PhD.

The study entitled “Invasion impact of greater club rush (Scirpus grossus
L. f) on wetland rice ecosystem” was conducted at College of Agriculture,
Vellayani during 2015-2017. The objectives were to study the invasive
characteristics, habitat affinity and distribution of greater club rush, to assess its
invasion impact on biodiversity and ecosystem functioning of the wetland
ecosystem and also to develop an ecofriendly management strategy for
recovering the invaded fields. To achieve the objectives, the study covered four
aspects, i.e. Part I. Weed mapping and habitat analysis, Part II. Study of invasive
characteristics of greater club rush, part III. Invasion impact on biodiversity and
ecosystem functioning of wetlands, Part IV. Management of greater club rush.
A preliminary survey on the presence of the emergent hydrophyte in
Thiruvananthapuram district indicated that the invasion was mainly concentrated
in some pockets in the wetlands adjoining the Vellayani fresh water lake. Hence,
the wetland ecosystem of the Vellayani lake watershed was selected as the
geographical unit for conducting the investigation.
Under Part I, the extent of greater club rush invasion in the wetlands
of Thiruvananthapuram district was assessed and mapped using Global
Positioning System (GPS) and Geographical Information System (GIS). As per
local enquiries, the invasion was noticed for the first time about 15 - 20 years
back and was spreading fast in waterlogged paddy fields. However, it was not
clear how this alien weed was introduced and naturalised in Kerala.
The results of the weed mapping studies revealed that greater club rush
invasion was concentrated mainly in the north - western side of wetlands
adjoining the Vellayani lake. From the drainage and land use maps, it was
inferred that there was a flood plain formed in the locality due to indiscriminate
wetland reclamation resulting in impeded drainage. The total area under invasion
was approximately 65 ha (47.70 per cent of the rice fallows). During the mapping
studies, it was also observed that the ownership of a major portion of these paddy
fields was vested with absentee landlords who leave the land uncultivated for a
long time thus providing open niches for further spread of the weed.
Correlation studies on weed density and dry weight with soil and water
parameters of the invaded fields indicated that the weed has high level of habitat
tolerance and ecological flexibility and was able to thrive under a wide range of
field conditions.
Under Part II, the invasive nature of greater club rush was assessed
by observing the growth and development characteristics of the weed under
field conditions. From the results, it was inferred that, tall stature (1.52 m),
dense growth habit (342.23 ramets m-2), low light infiltration (4.36 per cent),
high relative growth rate (0.028 g g-1 day-1), efficient propagation (rhizomes,
stolons and corms combine for multiplication and perennation), huge biomass
(30 t ha-1) and abundance of aerenchyma along with ecological flexibility
imparts intense competitiveness for greater club rush making it a successful
invader.
The major part of the study was to assess the impact of greater club rush
invasion on the wetland ecosystem (Part III). Based on the weed mapping and
habitat analysis, three heavily infested padasekharams, located within a distance
of 5 km were selected for the impact study and monitored for two years. The
methodology used was Multisite comparison of the co- existing flora and fauna
community, between invaded vs uninvaded sites. Alterations in nutrient cycling
and microbial activity due to the invasion were studied by collecting and
analyzing soil samples from selected sites during three seasons.
The results indicated that greater club rush invasion was depleting the
diversity of native vegetation in the wetland ecosystem in terms of species
richness and also density. Soil seed bank studies also confirmed the negative
impact of the invasion on flora diversity. However, the invaded fields were
observed to be rich in faunal diversity and an ideal home for several water birds
especially purple moorhen. The thick stand of the weed was found to provide
refuge to small animals like rats and snakes also; and these in general were
causing trouble to rice farmers in adjacent areas.
Contrary to several earlier reports, the impact on the ecosystem functioning
parameters was negligible, probably due to occasional flooding and
sedimentation in the selected locality.
To develop an eco-friendly strategy for managing the weed, an experiment
was undertaken in a heavily infested paddy field (Part IV) with eight
treatments and three replications and the design was RBD. The treatments were:
T1 - (Bensulfuron methyl + Pretilachlor) @ 660g ha-1 on the next day of tilling.,
T2 - (Bensulfuron methyl + Pretilachlor) @ 1320g ha-1 on the next day of tilling,
T3 –Azimsulfuron @ 35g ha-1 at 3-5 leaf stage , T4 -Azimsulfuron @ 70 g ha-1 at
3-5 leaf stage, T5 - (Glyphosate @ 1.0 kg ha-1 + 2, 4 – D sodium salt @ 2.0 kg
ha-1) at active growth stage, T6 - (Glyphosate @ 0.5 kg ha-1 + 2, 4 – D @ 1.0 kg
ha-1) at reproductive stage, T7 -Tillage (Farmer’s practice) and T8 – Control
(Uninterrupted weed growth). Among the various treatments, application of
Glyphosate @ 0.5 kg ha-1 + 2, 4 – D @ 1.0 kg ha-1 at the reproductive stage was
found to be the best, giving complete weed kill with no further regrowth.
The opportunities for utilising the weed and thereby managing it were also
investigated during the study. The results revealed that greater club rush with its
huge biomass production (>30 t ha-1) could remove many of the heavy metals
(arsenic, copper, zinc, lead, chromium, cobalt, nickel and chromium) from
contaminated soils more efficiently than water hyacinth which is a well known
phytoremediator. The nutritive value of the weed biomass (crude protein - 7.5
per cent, crude fibre- 26.79 per cent) was shown to be comparable with that of
guinea grass (crude protein - 8 to14 per cent, crude fibre - 28 to 36 per cent). The
K/(Ca+Mg) ratio was 1.39 which is considered to be within safe limits in a
livestock feed. However being a phytoextractor, further studies needs to be done
for justifying its use as a forage.
The study revealed that greater club rush has high level of ecological
flexibility and competitiveness which make it a successful invader. The weed has
already invaded considerable area of wetland rice ecosystem and is likely to
spread further in areas left fallow for long periods. It was found that the invaded
fields could be efficiently recovered by
application of a tank mixture of
glyphosate @ 0.5 kg ha-1 + 2, 4 – D sodium salt @ 1.0 kg ha-1 at the reproductive
stage. The major negative impact of the invasion on ecosystem was the decline
in species richness and density of native weed community. At the same time, the
thick weed growth provided ideal habitat for many faunal species. The weed was
found to have the potential to be used for phytostabilisation of heavy metal
contaminated soils and the luxuriant biomass had reasonable nutritive value also.
The overall conclusion is that greater club rush invasion can become a potential
threat to the wetland rice ecosystem, if left undisturbed.