halting the decline: strategic herbicide …...halting the decline: strategic herbicide application...

HALTING THE DECLINE: STRATEGIC HERBICIDE APPLICATION AND INFREQUENT FIRE LEAD TO REDUCED GAMBA GRASS AND INCREASED SAVANNA HEALTH Report for activity: Managing flammable high biomass grassy weeds Samantha Setterfield1,4, Natalie Rossiter-Rachor2,4 and Vanessa Adams3 University of Western Australia & NESP1, Charles Darwin University & NESP2, University of Tasmania3 and Bushfires and Natural Hazards CRC4

HALTING THE DECLINE| REPORT NO. 554.2020

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Version Release history Date

1.0 Initial release of document 16/03/2020

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Disclaimer: University of Western Australia, Charles Darwin University, University of Tasmania and the Bushfire and Natural Hazards CRC advise that the information contained in this publication comprises general statements based on scientific research. The reader is advised and needs to be aware that such information may be incomplete or unable to be used in any specific situation. No reliance or actions must therefore be made on that information without seeking prior expert professional, scientific and technical advice. To the extent permitted by law, University of Western Australia, Charles Darwin University, University of Tasmania and the Bushfire and Natural Hazards CRC (including its employees and consultants) exclude all liability to any person for any consequences, including but not limited to all losses, damages, costs, expenses and any other compensation, arising directly or indirectly from using this publication (in part or in whole) and any information or material contained in it.

Publisher: Bushfire and Natural Hazards CRC

March 2020

Citation: Setterfield, S. A., Rossiter-Rachor, N. A, and Adams, V. M. (2019). Halting the decline: Strategic herbicide Application and infrequent fire lead to reduced gamba grass and increased savanna health. Bushfires and Natural Hazards CRC, Melbourne.

Cover: Dense infestation of the gamba grass, Mary River National Park, NT. Photo: Patch Clapp, CDU.


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TABLE OF CONTENTS

ACKNOWLEDGMENTS 3

EXECUTIVE SUMMARY 4

END-USER STATEMENT 6

BACKGROUND 7

High biomass grass invasion in Australia’s tropical savannas 7

Invasion and control of gamba grass on Mary River National Park 10

Background to Invasion at wildman block, Mary River National Park 13

RESEARCH APPROACH 14

Benefits of fine-scale fire and gamba grass management 14

Benefits of large-scale strategic fire and gamba grass management 21

Costs of fire and gamba grass management 22

Modelling cost scenarios of gamba grass management 27

Validating and applying the management cost-model 27

Modelling future management using best practice management strategy 28

Modelling cost scenarios based on best practice management strategy 29

Conclusion 31

REFERENCES 33


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ACKNOWLEDGMENTS We thank the Parks, Wildlife and Heritage Division of the Northern Territory Government for access to Mary River National Park (formerly Wildman Reserve). We greatfully acknowledge the Mary River Rangers, both past and current. We particularly thank Steve Dwyer, Ian Bate, Ben Schumacher, Zac Pearce and Robert Goodman for advice on study sites, and insights into their highly effective weed and fire management on the park. We thank Chris Brock, Kris Brooks, Diego Alvarez Cortes, Michael Stauder, Fiona Freestone, and Ashley Setterfield for assistance with the floristic surveys and data entry. We also thank Lincoln Wilson, Julie Heran and Steve Dwyer for their feedback on the draft report.


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EXECUTIVE SUMMARY

• Mary River National Park Rangers have implemented a strategic approach to

managing gamba grass invasion including fine-scale application of herbicide, and fire exclusion where possible.

• Maintaining woody tree structure and diversity, and improving tree health, has

been a key focus of the park’s weed and fire management.

• The fire exclusion focus has resulted in a substantial decrease in fire frequency, with large areas of the park now burnt with a very low frequency (2 or 3 fires in 10 years).

• Infrequent fire in the northern Wildman block over the last 15 years has

resulted in: ‒ the density of live trees more than doubling ‒ the tree canopy cover increasing on average by ~ 70% to 110% ‒ the leaf litter depth and leaf litter cover almost doubling ‒ gamba grass cover halving without the use of herbicide ‒ Some sites previously dominated by gamba grass (2000-2005) are now

dominated by native grasses with a dense mid- and overstory of woody plants

• These results suggest that long-term implementation a low-frequency and low-intensity fire regime on Wildman block has:

‒ reduced the establishment and survival of gamba grass ‒ created conditions suitable for increased development of the savanna

woody tree component. ‒ increased carbon storage of woody plants ‒ reduced fire risk to tourists.

• We found that the park’s current weed management strategy reflects best

practice management.

• The total gamba grass management costs incurred over the 2018/2019 herbicide spray period was $70,150 (including equipment, labour and chemical costs). This also included the aerial application of herbicide from helicopter ($10,236). These costs represent an increased level of resources applied to gamba grass management over 2018/2019 season due to the lower rainfall and higher accessability of the areas with long-term infestation with medium and high density of gamba grass plants.


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• Modelling suggests that maintaining a high level of management in 2020 is required to effectively manage the gamba grass invasions, including a second year of aerial weed control. In following years, there is a large reduction in the costs in terms of chemical, labour and equipment.

• The aerial application of herbicide from helicopter was cost-effective for large, dense patches without tree cover. For example, in 5 ½ hours the helicopter treated ~370ha of gamba grass using ~450 litres of herbicide for a total cost of $10,236 (labour, helicopter hire, and chemical). Whereas ground-based spraying would cost almost four times as much ($38,262) and take 401 hours to spray a similar sized gamba infestation, using a similar amount of herbicide (labour, equipment, and chemical).

