0682-B4
A.S.T.B. Wijetunga 1 and S.P. Nissanka 2
Forest degradation is one of the main environmental and social problems in Sri Lanka. Pinus was introduced to Sri Lanka about a century ago, because their ability of growing on degraded sites was well understood. However, there have been so many unexpected consequences and complains after plating them. One of the major problems is very low undergrowth in Pinus plantations, due to a number of reasons.
A preliminary study was conducted to compare the Vesicular-Arbuscular Mycorrhizal (VAM) associations of ten selected plant species found as understorey species common to Pinus plantations and semi natural forest at Hantana Range, Sri Lanka. Pinus plantations at the selected site were surveyed to find out the under storey plant species. First 10 dominant species were selected based on their Important Value Index and screened for VAM associations. Mean Percentage Occurrence of VAM was compared with the same species from adjacent semi-natural forest to see whether there was a significant difference between them.
Vesicular-Arbuscular Mycorrhizal (VAM) associations were found in all the ten plant species studied from pine plantation habitat as well as from semi natural forest habitat. Mean Percentage Occurrences of VAM for all the ten species studied from Pinus plantations were significantly different when compared with the same species from the semi natural forest, and above values were significantly lower for all the species from Pinus plantations than the same species from semi natural forest.
According to the results of the previous studies on mycorrhizae, it is clear that the mycorrizal associations increase due to stress and adverse conditions. However, the results of the present study do not agree with those. This may be due to allelopathy effect of Pinus roots on mycorrhizae or due to some other reason or combined effects of several reasons.
It is widely accepted that deforestation is one of the main environmental and social problems in Sri Lanka. Until the turn of the 20th century, Sri Lanka was almost entirely covered by natural forests. Since, then the closed-canopy natural forest cover has dwindled rapidly, from about 80%, until in 1992 it was less than 24%. In view of the increasing demands placed upon the forestry sector, its diminished capacity to meet the needs of the people has become a major problem, which is being felt both at national and local level (Anon 1995a and Anon 1995b).
Despite past and continuing forestry activities, there is still net deforestation, with severe implications for the environment, the welfare of the rural people, agriculture, and other inter linked sectors, and the entire economy of the country.
As Anon 1995a and Anon 1995b state, the most serious consequences of deforestation and forest degradation are: reduction in bio-diversity due to destruction of wildlife and plant habitats, irregular water supply, shortened life spans of irrigation channels and dams, soil erosion and associated loss of soil fertility, reducing agricultural productivity in many regions, increased scarcity of wood, resulting in rapidly increasing prices paid by wood-using industries and consumers, fuel wood scarcity in some localities and contribution to greenhouse gas emissions. Accordingly, the overall impacts of forest degradation can be summarized simply as reduced well being of the present and future generations.
The forest cover in the Wet Zone of Sri Lanka (lowland and upcountry including Mahaweli catchment area) had dwindled to a critical level, according to the estimates made in 1979 (Bandaratilake 1989). Some studies have indicated that average annual transport of sediment in the river Mahaweli at Peradeniya for the period between 1950 - 1982 was around 486000 tons/year or soil loss of 417 tons/km2/year with respect to the Mahaweli catchment area above Peradeniya (Bandaratilake 1989). With the commencement of Mhaweli Development Scheme, the need to arrest this alarming situation was considered very urgent and of high priority. Therefore, it was recognized that some kind of vegetative cover, which could survive and grow under the adverse conditions of these sites, was immediately required to protect the soil from erosion and bring sufficient soil improvement to permit the establishment of vegetation of better composition.
Pinus caribaea and P. patula were selected as the main tree species in upcountry watershed reforestation programmes according to their remarkable ability of successful establishment and growth under the adverse conditions in lower elevations (P. caribaea) and higher elevations over 1223 m (P. patula). Pine planting was also extended to steep slopes, ridge tops and other inaccessible areas. As a result of this, over 11,736 ha of Pine plantations have been established since 1978 in the upper Mahaweli catchment areas in Kandy, Nuwara Eliya and Badulla Districts. Up to 1987, a total extent of 28,715 ha has been reforested with Pine species, mainly Pinus caribaea that occupied around 27,115 ha (80%), in Wet and Intermediate Zones of Sri Lanka (Bandaratilake 1989).
