Effects of grazing, trenching and surface soil disturbance on ground cover in woody encroachment on the Cobar Pediplain, south-eastern Australia

Journal of Arid Environments 96 (2013) 80e86

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Effects of grazing, trenching and surface soil disturbance on ground cover in woody encroachment on the Cobar Pediplain, south-eastern Australia Rhiannon Smith a, Matthew Tighe b, Nick Reid a, Sue Briggs c, *, Brian Wilson a, d a

Ecosystem Management, School of Environmental and Rural Science, University of New England, Armidale, NSW 2351, Australia Agronomy and Soil Science, School of Environmental and Rural Science, University of New England, Armidale, NSW 2351, Australia Institute for Applied Ecology, University of Canberra, Building 15, ACT 2601, Australia d NSW Office of Environment and Heritage, University of New England, Armidale, NSW 2351, Australia b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 6 September 2012 Received in revised form 14 January 2013 Accepted 22 April 2013 Available online 20 May 2013

This study investigated three possible reasons for low ground cover in an inter-patch in woody encroachment in semi-arid south-eastern Australia: (1) grazing by large herbivores, (2) competition with woody plants for resources, and (3) the smooth, crusted soil surface impeding litter lodgement and germination of seeds. Grazing exclusion, trenching (cutting roots of woody plants to 30 cm depth) and surface soil disturbance treatments were established in October 2008, and herbaceous ground cover and litter cover were measured after three, 16 and 30 months. Perennial grass cover in the ungrazed area was higher in trenched plots than in untrenched plots. Perennial grass cover in the grazed area was very low in trenched and untrenched plots. Herbaceous ground cover increased over time in ungrazed and trenched plots, much more than in grazed or untrenched plots. Soil disturbance did not affect herbaceous ground cover. Herbaceous ground cover was low in all treatments (<10%). Both grazing and cutting roots of woody plants affected herbaceous ground cover in this study. Herbaceous ground cover increased when roots of woody vegetation were severed (in the absence of grazing), indicating that herbaceous ground cover and woody vegetation compete for resources. Ó 2013 Elsevier Ltd. All rights reserved.

Keywords: Grazing Ground cover Inter-patch Semi-arid Woody encroachment Woodyeherbaceous competition

1. Introduction Many semi-arid ecosystems comprise mosaics of vegetation patches interspersed with inter-patches (Montaña et al., 2001). Inter-patches are characterised by biological or physical crusts with little or no vascular ground cover or litter, low water infiltration, and high erosion and sediment production; they are selfreinforcing, maintaining their existing condition (Greene and Tongway, 1989; Segoli et al., 2008). The soil surface in interpatches is smooth with high runoff, and with few or no microsites for seeds to lodge and germinate (Greene and Tongway, 1989; Segoli et al., 2008). Runoff removes plant propagules and litter, which depletes the seed bank and soil nutrient store (Harrington et al., 1979). The seeds that remain in inter-patches have difficulty germinating and establishing due to the physical barrier of the soil crust (Yates et al., 2000). Inter-patches interspersed with * Corresponding author. Tel.: þ61 2 6206 8609. E-mail addresses: [email protected] (R. Smith), [email protected] (M. Tighe), [email protected] (N. Reid), [email protected] (S. Briggs), [email protected] (B. Wilson). 0140-1963/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jaridenv.2013.04.006

vegetated patches are a natural feature of many arid and semi-arid ecosystems, but large areas of bare inter-patches indicate dysfunctional landscapes that do not retain water, nutrients and other resources (D’Odorico et al., 2012; Ludwig et al., 1997; Muñoz-Robles et al., 2011a). Woody encroachment is the increase in density, cover, extent and biomass of indigenous woody or shrubby plants, and occurs in arid and semi-arid biomes and in grasslands in mesic areas (D’Odorico et al., 2012; Eldridge et al., 2011; Ratajczak et al., 2012). Causes of woody encroachment include altered grazing and fire regimes, CO2 enrichment, variations in weather, and climate change (Eldridge et al., 2011). Woody encroachment has increased in the study region since European settlement (Gardiner et al., 1998; Noble, 1997). Details about patches and inter-patches in areas affected by woody encroachment in the study region are in MuñozRobles et al. (2011a,b). Grazing animals can increase the area of bare inter-patches, and exacerbate and reinforce the woody encroached state by removing herbaceous ground cover (Aguiar and Sala, 1999; Archer et al., 1999; Milchunas and Lauenroth, 1993; Walker and NoyMeir, 1982; Zaady et al., 2001). Reducing grazing pressure in arid

