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Australian Journal of Botany, 2004, 52, 629–637

Effects of soil disturbance, weed control and mulch treatments on establishment of Themeda triandra (Poaceae) in a degraded white box (Eucalyptus albens) woodland in central western New South Wales Ian ColeA,B,C , Ian D. LuntB and Terry KoenA A Department B The

of Infrastructure, Planning and Natural Resources, PO Box 445, Cowra, NSW 2794, Australia. Johnstone Centre, Charles Sturt University, PO Box 789, Albury, NSW 2640, Australia. C Corresponding author. Email: [email protected]

Abstract. Temperate grassy woodlands are endangered ecosystems in Australia, and many degraded remnants are in desperate need of understorey restoration. This experiment compared the effects of soil disturbance, weed control and mulch treatments on establishment of the original dominant grass, Themeda triandra Forssk., in a degraded white box (Eucalyptus albens Benth.) woodland at Cowra in central New South Wales (NSW). Awned Themeda seeds were surface-sown into replicated plots treated as follows: soil scalping, soil disturbance (by ripping), herbicide (simazine) application and retention of natural mulch. Scalping combined with soil disturbance best promoted Themeda establishment (47.8% after 40 days and 28% after 518 days), and also reduced broadleaf-herb densities. By contrast, scalping without soil disturbance had the worst effect on Themeda establishment (5.2% after 40 days and 4.5% after 518 days). Disturbance significantly enhanced Themeda establishment and decreased the density of annual grasses and the basal cover of non-Themeda species. By contrast, the retention of 500–800 kg of natural surface mulch had no apparent effect on Themeda establishment. Contrary to expectations, simazine reduced the density and basal cover of all species, including Themeda, which is normally resistant to this herbicide. All combinations of the mulched, disturbed and herbicide treatments (i.e. all treatments except scalping) gave similar results, ranging from 10.7 to 22.0 Themeda plants m−2 after 518 days. These results suggest that Themeda stands can be established in degraded box woodlands by using awned seed materials, with minimal seedbed preparation and simple sowing techniques. Further studies are required to determine whether established swards can resist weed invasion in the absence of ongoing weed management, and whether establishment success varies with soil conditions and landscape position.

Introduction Temperate grassy woodlands are among Australia’s most endangered ecosystems, owing to widespread clearing for agriculture (Hobbs and Yates 2000). Most woodland remnants are small, isolated and internally degraded, and contain a small subset of the original native flora plus a suite of exotic species (McIntyre and Lavorel 1994; Prober and Thiele 1995; Lunt 1997a). The original, dominant understorey grasses have been eliminated or greatly depleted from most remnants (C. Moore 1953; R. Moore 1973; Prober and Thiele 1995). Woodland function, composition and structure can often be enhanced by removing degrading processes, such as heavy stock grazing, but the degree of recovery is often rather

limited (Yates et al. 2000; Spooner et al. 2002). Many native grasses and forbs do not form persistent soil seed banks (Lunt 1997b; Morgan 1998); thus, once these species are lost from the standing vegetation, they cannot re-establish, regardless of changes to site management. Restoration of these species will require deliberate re-introduction of plants or propagules (Yates and Hobbs 1997; McDonald 2000). Themeda triandra1 (hereafter Themeda) was once the dominant grass in many grassy woodlands in south-eastern Australia, but has been greatly depleted by heavy grazing in most remnants (Moore 1953; Prober and Thiele 1995). Re-establishment of dominant native grasses such as Themeda is an important first step in woodland restoration, since dominant grasses can provide habitat for fauna,

1

Botanical nomenclature follows Harden (1993), except for Themeda triandra Forssk. and Austrodanthonia spp. which follow Linder (1997) and Austrostipa spp. which follows Jacobs and Everett (1996). For brevity, Themeda triandra is referred to as Themeda. © CSIRO 2004

