M. M. Khan1 and M. H. Khan2
1Punjab Forestry Research Institute,
Faislabad, Pakistan
2Pakistan Forest Institute, Peshawar, Pakistan
Pakistan has 4.5 million ha or 4.0% of the total land area of 87.98 million ha under forest. The forest resources, though meagre, contribute substantially to the national economy. Forestry sector employs half a million people and provides 3.5 million m3 of wood to meet the industrial requirements and almost one third of country's energy needs. Furthermore, forests and rangelands support about 30 million livestock, which contributes over US$ 400 million to Pakistan's annual earnings. The GDP contribution of the forest is approximately US$ 55 million annually.
Pakistan is not self-sufficient in timber production. The current annual production is around 14.5 million m3, and this is likely to decline as a result of population pressure, skewed land use practices, indiscriminate tree cutting, and high incidence of grazing. The present annual timber consumption is approximately 29.5 million m3 and is expected to increase to 50 million m3 by the year 2018. At present Pakistan imports 300,000 tonnes of wood pulp, paper and paper board products which are costing US$ 90 million annually, and this is expected to increase in the future.
The per capita consumption of fuel wood is 0.5 m3 per year and at present it is met from local sources. With a 2.7 % annual growth in population, traditional source of fuel wood will be insufficient. This is expected to pose a serious threat to the remaining forested land, especially the vulnerable upland watersheds and biodiversity conservation areas.
Dalbergia sissoo, known as shisham or tali in Pakistan, is renowned for its high wood quality, in terms of grain, texture, hue, tensile strength and other physico-mechanical. properties. The wood is also highly prized for heating and cooking due to its high calorific value. It is a very important raw material for a variety of wood-based industries, especially furniture and housing. It is now planted throughout the South Asian subtropical region, and extensively so in India, Pakistan and Nepal. No other timber species, except teak, is so extensively planted in India and Pakistan.
Sissoo was introduced in Pakistan in mid-1800s. In Attock, Rawalpindi and Hazara districts it occurs as a low tree at an altitude of 1500 m. In the Pothowar plateau, where protected, it grows into fairly large trees. In Islamabad it has profusely regenerated itself through root suckers and has flourished well due to protection. It has also been recorded in Jehlum and Gujrat districts along streams and fresh alluvium. In Baluchistan province sissoo occurs in the warm area of Harnai, District Sibi, along permanent and seasonal streams. In the irrigated tract of Punjab it is widely cultivated along roads, rail sides and canals as linear plantations.
Sissoo is also an important species used in agroforestry. The farmers in Punjab have been planting sissoo since decades in linear form around the fields. It is even retained in the agriculture fields, in isolation, for shade purposes. In the state forests it is planted extensively in irrigated plantations of Punjab, and in riverine sites in Punjab and Sindh.
As mentioned above, the major tree species in the irrigated plantation of Punjab is sissoo. In the mid-1900s these plantations were established at various sites in Punjab, mainly for production of fuel wood for steam engines – all of these plantations are located on the main railway lines. Some of the major plantations are Changa Manga, Daphar, Perowal, Bahwalpur, Kundian, Kamalia and Chichawatni. At present, the area under such plantations in Punjab is 154,886 ha, with an average annual production of 28,000 m3.
The plantation establishment techniques, from raising of seedlings to harvesting and utilisation have been worked out. In the irrigated plantation, stumps raised from 1-year old seedlings are planted. With a spacing of 10 x 6 m, 1740 stumps are planted in each hectare. After 5 years, the crop is mechanically thinned by removing alternate plants. The second thinning is selectively done after 10 years, followed by the third sanitation felling in the 15th year. The main felling is carried out after 22 years, by retaining approximately 36/ha as standards for future timber and seed production. It is the sissoo timber which is the main source of revenue of the Punjab Forest Department. The average sale price is around US$ 150/m3. These trees are removed during the successive main felling following which a few trees are left to be felled after 60 years. In some plantations Mulberry is being raised as an understorey crop with sissoo. The combination of both the species has become a great success, especially in Changa Manga Plantation.
The symptoms of die-back in Dalbergia sissoo begin with the thinning of leaves and crown, drying up of the ends of branches, table topped conditions, and stagheadedness in extreme case. Small dry twigs keep on falling continuously and the tree looks blunt stubbed, containing thick branches. There could be many causes leading to this situation. The species has grown for decades away from its natural habitat. It also undergoes heavy lopping and debarking for fuel wood on farmlands, and is grown in linear and compact public plantations. It is often raised in conditions where salinity, water-logging or acute shortage of irrigation water occur, and this can lead to physiological stresses making the tree vulnerable to various diseases.
Incidence of sissoo die-back particularly in central Punjab has been a matter of serious concern to foresters and farmers since its proliferation in recent years. Though the outburst has not attained epidemic proportions, yet the sporadic attacks have caused a great deal of damage and hundreds of trees have died on private farmlands. Punjab Forestry Research Institute at Faisalabad launched a systematic broad-based study to find out the possible causative agents and to prescribe preventive and curative measures.
The species, not being indigenous to the arid and semi arid regions, has gone through stresses ranging from faulty agricultural and irrigation practices to poor soil working, rendering the soil profile hard and compact. Lack of aeration, shallow root system, greater lateral root extension, temperature effects and sudden and greater fluctuations in moisture conditions could have further induced stress. The result has been deadly.
Mycologists have recorded some 62 pathogenic species of fungi in sissoo. No record of bacterial or viral pathogens has ever been reported. Most of the fungi have been studied from mycological point of view and a little has been mentioned on pathological aspects (Khan 1989). Ganoderma lucidum, a root rot, has been attributed to be the primary cause of stag headedness of sissoo, with secondary cause from pinhole and long horn beetles. Under stress termites attack the root system to accentuate the die-back. The study at Pakistan Forest Institute (Khan & Gul 1998) has verified the root rot fungus, G. lucidum as the primary pathogen, and insect borers which attack trees already under stress, as secondary invaders.
