Dense regeneration of floodplain 'Eucalyptus coolabah': invasive scrub or passive restoration of an endangered woodland community?
Clearing of native vegetation and changes to disturbance regimes have resulted in dense regeneration of native trees and shrubs in parts of Australia. The conversion of open vegetation to dense woodlands may result in changes to the composition of plant communities and ecosystem function if structure, composition and function are tightly linked. Widespread clearing of the floodplain tree 'Eucalyptus coolabah' subsp. 'coolabah' (coolibah), in New South Wales, Australia, has led to state and federal listings of coolibah woodland as an endangered ecological community. Dense regeneration of coolibah in the mid 1970s, however, also resulted in its listing as an 'invasive native species' in NSW, meaning it can be legally cleared under certain conditions. Dense regeneration could be a novel state dissimilar to the threatened community or it could represent the next generation of coolibah woodlands and may contribute to passive restoration of heavily cleared landscapes. This study investigated if dense stands are distinct from remnant woodland by comparing floristic composition of the ground-storey community and top-soil properties of four coolibah vegetation states: derived grassland, derived degraded grassland, dense regeneration and remnant woodland. Ground-storey composition was found to overlap broadly among states regardless of tree density. Most species were common to all states, although dense regeneration contained characteristic woodland species that were absent from grasslands. The carbon : nitrogen ratio of the soil was significantly higher in dense regeneration and remnant woodland than in either of the grassland states, indicating that the woody states are broadly similar in terms of nutrient cycling. The study demonstrates that structurally different vegetation states (grasslands, woodlands and dense regeneration) are not associated with distinct plant communities. The results also suggest that grazing management has a more pronounced effect on ground-storey composition of plant communities than tree density and that well managed derived grasslands and dense regeneration are floristically similar to remnant woodlands. Since dense regeneration and remnant woodlands are not floristically distinct from one another, dense regeneration could contribute to the conservation of endangered coolibah woodlands in cleared agricultural landscapes.
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.
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.
Comparison of seedling emergence and seed extraction techniques for estimating the composition of soil seed banks
1. Characterization of the seed bank is one of the most important demographic assessments that can be undertaken for a plant community. Overlapping generations, evidence of past above-ground vegetation and histories of invasion and disturbance are recorded in the seed bank. 2. Two broad approaches have been used to elucidate seed bank components - sifting-sorting techniques and germinability assays. The utility of these approaches varies with community type and habitat although a common theme among studies has been the quest for an efficacious method. Here, we compare the two approaches for semi-arid ephemeral wetlands: seed extraction through flotation and seedling emergence. 3. Species composition of the soil seed bank differed dramatically depending on the technique, with only 19 species common to both methods and a total of 66 species detected using both procedures. 4. Both techniques provided similar estimates of seed density and species richness of the seed bank in the top 5 cm of soil. However, samples collected from 5 cm to 20 cm had lower seed densities using the flotation technique than with the seedling emergence technique. 5. Differences in seed detectability between the two approaches may be related to seed size, seed dormancy and specific germination requirements. 6. The community composition of soil seed banks for ephemeral wetlands depends on the choice of technique.
Stage 2: Grazing / Landuse in the Macquarie Marshes and Gwydir Wetlands: Final Report to the New South Wales Department of Environment and Climate Change
The Gwydir Wetlands and Macquarie Marshes are both examples of variable and dynamic floodplain wetland ecosystems. Both systems have experienced a major alteration to their hydrology as a result of river regulation, and the two regions have a long history of floodplain grazing. As efforts are made to address the hydrological issues in these wetlands, there is also a need to better understand how other disturbances such as grazing may be impacting on the ecological functioning of these ecosystems. This study investigated five areas of wetland condition, namely vegetation composition, soil seed bank, soil chemistry, water quality and aquatic invertebrates, to determine the nature and the magnitude of any impacts that grazing by domestic stock may be having on these two wetland ecosystems.
Disturbance governs dominance of an invasive forb in a temporary wetland
Dominance of invasive species is often assumed to be due to a superior ability to acquire resources. However, dominance in plant communities can arise through multiple interacting mechanisms, including disturbance. Inter-specific competition can be strongly affected by abiotic conditions, which can determine the outcome of competitive interactions. We evaluated competition and disturbance as mechanisms governing dominance of 'Phyla canescens' (hereafter lippia), an invasive perennial forb from South America, in 'Paspalum distichum' (perennial grass, hereafter water couch) meadows in floodplain wetlands of eastern Australia. Water couch meadows (in the study area) are listed under the Ramsar Convention due to their significance as habitat for migratory waterbirds. In the field, we monitored patterns of vegetation boundaries between the two species in response to flooding. Under controlled glasshouse conditions, we explored competitive interactions between the native water couch and lippia subject to different soil moisture/inundation regimes. We did this using a pairwise factorial glasshouse experiment that manipulated neighbor density (9 treatments) and soil moisture/inundation (4 treatments). In the field trial, inundation increased the cover of water couch. Under more controlled conditions, the invader had a competitive effect on the native species only under dry soil conditions, and was strongly inhibited by inundation. This suggests that dry conditions favor the growth of the invader and wetter (more historical) conditions favor the native grass. In this system, invader dominance is governed by altered disturbance regimes which give the invader a competitive advantage over the native species.
Densely regenerating coolibah ('Eucalyptus coolabah') woodlands are more species-rich than surrounding derived grasslands in floodplains of eastern Australia
Woody plant encroachment - the conversion of grasslands to tree- or shrub-dominated ecosystems - occurs in rangelands and savannas worldwide. In eastern Australia, coolibah ('Eucalyptus coolabah' subsp. 'coolabah' Blakely & Jacobs) regenerated densely following floods in the mid 1970s, converting derived grasslands to dense woodlands. We compared soil and groundstorey vegetation attributes of dense coolibah regeneration to adjacent derived grasslands at three grazed sites in the northern riverine plains of New South Wales. Groundstorey species richness and diversity were significantly higher and groundstorey biomass was significantly lower in dense regeneration plots than in derived grassland plots. Soils from dense regeneration had higher C : N and pH, and lower Na than soils from derived grasslands. Although groundstorey species composition differed significantly between derived grasslands and dense regeneration within sites, variation among sites was more pronounced, indicating that site factors influence community composition more than dense regeneration of coolibah. Our findings suggest that, in contrast to other studies of woody plant encroachment, dense regeneration of coolibah does not result in a decrease in plant biodiversity or soil condition.
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.
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.
Stage 2: Grazing/Landuse in the Macquarie Marshes and Gwydir Wetlands: Final Report to the New South Wales Department of Environment and Climate Change
The Gwydir Wetlands and Macquarie Marshes are both examples of variable and dynamic floodplain wetland ecosystems. Both systems have experienced a major alteration to their hydrology as a result of river regulation, and the two regions have a long history of floodplain grazing. As efforts are made to address the hydrological issues in these wetlands, there is also a need to better understand how other disturbances such as grazing may be impacting on the ecological functioning of these ecosystems. This study investigated five areas of wetland condition, namely vegetation composition, soil seed bank, soil chemistry, water quality and aquatic invertebrates, to determine the nature and the magnitude of any impacts that grazing by domestic stock may be having on these two wetland ecosystems.