Manipulating livestock grazing to enhance native ... - CSIRO Publishing

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The Rangeland Journal, 2013, 35, 95–108 http://dx.doi.org/10.1071/RJ12074

Manipulating livestock grazing to enhance native plant diversity and cover in native grasslands J. A. Mavromihalis A, J. Dorrough A,B, S. G. Clark C, V. Turner A and C. Moxham A,D A

Arthur Rylah Institute for Environmental Research, Department of Sustainability and Environment, Melbourne, Vic. 3084, Australia. B Natural Regeneration Australia, PO Box 9103, Wyndham, NSW 2550, Australia. C Department of Primary Industries, 915 Mount Napier Road, Hamilton, Vic. 3300, Australia. D Corresponding author. Email: [email protected]

Abstract. Temperate perennial grasslands globally have been subject to extensive biodiversity loss. Identifying livestock grazing regimes that maintain and enhance the diversity and cover of native plant species in these ecosystems remains a key challenge. The responses of vegetation to different sheep grazing regimes were assessed over 3 years in grasslands of southeastern Australia. An open communal experimental design was used to assess the effects of varying season and duration of exclusion of grazing by sheep, replicated at three locations. Manipulation of season and duration of exclusion of grazing led to few major changes in the cover of native perennial grasses or forbs, although seasonal variation was considerable. Exclusion of grazing in the spring did increase the likelihood of occurrence of grazing-sensitive native forb species but also lead to an increase in the cover of exotic annual species. However, cover of exotic annual species tended to decline with increasing duration of exclusion, while the abundance of native, grazing-sensitive forbs and the cover of perennial grasses increased. Small-scale richness of native perennial forb species increased with a 3-month period of exclusion of grazing, but declined with year-round exclusion of sheep. Total species richness also declined in response to year-round exclusion of sheep and rates of decline were correlated with the rate at which herbage mass accumulated. While strategic grazing did not result in major vegetation changes in the short term, it is suggested that some grazing exclusion may enhance the survival of infrequent species most sensitive to sheep grazing. Caution, however, should be taken when grazing regimes implemented benefit both desirable (native forbs) and undesirable (exotic annuals) species. Having a mosaic of flexible grazing management regimes across the landscape is likely to be beneficial for native plant diversity. Additional keywords: biodiversity, conservation management, exotic species, grazing exclusion, native species, pasture, strategic grazing, temperate grassland, Themeda triandra grassland. Received 20 September 2012, accepted 22 February 2013, published online 18 March 2013 Introduction Temperate and Mediterranean-type grasslands of Australia, South America, North America and Europe are among the most highly modified and poorly conserved biomes on the planet (Kirkpatrick et al. 1995; Hoekstra et al. 2005). They are also predicted to be the most vulnerable biomes to future biodiversity loss (Sala et al. 2000). Over-grazing by livestock, passive and active nutrient enrichment, exotic plant introductions, cultivation and cropping have led to large losses of biodiversity and declines in vegetation cover, soil stability and water quality (e.g. Yates and Hobbs 1997; Dorrough et al. 2004b; Young et al. 2005; Harpole and Tilman 2007; Henwood 2010). In south-eastern Australia, large areas of native grasslands and grassy woodlands are managed extensively for livestock production (Yates and Hobbs 1997; Dorrough et al. 2004b; Kirkpatrick and Bridle 2007). Traditional management of these temperate native grasslands is year-long continuous grazing, with or without low to moderate levels of phosphorus fertiliser Journal compilation Australian Rangeland Society 2013