10.1071/BT04010

0067-1924/04/050629

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suppress exotic weeds and capture soil nutrients and water, thereby helping to reduce subsoil water intake. Whilst most woodland species appear to compete poorly against weeds, mature Themeda plants can vigorously compete against many weed species (Phillips 1999; Lunt and Morgan 2000). Several studies have documented the successful establishment of Themeda stands in a range of environments, including native grasslands in western Victoria (McDougall 1989; Phillips 1999), roadsides in the Adelaide Hills in South Australia (Stafford 1991) and mine sites in central New South Wales (Windsor et al. 2000). These studies all used seed-bearing hay as a seed source. This simple technique has a number of limitations, including the following: (1) only relatively small areas can be restored at a time and (2) it is difficult to accurately assess the viable-seed content of the hay, which means that optimal seeding rates and final establishment rates cannot be identified. Before Themeda understoreys can confidently be restored across wide areas, more on-ground experience needs to be gained in a wider range of environments, using more refined seed sources, such as seed-floret materials, rather than seed-bearing hay (Cole and Lunt, in press). Autecological studies have shown that several factors are important for the successful establishment of Themeda from surface sowings of awned seed. Soil disturbance is often required to maximise seed penetration, especially on crusted soils (Sindel et al. 1993). Competing plant species usually need to be controlled to maximise establishment and growth rates (Semple et al. 1999). Mulches may also promote establishment and growth by limiting evaporation and maintaining moisture around the seed, especially in hot, dry sites (Hagon and Groves 1977). The aim of this experiment was to compare the effects of soil disturbance, weed control and mulch treatments on Themeda establishment in a degraded white box (Eucalyptus albens) woodland in central New South Wales. This experiment differs from previous trials (e.g. McDougall 1989; Phillips 1999; Windsor et al. 2000) in that concentrated seed-floret materials were used, rather than the more conventional (but more problematic) seed-bearing hay. Materials and methods Site description The experiment was conducted on the NSW Department of Infrastructure, Planning and Natural Resources, Centre for Natural Resources, Research Centre at Cowra (33◦ 48 54 S, 148◦ 41 53 E) in a paddock typical of many grassy white box woodland areas across central NSW. The study site was in a mid-slope position and sloped gently (7◦ ) to the south. It contained a number of mature Eucalyptus albens above a degraded understorey consisting mainly of winter-growing, annual grasses, including Lolium rigidum, Hordeum leporinum, Avena fatua, Bromus spp. and broadleaf species, Trifolium subterraneum, Echium plantagineum, Arctotheca calendula, Chondrilla juncea and Polygonaceae spp. Native Juncus spp., Stipa spp. and Microlaena stipoides occurred sparsely. Summer-growing species included Panicum spp.,