• The helicopter application of herbicide extends the area of gamba grass control, without the need for front line contact with herbicide

• The total yearly cost for fire management was $19,500. The cost of fire management is minor but appears to be accelerating the effectiveness of on-ground control efforts compared to modelled outcomes and outcomes for other gamba grass invaded areas within the region.

FIGURE 1 DENSE GAMBA GRASS (ANDROPOGON GAYANUS) INVASION ON WILDMAN BLOCK (1999) (PHOTO: M. DOUGLAS).


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END-USER STATEMENT

Steve Dwyer, Senior District Ranger, Parks, Wildlife and Heritage Division, Northern Territory Government

Park management strategies should also report and adapt to the most up-to-date evidence about the outcomes of management actions. This project provided us with valuable data to support our fire and weed management activities and to inform future strategies.


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BACKGROUND

HIGH BIOMASS GRASS INVASION IN AUSTRALIA’S TROPICAL SAVANNAS

Australia’s savanna woodlands and open forests are the largest and the most intact expanse of savanna in the world (Hutley and Setterfield 2008). The savannas are typically considered in good condition with relatively little transformation through intensive agriculture (Douglas et al. 2011). The proportion of invasive plants in tropical Australia is relatively low (5 - 8% of the total plant species) compared to the Australian average (12%, Woinarski et al. 2007). However, invasive plants are increasing threatening the ecological values of the region. In the past two decades, large areas of the savannas and the wetlands have been invaded by ecologically transforming high biomass invasive grasses, resulting in major impacts on biodiversity and ecosystem function (Rossiter-Rachor et al. 2009).

One of the most significant invasive grass transformers is gamba grass (Andropogon gayanus Kunth.), a perennial African tussock grass, growing up to 4 m tall (Figure 2; Rossiter et al 2003). In northern Australia, widespread gamba grass is from the cultivar “Kent”; a cultivar was developed in the Northern Territory (NT) over several decades (Oram 1987). The cultivar was planted widely, as a pastoral grass and for minesite rehabilitation (Setterfield et al. 2010; 2018a). In 1985 it was planted on several pastoral properties in the Litchfield, and Adelaide River region (∼100 km south of Darwin; Petty et al. 2012) and also the Mary River region (∼100 km East of Darwin; Flores et al. 2005). By 1995, gamba grass had been documented in both Litchfield National Park (Petty et al. 2012), and also in Wildman Reserve (now Mary River National Park; Barrow 1995), both of which were adjacent to pastoral plantings of gamba grass. The rate of invasion since that time has been explosive (Petty et al. 2012) and gamba grass now covers 15,000 km2 of the NT, with the potential to invade 380,000 km2 in the NT alone in addition to other large areas in Queensland and Western Australia (Northern Territory Government 2018). The rapid invasion and the significant threat for conservation, aboriginal, pastoral, mining and defence land users is reflected by its status as one of Australia’s Weeds of National Significance (Australian Weeds Committee 2012) and a Key Threatening Process under the Commonwealth’s EPBC Act. Gamba grass invasion is considered of very high risk across northern Australia because of the resultant changes to fuel beds and fire behavior (Setterfield et al. 2013). Invasion results in a tall (~4 m), dense fuel bed of up to 25-30 t ha-2 that replaces the shorter (


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2010). If invaded sites are not burnt in one fire season, the fire fuel load increases resulting in more intense, high combustion fires in the following year (Setterfield et al 2010; 2013). Plant species diversity and density are substantially reduced after only a few gamba-fuelled fires (Figure 2; Brooks et al. 2010; Levick et al. 2015), thereby reducing above-ground carbon stores. This represents a financial risk to landholders engaging in the ERF savanna burning methodologies (Adams and Setterfield 2013).

High intensity gamba grass fires have substantial impact on fire management practices and costs (Neale 2017). Historically fires were low intensity native grass fires, which occurred in sparsely populated areas, and which could be managed using minimal fire fighting equipment (Setterfield et al 2010). Gamba grass invasion has resulted in hotter fires occurring in residential areas, requiring helicopters and water bombing planes to effectively protect people’s lives and properties. Setterfield et al. (2013) showed a nine-times increase in fire management costs in 10 years in a region of the NT due primarily to gamba grass invasion. Further economic impacts of Gamba grass fires are evident each year with damage to dwellings and infrastructure, and the cost of weed management activities to the landholders and government.


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FIGURE 2. EXAMPLES OF THE GAMBA GRASS INVASION PROCESS IN THE DARWIN REGION AND SUBSEQUENT CHANGES IN SAVANNA STRUCTURE DUE ALTERED FIRE REGIMES. PHOTOS SETTERFIELD AND ROSSITER-RACHOR. PHOTOS OF:

(A) NATIVE GRASS SAVANNA, (B) SCATTERED GAMBA GRASS PLANTS INVADING NATIVE SAVANNA, (C) MODERATE GAMBA GRASS WITH INTACT TREE OVERSTOREY, (D) DENSE GAMBA GRASS WITH INTACT TREE CANOPY, (E) DENSE GAMBA WITH DEGRADED TREE CANOPY, (F) DENSE GAMBA INVASION WITH NO TREE CANOPY

(a) (b)

(c) (d)

(e) (f)


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INVASION AND CONTROL OF GAMBA GRASS ON MARY RIVER NATIONAL PARK

We focus this report on approaches to managing gamba grass risk within one section of the Mary River National Park (MRNP; Figure 3 and 4). The MRNP (Figure 3) is located approximately 100 km east of Darwin and covers 112,000 hectares, including several separate land areas (sections) within the lower and middle Mary River Catchment. The main purpose of the Park is to protect and conserve its outstanding natural, cultural, historical and visitor values and to provide opportunities for the public to enjoy high quality experiences. MRNP is recognised as a Class 1 biodiversity National Park (most important) for its biodiversity values by the NT Government (Northern Territory Government, 2015, 2018). Invasive grasses threaten both savanna and wetland values in MRNP. One of the greatest threats in MRNP is invasion of savanna and wetland margins by gamba grass. In particular, the Wildman block has had areas of significant gamba invasion for several decades.