(Crosthwaite et al. 2008). The diversity and composition of these grasslands vary due to differences in available nutrients and variation in the intensity (McIntyre and Lavorel 1994a; Clarke 2003; Dorrough et al. 2006), frequency (Dorrough et al. 2004a; Dowling et al. 2005) and season (Kemp et al. 2000; Leonard and Kirkpatrick 2004) of livestock grazing. Continuous grazing has been shown to reduce perennial cover, increase bare ground and facilitate annual plant invasion (Noy-Meir et al. 1989; Kemp et al. 2000; Reeve et al. 2000). In landscapes with a short evolutionary exposure to ungulate grazing declines in patch-scale plant species richness and broader-scale diversity can be expected (Milchunas et al. 1988). There is much interest in the development of management strategies to enhance the diversity of these grasslands and halt what appears to be their inexorable decline (Dorrough et al. 2004b; Lunt et al. 2007). There have been numerous conservation schemes, including education, extension (e.g. Eddy 2005), direct and market-based incentives (Stoneham et al. 2003; Buchan www.publish.csiro.au/journals/trj

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2006) and land purchase for conservation and, while an increasing area of grassland has been secured for biodiversity conservation, there are often few data to guide their management (Wong and Morgan 2012). Variation in the intensity, duration and season of livestock grazing can greatly affect plant composition (e.g. Lodge and Whalley 1985; Earl and Jones 1996; Kemp et al. 1996). Increases in the abundance of dominant native perennial grasses have been achieved through manipulation of the duration and season of exclusion from grazing (Lodge et al. 1988; Jones 1996; Kemp and Dowling 2000; Kemp et al. 2000; Kahn et al. 2010). However, there are few experimental data on responses of the less abundant interstitial components of the native grassland flora (Kemp et al. 2003), and it remains unclear as to how best to manipulate the duration and season of grazing to better manage native grasslands for plant conservation (Lunt et al. 2007). Livestock are typically excluded from conservation reserves and, until recently, exclusion of grazing was the major management strategy promoted for the conservation of plant diversity on private land (Lunt et al. 2007). While complete exclusion of grazing may benefit some grazing-sensitive plant species (Leonard and Kirkpatrick 2004) and can potentially promote species diversity at a landscape scale (Olff and Ritchie 1998; Schultz 2012), at the patch scale a decline in species density is predicted, particularly in productive grasslands (Trémont 1994; Lunt et al. 2007). This decline in species density at high productivity has been supported by studies in Australia (Lunt and Morgan 1999; Schultz et al. 2011) and the northern hemisphere (e.g. Pykälä 2003). Furthermore, outside of conservation reserves, the complete exclusion of livestock is rarely a feasible or desirable solution to minimising negative environmental impacts and enhancing native plant diversity. If livestock are to be the main tool of management the question then is: which management regime might yield the best outcomes in terms of the diversity and abundance of native plant species, and in particular the abundance and persistence of those native plant species most sensitive to grazing? Seasonal periodic exclusion or deferment of livestock grazing is a key tool available to the grassland manager for modifying plant composition (Nie and Zollinger 2012). Periodic exclusion of livestock for weeks to months can increase the persistence of perennial grasses (Dowling et al. 1996; Kemp et al. 1996; Culvenor 2000; Garden et al. 2000b; Chapman et al. 2003). The season of exclusion is often crucial (e.g. Ash and McIvor 1998). Spring (September to late November in southern temperate Australia) is the main period of growth and flowering for many temperate annual and perennial grassland species. The removal of grazing during this time may enhance persistence and encourage future recruitment. Previous studies indicate that grazing exclusion during spring may benefit some native grasses and forbs (Garden et al. 2000b; Leonard and Kirkpatrick 2004). In a Themeda-Rytidosperma pasture in south-eastern Australia, Garden et al. (2000a) found Themeda triandra increased where grasslands were either rested in spring or year-long, while Leonard and Kirkpatrick (2004) found an increase in geophytes following spring rest from livestock grazing. In this study how manipulation of the duration and season of exclusion from grazing modifies vegetation structure and diversity in grazed native grasslands of south-eastern Australia is investigated. Three years of vegetation responses from a grazing

