Sansamma GeorgeGayathri Karthikeyan, PKAU2020-03-092020-03-092017http://krishikosh.egranth.ac.in/handle/1/5810145068Ph.DThe 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.ennullThesis