Across northern Australia, the main approach to manage fire risk resulting from gamba grass invasion to date has been to reduce the flammable fuel load using appropriate methods depending on land tenure, including slashing, hand pulling, grazing or burning as early in the fire season as possible (Northern Territory Government 2018). If fire is used as for fuel management, the sites are typically burnt early every one or two years to prevent the large increase in gamba grass biomass that occurs with successive fire-free years (Rossiter-Rachor et al. 2009), thereby reducing the intensity of both preventative and wild fires (Setterfield et al. 2010). Burning of gamba grass can also be undertaken prior to herbicide spraying to reduce the amount of leaf material that needs to be sprayed, and therefore reduce cost of herbicide and time taken in the application. Fire can therefore be part of an integrated approach to weed management (Northern Territory Government 2018).

The MRNP managers have implemented a strategic approach to gamba grass management including fine-scale application of herbicide, and fire exclusion. This aims to maintain woody tree structure and diversity, even if it is over a continuous ground layer of gamba grass. They have used strategic application of herbicide spray to create buffers around trees to protect their canopies, and to create fire exclusion buffers. They observed improvements in tree health with protection from fire and strategic control of gamba grass and have recently undertaken large-scale strategic control of gamba grass with the aim of long-term protection of the Park’s biodiversity, cultural and tourism assets.

The aims of this report are to:

• Determine benefits of gamba grass control on Mary River National Park, Wildman block.

• Determine costs of gamba control and fire management activities


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FIGURE 3. LOCATION OF MARY RIVER NATIONAL PARK, WILDMAN BLOCK, IN RELATION TO DARWIN AND NORTHERN AUSTRALIA.


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FIGURE 4. MARY RIVER NATIONAL PARK, WILDMAN BLOCK. NOTE THE THREE WEED AND FIRE MANAGEMENT ZONES: WILDMAN SOUTH, WILDMAN CENTRAL AND WILDMAN NORTH.

The three weed and fire management zones used by Park Rangers are noted on the map (Wildman south, Wildman central, and Wildman north) The largest gamba grass infestation is found in the wetland margins and woodlands of the Wildman north section of the park. (Source: Mary River National Park Annual Weed Management Plan, 2019).


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BACKGROUND TO INVASION AT WILDMAN BLOCK, MARY RIVER NATIONAL PARK

The most common vegetation type in Wildman block is savanna woodland and open forest (sensu Specht 1981) with an overstorey of Eucalyptus miniata Cunn. Ex Schauer and Eucalyptus tetrodonta F.Muell, and a ground layer of mixed annual and perennial grasses including Sorghum spp., Heteropogon spp., Pseudopogonatherum contortum (Brongn.) A. Camus, Chrysopogon fallax S. T. Blake. and Aristida spp. On the western side of the Wildman block, the savanna vegetation transitions along an increasing moisture gradient to a floodplain margin community dominated by Lophostemon lactifluus (F.Muell.) Peter G.Wilson & J.T.Waterh, Syzigium suborbiculare (Benth.) T.G.Hartley & L.M.Perry and mixed Melaleuca spp., which is inundated for 1–3 months during the wet season (hereafter referred to as ‘floodplain margin’). Further along the moisture gradient, the vegetation transitions to a Melaleuca-dominated swamp community, and then a grass- and sedge-dominated floodplain community, which is inundated for up to 9 months each year (Flores et al. 2005).

Within the Wildman block, gamba grass established across substantial areas of upland savanna, and in the floodplain margin communities. This National Park was one of the first sites of extensive, dense gamba grass in the NT (Flores et al 2005), invaded from plantings outside the National Park area. Gamba grass was planted on the adjoining station to the north of the block in 1985 (Flores 1999) and trial plantings occurred to the south east in the 1970s (Flores 1999). No evidence was found of gamba grass being planted in the Wildman block, despite extensive research and interviews about planting activity in the 1970s -1990s (Flores 1999).

Once established in the Wildman block, the spread of gamba grass was rapid. By 1998, Park managers were feeling that control of broadscale gamba grass control was not feasible and that available resources should be “spent protecting 1 or 2 small high conservation areas which were identified as Brian Creek rainforest and Mistake Creek billabong” (quoting from Senior Park Ranger, Internal memo, Parks and Wildlife Commission NT, NT Government 1998).

However, with time, Park managers decided to direct resources to increasing tree health in invaded and non-invaded areas by reducing fire frequency, including by actively excluding fire, and strategically spraying around trees. As described above, their expert opinion was that the health of the savanna improved with low fire frequency and strategic control of gamba grass, which reinforced their commitment to this management approach and its ability to deliver the Park’s aim of long-term protection of the biodiversity, cultural and tourism assets.