J. A. Mavromihalis et al.

experiment, replicated across three farms in the Victorian Volcanic Plain (VVP) in south-west Victoria, are reported. This study complements a previous publication that examined patterns in nutrient concentrations at the soil surface (depth of 0–100 mm) and richness and frequency of forbs (Zimmer et al. 2010). Data are presented from six grazing treatments and focus on small-scale temporal changes in vegetation cover and richness. It is examined whether: (1) richness and cover of native species will be enhanced following grazing exclusion; (2) long periods of exclusion from grazing will favour grazing-sensitive species; (3) cover of exotic annual species will be reduced with increased duration of grazing exclusion and (4) the season of exclusion modifies the responses? Methods Study region Experimental sites were established in native grasslands on the fertile, basalt-derived soils of the VVP bio-region of south-eastern Australia. The region of flat to undulating plains covers 23 000 km2 and ranges in altitude from 308C are common and were experienced during the study (Table 1). The dominant soil type of the VVP bio-region is a basalt-derived, acidic duplex with a hard-setting clay-loam surface soil over a medium-heavy clay subsoil (Conley 1984; Maher and Martin 1987). These soils are commonly waterlogged and poorly aerated in winter, forming extensive cracks in the summer (Conley 1984). Land-use history and vegetation Prior to European settlement, native grassland was estimated to cover ~1 000 000 ha of the VVP bio-region (Barlow and Ross 2001). Current estimates suggest ~5000–6000 ha remain and 0.1. It was tested whether the rate at which quadrat-scale species richness declined in ungrazed plots was correlated with the rates of herbage mass accumulation. For each plot, the average change in plant quadrat-scale richness between 2003 and 2005 (D quadrat richness) was estimated as the coefficient of a linear model. Similarly the annual rate of change in herbage mass was estimated as the linear coefficient of pasture stature (D pasture stature) using data from estimates collected every September, December and April. Vegetation stature is moderately well correlated with independently calibrated visual estimates of aboveground herbage mass [tonnes of DM ha–1 = 0.365 vegetation stature (cm) + 0.5821, R2 = 0.68] and was considered to be a more objective measure of herbage mass and density. A simple additive linear regression model, with a normal distribution, was fitted with D quadrat richness as the response variable and site and D pasture stature as the explanatory variables. Relationships between soil variables and D pasture stature were explored using linear regression with each soil variable modelled independently. Linear coefficients, their standard error and model and residual sum of squares were used to suggest those soil variables that were likely to be important in modifying rates of herbage mass accumulation. All analyses were undertaken using GENSTAT 8.1 (GENSTAT 8 Committee 2005). Results The three experimental sites differed in dominant plant species, cover of major life-form groups and overall species richness (see also Zimmer et al. 2010). Birregurra was dominated by Short Wallaby-grass (Rytidosperma carphoides var. angustior), Smooth Wallaby-grass (Rytidosperma laeve), Common Bogsedge (Schoenus apogon) and exotic annual species (e.g. Vulpia bromoides and Briza species; Table 2). Darlington had the highest native plant species richness (Table 1) and was dominated by Kangaroo Grass (Themeda triandra) and Common Wallabygrass (Rytidosperma caespitosa) (Table 2). Hamilton was dominated by Kangaroo Grass (Themeda triandra), Velvet Wallaby-grass (Rytidosperma pilosum) and Sweet Vernal Grass (Anthoxanthum odoratum). Ten grazing-sensitive species were recorded across all sites, including the endangered orchid, Prasophyllum suaveolens, and the more widespread orchid, Thelymitra pauciflora. One grazingsensitive species was recorded at Birregurra, eight at Darlington and four at Hamilton (Table 1). Average net primary productivity in 2003 was greatest at the Birregurra experimental site, followed by Hamilton and Darlington, and in 2004 was greatest at Darlington, followed by Birregurra and Hamilton (Table 1). Herbage mass and pasture stature peaked in December of both 2003 and 2004.


