Native Plant Richness Research Articles

Prescribed burning for control of yellow starthistle (Centaurea solstitialis) and enhanced native plant diversity

Two separate open grassland areas within Sugarloaf Ridge State Park, Sonoma County, CA, were burned for three consecutive years (1993–1995 [Site A] and 1995–1997 [Site B]) for control of yellow starthistle. Burns were conducted in late June to early July following seed dispersal and senescence of desirable grasses and forbs but prior to viable seed production in yellow starthistle. After the first year burn, there was no significant reduction in yellow starthistle cover the following spring and summer. Despite the lack of control, the first year burn reduced the yellow starthistle soil seedbank by 74% and the number of seedlings the following spring by 83%. However, total plant diversity and species richness increased dramatically in the burned areas. This was due primarily to an increase in the number of native broadleaf species. A second burn the next summer (1995–1997 site) reduced seedbank, seedling density, and summer vegetative cover the following year by 94, 92, and 85%, respectively, while maintaining significantly higher native plant cover and richness. A third consecutive summer burn decreased yellow starthistle seedbank and seedling density by 96, 98, and 85%, respectively, in the 1995–1997 burn site. Three consecutive years of burning in the 1993–1995 site reduced yellow starthistle seedbank and seedling density by over 99% and summer vegetative cover by 91%. These results indicate that prescribed burning can be an effective tool for the management of yellow starthistle and can have a long-term benefit on native broadleaf diversity and richness.

Exotic plant invasion and understorey species richness: a comparison of two types of eucalypt woodland in agricultural Western Australia

Exotic plants are a major threat to native plant diversity in Australia yet a generic model of the invasion of Australian ecosystems by exotic species is lacking because invasion levels differ with vegetation/soil type and environmental conditions. This study compared relative differences in exotic species invasion (percent cover, spp. richness) and the species richness of herbaceous native plants in two structurally very similar vegetation types, Gimlet Eucalyptus salubris and Wandoo E. capillosa woodlands in the Western Australian wheatbelt. For each woodland type, plant variables were measured for relatively undisturbed woodlands, woodlands with >30 years of livestock grazing history, and woodlands in road-verges. Grazed and road-verge Gimlet and Wandoo woodlands had significantly higher cover of exotic species, and lower species richness of native plants, compared with undisturbed Gimlet and Wandoo. Exotic plant invasion was significantly greater in Gimlet woodlands for both grazed (mean 78% cover) and road-verge sites (mean 42% cover) than in comparable sites in Wandoo woodlands (grazed sites 25% cover, road-verge sites 19% cover). There was no significant difference in the species richness of exotic plants between Wandoo and Gimlet sites for any of the three situations. Mean site richness of native plants was not significantly different between undisturbed Wandoo and undisturbed Gimlet woodlands. Undisturbed woodlands were significantly richer in plant species than grazed and road-verge woodlands for both woodland types. Grazed and road-verge Wandoo sites were significantly richer in plant species than communities in grazed and road-verge Gimlet. The percent cover of exotics was negatively correlated with total (native) plant species richness for both woodland types (Wandoo r = ?0.70, Gimlet r = ?0.87). Of the total native species recorded in undisturbed Gimlet, 83% and 61% were not recorded in grazed and road-verge Gimlet, respectively. This compared with 40% and 33% for grazed and road-verge Wandoo, respectively. Grazed Wandoo and grazed Gimlet sites had significantly fewer native plant species than did road-verge Wandoo and road-verge Gimlet sites. Ecosystem implications of differential invasions by exotic species, and the effects of grazing (disturbance) and other factors influencing susceptibility to exotic plant invasion (landscape, competition and allelopathy) on native species decline are discussed. Exclusion of livestock and adequate methods of control and prevention of further invasions by exotic plants are essential requirements for the conservation of these woodland systems.