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RESEARCH APPROACH

BENEFITS OF FINE-SCALE FIRE AND GAMBA GRASS MANAGEMENT AT MARY RIVER NATIONAL PARK

Each year Mary River National Park Rangers develop an annual fire action plan. This plan identifies the specific aims for each section of the park, the individual actions to achieve these aims, the resources required and the likely timeframe for action. The Wildman north is the section of the park with the greatest gamba grass invasion. The fire management aims in this block are to

“Maintain legislated fire breaks and exclude fire from North Wildman Block (burnt in August 2015 wildfire) and to increase fire management lines around heavily infested Gamba sites where fire is to be excluded to assist natural regeneration of fire damaged habitat including monsoon forest. Protection of monsoon forest habitats.” – Mary River National Park Fire Action Plan 2019 (Northern Territory Government, Parks, Wildlife and Heritage Division 2019)

Fire exclusion, where possible, has been a key change in the fire action plans in recent years. This has resulted in a substantial decrease in the fire frequency, with large areas of the part now burnt with a low frequency. Between 2000-2009 the majority of the Wildman block was burnt between 2-6 times in the 10 year period (Figure 5b). In more recent years, the fire frequency has declined, with the majority of the Wildman block being burnt 2 or 3 times between 2009-2018. This is a dramatic change in fire frequency and is it substantially lower fire frequency than neighbouring properties (Figure 5c) and other National Parks in the region.


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FIGURE 5: FIRE FREQUENCY ON MARY RIVER NATIONAL PARK, WILDMAN BLOCK (WWW.FIRENORTH.ORG.AU). (A) LOCATION OF WILDMAN BLOCK, (B) FIRE FREQUENCY 2000-2009, AND (C) FIRE FREQUENCY 2009-2018. NOTE AREA TO THE RIGHT OF THE MAP IS OFF PARK.

(a)

(b)

(c)

Wildman north

Wildman central

Wildman south

http://www.firenorth.org.au/


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In addition to low fire frequency, the Wildman block has typically experienced relatively low intensity fires, which has resulted in change in woody tree structure. The development of a mid-story and increase in overstory canopy are visually apparent (See Figure 6).

FIGURE 6. CHANGES IN NATIVE SAVANNA STRUCTURE AT MARY RIVER NATIONAL PARK DUE TO INFREQUENT BURNING. PHOTOS ARE OF A PHOTO POINT IN NATIVE GRASS SAVANNA IN: (A) AUGUST 2004, AND (B) JULY 2018 (PHOTOS: ROSSITER-RACHOR).

To quantify change in savanna structure within Wildman block, we undertook floristic surveys in 2018 to determine the change in woody tree density and canopy cover in

(a)

(b)

2004

2018


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sites that had been established and surveyed in 2005 within savanna areas with dense gamba grass and within savanna with a native grass understory. The invaded sites had likely had some herbicide application in the intervening period due to proximity to tracks, although they were not specifically identified for weed control action. The strongest land management activity was fire management which aimed for, and achieved, low frequency fires with relatively low intensity (2 or 3 fires in 10 years). We compared the change in woody tree cohort with stem diameter (dbh) greater than 8 cm. We found that in both the initially invaded and native grass sites, the woody vegetation density has increased markedly reflected by significant increases in the density of live trees (Figure 7a) and decrease in the basal area of the live trees (Figure 7b), which reflects an increase in the proportion of individuals in smaller size class (that is, mid-story stems with smaller stem diameter than canopy trees. Tree canopy cover more than doubled in that time (Figure 7c), and gamba grass cover significantly declined. Increased canopy cover reduces light penetration to the ground layer and decreasing the suitability of conditions for gamba grass growth (Setterfield et al. 2018).

A similar outcome was demonstrated by repeat surveys in 2018 of nearby savanna sites (also within the Wildman block) that had been established in 2001 in dense gamba grass understory (Rossiter-Rachor et al. 2008). The cover of gamba grass halved within 15 years (from approximately 45% in 2004, to 21% in 2018; Figure 8a), and the height of the gamba grass also declined significantly (from approximately 2.3 m in 2004 to less than 1m in 2018; Figure 8b; Figure 9). This is likely to be the consequence of unsuitable conditions for establishment and growth in the absence of fire. Fire and other disturbance agents increase the establishment of gamba grass (Flores et al. 2005 Setterfield et al. 2005). The results suggest that long-term implementation the low-frequency and low intensity fire regime at Wildman block has reduced the establishment and survival of gamba grass. There would be a reduction in microsites suitable for seed germination and seedling emergence due to the significant increase in thickness and continuity of the leaf litter layer, both of which doubled in between 2004 and 2018 (Figure 8c and d). Seeds would be less likely to move through this layer to the soil surface, where conditions (e.g. contact with the soil matrix) are suitable for germination, and emergent seedlings would have reduced success in growing through the leaf litter layer (Setterfield et al. 2005). Sites previously dominated by gamba grass (2000-3) are now dominated by native grasses with dense mid- and overstory woody plants (Figure 9).


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FIGURE 7. (A) MEAN DENSITY OF LIVE TREES (INDIVIDUALS >8CM DBH), (B) MEAN TREE BASAL AREA OF ALL TREES (INDIVIDUALS >8CM DBH), AND (C) MEAN TREE CANOPY COVER (%); OF NATIVE AND INITIALLY GAMBA INVADED SITES ON WILDMAN BLOCK IN JULY 2005 AND JULY 2018.