I. Cole et al.

Digitaria spp., Bothriochloa macra and Paspalidium jubiflorum (Windsor 1999). The soil is a non-calcic brown soil on Cowra granite. Texture varies from sandy loam in the surface horizons (0–20 cm), through sandy clay loam (20–30 cm) to light medium clay lower in the profile (30–90 cm). It is well drained but sets hard when dry. Raupach pH (measured using a barium sulphate–water field test) varied with depth from 5.0 (0–20 cm) through to 6.0 (20–30 cm) and 7.5 (30–90 cm). Seed material Themeda seed was harvested with a ‘Grasshopper’ brush harvester during the summer of 1998–1999 from near Wellington in NSW, about 200 km to the north and with similar rainfall to Cowra. This brush harvester produces a seed material comprised of awned seeds and chaffy seedheads (i.e. a seed-floret mix) but with minimal stem and straw. The seed mix is far more concentrated than the seed-bearing hay that has been used elsewhere (e.g. McDougall 1989; Phillips 1999). Germination tests conducted immediately prior to sowing determined that this material contained 3800 viable seeds kg−1 . Treatments The site was first slashed and treated with glyphosate (0.98 kg a.i. ha−1 ) in September 1999. It was then mown 1 week before sowing seed to simulate close grazing by sheep. This created a natural surface mulch, estimated to be between 500 and 800 kg ha−1 in total dry matter (Prograze 2000), consisting of dead grasses about 3.0 cm high above a thin litter layer on the soil surface. Ten different treatments were created, the first eight of which were factorial treatments. Two seedbed preparation treatments were used, mulch (maintaining the low natural mulch) and non-mulch (removing mulch by burning with a kerosene flame-thrower). These treatments were combined with ± disturbance (disturbed = ripping with light, spring scarifier tynes on a small tractor toolbar) and ± weed control (simazine, a root-absorbed herbicide that controls a wide range of annual grass and broadleaf weeds, applied at 1.8 kg a.i. ha−1 with a knapsack spray). Soil disturbance was rather crude, and the spring tynes often created deep open pits and large turf clods, which were loosely replaced over the rip lines prior to sowing. The remaining two treatments were produced by ‘scalping’ away the topsoil from the ninth plot with a shovel to an average depth of about 5 cm to remove the soil seedbank and then splitting this plot lengthwise to produce (after the disturbance treatment) scalped/disturbed and scalped/non-disturbed subplots. This scalped pair of treatments was randomly interspersed with the other treatment plots. Three replicate blocks were arranged contiguously down the hill slope. Plots were 4 m long × 2 m wide and were positioned end-to-end to form a 36-m-long block. Individual packets of the Themeda seed-floret material containing approximately 800 viable seeds were weighed for each plot, giving a sowing rate of 100 viable seeds m−2 . This was sown by hand on 22 October 1999. Although not planned, the trial was grazed down to about 5.0-cm height by wandering sheep 47 days after sowing. Several management decisions were made during the postestablishment phase, which, although considered necessary for the success of the trial, limit the claims that can be made for the effectiveness of the experimental herbicide treatment to control weeds in the long term. First, during January, mature Chondrilla juncea plants resprouted across the paddock, requiring control. As a result, all plots were sprayed with MCPA (1.25 L a.i. ha−1 ) at 94 and 214 days after sowing (DAS). Second, all plots were sprayed with simazine (1.8 L a.i.ha−1 ) at 144 DAS (in autumn 2000), to control large numbers of germinating annual grasses, in an attempt to maximise Themeda growth and survival rates. The experimental protocol therefore contained the following three distinct phases, each of which included a herbicide treatment as well as other associated activities (Table 1): (1) the pre-treatment phase when glyphosate was applied to control winter-growing annuals and

Establishing Themeda triandra in central western NSW woodlands


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Table 1. Experimental protocol used in the Themeda establishment trial DAS = days after sowing Experimental phase

Activity

Treatment events (DAS)

Pre-treatment (pre-sowing)

Slashing Herbicide (glyphosate) Mow + rake

−28 −28 0

Treatment (establishment)

±Mulch ±Disturbance ±Herbicide (simazine) Scalping ± disturbance

0 0 0 0

Post-establishment/growth phase (management)

Grazing Herbicide (MCPA) Herbicide (simazine)

47 94 144

Herbicide (MCPA)

214

Monitoring events (DAS)

40

147 278 518

to provide a natural mulch cover; (2) the treatment phase when the main experimental treatments, including a herbicide (simazine), were applied to selected plots; and (3) a post-treatment phase when all plots experienced a brief period of inadvertent grazing and herbicides (MCPA and simazine) to control post-establishment weeds. The aim of this experiment was to compare the treatments applied during the critical, early establishment phase within the first 3 months of seed sowing. The effects of these treatments were assessed during the establishment phase at 40 DAS and at three times during the subsequent post-establishment phase. Plant assessments and analyses Plants were assessed at 40 DAS (1 December 1999), 147 DAS (17 March 2000), 278 DAS (27 July 2000) and 518 DAS (22 March 2001).

Table 2. The number of days after sowing at which various plant attributes were assessed for the different species groups Attribute/species group 40 Density Themeda Broadleaf species Annual grasses Perennial grasses

x x x x

Number of days after sowing 147 278 517 x x

A

x

x

A

x

Basal cover Themeda Broadleaf species Annual grasses Perennial grasses Total non-Themeda species

x

x

x

x

B,C

B

B

B

D

x

x

x

High density and basal cover over all plots (too high to count accurately). B Very low density and basal cover across all plots, except for C below. C Moderate basal cover of Trifolium subterraneum mainly in simazine plots. D High basal cover over all plots.

