NATURAL REGENERATION OF SOME DRY ZONE FOREST SPECIES ASSISTED BY SILVICULTURAL MANAGEMENT IN A DRY ZONE WOODLAND AT DAMBULLA

N.D.R. WEERAWARDANE

(Forest Department)

Summary

Natural regeneration of forests is usually poor in degraded lands in the Dry Zone. Deforested lands covered with scrub vegetation usually do not allow sufficient natural regeneration to take place. Re-colonization of such lands with important local species can be accelerated up by appropriate silvicultural interventions. Such proven methods are not generally available, and as a consequence, only artificial regeneration by way of enrichment planting or replanting is practised using limited number of available species to convert these lands to woodlands. A simple but effective method to accelerate natural process of re-colonization of degraded lands and converting them to woodlands in the Dry Zone is discussed in the paper.

INTRODUCTION

The Dry Zone of Sri Lanka consists of a large area of natural forests, scrublands and shifting cultivated lands. Scrublands have been formed as a result of removing original forest cover for shifting cultivation and then abandoned. As a result of forest clearing, large extents of lands are available for reforestation in this climatic zone. Large scale reforestation programmes have been implemented in the past by the Government and will be continued in the future in these areas. These plantations have been established with the aim of increasing forest cover, timber and fuelwood supply in the country. Environmental benefits are also expected at least to some extent from these plantations. Natural forest enrichment by way of interplanting selected forest species in opened strips is also practised in degraded forests to upgrade the quality of them. In the past, these reforestation programmes have faced problems such as poor survival and growth associated with degraded site conditions, animal damage to growing seedlings and burning for land clearing etc. by humans.

Artificial regeneration of forests is usually done with single species which is beneficial for easy management operations carried out at later stages in plantations and also to increase production from a unit of land area. However, their impact on the environment is controversial and may have undesirable effects on the surroundings if they are not managed on a sustainable basis.

A successful project has been started in encouraging natural regeneratin of many valuable forest species in the Dry Zone at the Dambulla Arboretum and Woodlands which clearly shows the potential of these species to regenerate when they are silviculturally assisted.

MATERIALS AND METHODS

The Dambulla Arboretum and Woodlands are located in the Dry Zone of Sri Lanka where drought prevails for a period of about 5 months. Its location is close to the northern boundary of the Intermediate Zone. The Arboretum was established in 1963 with an extent of 3.5 ha and later extended to adjacent land creating a woodland totalling around 14.5 ha. The land was degraded scurbland and has been gradually converted to a mixed forest consisting of many valuable forest species. The method adopted was basically to assist trees to naturally regenerate by clearing away the original thorny scrub thicket and removing aggressive weed species and creepers which suppress the growth of tree seedlings on a regular basis. This enables naturally regenerated forest tree seedlings to gain dominance over the other species. No planting of seedlings or artificial fertilizer application were done. Pesticide application was also never done in the site. Thus, the method is a silvicultural management system nurturing the process of natural regeneration and is called the 'Popham method', one of whose principal tenets is to debar all hostile invaders. This technique involves human intervention only in order to allow the natural processes of regeneration their fullest scope, identifying the valuable species and protecting their growth from weed competitors and fire. Naturally growing seedlings survive drought better due to with their deep roots. A natural selection of better individuals also takes place within this process as poor individuals unfit for establishment die off during the dry periods. Thus the important aspects of the technique viz: development of undisturbed deep root systems of forest seedlings, natural selection of the fittest individuals during the establishment phase, providing a progressively increasing shade to suppress weed growth and reducing the impact of harsh environmental conditions in the Dry Zone, contribute to the success of the method. These aspects are generally not achieved in most of the normal reforestation programmes where artificial regeneration is practised by planting seedlings in an alien environment. The site under discussion is shown in the land plan in the Figure 1.

Soil in the study area is an Alfisol (Suborder: Ustalf) which corresponds to the Reddish Brown Earths in the Dry Zone. It has a sufficiently base-saturation with a partially decomposed organic layer close to the top soil. Chemically this soil type is considered a moderately rich soil which is biologically favourable for plant growth. Deep soil layers have a larger proportion of clay offering a physical barrier to deep root development. During the wet weather this clay swells closing the soil pores which leads to rising ground-water level probably causing stress to root development (Cramer, 1993). However, surface water logging is not usually observed in the study area. Soils which facilitates deep root development is usually preferred in order to withstand strong winds.

Regeneration and subsequent growth of selected tree species were studied in the Arboretum and Woodlands with the species mentioned in the Table 1. Out of these species only regeneration potential was studied in some species.

Regeneration was observed in these species by counting all seedlings in the area with the onset of rains when seedlings emerge from the ground. These were marked with stakes and numbers recorded in each species. Survivals were recorded annually for a period of three years. Their growth rates were monitored after their emergence. This was carried out in an area of 12.5 ha starting from November 1995 for a period of four years. Rainfall figures for Dambulla area are given in the Table 2 during the study period.