Pine plantations at Hantana Range are also a result of those reforestation programmes. Pinus caribaea plantations have been established on approximately 98 ha, in Hantana Range (i.e. 23% of the area). These plantations were established within the 1980's. There is a little regeneration beneath the Pine plantations; particularly they remain without thinning. In some areas broadleaf species have become established but have, for the most part, been suppressed by heavy shading. Annual fires through these plantations also inhibit regeneration (Bird 1995).
It is very clear that Pinus species are very popular tree species, is used in forestry, as they are fast growing trees under low nutrient and water stress conditions, i.e. specially at degraded sites or under poor site conditions. It is found that this ability is mainly due to the ectomycorrhizal associations and due to the deep tap root system of these species.
Many studies have been carried out on ectomycorrhizal associations of Pinus, in temperate regions as well as in tropical regions all over the world. Studies on such associations in Sri Lanka are restricted to a few. Adequate information on mycorrhizal associations of Pinus undergrowth species or associated species is not available in Sri Lanka. Only, a survey of endomycorrhizae in the unbernt 9 years old P. caribaea plantation and the lowland rain forest has shown that some species of endomycorrhizal genera occur in both sites. This indicates that the soil inoculum of endomycorrhizae similar to those found in the natural forest is available in the Pinus plantation for inoculation of broadleaf endomycorrhizal hardwood species, which may grow together with Pinus (Gunatillake 1989).
Research on mycorrhizal associations of plant species grown together with Pinus is very essential, because it can be assumed that if such associations are exists and they are very important for their regeneration, establishment, growth and development and survival under so called harsh and unfavourable conditions of Pinus plantations. If such associations are available, it may be very useful as an easy tool for future forestry programmes. Because, inoculation of seedlings with particular fungal species can be done at nursery stage to form micorrhizae and then, they can be introduced in forestry programmes like replacement of Pinus with indigenous forest flora, integration of other species with Pinus and other management practices etc.
Accordingly, a preliminary investigation was conducted on micorrhizal associations of some under storey plant species of Pinus plantations with the following three specific objectives; survey of the Pinus under storey species of selected sites, study of VAM associations in some selected under storey species recorded from the survey and comparison of these species for VAM with the same species exist as under storey species in semi-natural forest. For this study, some sites within the Pinus plantations and semi-natural forests at Hantana Range, Sri Lanka, were selected.
The Study Site: Hantana Range of Sri Lanka consists of a series of hills separated by valleys extending from south of Kandy to Galaha, more or less parallel to the Mahweli Basin. Hantana catchment forest is approximately about 432 ha and about 4 miles WSW of Kandy in the Kandy District in the Central Province of the beautiful island, Sri Lanka. The elevation ranges from 518 m - 1,100 m and is underlain by rocks of the Highland Series. Soils are reddish brown latasols. Climate can be characterized as hot humid and sub-temperate. The mean annual temperature is 24.1o C. The mean annual precipitation is 2,131 mm and rainfall is received from both North East and South West monsoons and is evenly distributed throughout the year.
In 1800, Hantana area was planted with coffee. Then it was converted to tea plantations and rubber plantations later. In 1950's the University of Peradeniya (University of Ceylon at that time) purchased this estate. Tea was cultivated on lease for sometime and later the area was allowed to regenerate naturally.
Out of 432 ha in Hantana Range, 253 ha are under forest cover. Around 132 ha are under semi-natural forest and the rest consists of grassland and Pinus plantations. The current status of the forest cannot be termed natural, as it has been cleared of natural vegetation for about 150 years. The semi natural areas although established over the last 50 years are exhibiting a fairly rapid succession towards natural vegetation. Some exotic species such as Alstonia macrophylla, Paraserianthus falcataria, Cedrela toona and Swietenia macrophylla together with indigenous and some endemic species are found in the forest.
The grassland and Pinus plantations are constantly set on fire during the dry spell of the year which between January to March. Fires were not confined to the grasslands and Pine plantations but were observed near the remaining forest patches also. Fires at Hantana Range are mostly due to anthropogenic activities (Goonetilleke 1996). These fires cause severe destruction to the forest (especially to the under storey) as it may retard the natural regeneration and establishment of vegetation, especially under Pinus plantations. Because, Pinus leaf litter rich with resinous ignition matter also subject to the fire very easily and it may further destroy the soil seed bank.