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Table 2. Total frequency of the 10 most abundant plant species at each of the experimental sites in spring 2003 Frequency is the proportion of all quadrats the species were observed in at each site (90 quadrats were sampled at each site)

Species Anthoxanthum odoratum Aphelia gracilis Rytidosperma spp. Briza minor Eryngium ovinum Haloragis heterophylla Hypochaeris radicata Juncus bufonius Juncus capitatus Leptorhynchos squamatus Plantago lanceolata Romulea rosea Schoenus apogon Themeda triandra Trifolium cernuum Vulpia bromoides

Experimental sites Birregurra Darlington Hamilton – 0.87 0.94 0.93 0.53 0.79 0.94 – 0.78 – – 0.94 0.96 – – 0.90

– – 0.98 0.93 0.87 – – – 0.77 0.94 – – 0.57 0.88 0.89 0.99

0.87 0.70 0.92 0.87 – – 0.98 0.46 0.46 – 0.53 – 0.70 0.48 – –

Season of exclusion Cover of annual exotic species and bare ground both varied in response to the season of grazing exclusion. Bare ground fluctuated seasonally and also in response to the season of grazing exclusion (sample date treatment, Wald statistic 24.2, d.f. = 14, P < 0.05), with bare ground peaking in autumn of each year, particularly in the winter exclusion plots (Fig. 1). Cover of annual exotic species was highest in the spring exclusion treatments (Fig. 1, treatment, Wald statistic 7.92, d.f. = 2, P < 0.05) and peaked in September to November of each year (sample date, Wald statistic 138.6, d.f. = 7, P < 0.001). Cover of native perennial grass species varied over the duration of the experiment (sample date, Wald statistic 19.8, d.f. = 7, P < 0.01, Fig. 2) but did not vary according to season of exclusion of grazing (treatment, Wald statistic 2.15, d.f. = 2, P = 0.34; Fig. 1). The cover of native perennial forbs was also unaffected by season of exclusion (treatment, Wald statistic 1.3 d.f. = 2, P = 0.60; Fig. 1) but cover did fluctuate seasonally, peaking in the spring of each year (sample date, Wald statistic 39.3, d.f. = 5, P < 0.001, Fig. 2). While there was a trend to suggest that the season of grazing exclusion affected native forb richness at the quadrat scale (Fig. 3), this was not statistically significant (Wald statistic 3.7, d.f. = 2, P = 0.16). Native forb richness increased between 2003 and 2005 regardless of the season of exclusion from grazing (Wald statistic 18.12, d.f. = 2, P < 0.001, Fig. 3). The proportion of quadrats supporting grazing-sensitive species varied between treatments (Wald statistic 15.1, d.f. = 2, P < 0.001) and years (Wald statistic 7.3, d.f. = 2, P < 0.05), although differences remained constant among years (sample date treatment, Wald statistic 1.6, d.f. = 4, P > 0.1). Grazing exclusion in spring resulted in close to a doubling of the proportion of quadrats predicted to support grazing-sensitive species compared with either winter or summer exclusion of grazing (Fig. 4).

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Fig. 1. Predicted proportion of (a) bare ground, (b) annual exotic species, (c) perennial forbs and (d) perennial grasses in plots with different seasons of exclusion from grazing by sheep. 95% confidence intervals around predictions are shown.

Duration of exclusion Cover of native perennial forb species was highest in response to complete exclusion of sheep and 3 months of exclusion of grazing during spring (treatment, Wald statistic 14.3, d.f. = 3, P < 0.01, Fig. 5), independent of seasonal variation. The cover of exotic annual species varied weakly as a result of an interactions between treatment and sampling date (treatment sampling date, Wald statistic 30.6, d.f. = 21, P = 0.08; Fig. 6). There was an overall trend for exotic annual species to peak in the spring of each year while treatment effects varied. At two of the three seasonal peaks, plots rested for 3 months in spring had the highest observed cover and in spring 2005 the 95% confidence intervals suggest little overlap in likely mean cover between spring exclusion (highest predicted mean cover) and continuous exclusion (lowest predicted mean cover) plots. At other times of the year trends were less consistent and few treatment differences can be inferred (Fig. 6). Cover of perennial grass species varied according to the duration of grazing exclusion and treatment effects remained constant throughout the experimental period (treatment, Wald statistic 27.3, d.f. = 3, P < 0.001; Fig. 5). Cover of perennial grass species was predicted to be highest in response to continuous

