Monthly and seasonal variations of biomass partitioning and macronutrients in the invasive grass Vossia cuspidata (Roxb.) Griff.
Vossia cuspidata (Roxb.) Griff. (Poaceae) is a serious invasive plant in Africa, capable to colonize several riverine wetlands. Controlling of any invasive plant is hampered by the lack of studies involving its growth ecology. Here, we aimed to evaluate the monthly and seasonal variations in the dry biomass partitioning of V. cuspidata and its macronutrients in three aquatic habitats (slopes, water edges and open water) along the banks of the River Nile in Cairo, Egypt. In the study area V. cuspidata started its growth in February 2018 by the formation of shoots, which sprouted from buds on the underground rhizomes, showed its maximum growth in autumn and entered the senescent stage in early winter. The monthly maximum biomass dry matter (BDM) of the root system (48.9 g DWm-2) occurred in October, and in April for rhizomes (110.4 g DWm-2). The BDM production of leaves was minimum in March at the three habitats while its maximum was obtained in September or October reaching 326.6 g DWm-2 on slopes, 303.6 g DWm-2 in water edges and 246.6 g DWm-2 in open water, respectively. The maximum monthly total BDM partitioning of V. cuspidata was 604.5 g DWm-2 on the slopes during September and reached 543.8 and 550.0 g DWm-2 in open water and water edges, respectively, during October. The highest total BDM production was obtained in autumn, while the mean of BDM of the four seasons was 1869.7 g DWm-2 ±150.9. Generally, the concentrations (% biomass DW) of the analyzed nutrient elements (K, Ca, Mg, N and P) showed mostly a gradual decrease with time during the year; starting from spring until summer or autumn, then increased again in autumn or winter. An important result of this study is that to conserve other native species, then the harvest of the aboveground organs of V. cuspidata should be carried out during late autumn in November after the translocation of reserved materials from the belowground organs to maximize V. cuspidata biomass removal.