Vegetation Survey: Thirty-eight 1m x 2m quadrats were surveyed for under storey species in the Pinus plantation at Hantana Range. Suitable quadrat size, i.e. minimal area of the quadrat for vegetation sampling at this site was determined according to the method described by Shimwell 1971. Samples were taken randomly from selected sites at Lower Hantana, Upper Hantana, Pudikattu, Mahakanda and Sarasavigama in order to represent the various parts of the Hantana Range under Pinus plantations. Differences in elevation and slope were considered to select sampling points within a site to avoid if there were any variation existed in vegetation as Pinus plantations are found on different elevations and slopes on Hantana Range.
Each plant within each quadrate was counted and most of them were identified at the site. Others were identified using standard texts for the flora of the country and herbarium specimens kept at National Herbarium, Peradeniya, Sri Lanka.
Basic ecological parameters such as Relative Density (RD), Relative Frequency (RF) and Important Value Index (IVI) were calculated for each species. To calculate IVI, only RD and RF were considered.
Important Value Index = Relative Density + Relative Frequency
Study of VAM Associations: Ten plant species were selected for screening the presence of VAM, based on their IVI rank (i.e. first 10 species with highest IVI), as considering the dominance. Fifteen individuals from Pinus plantations and another 15 from semi-natural forest were screened separately for VAM, from each species those which were selected according to their IVI. Each plant was uprooted and entire root system was removed from the plant and brought to the laboratory in sealed polyethylene bags separately. They were washed properly to remove soil particles, with running tap water without damaging to the fine rootlets. Then, 15 or more rootlets were fixed and stained according to the method described by Phillips and Haymen 1970. Fifteen stained root segments from each plant were examined for VAM. Presence or absence of VAM was recorded for each rootlet of each plant. For each plant, Percentage Occurrence of VAM was calculated as follows:
The mean value for 15 plants of each species from Pinus plantations and mean value for 15 plants from semi natural forest were compared using pooled t-test to see whether there were significant difference (p = 0.05) of Percentage Occurrence of VAM between plants from two habitats for all the ten species selected to study VAM, separately.
Under Storey Vegetation of Pinus Plantation: All together 923 individual plants were recorded within 38 quadrats sampled. Forty-five different plant species belonging to 41 genera and 25 plant families were identified. According to Wijesinghe 1994, 13 plants species were classified as woody perennials.
Out of 45 plant species recorded, 69% of them were woody species and the rest (i.e. 31%) was herbaceous species. Eighty percent of the plant species were perennial forms and the others were annuals (i.e. 20%). Considering both woody/herbaceous nature and perennial/annual nature of the 45 plant species found, they can be further categorized into three groups, viz. woody-perennials, herbaceous-perennials and herbaceous-annuals. Percentages of the species belong to each category are 31, 49 and 20 respectively. Most of the woody perennial plant species found during the study were in their seedling or sapling stages. Mature individuals of this group were very rare.
The first ten dominant plant species that were selected to study VAM associations based on their IVI (i.e. first ten species show highest IVI) are given in Table 1.
Table 1 - Plant species selected to study VAM associations
SPECIES FAMILY
1. Panicum maximum Poaceae
2. Psidium guineese Myrtaceae
3. Eupatorium odoratum Asteraceae
4. Hedyotis nitida Rubiaceae
5. Axonopus compresses Poaceae
6. Wikstroemia indica Thymaliaceae
7. Eleutheranthera ruderalis Poaceae
8. Mimosa pudica Fabaceae
9. Vernonia cinerea Asteraceae
10. Ageratum conyzoides Asteraceae
Occurrence of VAM Associations: All the ten plant species studied from both Pinus under storey habitat and semi natural forest habitat were associated with Vesicular-Arbuscular Mycorrhizae (VAM).
The Mean Percentage Occurrence of VAM in all the plant species studied from Pinus under storey were significantly different (p = 0.05) and significantly lower (p = 0.05) than those of all the respective plant species from semi natural forest under storey.
Out of 45 plant species recorded in Pinus plantations as under storey species at the study sites only 13 were woody perennials. Rest was herbaceous, and most of them were annual life forms. Almost all the species found were early successional species (i.e. pioneers). All the woody perennial species found were at their seedling or sapling stage, except Psidium guineese and Wikstroemia indica.
