tasmanian lentic wetland lawns are maintained by grazing rather than inundation

Tasmanian lentic wetland lawns are maintained by grazingrather than inundationaec_2168 303..309

CYNTHIA ROBERTS, J. B. KIRKPATRICK* AND P. B. MCQUILLANSchool of Geography and Environmental Studies, University of Tasmania, Private Bag 78, GPO,Hobart,Tasmania 7001, Australia (Email: [email protected])

Abstract Vertebrate grazers have been shown to be a critical element in maintaining lawns, although lawns canalso form in places without such herbivores. In Tasmania lawns are widespread in lentic wetlands. We usedenvironmental observations and exclosure experiments at two altitudinally contrasting lentic wetland lawns, andwaterlogging experiments, to test the hypotheses that their structure is maintained (i) periodic inundation; and (ii)grazing. Waterlogging experiments and field observations demonstrated that the two main invading shrubs wereindifferent to immersion for several months and that the distribution of the lawns was independent of inundationperiod, results inconsistent with the first hypothesis. The exclosure experiments showed that both woody andnon-woody plants became taller in the lawns when marsupial grazers and rabbits were excluded. It therefore seemsthat the lawn structure is maintained by grazing and that alternative structural states result from exclusion ofgrazing pressure in less than 2 years.

Key words: grazing, lawn, lentic wetlands, marsupials, short herbfield, shrub invasion, Tasmania, waterlogging.

INTRODUCTION

Lawns are short (<4 cm) swards of grasses, herbs andsoft sedges. Most contemporary lawns are human arte-facts, created by mowing or heavy stocking.The stronghuman propensity to create lawns for aestheticpurposes may relate to the association of naturallawns with large populations of game animals. Non-artefactual lawns have been widely hypothesized to bemaintained by continuous intense mammal grazingpressure (Vesey-FitzGerald 1960; Bell 1971; Kirk-patrick 1975; Newsome 1975; McNaughton 1984;Milchunas & Lauenroth 1989; Prins & van der Jeugd1993; Fuhlendorf & Smeins 1997; Mountford &Peterken 2003; McIvor et al. 2005; Kirkpatrick 2007;Archibald 2008; Stock et al. 2010).

However, lawns also occur in places, such as thealpine zone of mainland Australia (the short alpineherbfield of Costin et al. 2000), where there are novertebrate herbivores (Costin et al. 2000; Wahren et al.2001). In this case, the mechanical damage caused toupright plants by the movement of fire and water mayfavour a prostrate habit.

Lawns usually occur in places climatically andedaphically capable of supporting woody plants (Bond2008). Thus, one key to understanding processes oflawn maintenance lies in the causes of exclusion ofpotentially taller plants. These may be the species thatdominate the lawn, as with the marsupial lawns of

Newsome (1975), or they may belong to tree, shrub ortussock species that occur in the vegetation around thelawn, but are absent, rare or prostrate within it. Apartfrom grazing pressure and its possible interactionswith fire (Archibald et al. 2005; Kirkpatrick 2007),causes of exclusion of taller species could include sea-sonal inundation (Bell 1971; Milchunas & Lauenroth1989; Johnson & Rogers 2003; Kröger & Rogers2005), root competition (Fensham & Kirkpatrick1992) and abrasion by wind, water or ice (Costin1954).

In Tasmania, Australia, lawns, called marginalherbfields by Kirkpatrick and Harwood (1983), occuron the landward margins of a large number of lenticwetlands.They occur in wetlands with a wide range ofsalinity, pH and altitude (Kirkpatrick & Harwood1983), making inundation the most likely physicalfactor that might explain the exclusion of taller plants.Wetland communities dominated by shrubs or treesare widespread in seasonal lentic wetlands inTasmania(Kirkpatrick & Harwood 1983), suggesting that woodyspecies might be excluded from replacing lawns byfactors other than inundation.The most likely factor isgrazing pressure, as Tasmanian wetland shrubs andtrees are well-adapted to regenerate after, or recoverfrom, fire.

To establish whether inundation, by itself, willexclude woody plants from growing tall in lawns it isnecessary to demonstrate that inundation can eitherkill such plants or stunt their growth. To establishwhether grazing pressure prevents the growth of tallplants in the lawns it is necessary to test whether

*Corresponding author.Accepted for publication June 2010.

