Seedling growth of Shorea (Dipterocarpaceae) across an elevational range in Southwest Sri Lanka

Performance of seedlings of seven rain forest, canopy dominant Shorea species was studied in a transplant experiment ir! forest sites at three different elevations (low, mid and high) within the humid zone of southwest Sri Lanka. Five species generally inhabit low-to mid-elevations, one at mid-and lower montane elevations, and one exclusively at lower montane elevations. Temperature, rainfall and cloudiness varied with elevation. For each site seedlings were grown in pots under partial shade conditions using similar soils and evermoist conditions. All growth measures showed differences among elevation sites, among species and in the interaction between species and elevation sites. Performances of species collectively showed (i) decline in height and leaf number with increase in elevation, (ii) higher dry mass at low-and mid-elevation sites compared to that at high-elevation and (iii) a higher mass of single leaves at the mid-elevation site than at the high-elevation site. Rank order of species changed across elevations for both height and dry mass. Dry mass declined with elevation in four of the seven species studied. S. gardneri, the only exclusively lower montane species, increased dry mass with elevation. Height declined with elevation for six of the species with only S. gardneri showing no change. Changes with elevation in the rank order of species for total leaf number and mass of single leaves were small. However, total leaf number and masses of single leaves differed among species and among elevations. S. megistophylla and S. disticha had a few leaves with high individual masses, while S. gardneri, S. affinis and S. trapezifolia had many leaves with less mass per individual leaf. One group of species showed relatively little change in leaf number per seedling and large changes in mass of single leaves. The other group varied more in leaf number but mass of individual leaves remained constant. Growth allocation to leaf production versus individual leaf size appears related to the successional division of Shorea section Doona. Also all species grew better at the low-elevation site irrespective of their natural ranges except S. gardneri, whose natural range is restricted to high elevations, and exhibits markedly lower growth responsiveness than the other wider ranging species.