KARACHI: Once rated the fifth largest mangrove forest in the world with a cover as high as 250,000 hectares a few decades ago, mangroves of the Indus delta now rank lower than 15th on the (global) list and have decreased to 98,014 hectares, indicating two to three per cent annual loss.

No significant success has been achieved to increase its size despite attempts for mass mangrove plantation in the delta.

These observations are part of a paper published in an international journal. Titled The Effect of Global Warming (Climate Change) on Mangroves of Indus Delta with Relevance to other Prevailing Anthropogenic Stresses, A critical review, the paper has been published in July this year in the European Academic Research journalIt is authored by Dr Sye d Mohammed Saifullah, a retired professor of Karachi University`s botany department.

The paper begins with the importance of mangroves and states that the total mangrove cover of the world has been estimated to be 137,760,000 hectares with an economic value of $200,000 to $900,000 per square 1(ilometre and $1.6bn annually to ecosystem services.

`Besides, providing many goods and services to mankind, they also sustain about 80pc of global fisheries and serve as a sink of greenhouse gases.

They fight back global warming through carbon sequestration at a higher rate than any other ecosystems on a unit area basis; it is estimated that as much as 25.5bn tons of carbon are sequestered by mangroves annually, it says.

The mangroves in the Indus delta, itpoints out, are perhaps the most seriously stressed in the entire subtropical belt. Several anthropogenic stresses account for their drastic decline; the foremost among them is the sharp decline in the Indus river discharge into the delta.

`Some decades ago, 150MAF or more water used to reach the delta but now it is almost negligible except for occasional floods during monsoon.

This is due to the construction of several dams and barrages along the Indus river to meet the increasing water demands for agricultural and industrial purposes. Consequently, the alluvial flow of the river, important for mangrove`s growth, has also decreased. These issues have become severe over time,` it says.

These conditions along with massive groundwater extraction, the author says, are contributing to the subsidence of the Indus delta, a phenomenonalso affecting mangroves.

`It (the delta) is sinking continuously and has shrunk to about one tenth of its original size and cannot be restored to its original condition,` he says, citing other data.

Eight species of mangroves have been reported from Pakistan but now only four species occur in the delta with Avicennia marina the most dominant. The others are Rhizophora mucronata, Ceriops tagal and Aegiceras corniculatum.

The first two had almost disappeared from the delta but were reintroduced into the area by Sindh forest department and the International Union for Conservation of Nature (IUCN). Efforts for massive mangrove plantation, however, couldn`t produce significant results.

An estimated 0.54m saplings were planted in 2009 and more than 0.75m saplings in 2013 in the area of KetiBandar. But, the results were not promising and didn`t show significant increase in the mangrove cover.

`A 2015 satellite study showed that the plantations resulted in only 1.6pc increase in the total mangrove cover.

The reasons for this setback were mainly two; plantation was done in June in the midst of southwest monsoon characterised by high winds and high wave action, which may not have allowed proper rooting of seedlings.

`Secondly, only one species (Rhizophora mucronata) was planted, especially during 2009. The survival rate would have been better if A.

marina had been planted along with it.

Monocultures (the practice of growing a single species) are easily susceptible to environmental vicissitudes. Thirdly, the habitat and environmental conditions of the Indus delta do not favour the growth of Rhizophora mucronata, it says.On the effect of climatic changes, the author says, the phenomenon of sea level rise resulting from global warming might be contributing to subsidence of the delta. Citing various data,he argues that the inland retreat of mangroves along Karachi`s coastline in the face of sea level rise might not be possible owing to coastal installations. This won`t be the case, however, on the larger southern western part.

`The species` composition of mangroves will probably remain the same, though their relative proportions of occurrence and zonation may change.

Mangroves are resilient to climatic changes and in several areas (other countries) their growth has increased as a result of global warming.

(Similarly) The increase in temperature, precipitation and carbon dioxide may increase mangrove growth in the Indus delta,` the author says.