Austral Ecology (2011) 36, 303–309

© 2010 The Authors doi:10.1111/j.1442-9993.2010.02168.xJournal compilation © 2010 Ecological Society of Australia

exclusion of vertebrate herbivores results in verticalplant growth to the degree that lawns would bereplaced by other vegetation types. In the presentpaper we use experiments and observations to both ofthese ends in testing the hypotheses that: (i) inunda-tion by fresh water can exclude tall woody plants fromlawns; and (ii) grazing by vertebrate animals canprevent the plants in lawns from becoming tall enoughto cause a structural shift into taller vegetation types.

METHODS

Study sites

Two freshwater lentic wetland lawns near the altitudinalextremes of the vegetation type were selected for the estab-lishment of grazing exclosure experiments. We selected twoenvironmentally and floristically contrasting sites to give usmore confidence in the generality of the results of ourhypothesis testing than could be provided by an experimenton just one site.

The high altitude lawn (41°51′20″S, 146°30′54″E,altitude = 1151 m) is at the south west end of Lake Augustaon the Central Plateau (henceforth Augusta). The meanannual rainfall (1984–2007) at the Bureau of Meteorologystation at nearby Liawenee (altitude = 1062 m) is 1042 mm,distributed evenly between the seasons. At Liawenee, themean daily maximum and minimum temperatures for thecoldest month are 5.5°C and -1.5°C and for the hottestmonth are 18.7°C and 5.5°C. The mean daily maximumtemperatures are likely to be 0.6°C colder at the study site.Snow can fall at any time of the year, but there is no persis-tent snow cover during the winter months. Winds are pre-dominantly westerly and frequently strong.

The ephemeral wetland that the lawn largely occupies sitsin a depression between the arms of a parabolic sand dune.The sand particles are composed of Jurassic dolerite, groundby Pleistocene ice and sorted by waves and wind (Pharo &Kirkpatrick 1994). The drainage of water is slowed by aniron-rich indurated horizon. The surface soil in the wetlandhas less than 1% organic content, is weakly acid and low innitrogen and available phosphorus (Table 1). The vegetationsurrounding the lawn is alpine heath. The lawns and thesurrounding vegetation are grazed by Bennett’s wallaby(Macropus rufogriseus), the common wombat (Vombatusursinus), theTasmanian pademelon (Thylogale billardierii) andthe naturalized European rabbit (Oryctolagus cuniculus).

The low altitude lawn is on the property ‘Bangor’ on thenorthern end of the Forestier Peninsula (42°52′43.8″S,147°55′20.2″E, alt 6 m) in southeast Tasmania. The annualaverage rainfall (1966–2006, T. Dunbabin, pers. comm.,2007) at the Bangor homestead is 723 mm, distributedevenly among the seasons. The closest Bureau of Meteorol-ogy station recording temperature is Orford, 14 m in altitudeand approximately 22 km to the north, where the mean dailymaximum and minimum temperatures for the coldest month(1951–2007) were 13.1°C and 3.5°C and for the warmestmonth were 22.0°C and 12.1°C.

The lawn is part of a complex of dune-blocked wetlands.The wetlands are bounded inland by low dolerite hills. Thesoil under the lawn and the adjacent Leptospermum lanigerum– L. scoparium heath and Eucalyptus ovata open-forest is ablack silty clay loam with granular pedality at the surfacegrading to massive black clay at depth.The surface soil underthe lawn is weakly acid, has 3.5% organic content and mod-erate levels of phosphorus and nitrogen (Table 1). The lawnand the understorey of the adjacent vegetation are grazed byBennett’s wallaby, the common wombat, the Tasmanianpademelon, the common brushtail possum (Trichosurusvulpecula) and the European rabbit.

Inundation experiments and observations

To determine if waterlogging was affecting seedling survivalof the potential woody invaders, 60 L. lanigerum seedlings(<20 cm height) were taken from near the Bangor site and 60Ozothamnus hookeri seedlings from near the Central Plateausite in March 2006. These were the woody species that weremost common in each of the lawns. Seedlings were broughtback to the glasshouse and placed in pots (13.5 cm ¥14.5 cm) with a potting mix suitable for native plants ofseven parts fine composted pine bark, four parts coarsewashed river sand with fertilizer rates suitable for a barkbased medium (Handreck & Black 1984). The pH wasadjusted to around 6 with the addition of dolomite at a rateof 2.7 kg m-3. Major elements were supplied via the slow-release fertilizer Osmocote. Mean day and night temperaturein the glass house was 19.8°C and 14.2°C respectively. After4 weeks the seedlings were transfered to a shade house andleft for 2 weeks. Throughout the preparation period plantswere checked daily and watered when needed. The twowaterlogging experiments (one for each species) were started6 weeks after collection in the field (15 May 2006) to reflectthe start of possible inundation at the field sites.

