Abstract:
Since its introduction in Nigeria in the 1970s, Tithonia diversifolia (Td),an invasive species has posed increasing threats to crop production and native species diversity. However, the autecology of Td which plays a key role in providing information for its control is yet to be fully understood. This study thereforeinvestigated some autecological and reproductive traits of Td in Nigeria.
Principal Component Analysis-env was used tocompare the ecological niche of Td between its native range (Mexico) and its introduced range (Nigeria). The current and future geographical distributions of Td were modelled using Maximum Entropy principles. Impacts of Td were assessed on seed bank species diversity and soil physico-chemical properties using space-for-time substitution approach. Two Lowland Forests (LF), two Derived Savannas (DS) and one Jos Plateau Forest-grassland Mosaic sites were investigated. Nitrogen, Phosphorus and Potassium concentrations in soil and plant parts were determined using standard procedures in soil with highest Td. Mode of pollination, fecundity, germination and dormancy were assessed while seed bank behaviour and biomass were modelled in DS. Control of Td using paraquat dichloride, manual weeding and controlled agricultural burning were investigated using standard procedures. Data were analysed using descriptive statistics and Analysis of Variance (ANOVA) at α0.05.
Tithonia diversifolia occupies a different niche in Nigeria compared to Mexico (Schoener’s D=0.01, E=0.99). Maximum entropy models revealed that DS is most suitable for Td establishment. Tithonia diversifolia exerted no significant impact on seed bank diversity of invaded habitats. However, it significantly altered soil pH, cation exchange capacity, total N, inorganic PO4, organic C, available P, Fe, Zn and Cu. The leaves had significantly high levels of N, P and K compared to other plant parts. Reproductive allocation of nutrients in DS revealed that N ranged from 5.88-17.40%, P, 8.60-31.65% and K,7.73-22.53%. Tithonia diversifolia is facultatively xenogamous with 93% fruit set in open-pollinated capitula and a high pollen-ovule ratio (4,167±76). It produced 49±3 capitula/plant, corresponding to 454-8124 achenes/plant. Achenes of Td were permeable but showed morphological dormancy with low germinability (8.67%). Mechanical scarification and Gibberellic acid increased germinability by 40 and 65%, respectively. Tithonia diversifoliaformed a transient seed bank (<6 months) with 2811±201 achenes/m2. Seed bank density was best fit with exponential decay model (density=1712e^(-0.49time)+24), with an initial density of 1736 achenes/m2 at the rate of 0.49 achenes/week. Biomass of Td one month after emergence was 2.36±0.38 g/m2. This increased by 91% after two months. Biomass of Td followed a logistic model,biomass= 179.7⁄((1+855.4e^(-2.25time) ) ). Mature Td biomass was 179.56±22.54 g/m2, with the largest proportion (67%) allocated to shoots. Paraquat dichloride application was most efficient in controlling Td with over 80% seedling mortality and 50% reduction in plant height.
The prolific seed production and rapid vegetative growth of Tithonia diversifolia are responsible for its aggressive invasiveness. This species can be controlled using agricultural burning and systemic herbicide.
