5th December 1999
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Trees on Horton Plains are dying due to our unchecked air pollution

Death of a forest

By Thilo Hoffmann
About 30 years ago the slopes of Totupolakanda were covered on all sides by montane forests of the cloud forest type, the densely packed gnarled trees becoming smaller and stunted with increasing altitude, with the usual undergrowth of Nillu and associated plants.

There was not a bare patch anywhere, except where rocks protrude. The path would lead upwards in a dark tunnel of thick vegetation, the trees forming a closed canopy, not permitting any view, even from the top of the peak. Seen from above, the canopy of a healthy Sri Lanka mountain forest is as compact as the head of a cauliflower.

What we see now

Today all the higher slopes which are exposed to the south west monsoon have lost this compact forest cover, with only a few dying trees here and there, sticking out of the undergrowth. The trees are bent horizontally by the S-W winds, now skeletons of branches and twigs with the last living leaves in a small bunch at the end.

In the lower regions, at around 7000 ft, the Nillu is very high and closes over the path. Trees, however, are few and far between, all moribund. As one climbs further and the slope gets steeper even the undergrowth diminishes and becomes stunted, and at around 7300 ft. The view opens in all directions.

In between clouds you look down on the plains towards Kirigalpota and beyond, which was not possible 20 years ago. Occasionally you get a glimpse of the Nuwara Eliya basin, but the Uva side is obstructed by moving clouds. 

The entire slope is practically bare of trees, except in places where occasionally the path levels out towards a further ascent. Here, protected from the strong south-west blowing, small stands of trees remain, but they too are visibly weakened. This is also the situation at the top of the peak.

If one examines some of the remaining trees, their weakened, unhealthy condition is immediately noticed. The thick leathery leaves are spotted and often covered with black fungus, and bits along the margin have fallen off. But above all and significantly, most of the growing buds are dead and have dropped off. There are no seedlings, nor is there new growth or re-growth. 

Searching for a cause

Nearly 30 years ago when I first observed the dying of trees on the Horton Plains, and long thereafter, I did not think that air pollution in the low-country of Sri Lanka would be severe enough to cause the damage, especially as industrialisation was relatively undeveloped then, and vehicles were not so numerous. 

I was casting around for other causes, obviously most of them climatic in nature, i.e. changes in light intensity, excessive rain, frequent and severe droughts, extremely low temperature (frost), etc. Some of these factors alone or in combination may play a role and could have accentuated the ill-effects, but the main cause I am now sure is acid-rain (or cloud or mist).

About two years ago I was told (Dr Nimal Gunawardena, pers. comm.) that an automatic weather station had been set up at Horton Plains by a foreign agency in connection with a research project. 

For about three years this station measured at short intervals all relevant meteorologiacal data, including the pH of clouds, condensates and precipitations. It was found that during the SW monsoon the pH of the mist and fog and their condensate was as low as 3.5.

I first noticed the die-back in exposed situations at Totupolakanda and Kirigalpota, but soon it became visible in all parts of the plain, including the edges above Ohiya and at World's End. Today all the forest in and around the plains is affected, and the condition is spreading downwards beyond the rim of the plain. 

It has become gradually, noticeably and visibly worse from season to season, from year to year, until the present, when certain areas are practically denuded of trees. 

When the forest was intact, Baker's Falls could not be seen from the path, only heard; today it is very visible through the thinned out trees. The situation is similar at Hakgala, the entire Pedro range and parts of the Knuckles. 

Negligible factor

At times, mainly due to vandals and inattentiveness of the park staff, fires have swept through the plains during dry periods. In the past such fires did no damage to any part of the forest, not even at the edges. 

A healthy, natural Sri Lankan forest does not catch fire even under extreme conditions, but once the forest is weakened and there is dead wood, the fire spreads into it; this has happened in the Horton Plains, most visibly on a hillock just west of the resthouse.

Thus there are a number of secondary factors, but I think there can be no doubt that acid clouds and rain are the main agents. 

The dying of the forest coincides with the growth of the population, the number of vehicles and of industries.

Daily cooking with firewood or fossil fuel in millions of households as a source of pollution has been generally overlooked.

Destruction mechanism

As a layman I thought that very acid precipitations would directly affect sensitive tissues (e.g. buds) of the sensitive plants and lichens. But I have now learned that the pH of normal rain varies from a low 3.5 to 5 (7 is neutral). 

Acid-rain acts through the soil by lowering the soil pH. This releases free aluminium and other toxic elements which are taken up and damage the roots of plants. 

When the roots are damaged water uptake is curtailed, the trees become weak and more susceptible to other adverse influences, such as drought, pathogenic fungi and insects. These processes are very complex as many diverse factors are involved. The lowering of soil pH, say from 4.5 to 3.5, is a dramatic event which can easily account for the dying of our montane forests.