exclusion and 6 months of exclusion (Fig. 5). The effect of duration of exclusion of grazing on the cover of bare ground varied seasonally (treatment sampling date, Wald statistic 30.4, d.f. = 21, P < 0.1) peaking in autumn of each year particularly in continuously grazed plots (Fig. 6). Continuously excluded plots had little bare ground and little seasonal variation (Fig. 6). The duration of grazing exclusion affected small-scale species richness of native forbs and differences between treatments varied from year to year (Wald statistic 14.24, d.f. = 6, P < 0.05; Fig. 3). There was a tendency for species richness of native forbs to increase each year in continuously grazed, 6- and 3-month exclusion plots, but continuously excluded treatments had an opposing pattern declining between 2003 and 2005. By 2005, the highest species richness of native forbs was in 3-month exclusion plots and lowest in continuously excluded plots (Fig. 3). The proportion of quadrats supporting grazing-sensitive species was greater in plots that received some grazing exclusion compared with plots that received no exclusion (treatment, Wald statistic 9.3, d.f. = 3, P < 0.05; Fig. 4) although increasing the duration of exclusion beyond 3 months did not lead to an increase in the predicted number of plots that supported grazing-sensitive

Management of native grasslands


accumulation of pasture stature (average R2 = 0.61, range = 0.26–0.85, see Appendix Table 3). Variation in D pasture stature was correlated with plot estimates of total soil phosphorus and potassium but not with any other soil attributes (e.g. total nitrogen) (Appendix Table 2). Although rates of accumulation of pasture stature also varied according to site, this did not appear to be related to rainfall over the sampling period. Rates of accumulation were lowest at Birregurra, but rainfall there was similar to that of Hamilton, which had the highest rates of accumulation. There was a linear correlation between D richness and D pasture stature (F1,7 = 13.10, P < 0.01, Fig. 5). This model suggested that an increase in pasture stature of ~2–3 cm year–1 resulted in a decline of one species year–1 per 0.25 m2 while an increase in pasture stature of 6–8 cm year–1 led to declines of two to three species year–1 per 0.25 m2.

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species (Fig. 4b). A slightly greater proportion of quadrats supporting grazing-sensitive species were predicted in 2004, compared with either 2003 or 2005, but this trend was weak (sample date, Wald statistic 3.3, d.f. = 2, P = 0.19). Relationships between rates of change in quadrat species richness and accumulation of pasture stature Average quadrat species richness declined in every ungrazed plot with D richness ranging from 0.6 species year–1 up to 3.7 species year–1 (Fig. 7). Linear models fitted the changes in species richness well, six of the nine linear models had R2 estimates in excess of 0.80, while only one was less than 0.50 (Appendix Table 3). Annual D pasture stature ranged from 1.2 cm up to 6.8 cm. Despite seasonal variation in pasture stature, a linear model seemed to be adequate to describe the pattern of the 6

Seasonal exclusion of livestock can be a simple but effective means of modifying grassland plant composition and diversity. It was hypothesised that small-scale changes in vegetation would arise from varying both the season and duration of grazing exclusion in long-grazed productive temperate grasslands. While it was clear that inter-annual variation dominated patterns over the 3 years, minor treatment effects were also evident. Shortduration exclusion (3 months) from grazing increased the richness of native forb species and the likelihood of observing grazing-sensitive species, but also increased the cover of exotic annual species. In contrast, complete exclusion tended to result in declines in the richness of native perennial forb species, total plant richness and cover of exotic annual plant species but led to short-term increases in the cover of perennial grasses and forbs. The grasslands examined in this study had experienced over 150 years of livestock grazing, and so it is perhaps not surprising that after 3 years few major changes in vegetation cover and species richness were detected as a result of the effects of the treatments. Numerous short-term experimental studies in longgrazed grasslands have shown relative stability in vegetation