R. Smith et al. / Journal of Arid Environments 96 (2013) 80e86

and semi-arid ecosystems can cause herbaceous ground cover to increase (Archer, 1990; Ludwig et al., 1997), although ground cover can be controlled by resource availability rather than by grazing (Baez et al. (2006). Surface soil disturbance may also improve levels of herbaceous ground cover, by roughening the soil surface which allows seeds to lodge and water to enter the soil (Ludwig et al., 1997). Ground cover reduces run-off and erosion, and thus contributes to resource retention in these ecosystems (D’Odorico et al., 2012; Greene et al., 1994; Muñoz-Robles et al., 2011a). Competition for soil moisture can be severe in arid and semiarid ecosystems (Scholes, 1990). Several studies in semi-arid ecosystems have concluded that herbaceous vegetation and woody vegetation (trees and shrubs including woody encroachment) compete for resources, since herbaceous vegetation increased when woody vegetation was removed (Harrington and Johns, 1990; Tunstall et al., 1981; Walker et al., 1972), and woody biomass increased when competition from herbaceous plants was removed (Knoop and Walker, 1985). In contrast to these empirical findings, Canadell et al. (1996) and Schenk and Jackson (2002) suggested that herbaceous plants and woody plants do not compete for soil moisture in semi-arid ecosystems due to deep rooting depths of shrubs compared with herbaceous plants. This study investigated effects of the following treatments: (1) excluding grazing by large herbivores, (2) trenching (severing roots) to remove competition from trees and shrubs, and; (3) disturbance of the soil surface to increase roughness; on herbaceous ground cover and litter cover in an inter-patch in woody encroachment on the Cobar Pediplain in New South Wales, Australia. Carbon (C) and nitrogen (N) in surface soils were also measured to investigate if these soil attributes changed with the treatments. The hypotheses were: (1) herbaceous ground cover will be higher over time in ungrazed plots than in grazed plots; (2) herbaceous ground cover will be higher over time in trenched plots than in untrenched plots, and (3) herbaceous ground cover and litter cover will be higher over time in plots with surface soil disturbance than in undisturbed plots. We did not expect litter cover to differ over time between grazed and ungrazed treatments, or between trenched and untrenched treatments. We expected surface soil C and N to be lower in grazed, trenched and disturbed plots than in ungrazed, untrenched or undisturbed plots over time, because of mineralisation with disturbance (Dalal and Mayer, 1986a,b).

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2. Methods 2.1. Study site The study was conducted just north of Nyngan on the Cobar Pediplain in semi-arid New South Wales, in south-eastern Australia (31.54 S, 147.20 E). Monthly rainfall during the study period and average rainfall at Nyngan are shown in Fig. 1. Mean annual rainfall at Nyngan is 447 mm, and mean annual maximum and minimum temperatures are 26  C and 12  C, respectively (Bureau of Meteorology, undated). The experiment was conducted in a large inter-patch (the experimental area) at a site previously studied by Tighe et al. (2009) and Muñoz-Robles et al. (2011a,b). The soil type, ground cover and woody vegetation cover at the site were similar to those of the Cobar Pediplain generally. The vegetation state was woody encroachment, with poplar box (Eucalyptus populnea subsp. bimbil) and red box (Eucalyptus intertexta) trees, and a dense midstorey of Geijera, Dodonaea and Eremophila (>1200 stems/ha) (Muñoz-Robles et al., 2011a). Herbaceous ground cover at the site was sparse and consisted of native perennial grasses (Poaceae) and species of Chenopodiaceae and Asteraceae. Litter cover (dead, unattached plant material) was also sparse. Most of the soil surface consisted of biological crusts (dominated by cyanobacteria) or physical crusts devoid of vegetation. The soil type was a red dermosol (Isbell, 1996). The paddock containing the experimental area was usually grazed by domestic livestock (sheep and beef cattle), feral animals (goats, pigs, rabbits) and native herbivores (kangaroos). Domestic stock were not present in the experimental area during the 12 months before the experiment commenced in October 2008. The grazed part of the experimental area (grazed area) was grazed by sheep at 1 DSE (dry sheep equivalent)/ha from three months after establishment of the experiment until February 2010, at 2 DSE/ha between February 2010 and April 2011, and by cattle at 0.25 DSE/ha during April 2011. These stocking rates are average to just above average for the district (Russell, undated). 2.2. Experimental design Prior to the experiment, the site had little or no herbaceous ground cover, little to no surface soil roughness, a high proportion of cyanobacterial soil crust cover and bare ground, and no obvious impediments to overland flow of water. An ungrazed area and a

120

100

Rainfall (mm)

80

60

40

20

0 J F MAM J J A S O N D J F MAM J J A S O N D J F MAM J J A S O N D J F MA 2008

2009

2010

2011

Fig. 1. Monthly rainfall prior to and during the experiment (bars), and long-term monthly average rainfall (line) at Nyngan (Bureau of Meteorology, undated).

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R. Smith et al. / Journal of Arid Environments 96 (2013) 80e86