Entire above ground vegetation as well as the underground parts are subjected to fires during the dry spell (first quarter of the year). Number of species such as Axonopus compresses, Centrocema sp., Cucurligo orchioides, Cymbopogon nardus, Cyperus spp., Dioscorea sp., Eupatorium odoratum, Gloriosa superba, Hemidesmus indicus and Panicum maximum regenerate to a certain extent after fires with the commencement of rains, as they have underground parts. Some plant species are seems to be fire resistant. Psidium guineese and Wikstroemia indica always regenerate even after heavy fires, because they may have the ability of root cloning due to fire damage or due to their fire resistant underground root stocks or may be due to both reasons. However, it is very clear that these two species always survive and regenerate after annual fires and they can grow to a certain height and can achieve a mature stage within a one year period, but not to the reproductive stage.
Most of the herbaceous annual plant species recorded during the study bear light fruits or seeds and therefore they can easily be dispersed by wind. Therefore, it is very easy to spread those species from the adjacent semi natural forest or from roadside vegetation to the pine plantations annually. Seeds of some species may rest in the soil seed bank for a time and regenerate after fires, as some of them may need fire to break their dormancy. After fires, thick resinous pine leaf litter layer get burns out and expose the soil and this facilitate the contact of seed with soil to germinate. On the other hand, it allows the sunlight to contact with the soil and this may helps to break the dormancy of some seeds in the soil seed bank.
However, according to the results of the survey, the under storey plant community of pine plantations at the selected sites was very poor in number, diversity and growth conditions and as stated earlier, most of the plant species were at their seedling or sapling stage. Only a very few mature plants were recorded.
Results of the micorrhizal study showed that all the 10 plant species studied from both Pinus under storey habitat and semi natural forest habitat were associated with Vesicular-Arbuscular Mycorrhizae (VAM). But, the Mean Percentage Occurrence of VAM in all the plant species studied from Pinus under storey were significantly different and significantly lower than those of all the respective plant species from semi natural forest under story. This may be due to some reasons such as the allelopathic effect of Pinus roots on micorrhizal fungal species by the Pinus roots, any harmful effect of Pinus ectomycorrhizae or may be due to some other reasons.
According to Rice 1984, under aseptic conditions (in vitro), aqueous extracts from the inner bark of roots of old Pinus radiata trees inhibited the growth of mycorrhizal fungus (Rhizopogon sp.) and caused root necrosis and wilting of P. radiata seedlings. Accordingly Pinus root exudates can be inhibit the mycorrhizal associations of other plant species and other type of mycorrhizal associations such VAM.
The mycorrhizal associations may be essential for the germination, growth and survival (regeneration and establishment) of these plant species under the unfavourable conditions of Pinus plantations. As this is a preliminary investigation, the above statement cannot be generalized at this stage. However it gives some positive clues for future investigations. At this stage, it can be stated that the control experiments are the necessary to draw such conclusions. Further studies are needed to see whether the VAM associations recorded in the study are obligate or facultative. Studies on other types such as ectomycorrhizae and resting spores of micorrhizal fungal species are needed to get a better idea of the above phenomenon.
As stated earlier, not only the allelopathic effect, the fire may be the main factor to control fungal growth in Pinus soil as fires destroy fungal propagules. Micorrhizal fungi also may face this problem and this may be a reason for the low micorrhizal associations in Pinus under story species. After the dry spell, due to heavy rains in April onwards, most of the fungal spores survived in poor structured and highly exposed soil under Pinus plantations on slops of the Hantana hills subjected to heavy runoff and they removed from the original habitat. This may be another reason why VAM associations are lower in Pinus under storey species than semi natural forest under storey species as semi natural forest at Hantana is much healthier when compared with Pinus plantations, specially the soil.
The general belief is that "The mycorrhizal associations and activities are higher with the increase of stress". However, the results of the present study are contrary to this belief and the results of the previous studies on mycorrhizae. To check whether this is due to allelopathic effect of Pinus roots, fire damage, chemical factors of the soil, due to any other reason or due to a collection of factors, necessity of more research can be highlighted.
All the ten plant species studied from Pinus under storey habitat and semi natural forest under storey habitat from Hantana Range, Sri Lanka were with Vesicular-Arbuscular Mycorrhizal (VAM) associations. However, the performances of VAM associations were significantly higher in all the plants from semi natural forest than the plants from Pinus plantations. To find out the reasons for this difference, further research is recommended.
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1 Department of Botany, Faculty of Science, University of Peradeniya, Peradeniay, Sri Lanka
Email: [email protected]
2 Department of Crop Science, Faculty of Agriculture, University of Peradeniya, Peradeniay, Sri Lanka
Email: [email protected]