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following the implementation of alternative grazing management (e.g. Williams 1969; Sternberg et al. 2000). Despite the large inter-annual variation in the data, the data tend to suggest that neither continuous grazing, the typical management regime in many grasslands managed for sheep production, nor complete exclusion of sheep, the management regime implemented in many grasslands managed for conservation, are most likely to produce the best outcomes in terms of both cover of native plant species and species richness. Season of exclusion It was hypothesised that spring exclusion would favour native forbs but the results indicated that changes in richness and cover of forbs were slight. There was a tendency for exclusion of grazing in spring to promote the richness of native perennial forb species

compared with other treatments but this was not significant. Exclusion of grazing in spring did, however, increase the likelihood of occurrence of grazing-sensitive species. Spring is the main growth and flowering period for many annual and native perennial herbs and grasses. Grazing during this period may damage flowering and fruiting structures, and reduce flower production (Bridle and Kirkpatrick 2001; Dorrough and Ash 2004). Positive outcomes of exclusion of grazing in the spring on native grassland species are supported by anecdotal evidence from conservation managers but few empirical studies have been reported (Leonard and Kirkpatrick 2004). Leonard and Kirkpatrick (2004) observed that native species not commonly recorded in grassy vegetation in Tasmania, such as Liliaceae spp. and Orchidaceae spp., were associated with areas that were either rested or had a light grazing pressure during the spring. These conclusions support the observation in this study of higher likelihoods of grazing-sensitive species, which included five geophytes, in spring-rested plots. The cover of exotic annual plant species also tended to increase as a result of exclusion of grazing in the spring. In a study of naturalised pastures in eastern Australia, Dowling et al. (2006) found that the spring and summer peak in herbage mass was largely composed of exotic annual grasses. Intensive spring grazing followed by summer exclusion has been suggested as a management strategy for reducing the abundance of exotic annual grasses and increasing native perennials in native pastures of south-eastern Australia (Garden et al. 2000a, 2000b). In a survey of farms on the central, southern and Monaro tablelands of New South Wales, Garden et al. (2000a) reported a decline in exotic annual grass Vulpia sp. under increased grazing pressure in the spring. This raises a dilemma for managers using exclusion of grazing in the spring to increase the abundance and



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re-grow. While these sampling artefacts are worth consideration, it is important to note that trends in cover and richness of native forbs and abundance of annual exotic species were consistently greater with the spring (3-month) exclosure than the springsummer (6-month) exclosure, indicating that the shared rest from grazing before sampling was not sufficient to explain treatment effects (see also Zimmer et al. 2010).

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diversity of native perennial forbs including grazing-sensitive native species. Negative correlations between exotic annual plant cover and native plant diversity have been reported for temperate grasslands (e.g. Morgan 1998a). Whether negative effects of exotic plants will limit the potential for increases in the density and cover of native perennial species needs to be tested. Alternate years of grazing exclusion and targeted grazing in the spring may assist in the management of exotic annual cover in the native pastures examined in this study but this would need to be assessed. The seasonal exclusion treatments were partly confounded with the timing of the sampling (October–early November). In open plots sheep may have grazed preferred species before sampling, reducing their apparency. In spring and spring-summer (6-month) exclosure plots, the short rest (30–45 days) before sampling may have provided preferred species an opportunity to

It was hypothesised that the duration of the exclusion of grazing would be positively correlated with the cover of native perennial plants but that plant species richness was likely to decline with complete livestock exclusion. An absence of gaps in the grassland canopy from a lack of disturbance, following removal of livestock or fire suppression, can impede seedling survival and reduce small-scale diversity through competitive exclusion (Morgan 1998b, 1999; Hayes and Holl 2003). Resources, such as light, space, water and nutrients, become limited under such conditions and restrict seedling recruitment (Morgan 1997; Hofmann and Isselstein 2004). Native perennial forb richness and total plant richness did tend to decline following complete exclusion of sheep. However, the likelihood of occurrence of grazing-sensitive species and the cover of native perennial grasses and forbs were not reduced by the longer exclusion periods. In the case of grazing-sensitive species, it was predicted that diversity and density would be favoured by longer exclusion periods but it was evident that any length of exclusion positively influenced density. A period of grazing exclusion may allow individual plants to recover from negative impacts such as defoliation, physical damage to stems and roots, and changes to the physical and chemical environment such as soil compaction, nutrient enrichment and declining soil organic carbon (Yates et al. 2000; Ganjegunte et al. 2005). Native perennial grasses were also weakly favoured by treatments with greater than 6 months of exclusion of grazing. Continuous heavy livestock grazing has been singled out as a major cause of the