grazed area were established in the inter-patch. The grazing exclusion (ungrazed) area was fenced to 1.2 m high to exclude large herbivores and omnivores (sheep, cattle, goats, and pigs), and contained nine 4  3 m plots (five trenched, four untrenched); the grazed area was unfenced and contained eight 4  3 m plots (four trenched, four untrenched). The surface flow paths in both areas were checked with a carpenter’s level to ensure that runoff from the grazed area did not flow onto the ungrazed area or vice versa. The trenched and soil surface disturbance treatments were nested within the grazed and ungrazed areas. Plots were systematically allocated as trenched or untrenched in the grazed and ungrazed areas to avoid being trenched by default (i.e. an untrenched plot surrounded by trenched plots). Trenching consisted of manually digging a 30 cm deep, 10 cm wide trench around each trenched plot and cutting the roots of woody plants in the trench. A 30 cm wide section of commercial root matting was then placed vertically in the trench, the trench was refilled with soil, and the soil surface swept with a broom to ensure it was level with the surrounding soil surface. All trenched and untrenched plots were swept with a broom to remove litter. Each trenched and untrenched plot was divided into twelve 1 m2 subplots, and the soil surface was disturbed in six randomly selected subplots in each plot by breaking the soil surface to 5 cm depth with a mattock. The remaining six subplots in each trenched and untrenched plot were left undisturbed. Two of the trenched plots in the grazed area were 4  2 m in size to fit in the inter-patch. This provided eight 1 m2 subplots in each of these plots, four of which had their surface soil disturbed as described and four were left undisturbed. Duplicate samples of surface soil (2 cm depth) were collected from two sides of each plot prior to imposing treatments to check for any differences in soil chemistry at the start of the experiment 3 þ 2þ 2þ þ þ 3þ (C, N, C:N, NO 3 , PO4 NH4 , Ca , Mg , K , Na , Al , Ca:Mg, ECEC (effective cation exchange capacity), and percent Ca, Mg, K, Na, Al of ECEC). Values were very similar across the grazed and ungrazed areas (Appendix 1). These data are not discussed further.

herbaceous ground cover. Biological crusts (cyanobacteria) could not be distinguished reliably from physically sealed surface soil, and thus were not separately measured. Soil samples were air-dried and sieved to 2 mm prior to analysis for total C and N at the Environmental Analysis Laboratory, Southern Cross University, Lismore, New South Wales, Australia, using the LECO combustion method. A four-way partially nested mixed linear model (Neter et al., 1996) was used to test for differences in perennial grass cover, other herbaceous ground cover, total herbaceous ground cover, litter, and topsoil C, N and C:N between the combinations of grazing, trenching and soil surface disturbance over the three monitoring times. Analysis was carried out using Statistix 8 (Analytical Software, 2003). All assumptions of each model were met (transformation was necessary in some instances) and P < 0.05 was taken as statistically significant. Tukey’s HSD pairwise comparisons were used to analyse differences in ground cover components between treatments. 3. Results 3.1. Herbaceous ground cover and litter Herbaceous ground cover remained low in all experimental treatments throughout the study (<10%). Perennial grass cover and litter cover both showed grazing by trenching interactions (Table 1). Perennial grass cover in the ungrazed area was significantly higher in the trenched plots than in the untrenched plots (Fig. 2a). Perennial grass cover in the grazed area was very low in both trenched and untrenched plots (Fig. 2a). Trenched plots in the ungrazed area had lower litter cover than untrenched plots, but not in the grazed area (Fig. 2b). All changes in ground cover during the experiment were small in absolute terms. The ground cover components showed significant grazing by time interactions (Table 1). Perennial grass cover increased to almost 6% in the ungrazed area by April 2011 (after rising to almost 5% in February 2010), while perennial grass cover in the grazed area remained at <1% throughout the experiment (Fig. 3a). Other herbaceous ground cover in the ungrazed area was 4% by April 2011 (after being at just over 3% in February 2010) (Fig. 3b). Other herbaceous ground cover in the grazed area was only slightly higher in April 2011 than at the start of the experiment (after reaching 2% in February 2010). Litter cover in the ungrazed area was 8% in April 2011 (after reaching 9% in February 2010), while litter cover in the grazed area remained steady at almost 8% from January 2009 (Fig. 3f). Trenching by time interactions were significant for total herbaceous ground cover and for other herbaceous ground cover, but not for perennial grass or litter cover (Table 1). Total herbaceous ground cover in trenched plots reached 7% in February 2010 and remained at this level in April 2011, but only increased to 3% by April 2011 in untrenched plots (Fig. 2e). Other herbaceous ground cover in trenched plots was 3% by April 2011 (after reaching 4% in February 2010), and 1.5% in untrenched plots (Fig. 2c). Total

2.3. Data collection and statistical analyses Ground cover (herbaceous and litter) on the experimental plots was zero at the start of the experiment. The ground cover components (perennial grass, other herbaceous, total herbaceous, litter) were measured and soil samples collected for C and N analysis at three months (January 2009), 16 months (February 2010) and 30 months (April 2011) following establishment of the experimental treatments in October 2008. Two surface disturbed sub-plots and two undisturbed subplots were randomly selected in each combination of grazing and trenching treatments (grazed/trenched, grazed/not trenched, ungrazed/trenched, ungrazed/not trenched) at each monitoring time. Ground cover measurements and soil samples (0e2 cm depth) were taken in a 0.25 m2 quadrat in the centre of each subplot at each monitoring time. A photograph of each 0.25 m2 quadrat was taken and scored for cover of perennial grass, other herbaceous ground cover, and litter cover. Total herbaceous ground cover was the sum of perennial grass and other

Table 1 Significance of treatment by time interactions for the ground cover components. Ground cover component

