Constraints to Blady Grass (Imperata cylindrica (L.) P.Beauv.) Distribution on the New England Tablelands of New South Wales

Abstract

<p>Blady grass (<i>Imperata cylindrica</i>) is a major invasive weed that threatens agricultural systems, biodiversity and ecosystem function throughout tropical and sub-tropical to warm temperate regions around the world. It is able to reproduce both sexually via seeds and asexually via rhizomes, making it a difficult species to control or eradicate. It is adapted to a range of soil types, growing in poor acid soils with low fertility and organic matter, and is tolerant of fire and soil salinity. Once established, it is regarded as a strong competitor for nutrients, water and light. Although primarily a species of tropical and other warm environments, blady grass occurs in the New England region on the Northern Tablelands of New South Wales, Australia, in a cool temperature environment. Here, it occurs in patches, many of which appear to be expanding very little over time with few new patches being formed. The question arises as to how blady grass reproduces in this environment and what constrains its spread and distribution. However, there has been a lack of information about the ecology of blady grass in this region as well as more generally in Australia. Such information is critical for the design of effective future management programs. Therefore, this research focused on quantifying the size of the blady grass soil seedbank, assessing the longevity and germination of seeds, measuring flowering and growth responses to a range of environmental factors, and recording changes in rhizomes and their regrowth.</p> <p>The soil seedbank is a vital mechanism for the establishment, persistence, and re-invasion of many weedy species. Through a germination study, the soil seedbank was explored within and outside blady grass patches in the New England region. The results showed no germinable seeds of blady grass to be present. Potential explanations for this absence of germinable seeds included the presence of seed dormancy, a lack of seed production in this cool temperate environment, and a short period of seed persistence under field conditions. Most seeds of other weed and pasture species were concentrated in the top 5 cm of the soil profile and there were more seeds at the edge and outside of blady grass patches than in the centre of the patch. These results indicate that blady grass has a strong competitive or allelopathic impact on the growth and re-production of pasture species.</p> <p>In a series of laboratory and field experiments, the longevity and germination of seeds was found to vary according to storage and burial conditions. Seeds were not initially dormant and lost viability relatively rapidly (from 89% to 23% over 5 months) when stored at room temperature, while most seeds had either disappeared or were no longer able to germinate after 3 months of burial in the field. Seeds remained viable for a longer period of time when not buried. As a result, biological processes associated with soil moisture, such as germination and microbial decay, were thought to be responsible for the more rapid drop in seed viability when buried. The short life of seeds under field conditions is likely to constrain blady grass spread and persistence through the mechanism of seed dispersal. These results showed that regardless of their small size, light weight and ability to travel by wind over long distances, seeds may play a lesser role in the invasion of blady grass than rhizomes in cool temperate Australia.</p> <p>Changes in temperature, day length, fertilizer, foliage management (cutting and burning), and water stress failed to stimulate flowering of blady grass, while several of the treatments had a large impact on the growth of the weed. The results showed that cooler temperatures, as experienced in the New England region, constrained growth relative to warmer temperatures as found in more tropical areas. In the high-altitude New England region, blady grass is likely to be most active in summer, and has mechanisms for obtaining light and responding well to increased soil fertility, suggesting it is likely to be highly competitive against summerdominant native pasture grasses. However, both cutting and burning reduced the growth of blady grass and may offer some potential for control of the weed. A lack of flowering and viable seed production may well be a contributing factor in the limited spread and distribution of blady grass.</p> <p>The re-sprouting and growth of rhizome fragments depended on their size, colour, and presence of terminal meristematic tissue. The results showed that rhizome fragments with a greater number of nodes had more shoot and rhizome regrowth, while there was significant interaction between the effects of node number, rhizome colour, and presence of tips on regrowth. Rhizomes from different sources also varied in weight and diameter and this affected the degree of change they underwent following soil burial. While the capacity of rhizomes to contribute to the spread of blady grass is likely to vary, they are clearly responsible for the slow short-distance spread of blady grass patches in the New England region and may also have been responsible for longer distance spread when the weed was first introduced to the region.</p> <p>As far as can be determined, the patches of blady grass in the New England region that were included in this study have been in the same places for many years. How the local populations of blady grass were initially established is unknown. However, these studies support the hypothesis that these high elevation cool-climate populations of blady grass are 'relic' populations, that seeds do not play a significant role in the persistence of this species in the region, and that there is little evidence for long distance expansion under the current climatic conditions and land management regime. Whether climate change in future alters this situation remains to be seen. This research provides important information about the ecology of blady grass in the New England region that can help inform the design of effective and sustainable management strategies for its control.</p>