Individual study: Using Caribbean pine Pinus caribaea as a nurse canopy cover for establishing late-successional native tree species, Sinharaja Man and Biosphere Reserve, Sri Lanka
Ashton P.M.S., Gamage S., Gunatilleke I.A.U.N. & Gunatilleke C.V.S. (1997) Restoration of a Sri Lankan rainforest: using Caribbean pine Pinus caribaea as a nurse for establishing late-successional tree species. Journal of Applied Ecology, 34, 915-925
In the moist tropics, studies have demonstrated poor seedling establishment of late-successional trees on lands cleared of forest. In Sri Lanka, government policies aim to prevent erosion and restore degraded forests. This study examined the potential for establishing late-successional tree species that dominate the canopy of native rainforest by planting within and adjacent to experimental openings created within a Caribbean pine Pinus caribaea plantation in order to attempt forest restoration.
Study site: The study was undertaken in a Carribbean pine plantation bordering the Sinharaja Man and Biosphere Reserve (6˚21'N, 0˚21'E) in southwest Sri Lanka. Sinharaja supports one of the few remaining areas of intact native forest. The area is bordered mostly by tea and rubber plantations and abandoned agricultural land revert to shrubland. Some Pinus caribaea plantations have also been planted for the pulp industry and watershed protection.
The study site (on a 30˚ slope) had originally been cleared in the 1950s for swidden agricultural. In 1978 the Forest Department of Sri Lanka planted the area with pines at a 2 x 2 m spacing.
Tree species: Five native canopy tree species (Dipterocarpus zeylanicus, Mesua ferrea, Shorea disticha, S.megostophylla and S.trapezifolia) of tropical forest in south-western Sri Lanka were selected for planting based on knowledge gained from previous studies. Seedlings were planted in the pine plantation in August 1991, regularly weeded and monitored for 2 years. Four treatments were applied:
i) one row of pine removed (4 m strip width)
ii) three rows of pine removed (8 m strip width)
iii) under planting beneath three rows of pine adjacent to three rows removed
iv) Under-planting beneath a closed pine canopy (no pines removed)
Treatments were at least 20 m from one another and all tree removal rows (each 145 m in length) were in a north-south orientation. For each treatment a single seedling of each of the five species was planted together (1-m spacing) for comparison of performance. Groups of five trees were planted at 5-m intervals along the treatment transects. Seedlings were all one-year old at time of planting and had been grown locally at the Sinharaja field station.
Growth: Growth measurements (height, basal diameter, number of leaves) were taken at the end of the two years from surviving seedlings and mortality calculated. A sample of 12 seedlings of each species was taken to recorded dry mass of leaves shoots and roots.
Daily photosynthetic photon flux (DPPF): DPPF was measured using a data logger and quantum sensors on three sunny days for each treatment during January and February 1993.
Growth: The greatest growth and dry mass for all species were in the pine removal treatments ( i.e. where one or three rows of pine had been removed and the canopy opened up). S.trapezifolia and S.disticha exhibited the greatest height growth in these treatments. In the three-row canopy removal treatment, M.ferrea had a significantly lower dry mass than the other four tree species trialed.
There were no significant differences in seedling height among treatments and among species. For all species greatest height increases were observed in the 3-row removal (which opened up the canopy the most) and least under the closed canopy. The three Shorea species showed no height difference between the 3-row, 1-row and 3-row under-plantings. The greatest basal stem diameter for all species was also in the 3-row treatment (around 9 to 14 mm), marginally less in the other removal treatments and clearly lowest in the closed canopy (2 -4 mm).
Differences were shown in the number and area of leaves among species. S.trapezifolia and, to a lesser degree, S.disticha increased area by increasing leaf production. D.zeylanicus and, to a lesser degree, M.ferrea increased area by increasing the size of individual leaves.
Mortality: Almost all treatments had less than 10% mortality after 2 years. Mortality was greatest in S.trapezifolia in canopy planting (c.19%). S.megistophylla and D.zeylanicus had lower mortality (c<4%) than the other species in pine removal treatments, and for the latter species motality increased as DPPF decreased.
Conclusions: This experiment demonstrated that Caribbean pine used as a nurse facilitated the establishment of seedlings of native, late-successional, tropical forest tree species at a site adjoining the Sinharaja Man and Biosphere Reserve. Based on results from this study, tree seedlings establish and grow best when planted within openings created by the removal of three rows of Pinus canopy. Where planting without canopy removal is required, the authors recommend that of the five species grown, S.disticha and S.megistophylla should be selected because of greater shade and drought tolerance.