Aristida

Abstract: Summary 1. The influence of inter-annual rainfall variability and cattle grazing on the abundance (density, basal area), population dynamics (recruitment, growth, survival) and population structure of the dominant perennial grasses of an African savanna was investigated from 1984 to 1989. The unpalatable Aristida bipartite and the palatable Heteropogon contortus and Themeda triandra reproduce by seed; Bothriochloa insculpta reproduces mainly by seed, but sometimes by stolons; Digitaria eriantha can be stoloniferous; Setaria incrassata has rhizomatous growth. 2. Species abundance was more responsive to rainfall variability than to grazing. Density increased in all species in response to several successive dry years because of recruitment and tuft fragmentation, but eventually decreased in a severe final drought year because of plant mortality. The response of basal cover differed between species and sites, although all species (except Digitaria) declined in the final drought year. 3. Seedling recruitment, plant basal area, and mortality were likewise more strongly influenced by rainfall variability than by grazing. Differences in the population dynamics of species were related to their life history attributes. Aristida, a prolific seed producer, was favoured by grazing because high levels of seedling recruitment could be maintained. Seedling recruitment of palatable species (Heteropogon, Themeda) and of Bothriochloa was initially increased under heavy grazing because openings were produced within the vegetation, but this was reversed by the end of the study because of a decline in available seed. There was limited seedling recruitment of Digitaria and Setaria, but recruitment of Digitaria from stoloniferous growth increased population size, although there was no expansion of Setaria clones under the conditions of this study. Themeda and Aristida seedlings attained greater size under heavy than under light grazing.

Abstract: We studied the response of vegetation and vertebrate assemblages to fire and grazing, and their interacting effects, in Eucalyptus woodland in north-eastern Australia. In this vegetation type, many pastures remain free of cattle grazing due to the occurrence of a native shrub poisonous to livestock. Vegetation (floristic data and 22 habitat variables) and vertebrate fauna (birds, mammals, reptiles) were sampled in 29 standardized 50 × 50-m quadrats in the 2001 wet season, representing four treatments: sites burnt recently (within 2 y) and grazed by cattle (4-8 ha per livestock unit); sites unburnt (last burnt >2 y ago) and grazed; sites burnt recently and ungrazed; and unburnt and ungrazed sites. Fire and grazing had a significant influence on vegetation: both grazing and fire reduced ground cover (fire in grazed sites 51-23%, fire in ungrazed sites 68-39%) and increased the cover of forbs (8% in burnt and grazed sites, 3% if ungrazed) and tussock grasses (20% in grazed and unburnt sites and 5% when ungrazed). Grazing caused a shift in floristic composition from the perennial hummock grass Trioda pungens to tussock grasses (e.g. Aristida spp., Enneapogon spp.), forbs (e.g. Phyllanthus spp.) and shrubs (e.g. Acacia spp.). Of the vertebrate groups, birds responded more to fire effects (9 species), reptiles to grazing effects (6 species) and mammals to the interaction (2 species). Species reacted to increases in bare ground (e.g. crested pigeon Ocyphaps lophotes, hooded robin Melanodryas cucullatus, Ctenophorus nuchalis) and to the dominant ground cover (e.g. Ctenotus pantherinus) or change in vegetation architecture (e.g. singing honeyeater Lichenostomus virescens, variegated fairy-wren Malurus lamberti). The clearest example of an interacting effect was the cycle of complementary dominance between the rodents Pseudomys delicatulus and P. desertor, the latter's post-fire recovery becoming more muted in sites where cattle grazed (modelled time for population recovery twice as long as in ungrazed sites).

Abstract: A vegetational survey on the Jornada Experimental Range in southern New Mexico, taken annually from 1941 through 1957, is the basis for a study of the effects of the great drought of 1951 through 1956. Both cover and yields were studied. Observations were stratified into seven classes based on a consideration of land form and soil characteristics. Seasonal and annual precipitation during the drought averaged 55% of the pre—drought average. The most severe drought years were 1951, 1953, and 1956. Both the cover and yield of Bouteloua eriopoda (Torr.) Torr., the dominant species on the upland sandy soils, were greatly reduced by drought. However, drought damage was much more severe on the deep than on shallow sands. When the impermeable caliche layer occurred at shallow depths, moisture relations during drought were apparently much better than when caliche occurred at greater depths. Another result of drought was the invasion of Prosopis juliflora (Sw.) DC. in areas where grass stands have been thinned by drought. Sporobolus spp. and Aristida spp., minor components of the climax, were more susceptible to drought damage than Bouteloua eriopoda. Yields of perennial grasses per unit cover were as great during the drought as prior to the drought. Both winter—spring and summer precipitation are important in preventing death losses of black grama. In arid areas it seems necessary to consider both cover and species composition in arriving at a potential for a site.

Abstract: A B S T R A C T Aqueous extracts of fresh roots and shoots of Andropogon virginicus (broomsedge) were found to be inhibitory to the growth of seedlings of Amaranthus palmeri, Bromus japonicus, Aristida oligantha, and Andropogon scoparius. The first two species are often important in the pioneer stage of old-field succession in eastern Oklahoma, Aristida is prominent in the second stage, and Andropogon scoparius is important later in succession including the climax Quercus stellata-Quercus marilandica savanna. Sterile dilute extracts of roots and shoots of broomsedge were inhibitory to two test species of Azotobacter, a free living nitrogen fixer, and to two species of Rhizobium, a symbiotic nitrogen fixer. Small amounts of decaying shoots of broomsedge (1 g per 454 g of soil) were very inhibitory to the growth of the four test species listed above and to Amaranthus retroflexus, another species often important in the first stage of succession. Similar amounts of decaying material in soil also significantly inhibited growth and nodulation of the two most important species of legumes in old-field succession in eastern Oklahoma, Lespedeza stipulacea and Trifolium repens. Broomsedge is known to compete vigorously and grow well on soils of low fertility, so the inhibition of nodulation of legumes could help keep the nitrogen supply low and give broomsedge a selective advantage in competition over species that have higher nitrogen requirements. The combined interference of broomsedge against other species resulting from competition and allelopathy could help explain why it invades old fields in 3-5 yr after abandonment from cultivation and remains so long in almost pure stands. ANDROPOGON VIRGINICUS L. (broomsedge) has been reported by many researchers throughout the