Grazing by time

Perennial grass Other herbaceous Total herbaceous Litter

F4,180 F4,180 F4,180 F4,180

¼ ¼ ¼ ¼

12.33, P < 0.0001 22.65, P < 0.0001 35.87, P < 0.0001 6.30, P ¼ 0.0001

Trenching by time F2,180 F2,180 F2,180 F2,180

¼ ¼ ¼ ¼

1.08, 6.25, 4.81, 0.88,

P P P P

¼ ¼ ¼ ¼

0.3427 0.0024 0.0092 0.4152

Disturbance by time F2,180 F2,180 F2,180 F2,180

¼ ¼ ¼ ¼

1.84, P ¼ 0.1625 0.42, P ¼ 0.6559 0.43, P ¼ 0.6523 13.19, P < 0.0001

Grazing by trenching F1,180 F1,180 F1,180 F1,180

¼ ¼ ¼ ¼

5.01, 0.05, 2.35, 4.95,

P P P P

¼ ¼ ¼ ¼

0.0264 0.8188 0.1272 0.0274

b

b

Not trenched

Trenched

0 Trenched

Ungrazed

Grazed

ab

b

Trenched

b

2

a

(b)

Not trenched

4

a

Trenched

6

14 12 10 8 6 4 2 0

Not trenched

(a)

a

Litter cover (%)

8

Not trenched

Perennial grass cover (%)

R. Smith et al. / Journal of Arid Environments 96 (2013) 80e86

Ungrazed

Grazed

Fig. 2. Perennial grass cover (mean  1 SE) (a), and litter cover (b) in the untrenched and trenched plots in the ungrazed and grazed areas (combined data for January 2009, February 2010 and April 2011). Different letters denote significantly different means.

Perennial grass cover (%)

(a) 10 Grazed Ungrazed

8

a a

3.2. Soil carbon and nitrogen

4

Soil C and N were higher in untrenched plots than in trenched plots, and were higher in undisturbed subplots than in disturbed subplots (Fig. 4a and b). The C:N ratio did not differ significantly between the trenched and untrenched plots, but was higher in the disturbed than the undisturbed subplots (Fig. 4c). Grazing by time interactions were significant for C, N and the C:N ratio (Table 2), although changes in these soil attributes during the experiment were relatively small (Fig. 5aec). The results support the expectation that C and N will be lower in disturbed and trenched plots than in undisturbed and untrenched plots, but C was slightly higher in grazed than in ungrazed plots.

2

b b

b

b

0

Other herbaceous cover (%) Total herbaceous cover (%)

(c)10 Grazed Ungrazed

8

8 a

6

Trenched Untrenched

6

a

4

a

4

b

b 2

bc

c c

2

0

(d) 14

Litter cover (%)

herbaceous ground cover and its components did not differ between disturbed and undisturbed plots (all P > 0.3). The surface disturbance by time interaction was significant only for litter cover (Table 1). Disturbed and undisturbed subplots had similar amounts of litter cover in April 2011, although disturbed subplots had higher litter cover than undisturbed plots at the earlier monitoring times (Fig. 3g). Litter cover in the undisturbed plots increased during the experiment, but fell during the later part of the experiment in the disturbed subplots. The results support the hypothesis that herbaceous ground cover will be higher over time in ungrazed plots compared with grazed plots, partly support the hypothesis that herbaceous ground cover will be higher over time in trenched plots than in untrenched plots (only in the ungrazed area), and support the hypothesis that litter cover will be higher in disturbed plots than in undisturbed plots (although not in the final month of sampling). The results do not support the hypothesis that herbaceous ground cover will be higher over time in disturbed plots than in undisturbed plots. The expectation that litter cover would not differ over time between trenched and untrenched plots was met, although litter cover did differ between grazed and ungrazed plots in the first month of sampling.

6

(b) 10

(f)

83

12

a

Grazed Ungrazed

a

(e)14 12 10

8

Trenched Untrenched a

ab

8

6

6

b

4

bc

2

(g) Grazed Ungrazed a

a

a

a a

b

January February 2009 2010

April 2011

bc b

4

d d

0

18 16 14 12 10 8 6 4 2 0

4. Discussion

c c

0

10

2

b b

d d

0

18 16 14 12 10 8 6 4 2 0

Disturbed Undisturbed a a b

ab ab

c

January February 2009 2010

April 2011

Fig. 3. Perennial grass ground cover (mean  1 SE) (a), other herbaceous ground cover (b and c), total herbaceous ground cover (d and e), and litter cover (g and f) during the experiment (only significant interactions with time shown). Different letters denote significantly different means.

Grazing and cutting roots of woody plants affected herbaceous ground cover in this study. Trenched plots had higher perennial grass cover than untrenched plots, but only in the ungrazed area. Perennial grass cover was very low in trenched plots and in untrenched plots in the grazed area throughout the experiment. Total herbaceous ground cover and other herbaceous ground cover in the ungrazed area and in the trenched plots increased during the experiment much more so than they did in the grazed area or in the untrenched plots. Soil disturbance did not affect herbaceous ground cover. Herbaceous ground cover in the trenched plots above the filled-in trenches was not higher than away from the filled-in trenches, and thus was unlikely to be affected by soil disturbance from the trenching per se. Notwithstanding the positive effects on herbaceous ground cover of excluding grazing, and trenching to interrupt competition from roots of woody plants, levels of herbaceous ground cover remained low (<10%) in all treatments throughout the study. The results of this study are consistent with those of previous investigations in the study area and in similar systems elsewhere. Harrington and Johns (1990), Tunstall et al. (1981) and Walker et al. (1972) found that herbaceous vegetation increased when woody vegetation was removed, and they concluded that herbaceous vegetation and woody vegetation (trees and shrubs, including woody encroachment) compete for resources. Robson (1995) found that pasture biomass increased only in areas that were blade ploughed (which severs roots of woody plants at 20e30 cm depth)