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decline in perennial grass species over the previous few decades (Dowling et al. 2006) although the poor adaptation of introduced perennial cultivars in exotic pasture systems has also been suggested as a cause (Reeve et al. 2000). Studies have reported a decline in the cover of perennial grasses under continuous grazing as well as the benefits of exclusion periods when managing perennial pasture (Earl and Jones 1996; Culvenor 2000; Kemp et al. 2000; Chapman et al. 2003). However, attempts to generalise the response of perennial grasses as a ‘group’ may mask individual responses of species to differing grazing regimes (Kemp et al. 2000). The results of this study support previous findings that exclusion of grazing in long-grazed productive temperate grasslands can result in declines in inter-tussock plant diversity, particularly at finer scales (Lunt and Morgan 1999; Schultz et al. 2011). Also it was found that the rate at which fine-scale plant species richness declined in ungrazed plots was correlated with the rate at which pasture stature increased over the same period. Thus pastures that grew rapidly had progressively fewer plant species at fine scales. These observations match predictions from other studies (e.g. Proulx and Mazumder 1998; Schultz et al. 2011). Negative impacts on native diversity attributed to lack of disturbance are supported by earlier studies (Gilfedder and Kirkpatrick 1994; Lunt and Morgan 1999; Hart 2001; Kirkpatrick et al. 2005). Several native perennial forbs in lowland grassland in Tasmania, including the daisy Leucochrysum albicans, disappeared from areas considered to be insufficiently grazed (Gilfedder and Kirkpatrick 1994). The observations in this study relate only to fine-scale changes in species density and at broader scales total species richness may not be reduced by grazing exclusion (e.g. Dorrough et al. 2007). However, 2 years after the last fine-scale data collection (spring 2005) the plots were revisited (2007) and declines in native intertussock forb species densities were also apparent at a plot scale (225 m2) (Zimmer et al. 2010). While there was little variation in rainfall among the sites, total phosphorus and available potassium content in the soil were correlated with D pasture stature. Temperature, soil moisture availability and soil nutrients, in particular nitrogen content, have been found to be the main determinants of broad-scale variation in net primary productivity in grasslands (e.g. Webb et al. 1983; Lane et al. 1998; LeBauer and Treseder 2008; Schultz et al. 2011). In this case temperature and precipitation are unlikely to have contributed to the plot scale or inter-site variation as the sites experienced similar conditions. Evapotranspiration may have varied among sites and could have resulted in differences in soil moisture availability but it was not measured. Surprisingly no relationship with total nitrogen or nitrate-N contents was apparent. Conclusion This study suggests that management involving a period of exclusion from grazing may be beneficial for enhancing the conservation value of productive basalt grassland. Much anecdotal evidence suggests exclusion of grazing in the spring may be most beneficial and this study partially supports this view. Changes in species richness and cover were small, and exclusion of grazing in the spring also promoted exotic annual species. It is


important to emphasise the inter-annual variation in vegetation composition which dominated the patterns observed. Given such large year-to-year variation, it would seem inappropriate to develop fixed management strategies – flexible management regimes will be necessary to accommodate such temporal fluctuations. Specific management prescriptions are unlikely to match structural and floristic characteristics of remnant grasslands at a local scale and a mosaic of grazing management regimes, capable of responding to local climatic variation, is considered an advisable management strategy for native grasslands at a regional scale. Acknowledgements The authors would like to thank Kate and Tom Calvert, and Claire and James Dennis, for allowing experimental grazing plots to be situated on their properties and for information regarding management histories. Rhiannon Apted and Nathan Wong are also thanked for their assistance throughout the project, particularly with fieldwork. Fiona Coates provided useful comments on the manuscript. John Maggs is thanked for managing the Department of Primary Industries, Hamilton study site. This research was funded by the Victorian Department of Sustainability and Environment’s and the Department of Primary Industry’s Ecologically Sustainable Grassland Initiative (ESAI) project 2001–05 in collaboration with the Corangamite and Glenelg Hopkins Catchment Management Authorities, PlainsTender, Grain and Graze and the Australian Commonwealth’s Caring For Our Country program.