Plant densities were determined by counting and then averaging the number of plants in two 1-m2 quadrats, placed equidistant from each side and 0.5 m from the ends of each plot. Basal cover was estimated from 100 intersections of a 10 × 10-cm grid within each 1-m2 quadrat. Average data from the two quadrats were used in all analyses. Themeda density and the basal cover of total non-Themeda species were assessed on all dates. Densities of broadleaf herbs, annual grasses and other perennial grasses (excluding Themeda) were assessed at 40 and 147 DAS, after which it was impossible to accurately identify individual plants and basal cover was measured instead. However, at 278 and 518 DAS, annual-grass and broadleaf-herb densities were not high enough to result in meaningful data, perhaps as a result of the post-establishment herbicide-management regime (Table 2). Treatment effects on density and basal cover were examined at each assessment date by standard analysis of variance (ANOVA) methods. As usual with this approach, some interactions were inconsistent over time and difficult to interpret and only the main themes have been discussed. Treatment by time effects on Themeda density and non-Themeda basal cover were analysed by repeated measures ANOVA, with an antedependence structure of the covariance matrix of order one (Genstat Committee 1995). Single degree of freedom orthogonal contrasts were used to partition treatment effects. All data in both analyses were checked for evidence of lack of normality and heterogeneity of variances, and where necessary normalised with the (loge + 1) transformation.

Results Effects on Themeda establishment Themeda establishment varied significantly among treatments. The scalped × disturbed treatment best promoted Themeda establishment at 40 DAS, with 47.8% of the viable seed (i.e. 47.8 plants m−2 ) establishing successfully (Table 3). By contrast, the scalped × non-disturbed treatment resulted in the worst establishment (5.2%). Averaged over all treatment combinations, disturbance significantly promoted Themeda establishment at 40 DAS (16.1% cf. 9.4%, P < 0.01), mulch did not promote establishment (P = 0.15)

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Table 3. Comparison of treatment effects at 40 days after sowing Table shows structured comparisons and degrees of freedom used in ANOVA analysis and treatment means for density (plants m−2 ) of each species group and basal cover (%) for total non-Themeda. P-values for F-statistics are shown in parentheses, whereas significant effects are shown in bold. Treatment means for second-order interactions are not shown, as these were not significant; n = 2 Source of variation

d.f. Treatment Themeda grass

Interactions Mulch × disturbance × simazine Mulch × disturbance

1 1

Density (plants m−2 ) Perennial Annual grass grassA

Broadleaf herbs

Basal cover for total non-T. triandra (%)

Mulch + disturbance Mulch + non-disturbance Non-mulch + disturbance Non-mulch + non-disturbance

(0.85) 15.7 6.8 16.5 11.9

(0.43) 1.3 4.0 1.4 2.0

(0.31) 17.2 39.5 23.5 35.0

(0.14) 36.1 29.5 32.8 42.6

(0.55) 13.8 26.8 11.5 18.3

Mulch × simazine

1

Mulch + simazine Mulch + non-simazine Non-mulch + simazine Non-mulch + non-simazine

(0.29) 6.0 16.5 7.5 21.0

(0.39) 3.2 1.8 1.7 1.7

(0.30) 3.1 53.6 1.2 57.3

(0.16) 27.5 38.1 20.5 54.9

(0.26) 12.8 27.8 6.3 23.5

Disturbance × simazine

1

Disturbed + simazine Disturbed + non-simazine Non-disturbed + simazine Non-disturbed + non-simazine

(0.44) 7.3 24.8 6.1 2.7

(0.53) 2.1 0.8 2.6 3.1

(0.58) 2.1 38.6 2.2 72.3

(0.05) 24.3 44.6 23.7 48.4

(0.70) 7.7 17.6 11.3 33.8

Mulch

1

Mulch Non-mulch

(0.01) 11.3 14.2

(0.30) 2.4 1.7

(< 0.01) 28.3 29.3

(0.67) 32.8 37.7

(< 0.05) 20.3 14.9

Disturbance

1

Disturbed Non-disturbed

(0.15) 16.1 9.4

(0.43) 1.3 2.9

(0.86) 20.3 37.2

(0.39) 34.5 36.0

(0.06) 12.6 22.5

Simazine

1

Simazine Non-simazine

(< 0.01) 6.7 18.8

(0.13) 1.7 2.4

(< 0.01) 2.1 55.5

(0.77) 24.0 46.5

(< 0.01) 9.5 25.7

(< 0.01)

