Testing a facilitation model for ecosystem restoration: Does tree planting restore ground layer species in a grassy woodland?

Planning for the restoration of degraded ecosystems has a strong basis in facilitation successional theory, which, as applied in restoration practice, states that planting of structurally dominant tree species will assist the entry of other native species into a restored community. In Australia, tree planting has been widely applied in restoration of grassy woodland ecosystems. Trees have been postulated to reduce the cover and diversity of weed species, thus facilitating recolonization of native woodland species (indirect facilitation). The expected outcomes of this process include reduced species richness and abundance of exotic plant species and increased species richness and abundance/dominance of natives in areas beneath tree canopies, with these trends strengthening with time. To assess whether this was occurring, we carried out a comparative analysis of species assemblages found underneath and outside of planted tree canopies in sites replanted with juvenile canopy tree species 3–5 or 8–10 years previously. We sampled revegetated stands of Cumberland Plain Woodland, an endangered ecological community in Western Sydney, Australia. We found that neither the number nor abundance of native ground layer species beneath canopies increased as a result of trees being planted at sites of both ages. Where seed is limited, we predicted an increase in abundance of existing native species under planted tree canopies. On this point, the results were mixed and showed some natives with an increased abundance while others decreased. Exotic species richness showed the reverse of the expected pattern, being greater under tree canopies. These findings lend no support to the theory of indirect facilitation. We conclude that simple facilitation models may be inadequate to support planning of grassy woodland restoration and that those models incorporating successional time lags and restoration barriers are likely to be more informative about the development of communities initiated by tree planting.     

The influence of plant size on attack rates of Oxyops vitiosa,an introduced herbivore of Melaleuca quinquenervia

Oxyops vitiosa Pascoe (Coleoptera: Curculionidae) was released in Florida in 1997 as a biological control agent of the invasive tree Melaleuca quinquenervia (Cav.) S.T. Blake. Three years after release, the weevil’s dispersal into three replicated M. quinquenervia stands was monitored to determine if O. vitiosa preferentially selects larger trees during patch colonisation. Data indicated that tree size did not influence the likelihood of an attack by O. vitiosa and smaller individuals within host patches did not escape herbivory through enemy-free space.     

Inviting trouble: Introduced pasture species in northern Australia

Abstract I surveyed the history of exotic pasture introductions in northern Australia to compare the rate of introduction of useful species with that of weeds. Between 1947 and 1985, 463 exotic grasses and legumes in at least 2033 accessions were intentionally introduced into the region, the grasses predominantly from Africa, and the legumes from Central and South America. Of these, only 21 (5%) came to be recommended as useful, while 60 (13%) became listed as weeds. Seventeen of the useful plants were in fact also weeds, leaving only 4 species (<1%) that were useful without causing weed problems. They were far outnumbered by the 43 species (9%) that were weeds but had no recorded use. Of the 60 weeds, 21 were weeds of cropping, 20 were weeds of conservation and 19 were weeds of both sectors. Thirteen were listed as major crop weeds. Growth rate, maximum height, seed size, and continent of origin were unrelated to weediness. Good predictors of weediness were whether a plant was useful, a good performer in trials, or persisted at a field site. In addition, grasses were more likely, and legumes less likely, to be weedy than expected. It is argued that a new form of assessment is required before an exotic plant species is released into the Australian environment as a pasture plant. This would recognize that a successful introduction is almost certain to become a weed in some situations, and would attempt to assess the net national benefit of the proposed introduction.     

The use of herbicides for weed control in flooded rice in North Italy

The use of chemicals to control weeds in flooded rice in the Po Valley, Italy is restricted by limited knowledge of potentially useful herbicides. Different strategies of chemical weed control, including different application timings of several types of herbicides are compared based on effectiveness and selectivity. Echinochloa spp., the most common weed was, in the experimental conditions, better controlled with early applications (pre-sowing and preemergence) of new herbicides which were effective substitutes for molinate, the use of which is no longer allowed in some Italian areas. Alisma plantago-aquatica, Bolboschoenus maritimus and Schoenoplectus macronatus were controlled by different types of herbicides applied post-emergence. Cinosulfuron (sulfonylurea) showed the highest effectiveness and selectivity in controlling the more common Hetherantera spp., even when applied in very low amounts.     

Scale, Experimental Design and the Detection of Ineterspecific Competition within Plant Communities

Abstract We report preliminary results of a series of experiments designed to explore the importance of interspecific competition within arable weed communities at different scales. Competition hierarchies were apparent from a pot experiment with different levels of nutrients and water. Two field experiments looked at Bromus sterilis, Galium aparine and Papaver rhoeas in winter wheat in the field, in a range of combinations and management treatments, and a fourth field experiment included a wider variety of species. There was little effect of fertilizer on population behaviour in the the field. Bromus increased around ten fold per year on minimum-tilled plots, regardless of other treatments. Galium increased on organically-fertilized and minimum-tilled plots, but only in the absence of Bromus. Papaver densities remained low, but again were depressed in the presence of high densities of Bromus . Taken together, the experiments demonstrate the existence of competition between weed species. However, as the design of the experiment increased to include greater levels of environmental variation, so competition became more difficult to detect, and less useful for interpreting the results than knowledge of the biology of the individual species. At the scale of interest to the farmer, the level of competition is not a good predictor for weed population dynamics.     

Effect of environmental factors on disease development caused by the fungal pathogen Plectosporium alismatis on the floating-leaf stage of starfruit (Damasonium minus), a weed of rice

Starfruit, Damasonium minus (R. Br.) Buch, is a herbicide-resistant, aquatic-weed occurring in Australian rice fields. The fungal pathogen Plectosporium alismatis (Oudem) W.M. Pitt, W. Gams & U. Braun is a potential mycoherbicide for the control of this weed. Previous work showed that the fungus can suppress the growth of starfruit juvenile plants. This paper reports the effect of environmental factors on the disease development on starfruit at the floating-leaf stage. Fungal inoculation of plants in advanced phases of floating leaf stage reduced significantly (P<0.05) the green leaf area, but only when the percentages of diseased areas were deducted. However, inoculation when the first floating leaf was emerging reduced significantly (P<0.001) both, plant dry above-ground biomass and leaf area.     

Geigeria alata in the Namib desert: seed heteromorphism in an extremely arid environment

Abstract. Seed heteromorphism in annual Geigeria alata in the Namib Desert was investigated in relation to environmental conditions. Geigeria alata shows heteromorphism in seed dispersal distance depending on the position of the seeds on the parent plant. Seeds produced in capitula on branches (aerial) travel long distances, while seeds produced at the base of the plant remain in the vicinity of the parent plant. The ratio of aerial to basal seed production was investigated under different environmental conditions following an aridity gradient. Although there were significant differences in seed and capitula production between seasons and between different positions on the plant, no overall trend was found with increasing aridity.