Restoring Woodland Understories #4

themeda-2For 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

Fig 1. A healthy, forb-rich native understorey in box gum woodlands near Cowra, NSW.

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.

Field experiments

Fig. 2. Experimental fencing for pulse grazing experiment at Bakes TSR near Albury.

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.

Key findings

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.

Cumberoona sugar boundary Aug 2009

Fig. 3. Sugar application (right) dramatically reduced exotic annuals which dominated untreated areas (left) (Bakes TSR).

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.

Fig. 4. Cumulative treatment effects on exotic annuals at Cumberoona TSR. All treatments controlled annuals better than doing nothing. Note there were few broadleaf exotic annuals in 2011.

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.

Fig. 5. Sheep grazing plots at Cumberoona TSR, near Albury.

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.

Fig. 6. The dominant native grass, Austrostipa bigeniculata, increased after drought and release from heavy grazing, then declined (shown in green) if no treatments were applied (Cumberoona TSR).

Notably, all of the management options des­cribed above led to better outcomes by 2011 than doing nothing (Figs. 4, 6). 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
sugar good good moderate
spring burn good poor good
pulse graze moderate poor good

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.

Fig. 7. Sown sward of Kangaroo grass at Bakes TSR

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. 7), 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.

Fig. 8. Reduction in exotic annuals resulting from sowing different native grass species into swards initially dominated by Tall spear grass.

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. 8). 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

Acknowledgements

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

Related blogs

You can also download a pdf file of the information in this post.

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4 thoughts on “Restoring Woodland Understories #4

  1. Ian, what great results to observe after only three years of data. It was very interesting your note on climate over-riding the effects of treatments which reiterates the importance of a longer-term data set. I hope you continue studying these plots for years to come as they may well form a solid part of the ‘blueprint’ for grassy ecosystem restoration in this (SE Aust.) region. Please inform us when the scientific papers are published as this material is really exciting stuff that we have to get going on a broader scale in the field as common land management practice. Have your field days already been done and are there other opportunities to see the plots?

    • Hi Geordie, thanks very much for your feedback. We’re nearing the end of the current project, which was funded by the NSW Environmental Trust and Murray CMA and others, but we are hoping that we can continue the trials if we can attract more funding from now on. We’ve got lots of great infrastructure (fences, gates, water etc), so it’s a great set-up to look at how management activities affect exotic and native species composition. We’re hoping to use the plots to see how a different suite of management activities influence the vegetation in the future, but we are yet to decide on what management options will be used. The field day is on this Friday morning (starting at 9 am) if you can fit in a long drive north before then 🙂 Thanks again, best wishes Ian

  2. Pingback: Can livestock grazing benefit biodiversity?

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