Investigating the competitive ability of endemic Osbeckia octandra with its co- occurrence with invasive Sphagneticola trilobata and native weed, Urena lobata

Abstract

In Sri Lanka, Osbeckia octandra (L) DC. is a widely used valuable endemic herb in Ayurvedic medicine, popular natural remedy among Sri Lankans and an important pollinator plant. Their local populations are disappearing rapidly due to various anthropogenic activities. Further, local roadside habitats of some O. octandra populations are under the threat of invasion by several exotic and native weedy species. When the competition exerted by those competitively superior invasive and weedy species become severe, O. octandra populations can be threatened for extinction. Thus, the present study was carried out to investigate the competitive ability of O. octandra with its co-occurring exotic invasive Sphagneticola trilobata (L.) and native weedy Urena lobata (L.) species under soil water and nutrient gradients. Pot experiments were conducted with O. octandra monocultures as controls and O. octandra mixed-cultures with either S. trilobata or U. lobata as experimental pots to study the inter-specific competition for soil nutrients under three different nutrient levels (low, moderate and high) and for soil water, under two different water availabilities (well-watered and water-stressed). One set of 12 pots (4 pots of O. octandra/S. trilobata; 1:1 pots+ 4 pots of O. octandra/U. lobata 1:1 pots+ 4 pots of control pots) was prepared for every assigned nutrient and watering condition. After a five-month experimental period, the mean values of final plant heights, fresh weights, above and below ground dry biomasses of O. octandra plants were recorded and obtained data were analyzed by Analysis of Variance (Two- way ANOVA, P < 0.05) and Tukey’s pairwise comparison using the MINITAB 17 statistical software. According to the results, O. octandra plants in monocultures have performed better than the O. octandra from mixed cultures under all the tested different nutrient and water availability conditions, indicating that invasive S. trilobata and weedy U. lobata have significantly higher competitive abilities over endemic O. octandra along with the soil nutrient and water continua (P < 0.05). Having significantly lower root biomass of O. octandra in mixed cultures compared to the monoculture under low nutrient condition (P < 0.05), revealed that O. octandra has a low phenotypic plasticity. Also, our results revealed that non-invasive U. lobata was equally competitive as invasive S. trilobata against O. octandra under different soil nutrient conditions. Moreover, S. trilobata exerted a significantly higher competition than by U. lobata on O. octandra under well-watered condition (P < 0.05), while under water stressed condition U. lobata exerted a significantly higher competition than by S. trilobata (P < 0.05). Our results clearly shown, that the native weedy U. lobata has a potential to become invasive locally as S. trilobata in future, and suppress the growth of native, less competitive flora. Thus, further studies on invasiveness of native weedy species on endemic species will be needed to investigate.