In each of the two experiments, 10 seedlings were ran-domly assigned to each of four treatments: 4, 8 and 16 weeks(waterlogging) and control (no waterlogging). The seedlingsin the waterlogging treatments were placed in a 9.6-L bucketwith water 1 cm above the soil surface. The water level waskept constant throughout the experiments. Control seedlingswere placed in buckets and watered to keep them moist. No

Table 1. Means for surface soil variables and mean scatcounts during the uninundated months of the experimentalperiod for the Bangor and Augusta lawns

Variable Bangor Augusta

N (%) 0.49 0.06Extractable P (ppm) 8.40 2.62Loss on ignition (%) 3.51 0.99pH 6.10 5.72Macropod scats (ha day-1) 200 1053Wombat scats (ha day-1) 80 40Rabbit scats (ha day-1) 400 547Possum scats (ha day-1) 4 0

Soil nutrient data are the means of 10 surface soil samples(Roberts 2009).

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© 2010 The Authorsdoi:10.1111/j.1442-9993.2010.02168.xJournal compilation © 2010 Ecological Society of Australia

water was allowed to accumulate in the base of the controlbuckets.The buckets were rotated twice weekly.The height ofseedlings was measured at the start of the experiment andremeasured at 16 weeks.

Metre-deep peizometers (n = 10) were installed in each ofthe lawns within 10 m of their boundaries. The depth of thewatertable, or the height of water above the peizometer, wasrecorded monthly during the experimental period. A dumpylevel and Carr staff were used to survey the topography of theexperimental sites and adjacent vegetation.

Grazing experiments and observations

The lawn exclosure experiments at each of the two sites weredesigned to be independent, but can be regarded as treat-ment nested within site. An 80 ¥ 40 m expanse of each lawnwas divided into 5 ¥ 5 m squares. The lawn area at each sitewas then stratified into a zone 0–5 m from the edge of thelawn and a zone 5–20 m from the edge. Ten plots were thenrandomly selected from each of the zones.

An exclosure was paired with a control quadrat in each ofthe 20 randomly located lawn plots. The exclosures wereconstructed from wire mesh (25 ¥ 20 mm) with dimensionsof 0.5 m ¥ 0.5 m ¥ 0.2 m high, open on the bottom and heldin place with aluminium pegs at each corner and at themidpoint along each side. The location of the paired exclo-sure and control quadrats was determined by the presence ofshrub or tree seedlings. Each plot was searched for woodyseedlings which were then marked, numbered and randomlyselected for inclusion in the exclosure or control quadrat withthe constraint that the control plot should be no more than ametre from the exclosure. Where no seedlings were found,exclosure and control quadrats were placed in the north eastcorner of the plot with the exclosure or control treatmentrandomly assigned.

The experiments were established in June 2005, at whichtime height and species of all individuals of woody plantspecies within the quadrats were recorded, along with thecover and height of Schoenus spp., the most abundant taxonin the lawns.These measurements were repeated in February2007. Species nomenclature follows Buchanan (2007).

To gain an indication of grazing intensity, scats werecounted and cleared monthly from each of the 5 ¥ 5 m plots.The scats were grouped into macropod (Bennett’s wallabyand pademelon), wombat, rabbit and possum.

Statistical analysis

General linear models were derived in Minitab 14 to deter-mine the effects of site and exclosure nested within site on thedifference between 2005 and 2007 in woody seedlingnumbers, maximum height of woody seedlings and Schoenusspp. height in lawns. Treatment and place were treated asrandom variables. The set of diagnostic graphs produced inthis program were used to gauge consistency with theassumptions of the test. Treating the experiments at the twosites as independent, as was the original intent of the design,the paired t-test was used to determine the significance ofdifferences between 2005 and 2007 between exclosures andadjacent controls for each of the variables.

For each of the two waterlogging experiments, one-wayanova and Tukey’s family error rate were used to determinethe influence of the experimental treatments on the percent-age increase in seedling height between the commencementof treatment and 16 weeks after treatment.