(0.49)

(< 0.01)

(< 0.01)

(< 0.01)

47.8 5.2

0.0 0.0

12.2 7.2

20.5 21.8

7.0 8.0

(< 0.01)

(Undefined)

(0.55)

(0.78)

(0.84)

Scalped v. non-scalpedB Residual

1 16

Scalped 1

Residual Total A B

Disturbed Non-disturbed

2 29

Back-transformed means. The comparison scalped v. non-scalped confounds disturbance, mulch and simazine effects.

and, contrary to expectations, simazine significantly reduced establishment (6.7% cf. 18.8%, P < 0.01). A significant (P < 0.01) simazine × disturbance interaction occurred; simazine significantly reduced Themeda density in disturbed plots (from 24.8 to 7.3 plants m−2 ; P < 0.05), but was associated with a small, non-significant increase in Themeda density in undisturbed plots. Themeda seedling survival After 517 days, the scalped × disturbed treatment produced significantly (P < 0.05) more Themeda plants than did

all other treatments, with 27.7 plants m−2 . At the other extreme, the scalped × undisturbed treatment resulted in the lowest Themeda density, with only 4.5 plants m−2 , which was significantly (P < 0.01) less than in all other treatments. All of the non-scalped treatments gave similar results, ranging from 10.7 to 22.0 plants m−2 (Fig. 1). Amongst the non-scalped treatments, the mulch × disturbed × nonsimazine treatment gave the best outcome (22 plants m−2 ), but this was not significantly greater than that for the next-best and contrasting treatment, non-mulch × nondisturbed × simazine (17.5 plants m−2 ; Fig. 1).

Establishing Themeda triandra in central western NSW woodlands


Plants m–2 (or % establishment)

60

633

Mulch/non-disturbed/simazine Mulch/non-disturbed/non-simazine Mulch/disturbed/simazine Mulch/disturbed/non-simazine Non-mulch/non-disturbed/simazine Non-mulch/non-disturbed/non-simazine Non-mulch/disturbed/simazine Non-mulch/disturbed/non-simazine Scalped/disturbed Scalped/non-disturbed

50

40 30

20 10 0 0

100

200

300

400

500

600

Days after sowing (DAS) Fig. 1. Average Themeda density (plants m−2 ) for each factorial treatment combination at 40, 147, 278 and 517 days after sowing (DAS). Error bars indicate the least significant difference (P = 0.05) between any two means at that particular assessment.

In all treatments, Themeda density changed most dramatically during the first summer, between 40 and 147 DAS. Averaged over all treatments, Themeda density increased slightly from 13.5 plants m−2 at 147 DAS to 15.2 plants m−2 at 517 DAS, but these figures obscure substantial differences in trends amongst treatments. A number of significant main-order and first-order interactions

occurred over the experimental period (Table 4), most of which indicate that differences among treatments diminished in magnitude as time progressed (Fig. 1). Thus, Themeda density increased over time in most of the treatments that produced sparse initial establishment (25 plants m−2 ; Fig. 1). The

Table 4. Analysis of deviance table from repeated measures ANOVA of treatment effects on Themeda density and non-Themeda %basal cover over the whole trial period and change in treatment effects over time Significant effects are shown in bold Fixed term Scalped v. non-scalped (Sc v. NSc) Scalped + disturbed v. scalped + non-disturbed (ScDist v. ScUndist) Mulch (M) Disturbance (D) Simazine (S) M×D M×S D×S M×D×S Time (T) T × Sc v. NSc T × ScDist v. ScUndist T×M T×D T×S T×M×D T×M×S T×D×S T×M×D×S Deviance A

d.f.

Themeda density Wald statistic P(χ2 )

Non-Themeda basal cover Wald statistic P(χ2 )

1 1

14.42 177.69