R. Smith et al. / Journal of Arid Environments 96 (2013) 80e86

15

a

a

Trenched

a

b

10 5

Disturbed

0 Undisturbed

b

Untrenched

a

Disturbed

b

Undisturbed

a

Trenched

b

(c)

Untrenched

a

0.12 0.1 0.08 0.06 0.04 0.02 0

Disturbed

b

Undisturbed

a

Soil nitrogen (%)

(b)

Trenched

1.5 1.25 1 0.75 0.5 0.25 0

Untrenched

Soil carbon (%)

(a)

Carbon:nitrogen ratio

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Fig. 4. Soil carbon (%C, mean  1 SE) (a), nitrogen (%N) (b), and C:N ratios (c), in the untrenched and trenched plots in the ungrazed and grazed areas (combined data for January 2009, February 2010 and April 2011). Different letters denote significantly different means.

Table 2 Significance of the grazing by time interaction, and trenching and surface soil disturbance main effects for the soil variables. Soil attribute

Grazing by time

Trenching

Disturbance

Carbon (%)

F4,78 ¼ 3.30, P ¼ 0.0150 F4,78 ¼ 10.83, P < 0.0001 F4,78 ¼ 12.41, P < 0.0001

F1,78 ¼ 9.19, P ¼ 0.0033 F1,78 ¼ 8.47, P ¼ 0.0047 F1,78 ¼ 1.05, P ¼ 0.3089

F1,78 ¼ 4.57, P ¼ 0.0357 F1,78 ¼ 12.62, P ¼ 0.0007 F1,78 ¼ 11.22, P ¼ 0.0013

Nitrogen (%) Carbon:nitrogen

(a)

2.0

Soil carbon (%)

Archer (1997) and D’Odorico et al. (2012) reviewed interactions between herbaceous and woody vegetation in savannah and semiarid/arid ecosystems; they showed that interactions depend on local circumstances, including the rooting depths of woody plants. The lack of increase in herbaceous ground cover in the trenched plots in the later part of our experiment was possibly due to woody roots below 30 cm, that were not severed by the trenching, growing into the trenched plots to compete with herbaceous ground cover for soil moisture over time.

1.5

Grazed Ungrazed ab b

ab b

a

ab

1.0 0.5 0.0

Soil nitrogen (%)

(b) 0.2

Grazed Ungrazed a a

0.1

b b

b b

0.0

(c) Carbon:nitrogen ratio

and not grazed. Greene et al. (1994) showed the effects of grazing in inter-patches in western New South Wales, with zero herbaceous ground cover at stocking rates of 0.5 sheep/ha, and mostly 20% or higher ground cover at stocking rates of 0.2 sheep/ha. Encouraging and maintaining ground cover, including by managing grazing intensity, are essential for restoring and maintaining functional landscapes in semi-arid ecosystems such as in the study region (Aguiar and Sala, 1999; D’Odorico et al., 2012; Ludwig et al., 1997). Large areas of bare inter-patches indicate dysfunctional landscapes that do not retain water, nutrients and other resources (D’Odorico et al., 2012; Ludwig et al., 1997; Muñoz-Robles et al., 2011a). Harrington (unpublished data cited in Johns (1981)) showed that the roots of a poplar box tree extended horizontally more than 25 m, and the effect of ringbarking the tree on water availability in surface soil was detected 30 m from the tree. We have observed large, shallow roots of poplar box extending several metres from trees in eroded areas near the study site. Pressland (1975) showed that 50% of the fine roots (<5 mm in diameter) of mulga (Acacia aneura) were 15e30 cm below the soil surface more than 3 m from the plants. Johns (unpublished data cited in Johns (1981)) showed that Pressland’s (1975) findings applied to trees and shrubs at a site less than 50 km from our study site and with similar woody encroachment. Christie (1978) showed that up to 50% of the roots of herbaceous species on the same soil type and in the same environment as our study site were found in the top 20 cm of the soil. Overlap in distribution of roots of herbaceous and woody vegetation in the study region explains the findings of this and the other studies. The results are consistent with competition between herbaceous and woody vegetation for soil moisture or other resources. The results of this study and of others in the study region differ from several studies elsewhere that found that herbaceous and woody vegetation in semi-arid and arid environments did not compete for soil moisture (e.g. Knoop and Walker, 1985; Sala et al., 1989). The suggestions of Canadell et al. (1996) and Schenk and Jackson (2002) that herbaceous plants and woody plants do not compete for soil moisture in semi-arid ecosystems due to deep rooting depths of shrubs do not apply in the study area. Scholes and

20 15

Grazed Ungrazed

a ab

a a

b c

10 5 0 January February 2009 2010

April 2011

Fig. 5. Soil carbon (%C, mean  1 SE) (a), (b) nitrogen (%N) (b), and C:N ratios (c) during the experiment (only significant interactions with time shown). Different letters denote significantly different means.

