Abstract:
As light transmission is essential for plants in the forest under story, canopy openness
(CO) could be closely related to vegetation diversity in tropical rainforests. This
study investigated the variation of CO of wet zone rainforests in Sri Lanka along an
altitudinal gradient and its relationship to the Shannon-Wiener vegetation diversity
index (H’) and species richness (R). CO was quantified as the fraction of visible sky
(Vsky) in canopy hemispherical photographs (HCPs). Ten Permanent sampling plots
(PSPs) of one hectare was established along an altitudinal gradient from 117 m to
2132 m above mean sea level. Nine HCPs were obtained covering the entire 1 ha
plot and canopy properties were calculated by Hemiview image analysis software.
Plot-level means of Vsky and leaf area index (LAI) showed highly-significant
(P<0.0001) variation among PSPs. Sinharaja-Pitadenitya at 618 m had the lowest
Vsky and the highest LAI whereas Pidurutalagala at 2080 m had the highest Vsky and
the lowest LAI, with a highly-significant negative correlation (P<0.0001; r=-0.784)
between Vsky and LAI. Vsky and LAI showed respectively negative and positive
second-order polynomial trends with altitude across its whole range. Vegetation
diversity (H’ and R) showed highly-significant (P<0.0005) linear decreasement with
increasing altitude. Vsky showed negative second-order polynomial relationships
with H’ and R. Increasing H’ and R decreased Vsky from 2132 to 509 m as increasing
vegetation diversity increased LAI and reduced CO. However, this trend was
reversed from mid- (ca. 500 m) to low (ca. 100 m) altitude where both CO and
diversity increased. Vegetation diversity has probably increased at the lowest
altitude (Kanneliya) because of greater CO and the resulting transmission of light to
the ground level. Based on these results, it is concluded that whereas CO acts as a
determinant of vegetation diversity at lower (<500 m) altitudes, at higher altitudes
(>500 m), vegetation diversity determines CO.
