Global Positioning System (GPS) was used to both map the positions of individual nitrogen-fixing shrub plants and record individual plant height and breadth dimensions, within a small catchment of dry sub-humid unimproved short tussock grassland. The distributions of different species of shrubs varied with different landscape units within the catchment. The information will provide the base data for a first approximation of the role of the shrubs in the nitrogen balance of the ecosystem, which is pivotal to the sustainability of pastoral farming in the tussock grasslands

Boswell, C. C.

Espie, P. D.

Lowther, W. L.

English

University of Kentucky

University of Kentucky UKnowledge International Grassland Congress Proceedings XIX International Grassland Congress Mapping Nitrogen Fixing Shrubs in Dry Unimproved Tussock Grassland C. C. Boswell AgResearch, New Zealand W. L. Lowther AgResearch, New Zealand P. D. Espie AgResearch, New Zealand Follow this and additional works at: https://uknowledge.uky.edu/igc  Part of the Plant Sciences Commons, and the Soil Science Commons This document is available at https://uknowledge.uky.edu/igc/19/32/5 The XIX International Grassland Congress took place in São Pedro, São Paulo, Brazil from February 11 through February 21, 2001. Proceedings published by Fundacao de Estudos Agrarios Luiz de Queiroz This Event is brought to you for free and open access by the Plant and Soil Sciences at UKnowledge. It has been accepted for inclusion in International Grassland Congress Proceedings by an authorized administrator of UKnowledge. For more information, please contact UKnowledge@lsv.uky.edu. ID # 32-05MAPPING NITROGEN FIXING SHRUBS IN DRY UNIMPROVED TUSSOCKGRASSLANDC.C. Boswell, W.L. Lowther and P.D. EspieAgResearch, Invermay Agricultural Centre, P.B. 50034, Mosgiel, New ZealandAbstractGlobal Positioning System (GPS) was used to both map the positions of individualnitrogen-fixing shrub plants and record individual plant height and breadth dimensions, withina small catchment of dry sub-humid unimproved short tussock grassland.  The distributions ofdifferent species of shrubs varied with different landscape units within the catchment.  Theinformation will provide the base data for a first approximation of the role of the shrubs in thenitrogen balance of the ecosystem, which is pivotal to the sustainability of pastoral farming inthe tussock grasslands.Keywords: Carmichaelia petriei, Discaria toumoutu, legumes, nitrogen, nitrogen fixation,Sophora prostrata, tussock grasslands.IntroductionThe sustainability of dry and semi-arid tussock grasslands is greatly influenced byinputs of nitrogen (N) into the ecosystem.  Martin et al. (1994) believed grazing the grasslandto be unsustainable in the absence of fertiliser nutrient inputs.  It appears that indigenousshrub legumes such as native broom (Carmichaelia petriei), and kowhai (Sophora prostrata),and other N-fixing shrubs such as matagouri (Discaria toumoutu) and tutu (Coriaria arborea)were important N fixers historically (Wardle 1991).  In the 150 years following Europeansettlement, burning and grazing have reduced the numbers of N-fixing shrub plants in thegrasslands.  The present scarcity of legumes in New Zealand tussock grasslands, led Martin etal. (1994) and McIntosh (1997) to assume minimal current inputs of N.  However, Boswell etal. (2000) believe that a volunteer non-endemic annual (haresfoot clover, Trifolium arvense)may be an important N-fixer in parts of the ecosystem.As part of an effort to understand N cycling in these grazed natural grasslands, we havedetermined the incidence of individual shrub legume plants on different parts of the landscapewithin a small catchment.  Because of the repeatability of patterns of plant distributionespecially affected by sunny and shady aspect on the broken high country landscape, we canextrapolate the findings to other catchments in the same range of mountains and probably toother ranges with a similar climate.  Knowledge of the occurrence of different speciesindicates where future effort in measurement of N-fixation should be directed in order todetermine their importance in ecosystem nitrogen balances.Material and MethodsOur technique involved an initial survey of the catchment into land classes based onaspect, steepness, soil depth, and plant associations; and mapping the boundaries of landclasses using GPS.  The catchment is known as Tomahawk Gully in the Benmore Range,MacKenzie Basin, South Island New Zealand.  The gully runs approximately NW to SE sothat one side has a sunny aspect facing NE and the other has a shady aspect facing SW.  Thecatchment is approximately 0.4 km wide; the line of the drainage flow extends over 1.3 km; ithas a narrow gully floor and has steep sides (>30o); and rises from approximately 500 m asl to850 m asl.The natural vegetation is a short tussock association (Festuca novae- zelandiae, Poacita, Poa colensoi, Rytidosperma spp) with the scrub Melicytus alpinus along the gully floor.The catchment has been invaded by the flat weed mouse-ear hawkweed Hieracium pilosellain the past few decades, and the gully floor by sweet briar (Rosa rubiginosa).The distributions of N-fixers, including shrubs, clovers (especially haresfoot clover),and lichens within 5 m by 5 m quadrats at 25 m intervals along transects across land unitsfrom the valley floor to ridge top, were initially identified and quantified (Boswell et al.2000).  This technique proved unsuitable for the shrubs we were interested in.  On the shadyside of the catchment, the positions of individual shrubs (mainly native broom) within a 20 mwide transect from the valley floor to ridge top were located by GPS.  