R. Smith et al. / Journal of Arid Environments 96 (2013) 80e86

Soil surface disturbance resulted in higher litter cover at the earlier monitoring times (presumably from litter catching on the roughened soil surface), but not in higher herbaceous ground cover. Undisturbed soils and trenched soils had higher C and N than disturbed and untrenched soils. Disturbed soils had a higher C:N ratio than undisturbed soils. The differences in soil C, N and C:N with soil disturbance and trenching were probably the result of increased decomposition of soil organic matter from the soil disturbance (Dalal and Mayer, 1986a,b) imposed or caused by these treatments. Herbaceous ground cover was relatively low in all treatments (<10%) (average annual changes in ground cover in the region of the study area range from zero to around 30%, from a minimum of 20% ground cover (Grant, 2012)). The low herbaceous ground cover in this study may be due to low soil seed banks in inter-patches in the study area (M. Good et al., unpub. data) or to lack of seed sources in the area generally. Seeds are not usually retained in inter-patches because of their smooth soil surface (Belnap et al., 2001; Segoli et al., 2008; Shachak et al., 1998; Zaady et al., 2001). Seed banks of perennial grasses and other herbaceous vegetation can be depleted in areas of woody encroachment from decades of suppression of herbaceous growth by woody encroachment and high total grazing pressure (Hodgkinson and Harrington, 1985; Lett and Knapp, 2005). Herbaceous ground cover did not increase after soil disturbance. Rough soil surfaces provide ‘safe sites’ for seeds blowing around the landscape to lodge and germinate (Dunkerley and Brown, 1999; Tongway and Ludwig, 1996), and for litter to lodge in (as this study showed). Lack of seed sources in the vicinity of the experimental site may have contributed to the lack of herbaceous ground cover in the disturbed plots and possibly to low herbaceous cover overall (although Alemseged et al. (2011) found that perennial grasses established a few years after cropping in lightly or rotationally grazed paddocks in the study region). The soil in the disturbance treatment may not have been sufficiently roughened in this experiment, and possibly not for long enough, for seeds to lodge and water to be retained and infiltrate. There was visual evidence that soil in the disturbed plots “settled” after rain events, which would have reduced the effectiveness of the disturbed soil for retarding runoff and capturing seeds. Because excluding grazing, trenching (severing the roots of woody plants) and soil disturbance did not result in high levels of herbaceous ground cover (mean total herbaceous ground cover did not exceed 10%), other factors limiting germination or growth of herbaceous plants may be operating. Rainfall was generally above the long-term average during the latter part of the experiment, notably in the months prior to and including sampling in February 2010, and in spring 2010 (Fig. 1). Johns (1981) found herbage production was higher in a cleared area compared with an area of woody encroachment, particularly where livestock were excluded, but he concluded that herbage production was limited by more than soil water. He showed positive relationships between herbage production and soluble nitrogen, and water infiltration multiplied by soluble nitrogen. He concluded that competition between woody and herbaceous vegetation for nutrients and water is complex, and nitrogen could be a limiting factor for herbage production. This would be a useful topic for future investigation. Tighe et al. (2009) found that soil under woody encroachment in the study area was acidic (pH 5 or less at 5e30 cm soil depth). Low pH levels could adversely affect plant germination and growth of herbaceous ground cover. Allelopathic compounds produced by woody plants, including eucalypts, could also have affected germination and growth of herbaceous ground cover (see Franks, 2002; May and

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Ash, 1990) at the study site and in the region generally. Any negative effects of either or both these potential limiting factors on ground cover would be compounded by the smooth surface and low resource retention in the inter-patch, and the low seed bank in soil in inter-patches in the study area (M. Good et al., unpubl. data). 5. Conclusion This study found that herbaceous ground cover in a large interpatch in woody encroachment was higher on ungrazed plots and in plots where the roots of woody plants had been cut by trenching, than in grazed areas or untrenched plots in the inter-patch. The results suggest that herbaceous ground cover and woody vegetation (woody encroachment) compete for resources in the study area. Herbaceous ground cover in all experimental treatments remained relatively low over the 2.5 year study. Levels of overall ground cover (<20%, with <10% herbaceous) remained lower than the levels required to minimise runoff and erosion in these landscapes (Muñoz-Robles et al., 2011a). Low seed levels in the soil at the study site (M. Good et al., unpubl. data), as others have noted in inter-patches (Aguiar and Sala, 1999), and possibly also in the wider landscape (although see Alemseged et al., 2011), specific nutrient deficiencies (e.g. N: Johns, 1981), low soil pH (Tighe et al., 2009) and/or allelopathy may limit herbaceous ground cover in this system. These factors warrant investigation. Acknowledgements This research was funded by the Central West Catchment Management Authority, NSW Office of Environment and Heritage, and the National Action Plan for Salinity and Water Quality. We thank the landholders for allowing us to conduct this research on their property. Megan Good, Nick Schultz and Warren Furphy assisted with fieldwork. John Ludwig and several anonymous reviewers provided valuable feedback on earlier versions of the paper. Appendix A. Supplementary data Supplementary data related to this article can be found online at http://dx.doi.org/10.1016/j.jaridenv.2013.04.006. References Aguiar, M.R., Sala, O.E., 1999. Patch structure, dynamics and implications for the functioning of arid ecosystems. Trends in Ecology and Evolution 14, 273e277. Alemseged, Y., Hacker, R.B., Smith, W.J., Melville, G.J., 2011. Temporary cropping in semi-arid shrublands increases native perennial grasses. Rangeland Journal 33, 67e78. Analytical Software, 2003. Statistix 8 User’s Manual. Analytical Software, Tallahassee, USA. Archer, S.R., Mackay, W., Mott, J., Nicholson, S.E., Pando Moreno, M., Rosenzweig, M.L., Seligman, N.G., West, N.E., Williams, J., 1999. Arid and semiarid land community dynamics in a management context. In: Hoekstra, T.W., Shachak, M. (Eds.), Arid Lands Management: Toward Ecological Sustainability. University of Illinois Press, Champaign, USA, pp. 48e74. Archer, S.R., 1990. Development and stability of grass/woody mosaics in a subtropical savanna parkland, Texas, U.S.A. Journal of Biogeography 17, 453e462. Baez, S., Colins, S.L., Lightfoot, D., Koontz, T.L., 2006. Bottom-up regulation of plant community structure in an arid ecosystem. Ecology 87, 2746e2754. Belnap, J., Prasse, P., Harper, K.T., 2001. Influence of biological soil crusts on soil environments and vascular plants. In: Belnap, J., Lange, O.L. (Eds.), Biological Soil Crusts: Structure, Function, and Management. Springer-Verlag, Berlin, Germany, pp. 281e300. Bureau of Meteorology, undated. Climate Statistics for Australian Sites, NSW and Sydney Annual Climate Summary Archive. Bureau of Meteorology, Melbourne. http://www.bom.gov.au/ (accessed 19.08.12.). Canadell, J., Jackson, R.B., Ehleringer, J.B., Mooney, H.A., Sala, O.E., Schulze, E.D., 1996. Maximum rooting depth of vegetation types at the global scale. Oecologia 108, 583e595.

