Influence of Simulated Defoliation and N Fertilization on Compensatory Growth of Gmelina arborea Seedlings

To investigate the effect of artificial defoliation and N availability on growth of Gmelina arborea, seedlings were subjected to three artificial defoliation levels (0, 25, 50%) and four N regimes (unfertilized, 1 g N plant-1, 3 g N plant-1, 6 g N plant-1) in a field trial. The results showed that height increment was 24.09% lower in the 50% defoliation than the undefoliated and 25% defoliation treatments which were not significantly different from each other. On average, the 25% and 50% defoliation treatments reduced stem volume increment by 44.34%. Increments of diameter and biomass and leaf production were not reduced by defoliation. In terms of response to N, increments in height and stem volume rose from 8.98 cm and 8.23 mm at unfertilized to 11.39 cm and 12.13 mm at 3 g N plant-1, respectively, while number of new leaves increased by a margin of 1.51 from unfertilized to 6 g N plant-1. Total biomass increment that was unaffected by defoliation showed an increasing trend from 0.55 g at unfertilized and 1 g N to 0.83 g at 3 g N and 0.94 g at 6 g N plant-1. There was no significant interactive effect of treatments on any parameter, suggesting that the adverse effect of defoliation on growth of G. arborea seedlings may not be alleviated by N fertilization. It is encouraged that a similar study be conducted for a longer duration to ascertain if the responses are sustained or modified.

Methanogenic potential of commonly utilised South African subtropical and temperate grass species as influenced by nitrogen fertilisation

The aim of the study was to evaluate the effect of nitrogen (N) fertilisation on certain quality parameters and in vitro total gas and methane production of improved grass species commonly used as fodder species in South Africa. Treatments included seven grass species representing two photosynthetic pathways (C3 and C4) with three levels of N fertilisation (0, 50 and 100kg N ha–1). Plants were grown in a greenhouse and N was applied in a single application after a simulated defoliation. Sample material was harvested by hand after an 8-week regrowth period. Grass species and rate of N fertiliser both had effects (P<0.05) on the nutritive value and in vitro organic matter digestibility of the selected species. Crude protein concentration increased (P<0.05) and neutral detergent fibre concentration tended to decrease as the level of N fertilisation increased for both C3 and C4 species. Generally, no effect was found of N fertilisation on in vitro total gas or methane production; however, increasing the level of N fertiliser increased (P<0.05) the methanogenic potential (in vitro methane/in vitro total gas production) of D. glomerata, F. arundinacea and C. ciliaris after a 24-h incubation period but no significant effects were reported after a 48-h incubation period.

Effects of Simulated Defoliation on Growth and Photosynthetic Characteristics of an Invasive Liana, Ipomoea cairica (Convolvulaceae)

To better understand the response of Ipomoea cairica (Cairo morningglory) to herbivory, the compensatory growth and photosynthetic characteristics of plants were measured after simulated herbivory by leaf trimming at four intensities: 25, 50, 75, and 100% removal, starting from the apex. Defoliation at 25% had no significant influence on plant biomass, but the total biomass (−19 to −66%) and root biomass (−31 to −75%) of the plants decreased significantly when defoliation intensity was ≥ 50% (P < 0.05). Photosynthetic rates (Pn) increased with defoliation intensity (P < 0.01), and Pn values in the defoliated plants were 10 to 72% greater than those in the control plants, a relationship that could be attributed to a decrease in stomatal limitation (−11 to −34%) and the increase in rubisco content (9 to 18%) as well as higher photosynthetic efficiency and less light energy dissipated as heat. At defoliation intensities up to 50%, plants needed more energy to compensate photosynthetically, which could influence the plant photosynthetic characteristics as well as the allocation of assimilates, resulting in less root development. Since the spread of I. cairica depends primarily on clonal growth, smaller roots could limit uptake of nutrients from the soil. These direct and indirect effects indicate that leaf-feeding herbivores may have potential for biological control of I. cairica but to have any effect the herbivores would need to consume ≥ 50% of the leaf biomass.

Effects of Simulated Defoliation on Leafy Spurge (Euphorbia esula)-Infested Rangeland

The effect of annual defoliation on leafy spurge-infested rangeland was investigated over a 5-yr period. Artificial defoliation was conducted once or twice annually at various phenological growth stages of leafy spurge to simulate grazing by herded Angora goats. Single defoliation treatments did not reduce total leafy spurge stem densities. Defoliation twice in a growing season for 4 consecutive yr reduced total leafy spurge stem densities by 55% over nondefoliated controls. Grass foliar cover and yield increased in all defoliation treatments. Despite the increased grass yield on single defoliation treatments, cattle use would likely be limited in these treatments due to the high density of leafy spurge stems. Stem densities of leafy spurge in twice-defoliated treatments should not deter cattle from grazing these sites, thereby increasing the available forage supply. The data suggest that repeated grazing of leafy spurge-infested rangeland within a growing season would be required to reduce stem densities adequately and increase cattle use and production from these sites.