Plant Species Composition and Soil Seedbank in Parthenium Hysterophorus L. Invaded Grassland of Hetaunda, Central Nepal

Abstract

Impact of Parthenium hysterophorus L. invasion on the herbaceous plant species richness and composition of a grassland and its abundance in the total germinable soil seedbank were monitored for two years in Hetaunda Municipality of Makawanpur district, Central Nepal. The study also dealt with the impacts of defoliation caused by the leaf feeding beetle Zygogramma bicolorata on the germinable soil seedbank density of Parthenium. Field samplings were done in two steps; vegetation sampling and soil sampling. The vegetation samplings were done in September and the soil samplings in October for the two successive years 2009 and 2010 representing 1 vii

st and 2 nd year of defoliation by Zygogramma, respectively. A total of 30 transects, 10 on each of the three study sites, selected subjectively were sampled. On each transect, 3 quadrats of 1m  1m were sampled in such a way that they represented >90%, >40≤60% and 0-10% Parthenium coverage, indicating High, Medium and Low levels of Parthenium infestation, respectively. Altogether 90 quadrats (3 on each of the 30 transect) were sampled. In each quadrat, vascular plant species richness, Parthenium density, its coverage, maximum height and the coverage of other species were recorded. For the estimation of germinable soil seedbank density, soil samples were collected from the plots having high Parthenium infestations at two different depths (0-5 cm and 5-10 cm) by using soil core sampler, and thus collected soil samples were kept for germination in the greenhouse for eight months and soil seedbank was analyzed by counting germinating seedlings. There were no significant differences in the herbaceous plant species richness at different levels of Parthenium infestation but we found change in the species composition. Parthenium had the highest abundance on the germinable soil seedbank measured up to 10 cm soil depth and comprised about 4/5 th of the total soil seedbank. We did not find significant change in the germinable soil seedbank density measured for 2009 and 2010. The study suggests that Parthenium invasion is affecting the species composition of grassland Defoliation caused by the Zygogramma has not been so effective in reducing the soil seedbank density owing to the recent event and persistent nature of its germinable soil seedbank. Thus, for effective control of Parthenium, defoliation has to be continued for some more years associated with other long-term management programs. Keywords: Biocontrol, germinable soil seedbank, Species richness, Zygogramma bicolorata.