0

100

200

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400

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2005 2018

Live

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(tre

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a)

a) Density of live treesNativeInvaded

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b) Tree basal area (All trees >8cm DBH)

NativeInvaded

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2005 2018

Cano

py c

over

(%)

c) Tree canopy cover

NativeInvaded


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FIGURE 8. (A) MEAN GAMBA GRASS COVER (%), (B) MEAN GAMBA GRASS HEIGHT (M), (C) WOODY TREE LEAF LITTER COVER (%) AND (D) MEAN LEAF LITTER DEPTH (CM); AT WILDMAN BLOCK IN JULY 2004 AND JULY 2018

0

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2004 2018

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b) Gamba grass height

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FIGURE 9. CHANGES IN STRUCTURE OF GAMBA GRASS INVADED SAVANNA AT MARY RIVER NATIONAL PARK DUE TO INFREQUENT BURNING. PHOTO POINT IN AN INVADED SAVANNA IN: (A) AUGUST 2004, (B) JULY 2018 AND (C) MAY 2019 (SEVEN MONTHS AFTER LATE DRY SEASON WILD FIRE). NOTE THE DECREASED GAMBA GRASS COVER IN JULY 2018, AND MINIMAL GAMBA GRASS REGROWTH IN THE MAY 2019 PHOTO. (PHOTOS: N. ROSSITER-RACHOR).

a)

b)

c)

2004

2018

2019


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BENEFITS OF LARGE-SCALE STRATEGIC FIRE AND GAMBA GRASS MANAGEMENT TO BIODIVERSITY AND TOURIST VISITATION

Mary River National Park is visited by an average of approximately 4500 tourists each year (Table 1) to visit and use the many nationally and internationally recognised assets. The high conservation value of the park is related to the wide diversity of habitats including: freshwater wetlands, woodlands, open forest, monsoon rainforests, vine thickets, coastal and upland forests, rocky outcrops and their associated communities (NT Government 2017). These conservation assets are also key to the visitor experiences of local, interstate and international guests at the Wildman Wilderness Lodge, which is located on park. The lodge runs guided tours of the Mary River wetlands and floodplains, billabongs, cultural tours of the monsoon rainforests and woodlands, bushtucker tours and also offers scenic helicopter flights over the wetlands, and sunset tours of Leichhardt point. See www.wildmanwildernesslodge.com.au/explore/tours/. These visitor sites are all areas that historically have had some level of gamba grass invasion.

Large-scale strategic fire and gamba grass management is essential for maintaining these assets and minimising the risk to tourists. The highest fire risk for tourists coincides with the late dry season, and peak visitation numbers (July-Oct; see Table 1). Currently approximately 2800 people visit the Park in July to October. As gamba grass invades, the fuel loads in invaded habitats increase and the fire risk increases (Setterfield et al 2013). The Bureau of Meteorology currently use an elevated gamba grass fuel load of 11 t ha-1 when estimating daily fire risk in the MRNP, as an acknowledgement of the higher fire risk. Currently, visitor safety due to fire is not listed as a particular risk in the Park Management Plan (Northern Territory Government, 2015). However, if gamba grass invasion had occurred as predicted between 2000 and 2019, that is, if strategic fire and herbicide control programs had not been implemented and gamba grass dominated the savanna and wetland margin throughout the park, strong action would be required to prevent tourists moving through the area due to high fire danger. For example, in Victoria, on high fire danger days (Fire Danger Index >100 “Code Red-Catastrophic”) National Parks and forests are closed due to the risks to visitor safety (Victorian Government 2014). Current predictions are that this would be 1-5 days/yr in MRNP if strategic fire and gamba grass management had not been strongly implemented for the past 15 years (Setterfield et al 2013). However, if a more conservative approach of ‘extreme’ fire danger (Fire Danger Index >75) was chosen as the threshold for park closure in the NT, this could have resulted in park closures of 5-15 days/year. It should be noted that fire alerts and total fire ban days are triggered at Fire Danger Index >50, therefore a much more conservative approach could be considered if the savanna and associated tourist tracks and other visitation areas was heavily invaded by gamba grass given the flame height and fire intensity generated within dense gamba grass. The current large-scale strategic fire and gamba grass management have substantially reduced the gamba grass density on park, therefore reducing the fire risk.

http://www.wildmanwildernesslodge.com.au/explore/tours/


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TABLE 1. MONTHLY TOURIST NUMBERS MARY RIVER NATIONAL PARK, WILDMAN BLOCK. HIGH RISK FIRE MONTHS ARE HIGHLIGHTED.

2018 2017 2016 January 0 146 0 February 0 0 0 March 0 0 134 April 19 0 253 May 323 605 463 June 336 761 943 July 762 978 986 August 836 898 1014 September 390 682 862 October 264 392 468 November 222 509 274 December 0 158 0 Total 3152 5129 5398

Total visitors in high risk fire season (July-Oct)

2252 2950 3330 Note: Rockhole road is an unsealed road, and is closed during the peak wet season (Dec – March).

COSTS OF FIRE AND GAMBA GRASS MANAGEMENT ON THE WILDMAN BLOCK

The benefits of strategic fire and gamba grass management in MRNP are evidenced by the increased ecological integrity of the savanna ecosystems, increased carbon storage in woody plants, and reduced fire risk to tourists.

In this section, we estimate the dollar cost of fire and gamba grass management in the Wildman block, MRNP. The steps we undertook were:

(i) elicitation of information from Park rangers on the density and extent of gamba grass infestations within management zones

(ii) documenting equipment, labour, and chemical costs within these zones in 2018 and 2019, and

(iii) elicitation of fire management costs from the Park rangers.


