23 April, 2008 - The wilting tops of thousands of pine trees, starkly visible along the Thimphu-Paro highway, did not mean the forests were dying, say forestry officials.
“They’re not infected by any disease but withering from natural climatic conditions and changing seasons,” said the renewable natural resource research centre (RNR) programme director in Yusipang, Dr Lungten Norbu.
RNR officials explained this occurrence as a periodic “dieback”, a condition in trees where treetops or their peripheral parts die, due to climatic conditions. The phenomenon was first observed along the Wangchu-Pachu valley in 1994.
RNR officials in Yusipang, who have been studying this phenomenon among blue pine and cupressus pine species planted along this particular belt since 1994, said that the leaves of these trees turned brown because the soil on which they were planted did not favour their survival.
Visible symptoms of dieback, they explained, appeared between March and May and disappeared during the monsoon.
“It’s seen along the dry slopes of Babesa, Khasadrapchu, Chunzom along Wangchu and Jangsa along Pachu at altitudes between 2,100-2,300 metres above sea level,” said the centre’s entomologist, D B Chhetri, adding that the entire belt of trees was hit by a similar die-back in 1996, 1999, 2001, 2003 and 2008.
The entire Thimphu-Paro stretch (497 hectares), according to ecologist, Dr Pema Wangda, was barren, including the hill above Samaizingkha in Thimphu. “These trees were planted in the early 80s,” he said, adding that then more than 50 percent of those planted trees died. The forestry department replanted them.
While RNR officials are yet to establish a definite cause, they pointed out a few possibilities out of many for these trees wilting.
D B Chhetri said that the problem was confined to south, southeast and southwest facing slopes along the Wangchu-Pachu valley, which are affected by solar radiation, temperature, precipitation and soil formation.
“These affected sites have a superficial soil layer, which are sandy or rocky, affecting the trees’ ability to hold water,” said D B Chhetri, adding that rocks beneath prevented primary roots from penetrating into the earth, leading the roots to spread on the surface.
Under such circumstances, during cold dry weather, abrasion from wind-blown ice crystals removed the wax coating from the needle-like leaves of the pine trees, and the frost chilled the soil and the moisture within.
As a result, certain parts of the tree died from excessive transpiration, while the roots struggled for moisture from the frozen soil and failed to replace the lost moisture.
“In the first place, the tree species planted by the forestry department were not checked whether or not they would survive the adverse affects of the weather in that area,” said D B Chhetri. “Many were saplings found in the country’s different zones and a few were exotic trees brought from outside the country.” Some trees, he added, were those that required about 1,000 mm of rain annually.
RNR officials also found that only the trees on the peripheries were hit by dieback, while those in the interiors were unaffected because they were protected from direct exposure to cold wind and solar radiation.
While the RNR officials are still studying the ring growth of some of the sample trees to find out if they could correlate past climatic patterns with the present and monitoring extreme climate conditions in the affected sites, they said it was better to let nature take its own course to mend these affected sites.
The surviving branches below the wilted treetops, they said, rejuvenated and grew up to form the tree’s crown again.
Dr Lungten Norbu pointed out that the stretch lay on a solid rock foundation and since there had been no landslides or erosion, it would best be left untouched.
“There’s no point in planting new trees which entails heavy expenses,” said Dr Lungten Norbu. “Rather leave the area on its own and free from grazing. Should the soil improve, it will allow some natural growth.”
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