Abstract

Media summary

Key Words

Introduction

Deforestation is one of the main environmental and social problems in Sri Lanka. The closed-canopy natural forest cover has dwindled rapidly, from about 80% in 1900, to less than 24 % in 1992. In view of the increasing demands placed upon the forestry sector and also to enhance natural regeneration, forest plantations were established mainly using exotic species. Pinus caribea and P. patula were selected as the main tree species in watershed reforestation programmes according to their remarkable ability of successful establishment and growth under adverse conditions. In lower elevations P. caribaea and in higher elevations over 1223 m amsl P. patula were planted. Pinus planting was extended to steep slopes, ridge tops and other inaccessible areas covering a total extent of 28,715 ha, mainly in Wet and Intermediate Zones of Sri Lanka by 1987 (Bandaratilake 1989). Replanting with Pinus has been abandoned later due to growing public and environmentalists concern on negative impacts of Pinus on natural regeneration, wildlife habitats, ground water extraction, forest fire and soil erosion etc, however, without proper investigation. Studies have shown that poor vegetation regeneration and biological diversity under some Pinus species are associated with allelopathic effects of Pinus. Severities of allelopathic effects of most species vary based on soil types, climatic and management conditions etc. (Rice 1984). Therefore, this study was initiated to investigate the impact of Pinus plantation on vegetation regeneration and soil biological diversity compared with near by natural vegetation.

Materials and Methods

This study was carried out at two locations; namely Loolkandura (elevation 1100 m amsl) in the upcountry region and at Hantana (elevation 518 – 1100 m amsl) in the mid country region, both in wet zone (> 2500 mm annual rainfall) where natural forests and Pinus plantations are located close by. At Loolkandura, random soil samples were taken from 0-20cm depths from both Pinus and natural forests. Soil organisms such as bacteria, mycorrhiza, other fungi, nematode and earthworm populations, and soil microbial activities were determined. Amount of CO₂ evolution rate in soil was determined using in vivo method and mycorrhizal population were determined using sucrose centrifugation method as described by Anderson (1982). Nematodes population was determined by using Baermann funnel as described by Hooper (1986). Bacteria and Fungi Populations were determined by growing them in plain agar and measuring their growth area. Earthworm population was determined from 1 m² area in each site. At Hantana, randomly selected thirty-eight quadrates (each 1m x 2m) were surveyed for understorey species in the Pinus plantation. Basic ecological parameters such as Relative Density (RD), Relative Frequency (RF) and Important Value Index (IVI) were calculated for each species. Ten plant species were selected for screening the presence of VAM, based on their IVI rank. Same plant species were also collected from semi natural forest for comparison. Plants were uprooted and entire root system was removed. Root tips were examined for VAM as described by Phillips and Haymen (1970). Data analysis was done using SAS statistical package.

Results and Discussion

Number of plant species, families, total number of individuals per unit area and canopy height and number of strata’s in the canopy were greater in the natural/semi-natural forests at both locations than in Pinus forests. Results obtained from Loolkandura showed that the soil micro floral density measured as mycorrhizal spore counts and fungus population was reduced by 54% and 60% respectively in the Pinus soil compared to natural forest. However, the soil bacterial population was similar in both forest types. Soil faunal diversity and density were also reduced significantly under Pinus vegetation compared to natural forests (Table 1 and Table 2). Earthworm and non-parasitic nematode populations in soil were also reduced by 78% and 40% respectively under Pinus forests. However, the parasitic nematodes were suppressed under both forest types. Microbial activity was four times higher in the natural forest compared to Pinus forest, indicating very high biological diversity in the soil under the natural forest cover (Table 2).

Ten dominant plant species selected to study VAM associations based on their IVI at Hantana are given in Table 3. All ten-plant species studied from both Pinus understorey and semi natural forests were associated with VAM. The Mean Percentage Occurrence of VAM in all the plant species from Pinus understorey was significantly lower than the respective plant species from semi natural forest.

Table 1. Influence of Pinus and natural forests at Loolkandura on soil mychorrhizal, fungus, bacterial and earthworm populations.

Means with same letters are not significantly different at 5% probability level.

Table 2. Influence of Pinus and natural forests at Lookanduara on parasitic and non parasitic nematodes, and microbial activity of soil

Means with same letters are not significantly different at 5% probability level,
ND: (not detected).

Table 3. Plant species selected to study VAM associations, their Important Value Index (IVI) and Mean Percentage of Occurrence of VAM of dominant understorey species in Pinus and semi natural forests at Hantana range.

Greater mycorrhizal, fungi and earthworm populations and mean percentage occurrence of VAM in roots of understorey species in natural forests may be associated with the higher species diversity, moisture contents in the litter and soil, aeration, organic matter contents and less allelochemicals presents. Release and accumulation of allelochemicals and lack of understorey vegetation may have reduced soil faunal and floral diversity in the Pinus forests. In agricultural systems where crops are often grown as monocultures, plant parasitic nematode tends to predominate, but suppress their growth in forest ecosystems (Stirling 1991). Overall results suggest that very low diversity and population densities of floral and faunal species may be associated with the allelochemical released by Pinus plants. 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 inhibit the mycorrhizal associations of other plant species and other type of mycorrhizal associations such as VAM. Coder and Warnell (1999) also reported that Pinus litter has an inhibiting effect on some species and its own seed germination and seedling growth as well.

Conclusion

Vegetation diversity was lower, and soil organism populations of mycorrhizae, fungi, bacteria, free living nematodes and earth worms were reduced by 2.3, 2.4, 1.6, and 4 folds respectively in Pinus compared to natural forest. Soil microbial activities were also reduced 4 fold in Pinus forest indication lower soil biodiversity and activity. VAM associations were found in all ten-understorey plant species in both Pinus plantation and natural forests. However, Mean Percentage Occurrences of VAM for all the ten species were significantly different and number of VAM present were lower in all species in Pinus plantations than the same species from natural forests. These lower vegetation diversity and soil biodiversity under Pinus plantation may be associated with their allelopathic effects compared to the natural vegetations.

References

Anderson JP (1982). Method of soil analysis part 2; Chemical and microbial properties, Agronomy Monograph. 2^nd ed. No. 9.

Bandaratilake HM (1989). Development of Pine Plantations in Sri Lanka. In ‘Reforestation with Pinus in Sri Lanka’ (Eds. Gunasena HPM, Gunatillake S and Perera AH). Proc. of a Sympo. organized by University of Peradeniya and British High Commission on behalf of ODA, July 1988. 10 – 18.

Coder Kim D and Warnell DB (1999). School of Forest Resources, University of Georgia 4/99 Potential Allelopathy in Different Tree Species.

Hooper DJ (1986) Extraction of free living stage from soil, in laboratory method for work with plant and nematodes (Ed Southey AF). Reference Book 402.

Phillips JM and Hayman DS (1970). Improved Procedures for Clearing Roots and Staining Parasitic and Vesicular-Arbuscular Mycorrhizal Fungi for Rapid Assessment of Infection. Transactions of British Mycological Society 55, 158 – 161.

Rice EL (1984). Allelopathy. 2^nd ed. Academic Press, New York and London.

Stirling GR (1991). Antagonists of nematodes in biological control of plant parasitic nematodes, progress problems and prospects (Ed. Stirling GR). CAB International, Wallingford.UK.