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On the basis of this information, we also provided cost-estimates of weed management for the next five years to achieve local eradication of low density infestations, and substantially reduce size and density of the large, high-density infestations. To achieve this, we used a weed management-cost model developed by Adams and Setterfield (2013). The model provides cost ($) estimates of weed control and eradication based on input information on the size and density of infestations (Adams and Setterfield 2013). We used the data generated in step (i) and (ii) above to update the weed management-cost model parameters and then to forward predict cost of longer-term weed management activity on MRNP.

(i) Documenting and costing gamba grass management across Wildman block

The cost of gamba grass management was determined from information provided by Park rangers. In consultation with the rangers, we initially defined the boundaries of a range of gamba grass infestation zones based on infestation size and density, access, and the appropriate control strategy for that zone. We then asked the rangers to estimate the aspects of weed management, specifically providing information that would provide direct cost of activity and could also be used to parameterise the weed management cost-model. The information sought from rangers was focused on the costs of labour (L), equipment (E) and chemical (C), provided in response to the following questions:

• Labour: please list the number of personnel, number of days and average hours per day spent managing each infestation in the last year.

• Equipment: please list the equipment used (4x4 quickspray, quads, backpack sprays) to manage each infestation in the last year.

• Chemical (litres of herbicide per hour used): please provide the average chemical litres per hour applied of glyphosate to each infestation, the concentration, and indicate if this rate differs between equipment type used.

Lastly, we asked how many years (t) each infestation had been treated for and how effort had varied over time. For each gamba grass infestation that managers were actively managing they answered the above questions, detailing the history of management of the infestation (t) approach taken to manage the infestation (equipment, labour and chemical costs) as well as the time of year treatment occurs.


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FIGURE 10. MAP OF MAIN GAMBA GRASS MANAGEMENT SITES ON THE WILDMAN NORTH BLOCK, AND IDENTIFICATION OF DENSITY CLASSES:

• YELLOW AREAS ARE HIGH DENSITY GAMBA GRASS MONOCULTURES WITH FEW TREES • RED AREAS ARE MEDIUM TO HIGH DENSITY GAMBA GRASS WITH MORE SOME CANOPY • GREEN AREAS ARE LOW DENSITY (SCATTERED PATCHES) GAMBA WITH A FULL TREE CANOPY

This activity identified five actively managed infestation zones in the Wildman North block of varying gamba grass density, and two other zones in the Wildman South block and Point Stuart Road reserve both of which have scattered infestations (Figure 10). The zones vary in size, area and in the density of gamba grass infestations. As a consequence, the management resources varied greatly between zones, from 1 hour of on-ground searching and spraying of isolated gamba grass plants (zone 6) to intensive chemical spraying of dense infestations from helicopter (zone 4).

Our calculated gamba grass management cost for each zone (based on the equipment, labour and chemical costs is detailed in Table 2), ranged from approximately $400 to over $38,000. A total of 90 days (1.0 FTE, 675 hours) of labour were spent on weed management in 2019. The calculated total cost of weed management in 2019 was $70,150 (Table 2).


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TABLE 2. DETAILS OF THE COST COMPONENTS (EQUIPMENT, LABOUR, CHEMICALS) AND TOTAL COST FOR GAMBA GRASS MANAGEMENT IN DEFINED ZONES ON WILDMAN BLOCK, MARY RIVER NATIONAL PARK. DATA REPRESENTS ACTIVITY IN 2019.

ID Density

in 2019 Area (ha)

Equipment used

Labour description

Labour (hrs)

Chemical used (L)

Total Cost

($, 2019) NORTHERN WILDMAN BLOCK – ON GROUND 1 scattered 12 4x4 with spray

unit Team of 2 staff 10 3

639

2 scattered 24 4x4 with spray unit

Team of 4 staff 36 27 2,989

3 medium 24 4x4 with spray unit

Multiple field days, team size varied (typically 2 - 4)

64 45 4,966

4 dense 370 4x4 with spray unit


401 450 38,262

5 Scattered - containment

246 4x4 with spray unit


76 81 7,035

NORTHERN WILDMAN BLOCK – AERIAL SPRAYING

4 Dense 376 aerial spray Team of 2 staff + 1 volunteer and helicopter pilot

21 460 10,236

SOUTHERN WILDMAN BLOCK

6 scattered 6.25 4x4 with spray unit

Team of 2 staff 4 1 403

POINT STUART ROAD RESERVE

7 scattered 100 4x4 with spray unit

Team of 4 staff 63 50 5,618


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(ii) Documenting and costing fire management across Wildman block

To estimate the cost of fire management we consulted fire management staff to determine the aims, approach and resources used across the Wildman block for fire management activities. We elicited the cost inputs for each focus area for fire management. For each area, we first asked for a description of the management aim, and then asked the managers to estimate the components of the cost function: labour (L), and equipment (E) according to the following questions:

• Labour: please list the number of personnel, number of days and average hours per day spent on fire management in the last year (including volunteers).

• Equipment: please list the equipment used (tractor with blade, slasher, 4x4, quads) to manage each site in the last year.

This activity identified four main fire management zones across the Wildman block: (1) Wildman south (2) Wildman central, (3) Wildman north and (4) the Point Stuart Road reserve (Figure 11). The fire activities varied between the zones, from:

• Wildman north – Grading a firebreak and spraying to create a gamba free buffer along the northern boundary of the park to maintain the Park’s legislated fire breaks. This also aims to protect the woodlands and monsoon forests of the northern block from wildfires.

• Wildman central- Fuel reduction burns along the management track to create a wildfire for later in the year, and to protect the monsoon forest pockets.

