For the past four years, Ian Cole, Suzanne Prober and I have been busy running an experiment to work out how to restore native plants and control weeds in degraded woodlands. I’ve posted a number of blogs on this project before (see the links below). Later this week, we’re hosting a field day to show interested woodland managers how well – and not so well – the different treatments worked in our trials.Everyone at the field day will receive a handout that summarises the key findings from the study. It’ll be a long while before the scientific papers from the study get finished and published. In the meantime, we hope you enjoy the field day summary below. Please contact us if you would like more information on the project or results.
Restoring Woodland Understoreys Project Summary, November 2011
Box-gum woodlands, dominated by White Box (Eucalyptus albens), Yellow Box (E. melliodora) and Red Gum (E. blakelyi) are one of Australia’s most endangered ecosystems. They once formed park-like landscapes with scattered trees and a wide array of native grasses and forbs in the understorey (Fig. 1). These ecosystems have been cleared for agriculture, and most remnants are small, degraded and weed-invaded.
In the past 20 years there has been a groundswell of activity to restore these ecosystems and their landscapes. One of the biggest challenges faced by on-ground projects is to find practical ways to improve the quality of degraded understoreys.
This summary describes the key findings of studies undertaken by CSIRO Ecosystem Sciences and Charles Sturt University, that have focused on restoring the native grassy sward in Box-Gum woodlands.
We established field experiments in a number of remnants on the south-west slopes of NSW, to trial different options for encouraging native grasses and reducing the most prominent weeds, particularly exotic annuals such as Wild oats (Avena spp.) and Paterson’s Curse (Echium plantagineum) (Fig. 2). Building on earlier research, we used pulse grazing and spring burning to prevent annual weeds from seeding, and added carbohydrate (sugar) to reduce available soil nutrients and weed growth. We sowed native grasses to improve native grass cover and increase long term resistance to weed invasions.
Success may not be immediate
Confirming what land managers already know, we found that climatic variability can over-ride management inputs. Thus, management activities will have better outcomes in some years than others. There is also a high level of variation in the abundance of different exotic annuals during wetter compared with drier cycles, with exotic annual grasses more common in wetter periods. Native C4 grasses such as Bothriochloa macra and Themeda triandra establish particularly well following wet summers.
Exotic annuals can be suppressed by managing nutrients
A very reliable method for suppressing exotic annuals is to add sugar to the soil surface, from the time the exotic annuals are establishing (Figs 3&4). Sugar feeds soil micro-organisms, which in turn use up soil nutrients and limit the growth of the nitrogen-loving exotic annuals.
Sugar only offers a short-term solution, as each application lasts about three months. It is best used to provide a window-of-opportunity for establishing native species, possibly in small nodes across a site to minimize costs. We applied about 0.5 kg/m2 of sugar about 3 times per year (May, August, November). Cheaper alternatives like molasses may also be effective. Sugar should never be applied near waterways.
Grazing and burning can suppress some (but not all) exotic annuals
Burning and grazing can be used to limit seed production of exotic annuals, alter seed-bed conditions and renew the vigour of native grasses. Exotic annual grasses such as Wild Oats (Avena spp.) can reliably be suppressed over small areas by burning in spring (prior to the weeds’ seeding) with a weed burner (Fig. 4). This method does not control broad-leaf exotic annuals such as Paterson’s Curse (Echium plantag-ineum), that have longer-lived seed banks and germinate well on bare ground.
For larger areas, one to two weeks of intense grazing by livestock (Fig. 5) each year in spring (‘pulse grazing’) may achieve a similar result. In our experiment it took three years to obtain a detectable effect from pulse grazing on exotic annual grasses (Fig. 4). As for burning, pulse grazing is less effective for controlling broadleaf annuals. We hope to continue to apply grazing treatments in our experiments to evaluate the longer-term reliability of grazing strategies.
Anything is better than doing nothing for extended periods
Notably, all of the management options described above led to better outcomes by 2011 than doing nothing (Figs. 4, 5). In particular, in sites dominated by Tall spear grass (Austrostipa bigeniculata), the native grass sward tended to die back and become swamped by weeds when untreated for several years, whereas burning, grazing and/or sugar allowed it to remain vigorous. We hope to continue monitoring the untreated swards to see if they recover naturally after these die-back events.
While all three treatments had positive outcomes, the reliability and effectiveness of each treatment for controlling exotic annuals and promoting native grasses varied, as shown in the table below.
|Treatment||Annual grasses||Broadleaf annuals||Native grasses|
Controlling exotic annuals can improve establishment of native grasses
Our results confirmed a common observation that native grass establishment is erratic and likely to fail in dry seasons. Nonetheless, establishment was often more successful in treated plots where exotic annuals were suppressed.
Establishment success also varies between species. For seed sown on the surface, we had greatest success with C4 grasses, particularly Kangaroo grass (Themeda australis, Fig. 6), the likely original dominant of these woodlands, and the disturbance-increaser, Red grass (Bothriochloa macra). Over the period of our studies, which were mostly dry years, we recorded minimal establishment of the C3 grasses, Snow Tussock (Poa sieberiana) and Wallaby grasses (Austrodanthonia spp.), despite providing ample seed.
Native grasses are themselves a restoration tool
Our studies have shown that native grasses help to make degraded woodlands function more naturally, and that this is ultimately the key to sustainable restoration of box-gum woodlands. In particular, native grasses help to maintain low soil nutrient availability and can strongly suppress exotic annuals over the longer term. This in turn should benefit re-establishment of a wider diversity of woodland plants.
Kangaroo grass (Themeda australis) is the most effective species for reducing soil available nutrients and minimizing growth of exotic annuals (Fig. 7). Other native grasses such as Tall spear grass (Austrostipa bigeniculata) and Red grass (Bothriochloa macra) provide similar but weaker benefits. These species are more tolerant of livestock grazing than Kangaroo grass, so may be more appropriate in restoration areas that undergo more prolonged grazing. Figure 6 also emphasizes that it is important to maintain the vigour of the native swards. This can be achieved through pulse disturbances such as fire or grazing every few years.
Three key approaches to restoring weed-invaded woodland understoreys are to:
- Control nutrients to discourage weeds.
- Use pulse grazing or burning in spring every few years to maintain a healthy native sward and suppress exotic annual grasses.
- Re-introduce other native grasses, particularly Kangaroo grass, to suppress nutrients and weeds.
By Suzanne Prober, Ian Lunt & Ian Cole, November 2011
We gratefully acknowledge support for our research from the NSW Government through its Environment Trust, the Murray CMA, CRC for Future Farm Industries, Sugar Australia, Hume LPHA and local graziers. Neil Padbury and many helpers provided invaluable field assistance.
If you would like more information about this project, please contact:
- Dr Suzanne Prober, Suzanne.Prober [at] csiro.au
- Assoc. Professor Ian Lunt, ilunt [at] csu.edu.au
- Mr Ian Cole, Ian.Cole [at] environment.nsw.gov.au
You can also download a pdf file of the information in this post.
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- Taking stock of fencing
- Woodlands on TV!
- From Weeds to Wildfire: Restoring Woodland Understoreys #3