RESULTS

Inundation

At Bangor, where inundation of the lawn and adjacentvegetation occurred for less than 2 months of theexperimental period, the mean water in the lawn was-54.2 � 3.1 cm. The part of the lawn on which theplots were located was elevated above the adjacenttaller vegetation. At Augusta, where the lawn was inun-dated between July 2005 and February 2006, themean water table level was above the ground(15.1 � 3.6 cm). The boundaries of the lawn largelyfollowed the contour, although shrub-dominated veg-etation occurred at the same elevation of the lawn tothe southeast.

All L. lanigerum seedlings survived the first experi-ment and put on growth. There was differentiation ingrowth by treatment (F = 4.82, d.f. = 3, P = 0.006,R2 = 28.67%), with the 4-week waterlogging treatmentresulting in significantly less growth than the 8- and16-week waterlogging treatment (Fig. 1).

The O. hookeri seedlings all survived the experimentand put on height during its course. There was nodifference in growth of O. hookeri between any ofthe treatments and the control (F = 2.14, d.f. = 3, P =0.113, R2 = 15.11%).

Grazing

Scat counts confirmed the constant presence of verte-brate herbivores at the times when the lawns were notinundated. Grazing intensity, at least from macropods,appeared greater on the lawns at Augusta than on theBangor lawns (Table 1). However, the Augusta lawnwas under water during the least favourable times forplant growth.Wallaby and wombat scat counts peakedduring late summer-early autumn at both sites, whilein the adjacent vegetation the peaks were in winter(Roberts 2009).

There was no effect of site (F = 0.57, d.f. = 1, P =0.530) or exclosure nested within site (F = 3.02, d.f.= 2, P = 0.06) in the general linear model for change

in the number of seedlings (R2 = 16.25%), althoughseedling numbers did increase more within the exclo-sures than the paired controls at Bangor (pairedt1,19 = 4.06, P = 0.002) (Fig. 2).

There was no effect of site (F = 1.85, d.f. = 1, P =0.307) and a strong effect of exclosure nested withinsite (F = 14.19, d.f. = 2, P < 0.001) on change in the

GRAZING MAINTAINS LENTIC WETLAND LAWNS 305


maximum height of woody plants between 2005 and2007 (R2 = 57.69%), with significant differencesbetween control and exclosure at both Bangor (pairedt1,19 = 3.74, P = 0.004) and Augusta (paired t1,19 =3.59, P = 0.005) (Fig. 3).

The model for the change in height of Schoenus spp.between 2005 and 2007 was not significant for site(F = 0.0, d.f. = 1, P = 0.957), but highly significant fortreatment nested within site (F = 27.36, d.f. = 2, P <0.001) (R2 = 41.9%). Controls and exclosures werewell-differentiated at both sites (Fig. 4).

Only one individual woody plant, at Bangor, wasfound in quadrats more than 10 m from the boundary

of the lawn with adjacent vegetation. At both sites, theseedlings that successfully established in either theexclosures or controls on the lawns during the courseof the experiment were within 5 m of the lawnboundary.

DISCUSSION

Our results are not consistent with the hypothesis thatfresh water inundation can prevent woody plants fromgrowing tall enough to eliminate lentic wetland lawns.The results of the pot experiment showed that the

Fig. 1. Box plots of the percentage height growth of Leptospermum lanigerum between the initiation of the experiment and16 weeks in response to different periods of inundation. The star represents an outlier.

Fig. 2. Mean and 95% confidence limits for change in number of woody seedlings per quadrat between 2005 and 2007 by siteand treatment.



most common shrub species in the two lawnsincreased in height with their roots underwater for4 months. Shrubs also survived prolonged immersionin the field at Augusta. Leptospermum scoparium, aspecies that occurred in the Bangor lawns with L.lanigerum, has previously been demonstrated to beindifferent to waterlogging (Cook et al. 1980; Pryoret al. 2006). Eucalyptus ovata, the dominant of theforest adjacent to the Bangor lawns, and present as anoccasional seedling on them, also grows well with itsroots immersed (Kirkpatrick & Gibson 1998).

The fact that all but one individual woody plant waslocated within 10 m of the lawn boundary is morelikely to be related to disseminule dispersal range thanan inundation gradient. At Bangor, the areas of thelawn close to its boundary had the same elevation asthose further away, discounting more prolonged inun-dation as an explanation for the lack of woody plantseedlings.

