Prolonged summer flooding switched dominance from the invasive weed Lippia ('Phyla canescens') to native species in one small, ephemeral wetland.
Lippia ('Phyla canescens', Verbenaceae) is an invasive perennial forb from South America that is now widespread throughout the Murray Darling Basin in Australia, threatening the biological values of internationally significant wetlands (Ramsar sites) and the productivity of the grazing industry (Earl 2003). In the Gwydir Wetlands in north western NSW, the spread of Lippia has been associated with large and widespread flooding that occurred in the late 1990s (McCosker 1998). Lippia dominance is believed to be favoured by the altered flood regimes associated with river regulation, namely a reduction in the frequency and duration of flooding. Studies have found Lippia to occur in infrequently flooded sites (Blanch et al. 1999; Mawhinney 2003) and at higher elevations in the floodplain (Blanch et al. 2000). Glasshouse studies have found that Lippia growth is stunted at inundation depths of 20 cm, although no mortality was reported (McCosker 1994; Hobson 1999). This has lead to a widespread belief that restoration of a more 'natural' flood regime may assist in Lippia control. However, few data are available to support this notion.
Periodic rest from grazing provided no control of an invasive perennial forb
This study examined if one grazing strategy (namely seasonal rest) was effective in the control of the invasive forb 'Phyla canescens' (Kunth) Greene (hereafter lippia). We examined if rest from grazing could increase the competitiveness of native palatable species by allowing time to recover from defoliation, thereby altering competitive interactions between native species and lippia. In a field trial, we manipulated cattle grazing to determine its effects on the biomass of lippia and native species. We compared rest from grazing at different times of the year with year-long grazing (low intensity continuous and high intensity short duration) and no grazing (permanently excluding large grazing animals). Experimental plots were stratified into different hydrological areas (approximately annual flooding and flooded less than once every 5 years) to include flood dynamics in the management scenarios. We detected no negative impacts of seasonal rest on lippia, but some positive effects on native species. We found that complete exclusion from grazing in areas that already have substantial lippia invasion (and no flooding) may actually favour the expansion of lippia (at certain times). This study does not suggest that grazing management cannot be used as a tool for lippia control - simply that seasonal resting had no effect over a 3-year period. This is likely due to the dynamics of a boom-and-bust landscape in which if the abiotic conditions are not suitable for growth, then native species will not grow whether grazed or rested.
Vegetative reproduction facilitates early expansion of 'Phyla canescens' in a semi-arid floodplain
'Phyla canescens' (Verbenaceae, hereafter lippia) is an invasive perennial forb from South America that is now widespread in Australia, threatening the biological values of internationally significant wetlands (Ramsar listed) and the productivity of the grazing industry (Crawford 2008; Earl 2003). Under certain conditions, lippia can displace native herbaceous vegetation and form mono-specific stands, but little is known about the process of colonization and early invasion. Lippia has a dual reproductive strategy, recruiting via seedlings and vegetative fragments in periods following inundation (Macdonald 2008). Determining the relative contribution of vegetative and sexual reproduction for lippia across environmental gradients is important for management action - if spread is primarily by vegetative fragments then control efforts should be focussed on managing vectors (e.g. movement of stock and vehicles), whereas if spread is mostly by seeds then management should focus on reducing the conditions favourable to seed production and seedling establishment. The predominant reproductive mode also has implications for the selection of biocontrol agents.
Using strategically applied grazing to manage invasive alien plants in novel grasslands
Introduction: Novel ecosystems that contain new combinations of invasive alien plants (IAPs) present a challenge for managers. Yet, control strategies that focus on the removal of the invasive species and/or restoring historical disturbance regimes often do not provide the best outcome for long-term control of IAPs and the promotion of more desirable plant species. Methods: This study seeks to identify the primary drivers of grassland invasion to then inform management practices toward the restoration of native ecosystems. By revisiting both published and unpublished data from experiments and case studies within mainly an Australian context for native grassland management, we show how alternative states models can help to design control strategies to manage undesirable IAPs by manipulating grazing pressure. Results: Ungulate grazing is generally considered antithetical to invasive species management because in many countries where livestock production is a relatively new disturbance to grasslands (such as in Australia and New Zealand as well as Canada and the USA), selective grazing pressure may have facilitated opportunities for IAPs to establish. We find that grazing stock can be used to manipulate species composition in favour of the desirable components in pastures, but whether grazing is rested or strategically applied depends on the management goal, sizes of populations of the IAP and more desirable species, and climatic and edaphic conditions. Conclusions: Based on our findings, we integrated these relationships to develop a testable framework for managing IAPs with strategic grazing that considers both the current state of the plant community and the desired future state - i.e. the application of the principles behind reclamation, rehabilitation, restoration or all three—over time.