Forest die-back overwhelms Sri Lankan Montanes

by Sajitha Prematunge

For decades, Trees of all age groups, classes and sizes in Tropical montane forests like Horton Plains and Haggala, have been dying due to unknown reasons.

This is cited by many names in many categories such as canopy die-back, forest decline, species level die-back, cohort senescence, stand level die-back, in German "waldsterben" (forest dying), "waldschaden" (forest damage), and "neuartige waldschaden" (new type forest damage) but is primarily known as forest die-back. During the "Decline" stage, as indicated by the names, the forests lose their vivacity, trees cease growing and leaves discolour and fall off leaving a bare framework of branches. This could either lead to die-back or cause what's called stag headedness.

As suggested by the names it's not restricted to Sri Lanka; forest die-back has become a major problem for countries like North America, Hawaii, Croatia, Germany, Russia, and New Zealand.

When it was first discovered in Sri Lanka in the 1960s the research community came up with numerous hypotheses such as low absorption of nutrients by plants, the dropping of ground water table, decease, acid rain, damage caused by Sambar, climatic change and ultimately lead toxicity.

Other than in Horton Plains and Haggala, Pidurutalagala ridge, Kobonilgala near Corbet's gap in Knuckles have been pronounced as affected by forest die-back. First observed in the western slope of Thotupolakanda in 1978 this version of die-back has only affected trees of the Calophyllum genus. It was pronounced as due to severe drought, since the plant community recovered after the rains.

Die-back has been defined as the possible result of various deceases. Researchers state that plant populations of such large scale have been known to succumb to deceases only after a stress factor, like prolonged droughts, excess rain, high winds, moisture, fungal or insect infestations, takes hold.

Die-back is most severe in the face of the wind and it's believed that these, together with wind and fog, operate as facilitators making way for deceases that are incapable of attacking healthy tissue.

Two of the deceases that are reported to afflict the montane species of Sri Lanka are "Pink decease" and "Stripe canker decease" named for its' major symptom of bark peeling off in stripes. Two out of the four plant species that contracted the "Pink decease" were endemic. However in both cases the effects were slow and therefore not aggressive.

More over the ecosystem is naturally designed to keep the level of pathogens on check, preventing it from becoming an epidemic. Therefore could not be considered as a contributory cause of die-back.

"A study of phytosociology and forest health" by Professor N. K. B. Adikaram (for "Forest Fire-back in Horton Plains National Park" - Global Environmental Facility Project Management Research Grant) in the Horton Plains prove that 38% of all plant species in the plains are either dead or dying.

Areas with severe die-back were the western slope of Thotupolakanda: the wind face. The peaks of the area are frequently covered with fog and this is known to be another instigating factor for die-back. Although some plant species of the world are found to be highly susceptible to die-back, the studies conducted in Horton Plains show that forest die-back is not species specific.

In this sense Horton Plains is unique. Unfortunately though, 37 species of all species studied, were identified as die-back prone and 26 (70%) of this are endemic to Sri Lanka!

Die-back can be highly fatal, when a cluster of trees of the same species contract it. The whole cluster could die at the same time. Syzygium species and C. walkeri in Thotupolakanda are examples for such clusters and they have literally disappeared from the area.

In nature plants have to continuously fight for space, water, light and nutrients and the strongest always wins. Especially since primary species are known to be more susceptible to the die-back phenomenon, when primary species die secondary species are there to take over. The result: a forest consisting mainly of secondary species.

Therefore the prevailing forest cover could be much different from what used to be and it keeps changing since even the secondary species are not immune to die-back. And the picture of future forests could be much diverse.

The Sambar is an animal of the forest and is 11 animals per sq.km strong at Horton Plains. "Damage by herbivores, seedling regeneration and Extent of Die back" studies by Mr. K. B. Ranawana have been conducted on the Sambar ecology, damage caused to vegetation and its' effects on die-back. Most significant effect they have on Horton Plains plants are bark damage.

Sambar eats the bark of many montane plant species and this causes the rapid decomposition of the trunk. Most of the plant species highly preferred by the Sambar are the trees most susceptible to die back. Therefore a connection between die-back and Sambar damage is a possibility.

The damage to bark of Microtopic zeylanic, which has been recognized as a very rare species, is cause for much concern among the researchers. More over it also browses on creepers, grasses and seedlings and saplings of shrubs. Therefore regeneration could be very slow. But irreversible and lasting damage could only be done if there is an over population of Sambar and Sambar over population does not always correspond with the areas of forest die-back.

The hypothesis on die-back due to dropping of ground water table has been proven unlikely because it could not happen in such large scale every where forest die-back was taking place.

The rate at which the forests are succumbing is too fast to be interpreted as a result of a natural cycle of climate change. Leaving the only plausible explanation; as high amounts of lead in the soil. The major obstacle in proving this theory is that there is no data indicating the level of lead that could cause die-back, neither in Sri Lanka nor the world.

The case study "The relationship between soil geochemistry and die-back of montane forests in Sri Lanka" by P. N. Ranasinghe Geological Survey and Mines Bureau, C. B. Dissanayake, Department of Geology, University of Peradeniya, D. V. N. Samarasinghe Coconut Development Authority and R. Galappatti, with the backing of their field samples measuring the level of lead in trees and soil, shows that lead amounts in the soil is indeed higher than usual specially in the wind face of the forests.

This coincides with the characteristics of die-back, since die-back is most acute in the wind face. Both Horton Plains and some parts of Haggala were sampled for nutrients, toxic elements and Physical parameters such as moisture content, and conductivity.

A high concentration of Iron, Manganese and Aluminum detected in the soil can be toxic to some species. More over high amounts of iron reduces the amount of Nitrogen absorbed by plants. These scenarios could have been made worse by the detected acidity of the soil. These factors can instigate die-back, but can not be the sole causes.

On the other hand Led is a highly toxic element. Especially in the ridges of Thotupolakanda and Kirigalpoththa, where die-back has done major damage, led levels are significantly high. The amount of Led in trees of Haggala are 15-30 ppm (parts per million). The study shows that the source of such high amounts of Led can not be of a natural sub surface origin.

The most plausible explanation seems to be that led generated by industrial activities in the south-western cities of the country, was transferred to the forests of Horton Plains and Haggala by strong South-western monsoons. Although some, question how a heavy metal such as led could be transferred by monsoons to the destinations in questions. But in reality the distance between the western coastal cities of Sri Lanka to Horton Plains and Haggala is only about 50 kms in a straight line.

Similar studies in led levels were conducted in different parts of the country and the results indicate that the led levels of Horton Plains is as high as the levels in towns close to main roads, like Pannipitiya and Dombagaskanda.

This supports the hypothesis that led is a key contributory factor for forest die-back. Since the plant species found in Horton Plains and Haggala are mostly endemic, they have a greater risk of succumbing to the effects of foreign elements. The researchers suggest that more of the research conducted, should focus on the source of Led.

Human beings rarely think of the consequences of their actions until they are too late. Many species have become extinct and the world endures irrevocable damage due to Mans ignorance.

There is no remedy for forest die-back. But most of the evidence collected by the researchers point to industrial pollution as the culprit. If we know the cause it's high time we tried to reduce its' severity, if not completely terminate it.

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