove ecosystems,” he said.

CIFOR research published last year revealed just how critical mangrove forests could be in the fight to slow climate change. Like tropical peatlands, mangroves absorb huge amounts of carbon dioxide from the atmosphere, storing the carbon in the trees and in the soil.

If the forests are converted for other uses, however, all that carbon is released into the atmosphere, contributing to global warming.

The research found that mangrove forests contain an average of 1,000 tons of carbon per hectare, more than three times as much as terrestrial tropical forest.

That means that mangrove deforestation generates around 10 percent of carbon emissions from deforestation globally, despite accounting for just 0.7 percent of the world’s forest cover.

Exploring the underground

The CIFOR scientists found that between 49 and 98 percent of the carbon in mangrove forests is stored below ground in the rich, tidally submerged soil where organic material decomposes very slowly.

They are now trying to find out more accurately just how much carbon is stored underneath these forests – not just in the soils, but in the roots as well.

To do this, they’ve come to the vast estuaries of the Kubu Raya district in West Kalimantan, where they have been given access to a logging concession belonging to PT Kandelia Alam – a company that aims to sustainably harvest mangrove wood for charcoal and woodchips.

It’s the start of a new research project that aims to measure the carbon content of every part of the mangrove ecosystem – leaves, branches, trunks, soil, woody debris lying in the mud – and roots.

The mangrove species in this ecosystem (there are several, but the dominant one is Rhizophora apiculata) have two kinds of roots: aerial prop roots that extend from the trunk into the mud, and sprawling underground root systems.

While more is known about the prop roots, accurate measurements for these underground roots systems at the ecosystem level (that is, including all the species in a given area) do not yet exist.

Importantly, the current guidelines used by the Intergovernmental Panel on Climate Change (IPCC) to calculate carbon stocks suggest these below ground roots account for just 10 percent of the total biomass of the tree.

“But we believe this is an underestimate,” says Purbopuspito. “That figure is taken from the measurements of upland forests – and mangroves are different.”

“In wetlands, the soft muddy soils mean that the trees need large underground root systems to support them,” he said.

So Purbopuspito and the joint CIFOR/FORDA team are gathering and measuring every scrap of organic material in a circular plot with a radius of seven metres – one of six plots along a 150 metre transect extending into the flooded forest.

This means cutting the trees down with chainsaws, separating the leaves from the twigs, chopping up the branches and aerial roots, counting debris, and pulling up the underground roots by hand out of the mud.

They then weigh everything on site, and collect small samples of each type to take back to the laboratory, where they will be analysed for their carbon content.

Knowledge gap

With around three million hectares of mangroves, Indonesia boasts an estimated 21 percent of the world’s remaining supply.

In addition to storing carbon, they support endangered animals, including the Kalimantan proboscis monkey; provide nurseries for the fish that feed coastal communities; and protect coastlines from storm surges, tsunamis and other effects of climate change.

But Indonesia’s green fringes are being rapidly lost, converted to shrimp ponds and oil palm plantations.

Unfortunately, despite the threats, the country’s mangroves have not received enough research attention, says Adi Susmianto, director of FORDA’s Research and Development Center for Conservation and Rehabilitation.

“Frankly speaking, mangrove research is a little bit behind compared to other tropical forest issues,” he said.

“There’s not enough expertise in mangrove issues – very few scientists have capacity in studying mangrove ecosystems.”

“Technically also, research in the mangroves needs more resources. It’s an expensive place to do research,” he said.

Senior CIFOR scientist Daniel Murdiyarso is working with the IPCC to develop new guidelines quantifying the storage of greenhouse gases in wetlands – which will hopefully include the results of the Kalimantan research.

Murdiyarso says many nations and policy-makers lack even basic data about mangroves.

“This ecosystem is very much neglected. It has been considered as waste-land, or marginal land – and people tend not to look at it more carefully.”

But, he says, the recent discoveries have shown mangroves are too important to ignore.

“This is urgent,” he said.

“We are losing mangroves very rapidly, so we need to know exactly what’s happening in terms of carbon storage, and how it will affect climate change.”