• Wildman south- Grading/slashing a firebreak (management track) along the southern boundary of the park, to maintain the Park’s legislated fire breaks, and small fuel reduction burns along the road; and to create a buffer from wildfire that may threaten the central Wildman block later in the fire season;

• Point Stuart Road Reserve- Grading firebreaks along the road reserve and burning the road reserve. This is to reduce the likelihood of opportunistic arson or accidental ignitions along Point Stuart Road, and to protect northern, central and southern Wildman blocks from wildfires.

The total yearly cost for fire management is $19,500. This includes all labour and equipment costs.


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FIGURE 11. THE THREE WEED AND FIRE MANAGEMENT ZONES ON MARY RIVER NATIONAL PARK, WILDMAN BLOCK: WILDMAN SOUTH, WILDMAN CENTRAL AND WILDMAN NORTH, AS WELL AS THE POINT STUART ROAD RESERVE. SOURCE: MARY RIVER NATIONAL PARK FIRE ACTION PLAN 2019 (NT GOVERNMENT, PARKS, WILDLIFE AND HERITAGE DIVISION, 2019)

MODELLING COST SCENARIOS OF GAMBA GRASS MANAGEMENT BASED ON BEST PRACTICE STRATEGY

Validating and applying the management cost-model

Prior to using the weed management cost-model to compare the cost of alternate gamba management scenarios, we needed to validate the model by comparing the actual per area costs (based on data provided by management staff) to that predicted by the management cost model. We first updated the cost parameters within the model with 2019 labour rates and chemical costs. We then ran the model for the weed management zones, because this activity is not accommodated by the model. The model generates the weed management costs based on mapped pixels of 6.25 ha, each of which starts with a defined invasion density (Adams et al. 2015; Adams and Setterfield 2013). We compared the 2019 on-ground data (Table 2) to the


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equivalent year in the model (i.e. year 4 cost estimates). We found that the management cost model provided robust cost estimates across the infestation zones (Table 3; R2=0.805 for actual and modelled costs), and therefore can be used to provide reliable estimates of longer-term weed management activity.

TABLE 3. ESTIMATED COSTS AND MODEL COSTS FOR WEED INFESTATIONS. BY INFESTATION RELEVANT DETAILS FOR THE COST MODEL ARE GIVEN AS WELL AS ACTUAL AND MODELLED COSTS FOR COST COMPARISON (R2=0.805 FOR ACTUAL AND MODELLED COSTS).

Modelling future management using best practice management strategy

The weed management cost-model predicts costs of activity required to meet the management aim defined for any defined management zone. Therefore, based on the mapped information on gamba grass infestations within the Wildman block, we used best practice approaches to spatially define weed management zones (shown in Figure 12) with the aims of (i) managing density within core infestations and containing spread into adjacent land, (ii) a buffer around the core that is managed (i.e. search and control of infestations) and (iii) local eradication zones, within which the infestations are scattered and the focus is to manage or eradicate plants (Figure 12). This defined zones and activities aligned closely with those being undertaken by MRNP weed managers. As shown on Figure 12, the Wildman North A area is identified as a core infestation to be contained by a ‘containment buffer zone’ in which all infestations are eradicated, and the buffer is then patrolled each year to ensure no infestations escape the containment core (Figure 12). The remainder of the area (Wildman North C) is currently low density or satellite infestations which are prioritised for eradication. This is consistent with best practice and also reflects optimization

ID Density Area (ha)

Actual cost ($)

Actual cost per 6.25 ha ($)

Modelled cost per 6.25ha ($)

NORTHERN WILDMAN BLOCK ON-GOUND

1 scattered 12 639 319 592

2 scattered 24 2,989 747 592

3 medium 24 4,966 1,241 1220

4 dense 370 38,262 700 1220

5 Scattered -

containment 246 7,035 152 150

SOUTHERN WILDMAN BLOCK

scattered 6.25 403 403 592

POINT STUART ROAD RESERVE

scattered 100 5,618 351 592


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approaches that regularly identify that containment of core zones complemented with eradication of satellite infestations is the most cost-effective strategy. It is also consistent with asset management approaches which would recommend protection of monsoon forest assets.

FIGURE 12. RECOMMENDED MANAGEMENT STRATEGY REFLECTING BEST PRACTICE WEED MANAGEMENT MODELS (SEE ADAMS AND SETTERFIELD 2016).

• CORE INFESTATION- RED BLOCK, WILDMAN NORTH A. GAMBA INFESTATION REMAINS BUT IS CONTAINED TO THIS CORE ZONE. POSSIBLY TO REDUCE SIZE INCREMENTALLY OVER TIME FROM CONTAINMENT BUFFER

• CONTAINMENT ZONE – GREY SHADED PERIMETER OF THE CORE INFESTATION. ERADICATE PLANTS WITHIN THIS ZONE AND MAINTAIN AS CONTAINMENT BUFFER.

• ERADICATION ZONE- WILDMAN NORTH C AND ADJACENT TO CONTAINMENT BUFFER, ERADICATE ALL SATELLITE INFESTATIONS, KEEP CLEAN.

Modelling cost scenarios based on best practice management strategy

We estimated the future costs of the best practice management strategy using the cost model from Adams and Setterfield (2013). This included all existing gamba grass weed management activities as well as:

1. Initiating on ground eradication efforts in the remaining untreated dense site in the northern block (currently being tackled on an incremental basis through expansion eastward of site 4). We assumed that budget would be available for the full site to be under treatment from 2020 onwards.