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R. Smith et al. / Journal of Arid Environments 96 (2013) 80e86

Christie, E.K., 1978. Ecosystem processes in semi-arid grasslands. I. Primary production and water use of two communities possessing different photosynthetic pathways. Australian Journal of Agricultural Research 29, 773e787. Dalal, R.C., Mayer, R.J., 1986a. Long-term trends in fertility of soils under continuous cultivation and cereal cropping in southern Queensland. I. Overall changes in soil properties and trends in winter cereal yields. Australian Journal of Soil Research 24, 265e279. Dalal, R.C., Mayer, R.J., 1986b. Long-term trends in fertility of soils under continuous cultivation and cereal cropping in southern Queensland. II. Total organic carbon and its rate of loss from the soil profile. Australian Journal of Soil Research 24, 265e279. D’Odorico, P., Okin, G.S., Bestelmeyer, B.T., 2012. A synthetic review of feedbacks and drivers of shrub encroachment in arid grasslands. Ecohydrology 5, 520e530. Dunkerley, D.L., Brown, K.J., 1999. Banded vegetation near Broken Hill, Australia: significance of surface roughness and soil physical properties. Catena 37, 75e88. Eldridge, D.J., Bowker, M.A., Maestre, F.T., Roger, E., Reynolds, J.F., Whitford, W.G., 2011. Impacts of shrub encroachment on ecosystem structure and functioning: towards a global synthesis. Ecology Letters 14, 709e722. Franks, A.J., 2002. The ecological consequences of buffel grass Cenchrus ciliaris establishment within remnant vegetation of Queensland. Pacific Conservation Biology 8, 99e107. Gardiner, D.B., Tupper, G.J., Dudgeon, G.S., 1998. A quantitative appraisal of woody shrub encroachment in western New South Wales. Rangeland Journal 20, 26e40. Grant, R., 2012. Feral Goat Management to Achieve Health Groundcover: Discussion Paper. Western Catchment Management Authority, Cobar, Australia. Greene, R.S.B., Tongway, D.J., 1989. The significance of (surface) physical and chemical properties in determining soil surface condition of red earths in rangelands. Australian Journal of Soil Research 27, 213e225. Greene, R.S.B., Kinnell, P.I.A., Wood, J.T., 1994. Role of plant cover and stock trampling on runoff and soil erosion from semi-arid wooded rangelands. Australian Journal of Soil Science 32, 953e973. Harrington, G.N., Johns, G.G., 1990. Herbaceous biomass in a Eucalyptus savanna woodland after removing trees and/or shrubs. Journal of Applied Ecology 27, 775e787. Harrington, G.N., Oxley, R.E., Tongway, D.J., 1979. The effects of European settlement and domestic livestock on the biological system in poplar box (Eucalyptus populnea) lands. Australian Rangeland Journal 1, 271e279. Hodgkinson, K.C., Harrington, G.N., 1985. The case for prescribed burning to control shrubs in eastern semi-arid woodlands. Australian Rangeland Journal 7, 64e74. Isbell, R.F., 1996. The Australian Soil Classification, revised ed. CSIRO Publishing, Collingwood, Australia. Johns, G., 1981. Hydrological processes and herbage production in shrub invaded poplar box (Eucalyptus populnea) woodlands. Rangeland Journal 3, 45e55. Knoop, W.T., Walker, B.H., 1985. Interactions of woody and herbaceous vegetation in a southern African savanna. Journal of Ecology 75, 235e253. Lett, M.S., Knapp, A.K., 2005. Woody plant encroachment and removal in mesic grassland: production and composition responses of herbaceous vegetation. American Midland Naturalist 153, 217e231. Ludwig, J., Tongway, D., Freudenberger, D., Noble, J., Hodgkinson, K., 1997. Landscape Ecology, Function and Management: Principles from Australia’s Rangelands. CSIRO Publishing, Collingwood, Australia. May, F.E., Ash, J.E., 1990. An assessment of the allelopathic potential of Eucalyptus. Australian Journal of Botany 38, 245e254. Milchunas, D.G., Lauenroth, W.K., 1993. Quantitative effects of grazing on vegetation and soils over a global range of environments. Ecological Monographs 63, 327e366.

