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
W A J M De Costa and A G Chandrapala
University of Peradeniya
Stomatal conductance is a primary determinant of the net carbon balance
and growth of different plant species in a forest community. Because of
its influence on transpirational water loss, stomatal conductance determines
the water balance of a plant. As both carbon and water balances play important
roles in survival and establishment of seedlings of different plant species
in a forest community, stomatal conductance has a direct influence on
species composition and succession of a forest. Therefore, the main objective
of the present study was to describe the inter-species variation of stomatal
conductance in a wet, evergreen forest in the mid-country wet zone of
Sri Lanka. A secondary objective was to examine the variation of two parameters
which could influence stomatal opening, namely incident light intensity
and leaf temperature, and to see whether any direct correlations 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 plant species 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 also recorded
the incident light intensity ( in terms of photosynthetically-active radiation)
and leaf temperature. Conductances of both upper and lower leaf surfaces
were measured and total leaf conductance was obtained by summing the two
above measurements. In a given species, at least three plants were sampled.
There were significant differences between species in lower surface conductance
(gl), upper surface conductance (gu ) and total leaf conductance (g).
Except in one species, gl was greater than gu in all other species examined.
Lower Surface Conductance(gl ) ranged from 17 to 1280 mmol m-2 s-1 with
a mean of 175.8 += 8.9 mmol m -2 s-1 whereas gu ranged from 2.36 to 252
mmol m-2 s-1 with a mean of 25.15 += 1.58 mmol m-2 s-1 . The range of
total conductance (g) was 20.45 - 1309.8 mmol m-2 s-1 with a mean of 201.41
+= 9.44 mmol m-2 s-1 . The frequency distributions of all three conductance
parameters (i.e. lower, upper and total) were skewed to the left with
the respective median values being 132.5 (for gl ), 18.25 (gu ) and 149.9
(g) mmol m-2 s-1 . Normality of the distributions when tested using the
box plot technique, showed significant skewness to the left. Deletion
of extreme values did not improve normality significantly.
The light intensities experienced by the leaves ranged from 2.5 to 1830
Ámol (PAR) m -2 s-1 and the leaf temperatures ranged from 24.55 to 34.40
C. When all species were considered together, total leaf stomatal conductance
(g) had a significant positive correlation with light intensity. But,
there was no significant correlation between g and leaf temperature. Similar
results were obtained when lower and upper surface conductances were considered
separately as well. There was a highly significant positive correlation
between light intensity and leaf temperature. A multiple regression analysis
showed that these two variables explained only 8% of the overall variation
of leaf stomatal conductance. This infers species differences, interactions
between species, and their response to variations in light intensity and
leaf temperature are more significant. Stomatal conductance of different
species respond in different degrees to light and temperature variations.
A cluster analysis allowed the 108 plant species examined to be grouped
in to different categories on the basis of their total leaf stomatal conductance
(g). The different species groups could be broadly related to their position
in the successional sequence. It was found that the species established
under the canopy on the forest floor generally had lower levels of g than
those establishing in canopy gaps. While many species of the latter group
(growing in gaps) had the ability to raise their stomatal conductance
in response to increasing light intensity, most species of the former
group (growing under the canopy shade) could not respond to the occasional
sunflecks that are incident on them at different times of the day.
Department of Forestry and Environmental Science,
of Sri Jayewardenepura,Sri Lanka. 1999. All rights reserved.