2. As site 4 is fully managed, with most plants eradicated (in ~3 years), resources


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are reallocated to containment activities (patrol of border searching for any newly established plants and minor weed spraying to remove any plants detected).

3. Additional containment borders along the property boundary could be added to ensure no incursions from outside the property.

We estimated the annual weed management costs for the duration of eradication efforts (2020-2025) and the ongoing containment costs for three cost scenarios which provide a realistic baseline estimate as well as lower and upper bounds.

Cost Scenarios:

1. Scenario 1 – Maintain high level control effort

The current density shown in Table 2 is the starting density. In 2020, there would be a continuation of the effort applied in 2019, including aerial control in 2020. Use cost model from Adams and Setterfield 2013 to project costs over duration of weed management program. This is an optimistic scenario to provide a feasible lower bound possible cost.

2. Scenario 2 – Starting position if all infestations in 2019 were one density class higher than shown in Table 2

This is assuming that previous control hadn’t been undertaken effectively and the starting position was worse than current. The cost of reducing the density increases in 2020 and then declines.

3. Scenario 3 – Starting position if all infestations in 2019 were assumed to be high density

Use cost model from Adams and Setterfield 2013 to project costs over duration of weed management program. This is a conservative scenario to provide an upper bound if all the infestation zones had not been managed and had reached their invasion potential prior to control.

The total cost of the eradication phase of the program varied from $187,400 to $250,700. Containment for the internal boundary will cost $7,300 annually ongoing and the external containment boundary of the Park will cost $3,600 ongoing annually (Table 4).

The model demonstrates the cost-efficiency of maintaining high-level control into 2020 and beyond (Scenario 1). For example, cost in 2020 remains high in terms of labour, chemical and use of helicopter for aerial spray, but then drops rapidly. This represents a major freeing up of staff time in 2021 and future years to allocate to other activities. By contrast, the cost of annual management increases in 2020 in Scenario 2 and 3 as the effort of dealing with dense infestations increases. For example, initially chemical control would be restricted to a 100m zone from an access track, and the rest of a


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dense infestations is not managed. In the following year, the cost increases as the initial 100m has been reduced in density but still requires chemical treatment, but an additional 100m in from the track is now accessible and therefore starts being treated. In addition, as areas move from medium to scattered density, the labour cost effort per ha increases as more time is spent searching and controlling the remaining, and any new, infestations while the chemical costs drop due to reduced number of plants (Figure 13). TABLE 4. TOTAL COSTS OF FUTURE WEED MANAGEMENT FOR THREE SCENARIOS.

*Boundary of the core infestation on Wildman north A, ** Maintain current high level of control effort

FIGURE 13. ANNUAL GAMBA GRASS WEED MANAGEMENT COSTS FOR BEST MANAGEMENT STRATEGY UNDER THREE COST SCENARIOS.

CONCLUSION

The strategic fire management on Wildman block over the past 15 years has created conditions suitable for increased development of the savanna woody component and reduced gamba grass vigour.

$-

$20,000.00

$40,000.00

$60,000.00

$80,000.00

$100,000.00

$120,000.00

2019 2020 2021 2022 2023 2024 2025 2026

Modelled Weed Management Annual Costs

Scenarios

1** 2 3 Main control/eradication costs $ 187,423 $ 233,827 $ 250,695 Containment - internal boundary* $ 7,344 $ 7,344 $ 7,344 Containment - external boundary $ 3,672 $ 3,672 $ 3,672

Scenario 1

Scenario 2

Scenario 3


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We found that the current weed management strategy reflects best practice management and also combines a unique approach to excluding fire. The cost of fire management is minor but appears to be accelerating the effectiveness of on-ground control efforts compared to our cost models and effectiveness models developed for other regions.


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Douglas, M.M, Jackson, S., Setterfield, S.A, Pusey, B., Davies, P., Kennard, M., Burrows, D., and Bunn, S. (2011) Northern futures: threats and opportunities for freshwater ecosystems. In: Pusey, B.J., (ed.) Aquatic Biodiversity in Northern Australia: Patterns, Threats and Future. CDU Press, Darwin, NT, pp. 203-220.

Flores, T.A. (1999). Factors affecting the recruitment of Andropogon gayanus (gamba grass). Honours thesis, Charles Darwin University, Darwin.

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NT, Darwin. https://dtc.nt.gov.au/__data/assets/pdf_file/0006/260493/Mary-River-final-JMP_March2015_sml.pdf

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invasion on fire behaviour in northern Australian savannas. Diversity and distributions, 16(5), pp.854-861.

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Whitehead, P., and Wilson, C. (2000). Exotic grasses in Northern Australia: species that should be sent home. In: Proceedings of the Northern Grassy Landscapes Conference, Katherine, August 29–31, 2000. pp 83–87. Tropical Savannas CRC, Darwin.

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ACKNOWLEDGMENTSEXECUTIVE SUMMARYEnd-User StatementBackgroundHigh biomass grass invasion in Australia’s tropical savannasInvasion and control of gamba grass on Mary River National ParkBackground to Invasion at wildman block, Mary River National Park

REsearch approachBenefits of fine-scale fire and gamba grass management at Mary River National ParkBenefits of large-scale strategic fire and gamba Grass management to biodiversity and tourist visitationCosts of fire and gamba grass management on the wildman blockModelling cost scenarios of gamba grass management based on best practice strategyValidating and applying the management cost-modelModelling future management using best practice management strategyModelling cost scenarios based on best practice management strategyConclusion

References

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