Montaña, C., Seghieri, J., Cornet, A., 2001. Vegetation dynamics: recruitment and regeneration in two-phase mosaics. In: Tongway, D., Valentin, C., Seghieri, J. (Eds.), Banded Vegetation Patterning in Arid and Semiarid Environments. Springer-Verlag, New York, USA, pp. 132e145. Muñoz-Robles, C., Reid, N., Tighe, M., Briggs, S.V., Wilson, B., 2011a. Soil hydrological and erosional responses in areas of woody encroachment, pasture and woodland in semi-arid Australia. Journal of Arid Environments 75, 936e945. Muñoz-Robles, C., Reid, N., Tighe, M., Briggs, S.V., Wilson, B., 2011b. Soil hydrological and erosional responses in patches and inter-patches in vegetation states in semi-arid Australia. Geoderma 160, 524e534. Neter, J.M., Kutner, H., Nachtsheim, C.J., Wasserman, W., 1996. Applied Linear Statistical Models. Irwin, Chicago, USA. Noble, J., 1997. The Delicate and Noxious Scrub. CSIRO Wildlife and Ecology, Canberra, Australia. Pressland, A.J., 1975. Productivity and Management of Mulga in South-eastern Australia. PhD thesis. Australian National University, Canberra, Australia. Ratajczak, Z., Nippert, J.B., Colins, S., 2012. Woody encroachment decreases diversity across North American grasslands and savannas. Ecology 93, 697e703. Robson, A.D., 1995. The effects of grazing exclusion and blade-ploughing on semiarid woodland vegetation in north-western New South Wales over 30 months. Rangeland Journal 17, 111e127. Russell A., undated. Using DSEs and Carrying Capacities to Compare Sheep Enterprises. NSW Department of Primary Industries, Sydney, Australia. http://www. dpi.nsw.gov.au/agriculture/farm-business/budgets/livestock/sheep/ background/dse#Table2 (accessed 12.08.12.). Sala, O.E., Golluscio, R.A., Laurenroth, W.K., Soriano, A., 1989. Resource partitioning between shrubs and grasses in the Patagonian steppe. Oecologia 81, 501e505. Schenk, H.J., Jackson, R.B., 2002. The global biogeography of roots. Ecological Monographs 72, 311e328. Scholes, R.J., Archer, S.R., 1997. Treeegrass interactions in savannas. Annual Review of Ecology Systematics 28, 517e544. Scholes, R.J., 1990. The influence of soil fertility on the ecology of southern African dry savannas. Journal of Biogeography 17, 415e419. Segoli, M., Ungar, E.D., Shachak, M., 2008. Shrubs enhance resilience of a semi-arid ecosystem by engineering and regrowth. Ecohydrology 1, 330e339. Shachak, M., Sachs, M., Moshe, I., 1998. Ecosystem management of desertified shrublands in Israel. Ecosystems 1, 475e483. Tighe, M., Reid, N., Wilson, B., Briggs, S.V., 2009. Invasive native scrub and soil condition in semi-arid south-eastern Australia. Agriculture Ecosystems and Environment 132, 212e222. Tongway, D., Ludwig, J., 1996. Rehabilitation of semiarid landscapes in Australia. I. Restoring productive soil patches. Restoration Ecology 4, 388e397. Tunstall, B., Torsell, B., Moore, R., Robertson, J., Goodwin, W., 1981. Vegetation change in a poplar box (Eucalyptus populnea) woodland: effects of tree killing and domestic livestock. Rangeland Journal 3, 123e132. Walker, B., Noy-Meir, I., 1982. Aspects of the stability and resilience of savanna Ecosystem. In: Huntley, B.J., Walker, B. (Eds.), Ecology of Tropical Savannas. Springer-Verlag, New York, USA, pp. 556e590. Walker, J., Moore, R., Robertson, J., 1972. Herbage response to tree and shrub thinning in Eucalyptus populnea shrub woodlands. Australian Journal of Agricultural Research 23, 405e410. Yates, C.J., Norton, D.A., Hobbs, R.J., 2000. Grazing effects on plant cover, soil and microclimate in fragmented woodlands in south-western Australia: implications for restoration. Austral Ecology 25, 36e47. Zaady, E., Yonatan, R., Shachak, M., Perevolotsky, A., 2001. The effects of grazing on abiotic and biotic parameters in a semi-arid ecosystem: a case study from the northern Negev Desert, Israel. Arid Land Research and Management 15, 245e261.