Biodiversity indicators in semi-arid, agricultural Western Australia

The predicted future loss of native Australian species of plants and animals, in part as a result of adverse land management strategies, has led to attempts to identify areas of high biotic richness (numbers of species). Bioindicators are measures of the physical environment, or of a subset of the plants or animals, that best predict biotic richness. Ideally, bioindicators should aim at predicting as large a component of the plant or animal fauna as is possible at minimum cost. For two contrasting vegetation types, we examined remnant area, vegetation structural diversity, species richness of plants, lizards and terrestrial arthropods, and the relative abundance of individual arthropod species, as indicators of faunal richness, using correlation, principal component regression and stepwise regression analyses. The study was carried out in gimlet Eucalyptus salubris woodlands (29 sites) and shrublands (27 sites) in semi-arid, agricultural Western Australia. Sites varied considerably in grazing history (woodland) and in farming history (shrubland). Fauna sampled were lizards (woodland), scorpions (woodland), isopods (woodland), cockroaches (woodland), termites (woodland, shrubland), earwigs (woodland), hemipterans (shrubland), beetles (woodland, shrubland), butterflies (shrubland) and ants (woodland, shrubland). None of the indicator variables in any analyses effectively predicted total faunal richness for either vegetation type (<35% of variation in total richness explained). In correlation analyses for woodlands, vegetation structural diversity and plant richness, but no fauna variable, explained a high percentage of the variation in the richness of lizards (56% explained by richness of native plants, +ve), scorpions (48%, richness of native plants, +ve), termites (55%, vegetation structural diversity, +ve) and beetles (59%, litter, –ve). The richness of the shrubland fauna was poorly predicted by all indicator variables (<25% explained). When using the total richness and abundance of ant functional groups, the abundance of a subset of species within ant functional groups, and of termite and beetle species, in principal component regressions, various ant functional groups explained 42% each of the richness of scorpions and beetles, and eight beetle species explained 50% of termite richness. When remnant area, vegetation structural diversity and the richness of native plants in woodland were tested in step-wise regressions as indicators of total faunal richness, remnant area was the only significant indicator variable, explaining 33% of total richness. The richness of native plants and vegetation structural diversity explained a total of 76% of the pooled richness of lizards + scorpions + termites. No significant indicator variable was found by regression procedures for total richness, or for a subset, of the shrubland fauna. We argue that differences in the predictive qualities of vegetation structure and plant richness between the vegetation types was due, in part, to differences in the spatial heterogeneity of biotic richness, and possibly the scale at which structure was measured. The use of structural diversity or plant richness as predictors of faunal richness for different woodland types, or those with different disturbance histories, or in different geographic or climatic regions, should not be adopted without verification of their efficiency at predicting the richness of the local fauna.

Predicting Richness of Native, Rare, and Exotic Plants in Response to Habitat and Disturbance Variables across a Variegated Landscape

Species richness of native, rare native, and exotic understorey plants was recorded at 120 sites in temperate grassy vegetation in New South Wales. Linear models were used to predict the effects of environment and disturbance on the richness of each of these groups. Total native species and rare native species showed similar responses, with richness declining on sites of increasing natural fertility of parent material as well as declining under conditions of water enrichment (resulting from human‐induced changes in drainage characteristics, leading to increased run‐off), severe livestock grazing, and soil disturbance. The response of rare native species to water enrichment, however, was significantly greater than that of all native species. Exotic species richness varied in reverse to that of native species with positive responses to water enrichment and soil disturbance. The contrasting behaviors are attributed to differences in the evolutionary history of native and exotic assemblages and their resulting preadaptations to a landscape recently subjected to agricultural settlement. It would appear that for exogenous disturbances, the intermediate disturbance hypothesis is not supported by our data. In the sampled region, pastures represent the major land‐use in terms of area, but have relatively low densities of native and rare species compared with more lightly grazed areas. However, their management is considered to be essential to the maintenance of diversity on a regional scale.

Development of vegetation in small created wetlands in southeastern Wisconsin

We examined the natural colonization by vascular plants of 11 created wetlands in southeastern Wisconsin. The wetlands studied were small depressional wetlands that were isolated from other wetland sites. Wetlands were sampled over a two-year period, providing samples of wetlands aged one to three years. The development of wetland vegetation in these 11 naturally colonized sites was compared to that in five wetlands to which we introduced 22 species of native wetland plants. We identified 142 species of vascular plants in the naturally colonized wetlands. Of these, 82 (58%) were native, obligate or facultative wetland plants. The diversity and richness of native wetland plants and the proportion of total plant cover that was comprised of native wetland species increased from one-to three-year-old wetlands. The diversity and richness of native wetland species increased with wetland age, wetland size, and with proximity to the nearest established wetland. Distance to the nearest seed source had a particularly strong effect on the number of native wetland species present. Cattail (Typha spp.) accounted for 15% of the cover of native wetland species in one-year-old wetlands; this increased to 55% in three-year-old wetlands. We predict that the naturally colonized wetlands will develop into near monocultures of cattail with a fringe of willows (Salix spp.) and cottonwood (Populus deltoides) at the wetland/upland margin. Wetlands seeded with native wetland species had much higher diversity and richness of native wetland species than unseeded wetlands after two years. Seventeen of the 22 seeded species became established in at least two wetlands after simple introduction of seed to the sites. Cattail cover after two years was lower in seeded sites, both as an absolute cover and as a proportion of native wetland plant cover. Early introduction of a diversity of wetland plants may enhance the long-term diversity of vegetation in created wetlands.