PhD

Altitudinal gradients are among the most powerful ‘natural experiments’
for testing ecological and evolutionary responses of biota and has strong
influence on structure of vegetation.The present study was undertaken in a west
coast tropical forest located along a gradient viz. Zone I (350-900m above msl)
and Zone II (900-1450m above msl) in Mankulam Forest Division of Western
Ghats region in Kerala, India. The aim of the study was to assess and compare the
floristic composition, structure and diversity along this gradient. Additionally the
changes in the physico-chemical properties of soil i.e.,Bulk density (BD)
,pH,Electrical conductivity (EC), total nitrogen (N), available phosphorus (P),
available potassium (K), organic carbon (C) and available calcium (Ca) along this
altitudinal gradient were also studied. Sampling of vegetation and soil was done
using fifteen, 10 m x 10 m plots (covering 0.15ha in each altitudinal zone).
Within each plot, all tree individuals (≥ 10cm GBH) were identified and their
height was measured. In total, 200 individuals belonging to 88 tree species
representing 34 families were recorded across the altitudinal gradient.In each
sample plot, soil samples were analyzed at two depths viz. ‘upper’ (0–20 cm) and
‘lower’ (20–40 cm) along the altitudinal gradient.
The diameter frequency distribution showed a “bell shaped pattern” in
both altitudinal zones. The vegetation profile diagram at both altitudes showed a
typical three layered structure which is a characteristic feature of evergreen forest
with majority of individuals in lower storey (<10 m ) height classes in both zones.
The maximum tree abundance (67.09) was recorded in Zone II. Tree density
(713.33 trees ha-1) was also higher in zone II compared to zone I (613.33 trees ha-
1).Similarly highest tree percent frequency (546.6%) was recorded in zone II.
Vateria indica (IVI= 24.09) was found as dominating tree species in zone II,
while in zone I dominating tree species was Dysoxylum malabaricum (IVI=
34.85). However, the tree basal area (20.31 m2 ha-1) was estimated higher in zone
I compared to zone II with basal area (12.31 m2 ha-1).Rank abundance curve
indicated highest abundance for Gordonia obtusa in zone II, while in zone I
Dysoxylum malabaricum recorded the highest abundance. Physico- chemical
properties of soil viz. soil pH, C, N, P, K and Ca showed an increasing trend
along the gradient. The highest values for soil pH (4.12-5.59), C (1.82-8.36%),
TN (0.08-0.37 %), P (4.36- 1098.49 Kg ha-1), K (398.24- 1628.48 Kg ha-1) and
Ca (320 - 1160 mg Kg-1) was recorded in zone II. BD (0.77-1.33g cm-3) and EC
(0.30 – 1.82 dS m-1) values were higher in ‘upper’ (0–20 cm) layer of zone I.
Canonical correspondence analysis (CCA) showed a complex
interrelationship amongst species clustering, mountain ranges and soil properties.
The study revealed that there is gradual increase in species abundance as the soil
properties increases along the altitude, in both zones. Correlation analysis clearly
indicated that in zone I AK is strongly and positively correlated with C because
layer of organic matter significantly improves the retention of K in the soils.
Whereas, N is negatively correlated with BD. Similarly, in zone II, TN was
observed to be negatively correlated with BD while K was strongly and
positively correlated with N.
The floral diversity indices viz Simpson index, Shannon index and Pielou index
of evenness showed marginal variation along the gradient. However, there was
variation in species composition along altitude. Actinodaphne bourdillonii, Litsea
keralana, Gordonia obtusa and Turpinia cochinchinensis were the characteristic
species in zone II. In zone I Clausena anisata, Memecylon talbotianum and
Madhuca neriifolia were observed to be prominent.
Species abundance, diversity and vegetation structure and the physicochemical
properties of soil showed a ‘‘humped’’ pattern along this altitudinal
gradient at Mankulam. In Zone II, species diversity was marginally better with
higher species richness and better floristic structure and species composition. Soil
properties were also observed to be better in zone II. The possible reason for this
could be attributed to several reasons including the presence of luxuriant and
comparatively undisturbed vegetation which could have contributed to a higher
soil organic matter content rich in humus.