RESULTS AND DISCUSSION

Regeneration and survival

The greatest number of seedlings recorded is Damunu followed by Neem, Ketekela, Rathuwa, Welang, Satin and Palu. Damunu and Rathu-wa being pioneer and low density timber trees, a higher regeneration potential can be expected. Nevertheless regeneration of important timber species such as Ketakela, Welang, Satin and Palu are encouraging. The number of seedlings emerged annually in each species was up to about 200 in the total study area.

The values recorded for Neem (number of seedlings emerged) vary from a very high number in the first year to very low numbers in other two years indicating some unsteady state of natural regeneration capacity. Comparatively moderate regeneration potential of about 50-100 seedlings per 12.5 ha was recorded in species of Weera, Panakka, Ebony, Mee, Milla, Hulanhik, Tammenna, Madan, Ehela, Bo mee, Pamburu and Thumpath Kurundu. The poorest regeneration was shown by Dambu, Daeduwa and Bokutu. Appreciable regenerations in valuable hardwood species such as Milla, Panakka, Ebony, Hulanhik and Madan are noteworthy and deserve more attention in future studies.

Survival records are given in Table 4 in three seedling batches in three consecutive years as that of Table 3. Considering average values of all species they show that survival rate is more or less stabilized by the end of the third year. However, as shown in the same Tables that there are species differences in the survival. In species such as Palu, Weera, Panakka, Ebony, Mee, Milla, Hulanhik, Halmilla, Thammenna, Damunu, Pamburu and Thumpath kurundu it seems that survival rate is stabilized even at the age of 2 years.

Results of both studies show that most of the local species have the ability to naturally regenerate well if they are properly taken care of by simple silvicultural methods. Under natural conditions of scrublands, without any interference of man, the survival rates have been very much lower due to grass and other hostile weed competition effects. The management practices adopted here was, as mentioned above, only cutting scrub vegetation initially and slashing of weeds (including both grass and broadleaved species) subsequently which suppressed the growth of those forest species.

No other care was given such as fertilizer application and irrigation. In the woodlands no artificial fertilizer is applied and plants are allowed to grow only under natural conditions. Nutrients required for plant growth is supplied by decomposing organic matter as in natural forests. Possibly the survival would have improved in later stages by an application of mulch with decomposed leaves due to better soil moisture conservation and subsequent improved growth of roots so that water and nutrient uptake will be more efficient. This can be done simply by collecting fallen leaf material and putting them around the plants which can delay soil moisture evaporation from the root zone in the dry periods. This can also concentrate nutrient supply around the root zone of the seedlings, helping plants to grow better. In other studies, it has been shown that such mulching can encourage fine root development near the soil surface which facilitates faster nutrient absorption. However, if a sufficient leaf mulch is already available by natural leaf fall on the soil surface, artificial mulching may not be necessary. It could also be expected that when forest tree-cover is established, noxious weed competition effects will be minimal and as a result forest seedlings would survive better, provided sufficient light is available. Light environment in the present study site was moderate, falling to about 50% of full sunlight. Low light levels are generally preferable for most local hardwood species during the establishment period, except for pioneer species.

Growth rates

Growth rates were studied annually, until the fourth year after the emergence of seedlings. Different species showed different rates of growth as shown in the Table 5.
Table 5: Plant height of different species at different stages of development
According to the above Table the fastest growth rate was recorded by Hulanhik from the second year up to the fourth year. Growth was 67 cm in this species at the end of the fourth year. Neem and Ebony had the second highest growth rate followed by Satin. Panakka, Palu, Mee and Milla where ranked next. Weera and Kon had fairly slow growth rates, while Halmilla had the lowest recording only 14 cm even after four years. The growth rate of Halmilla in the study area is disappointing since it is a fast growing species elsewhere in the Dry Zone. It is noteworthy that species like Ebony and Satin are usually recorded as slow growers but ranked higher place in this study in species assessed.

of a bare land provided sufficient seed sources are available in surrounding areas (Popham and Neil, 1994). When sufficient shade is established, it suppresses weed growth reducing labour cost. However, the cost should not be held as a discouragement where sufficient regeneration and successful growth of seedlings are observed which generally cannot be achieved through conventional planting methods. Usually planted seedlings of slow-growing local species do not survive long dry periods in the dry zone due to insufficient root development at the time of planting. Root deformation in the nursery containers is usual when plants are raised in the nursery, which also affects later root development and hence plant growth. Naturally grown seedlings develop strong root systems with a long tap root which enables them better survival and growth under dry conditions. The present method can be used as a model for converting abandoned lands to mixed species forest containing valuable local species. Further research work is needed to understand the basic ecological requirements of individual species in order to enhance their establishment and growth rates.

Dambulla Arboretum and Woodlands developed by the above mentioned method is now a sanctuary for about 70 forest tree species, favouring the native hardwoods and of more than 20 plant species altogether (Stewart-Cox, 1998). The method has shown that it is possible to restore degraded and abandoned forest lands with a full complement of tall trees using a cheap and simple method. A homestead in the nearby area has started to follow the same principal to reforest their lands with indigenous species.