Polyborus gilvus and Fusarium oxysporum cause root rot and wilt, respectively. The cracks and galleries may be flocked with hyphae attacking wood of dying trees in association with some other causes. Poria ambigua, another root and butt rot fungus is widely identified associated with Sinoxylon beetles. The tunnels are with the fungus growth. Rhizoctonia a soil fungus, has also been reported as causal agent to the root system of sissoo at higher soil moisture levels (Khan 1989).
A similar study conducted by a panel of multidisciplinary experts at UAF (1998) has indicated pinhole bark beetle (Coleoptera: Scolytidae) as a primary disease causing agent, and the long horn beetle (Coleoptera: Cerambycidae) as a secondary invader in association with saprophytic fungi in the galleries.
Xyleborus spp (Coleoptera: Scolytidae), a pinhole beetle, both of sapwood and heartwood with larvae feeding on Ambrosia fungus, has been identified as a causal agent in the plains of Punjab (Tiwari 1994). Batocera and Dorysthenes spp (Coleptera: Cerambycidae) have also been reported as root and bark feeding insects particularly on unhealthy and decaying bark throughout' the plains of India and Pakistan. Eggs are laid under bark and on wounds of stem, shoot and twig. Emergence is most common in May, June, and July. Batocera rubus beetle usually emerges in March-April. Life cycle for Batocera spp varies from 1 to 2 years while that of Dorysthenes spp is 3 to 4 years or even more. The leaves of the attacked trees turn yellow and the tree dies slowly after 2 to 3 years. Agrillus dalbergiae (Ismail & Shah 1974) and Perissus dalbergiae have been reported as causing girdling of the stem particularly in the northern part of the Indian sub-continent (Tiwari 1994). Since the pathogenic insects complete a phase of their life cycle on the host tree, the damage caused is usually reversible on reduction of their numbers. However, damage caused by fungi is insidious and usually irreversible (Khan 1989).
The broad-based diagnostic study was laid out in the five agro-ecological zones of Punjab as classified by Pakistan Agricultural Research Council (PARC). The selected agro-ecological zones/sub zones were: I) III-A & B – Sandy Deserts; ii) IV-A – Northern Irriigated Plains; iii) V – Barani (Rainfed) Lands; and iv) X – Sulaiman piedmont (Map). The following areas were selected for each of the agro-ecological zones:
The agro-ecological sample sites representative of different soil conditions were identified and subsequently about 20 samples of roots, soil from rhizosphere, collar bark, affected shoots and larvae/grubs from the affected trees were collected. The identification and laboratory analysis were done at NIAB University of Agriculture, Faisalabad (UAF) and Ayub Agricultural Research Institute (AARI).
The following possible causative factors were investigated:
During the course of site identification and sample collection, a preliminary reconnaissance survey taking the advantage of Rapid Rural Appraisal (RRA) and participatory Rural Appraisal (PRA) was also simultaneously conducted covering an area of about 2 % of the total land mass of the province.
Control measures were to be applied primarily as a diagnostic approach and confirmatory backup in the representative sites of the study area. Biological treatment with antagonists, Trichopak and Asperpak, was applied at selected sites.
Rapid Rural Appraisal revealed that D. G Khan canal in Piedmont agro-ecological zone was most severely affected. Sandy deserts and rainfed zones were least affected while irrigated public plantations were mostly free from the disease. The damage appeared to be severe in the central part of the irrigated plains of the Punjab. The symptomatic damage approximated to about 20 % on average, ranging from 10-40%, while unsymptomatic infection may be higher in proportion.
The initial investigations have identified Phytopthora, Fusarium, Phoma and Botrytis fungi as probable causal agents. The pathogenic confirmation is under trial at AARI. It may take longer time to establish pathogenicity, partly due to the woody nature of the species. Depletion of organic matter and phosphorus contents of the soil seemed to have an inverse relationship with the spread of the disease.
Biological treatment with Trichopak and AspergoPak was simultaneously applied. This resulted in a significant response only at the early stage of the disease. Physical and silvicultural prophylactic measures including isolation trenching around the trees to inhibit any zoosporic activity activating the antagonistic fungal growth in the rhizosphere, planting an admixture of species, and appropriate subsequent siliviculatural management practices remain the only viable economic option to restrict the incidence of sissoo die-back. Other edaphic parameters including soil pH, Total Soluble Salts (TSS), organic matter, saturation percentage, soil texture and general nutrient availability have not shown significant relationship with the incidence during the preliminary investigations. The level of organic matter in most of the farmlands was low, indicating possibility of poor nodulation.
The initial investigations have identified the presence of Phytophthora spp., Fusarium spp., Phoma sp. And Botrytis sp. Pathogenicity trials are presently being experimented at AARI. The pathogens seem to have been activated due to decreased useful antagonistic microbial activity as a result of faulty irrigation and agricultural practices in the form of excessive use of fertilisers and pesticides which created favourable microenvironment for proliferation of the disease.
Biological control measures presently seem to have no impact, especially since it is at a late stage of the infection. The chemical control measures may not be environmentally desirable and economically feasible in the long run. IPM measures shall be administered on confirmation of the pathogenicity.
The physical and silvicultural preventive measures, including isolation trenching around the trees, planting admixtures, and appropriate cultural operations and silvicultural management remain the only economically feasible option to restrict the incidence of die-back.
The introduction of a suitable substitute, Dalbergia latifolia (rosewood) would require prior ecological adaptability trials. It is being tested at PFRI.
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