VARIATION OF STOMATAL CONDUCTANCE AND ITS CONTROLLING FACTORS OF DIFFERENT SPECIES IN A WET EVERGREEN FOREST IN THE MID·COUNTRY WET ZONE OF SRI LANKA

Abstract

Stomatal conductance is a primary determinant of the net carbon balance and growth of differentplant species in a forest community. Because of its influence on transpirational water loss, stomatalconductance determines the water balance of a plant. As both carbon and water balances playimportant roles in survival and establishment of seedlings of different plant species in a forestcommunity, stomatal conductance has a direct influence on species composition and succession of aforest. Therefore, the main objective of the present study was to describe the inter-species variationof stomatal conductance in a wet, evergreen forest in the mid-country wet zone of Sri Lanka. Asecondary objective was to examine the variation of two parameters which could influence stomatalopening, namely incident light intensity and leaf temperature, and to see whether any directcorrelations can be established between stomatal conductance and the above parameters.

This study was conducted in the Gannoruwa forest reserve in Kandy in September, 1997. 108 plantspecies which included all growth forms (i.e. trees, shrubs, herbaceous plants, vines and grasses)were found. Stomatal conductance was measured using a portable diffusion porometer which alsorecorded the incident light intensity ( in terms of photosynthetically-active radiation) and leaftemperature. Conductances of both upper and lower leaf surfaces were measured and total leafconductance was obtained by summing the two above measurements. In a given species, at leastthree plants were sampled.

There were significant differences between species in lower surface conductance (gi), upper surfaceconductance (gu) and total leaf conductance (g). Except in one species, gl was greater than gu in allother species examined. Lower Surface Conductance/g, ) ranged from 17 to 1280 mmol m,2 S'I with amean of 175.8 += 8.9 mmol m ,2S'I whereas gu ranged from 2.36 to 252 mmol m,2 S'I with a mean of25.15 += 1.58 mmol m,2 S'I. The range of total conductance (g) was 20.45 - 1309.8 mmol m,2 S'Iwith a mean of 20 1.41 += 9.44 nunol m,2 S'I. The frequency distributions of aH three conductanceparameters (i.e. lower, upper and total) were skewed to the left with the respective median valuesbeing 132.5 (for gr ), 18.25 (gu) and 149.9 (g) mmol rn" S'I . Normality of the distributions whentested using the box plot technique, showed significant skewness to the left, Deletion of extremevalues did not improve normality significantly.

The light intensities experienced by the leaves ranged from 2.5 to 1830 umol (PAR) m ,2S'I and theleaf temperatures ranged from 24.55 to 34.40 C. When all species were considered together, totalleaf stomatal conductance (g) had a significant positive correlation with light intensity. But, therewas no significant correlation between g and leaf temperature. Similar results were obtained whenlower and upper surface conductances were considered separately as well. There was a highlysignificant positive correlation between light intensity and leaf temperature. A multiple regressionanalysis showed that these two variables explained only 8% of the overall variation of leaf stomatalconductance. This infers species differences, interactions between species, and their response tovariations in light intensity and leaf temperature are more significant. Stomatal conductance ofditTerent species respond in ditTerent degrees to light and temperature variations.

1\ cluster analysis allowed the 108 plant species examined to be grouped in to different categorieson the basis of their total leaf stomatal conductance (g). The different species groups could bebroadly related to their position in the successional sequence. It was found that the speciesestablished under the canopy on the forest floor generally had lower levels of g than thoseestablishing in canopy gaps. While many species of the latter group (growing in gaps) had the abilityto raise their stomatal conductance in response to increasing light intensity, most species of theformer group (growing under the canopy shade) could not respond to the occasional sunflecks thatare incident on them at different times of the day.