The main tree species there (e.g. Kina) are obviously very susceptible.

Exposure to the S-W monsoon is also a factor, because in such situations the trees weaken and die first. In sheltered positions the process takes longer. During the monsoon the area is permanently saturated and dripping with moisture condensed from clouds and mist.


A recent study by the University of Peradeniya has confirmed "acid pollution" of soils in the Horton Plains, but no connection was made to the dying of the forest trees. The blame was mainly put on pollutants "drifting from the coal-fired power plants in South India". 

However, there is little, if any, air movement between South India and our Central Hills; throughout the year winds blow mainly from SW to NE and vice versa.

A more likely threat comes from the newly (February/March 1999) discovered "veil" of brown haze of pollutants (aerosols, soot, sulphate, nitrate and mineral particles) which covers about 10 million square kilometres over the Indian Ocean. 

It is assumed that the pollutants from industrial plants and from the burning of biomass are blown out from the subcontinent during the N-E (winter) monsoon and concentrated over the ocean. The S-W monsoon may bring back part of the pollutants and drop them over the land as acid-rain. 

The haze also filters out sunlight which may reduce plant production (photosynthesis) in affected areas. It is, however, unlikely that the dying of our forest trees (which started three decades ago) would be due to this new haze over the sea. One of the German scientists involved in the study has confirmed this.

Real reason

No, the real reason, I believe, is our very own air pollution! The clouds which during the S-W monsoon sweep in low over the coast of Sri Lanka are loaded with toxic fumes emanating day and night from an ever-increasing fleet of vehicles - petrol and diesel-driven- and from factories, the harbour, the refinery, power-generating plants and untold household fires. Large-scale jungle clearing adds to the load.

Effective remedy

The quickest and most effective remedy would be the use of catalytic converters in all motor vehicles. Such converters are compulsory for cars in many countries and have helped to reduce air pollution substantially, but they are irreparably damaged if leaded petrol is used. In Switzerland, for instance, leaded petrol will be a thing of the past, as from the beginning of next year; 96% of current consumption is lead-free. 

The engines of the few remaining cars (mostly old crocks) without converters can easily be adjusted (ignition and carburettors) for use with lead-free petrol, and where additives are available to replace the lead. Thus there is no reason to avoid the use of converters. But in Sri Lanka they are not used because there are no laws and regulations and because the lead-free petrol essential for their proper functioning is - only available at a handful of petrol pumps in Colombo. 

Most of the new imported cars are factory-equipped with converters. In Sri Lanka these are either replaced with through-pipes or allowed to break down rapidly with leaded petrol. 

The hundreds of thousands of cars which clog our roads produce the poison gases which not only cause health problems to the people, but have effectively killed most of our montane forests. The monopoly Petroleum Corporation has failed in its clear duty to provide lead-free petrol and to educate the public. The Government has failed to produce the necessary laws and rules and norms. 

It has been argued that developing countries cannot afford converters which cost a few thousand rupees; but can we afford to lose our watershed forests? In any case, money does not appear to be the problem, judging by the great number of luxury cars on our roads. It is more likely apathy to environmental concerns and lack of awareness.

Urgent need

Strange is the apparent lack of interest by the scientific community in the dramatic changes taking place in our montane forests. There is no visible rejuvenation and the forest may not recover at all. The ordinary visitor to the Hortons Plains probably does not notice the calamity, because of lack of experience with the former intact forest and because of the camouflage effect of the undergrowth, especially of the Nillu, which can reach considerable heights. Monitoring and studies could well have started 30 years ago when the first signs began to appear. Today, corrective measures such as the compulsory introduction of catalytic converters may come too late. 

Given the unchecked destruction of natural wetlands, grasslands and forests, together with the ballooning pollution and general degradation of the environment, our paradise island is only decades away from a major environmental disaster. The tea industry could well become the next victim.

Thilo W. Hoffmann, came to Sri Lanka in 1946. He has a master's degree in agronomy and has been closely involved with the study and conservation of nature and wildlife in Sri Lanka. He has written innumerable papers and articles on wildlife and conservation subjects, been a committee member of the Wildlife and Nature Protection Society for over 30 years, and an honourary office-bearer for 18, including 11 years as President. 

Hoffmann is also an ornithologist and for the last 20 odd years has been the sole Hony-officebearer of the Ceylon Bird Club. He was a member of the 10-man Presidential Task Force which developed the National Conservation Strategy of Sri Lanka (1982-88). He was also Hony. Project Manager of the Mahaweli Environment Project which set up a string of new conservation areas along the river. In 1989 he was elected by UNEP to the "Global 500 Roll of Honour" in recognition of his lifelong honorary work in Sri Lanka. 

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