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Zimmer, H. C., Turner, V. B., Mavromihalis, J., Dorrough, J., and Moxham, C. (2010). Forb responses to grazing and rest management in a critically endangered Australian native grassland ecosystem. The Rangeland Journal 32, 187–195. doi:10.1071/RJ09069

Nitrate N Available P S K Total N Total P Organic C

Variable 0.23 4.38 0.16 19.67 4.98 17.64 2.89

Model SS 36.95 32.80 37.02 17.51 32.20 19.54 34.29

Residual SS 0.024 0.493 0.113 0.023 12.8 0.026 0.81

Estimate

0.116 0.51 0.652 0.008 12.3 0.010 1.06

s.e. of mean

Appendix Table 2. Results of separate linear regressions of D pasture stature with each of a range of soil variables : [nitrate N (mg kg–1), available P (mg kg–1), S (mg kg–1), K (mg kg–1), total N (%), total P (mg kg–1), and organic C (%)] For each model the model sum of squares, residual sum of squares, parameter estimate and s.e. of mean are shown

www.publish.csiro.au/journals/trj

Birregurra Birregurra Birregurra Darlington Darlington Darlington Hamilton Hamilton Hamilton

Site

1 2 3 1 2 3 1 2 3

Block

5 4 1 5 1 3 3 6 4

Plot

y = 1.206x + 7.453 y = 1.936x + 7.661 y = 2.270x + 6.077 y = 6.945x + 3.948 y = 2.981x + 7.949 y = 6.840x + 4.069 y = 3.262x + 6.421 y = 3.848x + 7.664 y = 5.959x + 7.384

Sward stature R = 0.366 R2 = 0.432 R2 = 0.692 R2 = 0.805 R2 = 0.257 R2 = 0.853 R2 = 0.785 R2 = 0.588 R2 = 0.735

2

y = –0.8x + 10.40 y = –1.3x + 10.80 y = –1.2x + 11.0 y = –2.1x + 19.53 y = –0.7x + 18.20 y = –3.7x + 21.87 y = –0.6x + 12.67 y = –2.4x + 13.80 y = –2.9x + 13.33 R = 0.842 R2 = 0.983 R2 = 0.923 R2 = 0.579 R2 = 0.119 R2 = 0.830 R2 = 0.628 R2 = 0.923 R2 = 0.990

2

Quadrat richness 8 3 6 2 2 1 6 15 22

Nitrate N

9 8 10 8 15 8 15 17 22

Ammonium N

5 6 5 7 9 7 4 5 6

Available P

129 152 158 338 279 247 161 182 201

Available K

4.5 4.8 5.5 4.4 5.2 3.9 5.5 6.5 8.0

S

2.22 2.32 2.4 2.57 2.78 2.65 3.95 3.91 3.94

Organic C

0.17 0.18 0.15 0.22 0.20 0.20 0.31 0.30 0.30

Total N

137 164 133 237 211 230 251 276 291

Total P

1.80 1.87 1.97 1.81 2.10 2.36 3.04 3.18 3.04

Total C

Appendix Table 3. Linear equations for the change in sward stature (cm) and mean quadrat species richness for each ungrazed plot Soil nutrient contents (nitrate N (mg kg–1), ammonium M (mg kg–1), available P (mg kg–1), K (mg kg–1), S (mg kg–1), organic C (%), total N (%), and total P (mg kg–1) for each plot are also shown

Arthropodium strictum Brachyscome basaltica Bulbine bulbosa Glycine latrobeana Microseris sp. 1 Microtis unifolia Prasophyllum suaveolens Thelymitra pauciflora Tricoryne elatior Velleia paradoxa

List of species

Appendix Table 1. Species selected a priori as grazing sensitive

108 The Rangeland Journal J. A. Mavromihalis et al.