The results of the field experiments at both sites arestrongly consistent with the hypothesis that vertebrateherbivores prevent the replacement of lawns by woody

Fig. 3. Mean and 95% confidence limits for change in the height of the tallest woody plant in quadrats between 2005 and 2007by site and treatment.

Fig. 4. Mean and 95% confidence limits for change in the height of Schoenus spp. in quadrats between 2005 and 2007 by siteand treatment.



or tall herbaceous vegetation by repeatedly removingthe upright shoots of seedlings. The increasingnumbers of individuals of woody plants in the exclo-sures at Bangor over the experimental period indicatethat their establishment can also be impeded bymammal grazing.

While the exclosures are likely to have moderatedclimatic extremes to some minor degree, the damageto woody plants in the control quadrats was visibly theresult of teeth, with no woody plants on the lawns ateither site showing evidence of wind, frost or droughtdamage.The mesh size used in the exclosures allowedaccess for invertebrates, which are therefore unlikely tohave been major contributors to the difference inwoody plant heights between controls and exclosures.

The importance of vertebrate herbivores in prevent-ing woody plants from growing tall on lawns has beenindicated previously by Prins and van der Jeugd(1993) who observed woody plant invasion of lawns inEast Africa after a vertebrate population crash. Theconversion of lawns to taller herbaceous vegetationtypes as a result of release from grazing pressure, ashappened in our exclosures, has been more widelyobserved (Archibald 2008; Stock et al. 2010).

Herbivore grazing has been shown to increase attimes of high lawn productivity (McNaughton 1985;Kröger & Rogers 2005). In Tasmania, summer as awhole is likely to be the time of peak productivity, asday lengths are long, temperatures are high and soilsare moist but not inundated. The observed peakgrazing pressure in February and March therefore sug-gests that mammals concentrate on the lawns whenfodder production is low in the surrounding vegeta-tion, which suffers peak moisture stress in late summerand early autumn, when the soils of the lawns remainmoist (Roberts 2009).

Thus, grazing appears to be the cause of mainte-nance of the lawn structure in the two wetlands. Wesuggest that a mixture of herbivore species, each ofwhich has different feeding preferences (Sprent &McArthur 2002; Evans et al. 2006), and the totalintensity of grazing, might ensure that no plant speciescan escape the sward by developing older unpalatablefoliage (McNaughton 1984). However, environmentalfactors may interact with grazing pressure to maintainlawns. Constant moistness, and at least moderate soilfertility, may be necessary to create a sward that iscontinuously attractive to grazers in comparison toother options. The wetland situation of our lawnsmakes them more constantly moist than the surround-ing country, although they are not necessarily morefertile (Roberts 2009). If grazing animals wereattracted elsewhere for a substantial period, ashappens after extensive fires in the South Africansavannahs (Archibald et al. 2005), lawns might rapidlychange into taller vegetation, as happened in less than2 years in our exclosures.

The sward might also need to be continuous enoughto prevent the least palatable species escaping grazingby their chance occurrence in bare patches. Thedietary preference of the major herbivores at our sitesis for herbs and grasses (Sprent & McArthur 2002;Evans et al. 2006).They may eat the shrub seedlings inthe lawns when intimately intermixed with more pal-atable forage, but not necessarily when seedlings areisolated from preferred forage. If there was a strongnegative relationship between the cover of preferredplants and the degree to which non-preferred plantswere grazed, the drying of easternTasmanian wetlandsthat has occurred since the climatic shift to latesummer-early autumn dryness in the late 1970s (Kirk-patrick et al. 2000; Calder & Kirkpatrick 2008) shouldhave resulted in the invasion of the more elevated partsof marsupial lawns by shrubs. This hypothesis will betested in a later paper.

ACKNOWLEDGEMENTS

The research reported in this paper was support by aCommonwealth Postgraduate Scholarship to CynthiaRoberts and an Australian Research Council Discov-ery Grant (DP0665083) to Jamie Kirkpatrick. We areextremely grateful to the following people for assis-tance and advice: Roz Heinz, Margaret Brock, StuartHuxtable, Sue Baker, Nicky Meeson, Emile Verd-uarand, Beatrice Wiggenhauser, Maj-Britt di Folco,SarahTaylor, KerryWhyte, Jenny Scott, Roland Payne,Steve Leonard, Mick Russell, Jon Marsden-Smedley,Denis Charlesworth, Kate Charlesworth, Tom andCynthia Dunbabin and Darren Turner.

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tasmanian lentic wetland lawns are maintained by grazing rather than inundation

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