Similarly the individualnative broom shrubs were also mapped in a 20 m wide transect in the top of the catchment.Plant dimensions (height and two width/breadth dimensions) were also recorded on the GPSfiles for subsequent biomass and N balance estimations.  Kowhai was associated with rockyoutcrops near ridge tops in the sunny face of the catchment land units. The shrub wassurveyed in detail on a representative part of the ridge, with individual plants again mappedby GPS and plant dimensions recorded.  One small area of very old matagouri plants andother individual plants were mapped wherever they were seen in and outside of the samplingareas.Results and DiscussionFig. 1 shows the land units identified within the catchment and the distribution ofindividual or dense associations of kowhai, and matagouri plants in the catchment.  Locationsof the wide band transects from which data for the distribution of the more regularly dispersednative broom was obtained are also shown.Kowhai was mainly associated with rock outcrops at the top of the sunny aspect of thecatchment.  A total of 98 plants were recorded in a single well-defined ridge top location (seeFig.1).  Mean plant size (height 58 cm, 53% in the range 40-80cm; mean volume 0.122 m3,52% in the range 0.01-0.1 m3) was much less than the mature matagouri plants. The biggestkowhai shrub recorded was 1.6 m tall and had a volume of 1.15 m3.  Kowhai appears moresusceptible to sheep browsing than matagouri.  The estimated maximum kowhai plantpopulation for the catchment (Table1) was calculated from the plant density recorded in thesample, and the area of rock outcrops on the extended ridge of the sunny aspect.Matagouri occurred as a single patch (0.015 ha) of a few old trees about 2.5 m tall,grouped shrubs, and sparsely scattered outlying plants.  Three plants close by the patch, on thelower sunny face were 1 m-2 m tall and 0.9 m-2 m across.  The mean volume of these shrubswas 1.8 m3.  Two plants were recorded within the 20 m wide (0.5 ha) transect area on thelower shady face were taken as representative of plant density on this landscape unit.  Otherplants mapped were taken as isolated groups or single plants.  Three plants were mapped inthe upper part of the gully floor land unit, and a group of three and a separate single plant inthe top of the catchment.  The spiny growth habit of matagouri protects it from grazing.  Webelieve the low density of matagouri reflects a past history of burning and seedling grazing.Native broom was not recorded on the lower sunny slopes. Twenty-six individual plantswere recorded within the 20 m wide transect on the lower shady slope, and 46 plants in asimilar transect at the top of the catchment.  Native broom occurs more frequently on thehigher parts of the slopes e.g. within about 40 m of the ridge tops on the shady slope.  Lowplant heights (mean 33 cm, 54% of plants between 5 and 20 cm) and small individual plantvolumes (mean  0.06 m3, 51% of plants < 0.01 m3 ) reflect close-grazing by sheep.  Thespecies has the potential to grow over 2 m tall (Poole and Adams 1980) but only one plantapproached this in the experimental catchment.GPS offers a useful technique for the measurement of the spatial distribution,frequency, and size of individual N-fixing shrub plants and groups of plants with identifiableboundaries; especially in ecosystems featuring difficult terrain.  The survey showed theyoccur in localised sites within the catchment, especially near the tops of the ridges.  Repeatsurveys offer the capability of charting the survival of individual plants or groups of plantsover time, and mean changes in physical size of the shrubs.  Net growth or browsingdepredation by merino sheep and/or rabbits, will provide one of a suite of indications ofecosystem sustainability.The low numbers of matagouri plants counted, and higher numbers of kowhai plantsbelied conventional thinking of their respective importance in such tussock grassland.Constraints put upon extrapolation from the published indicative figures for N-fixation bymatagouri (Daly 1969) and tutu (Silvester et al 1979) make them unsuitable for modelling;while N-fixation in kowhai and native broom has never been measured.  This study reinforcesthe requirement for such measurements. Shrub distribution suggests N-input to the ecosystemmay be spatially limited but may be significant within small areas of the catchment.  Theeffectiveness of shrub-N to the ecosystem will be limited by the extent of N transfer bybrowsing merino sheep, hares and rabbits either directly from the shrubs, or from understoryplants which benefit from the shrub N-fixation.ReferencesBoswell C. C., Lowther W. L. and Espie P. R. (2000). Importance of Trifolium arvense as anitrogen fixer in semi-arid tussock grasslands of New Zealand. Proc. 19th. Int. Grass. Cong.,Brasil, (this proceedings).Martin G., Garden P., Meister A., Penno W., Sheath G., Stephenson G., Urquhart R.,Mulcock C. and Lough R. (1994). South Island High Country Review. Final Report fromThe Working Party on Sustainable Land Management, April 1994: South Island HighCountry Review Working Party, Wellington, 184 pp.McIntosh P. (1997). Nutrient changes in tussock grasslands, South Island, New Zealand.Ambio 26: 147-151.Poole A .L. and Adams N. M. (1980). Trees and shrubs of New Zealand. P.D.  Hasselberg,Government Printer, Wellington, New Zealand.Silvester W. B., Carter D. A. and Sprent J. I. (1979). Nitrogen input by Lupinus andCoriaria in Pinus radiata forest in New Zealand. Pages 253-265 in J. C. Gordon, C. T.Wheeler and D. A Perry, eds. Symbiotic Nitrogen Fixation in the Management of TemperateForests. Orgeon State University Press, Corvallis, OR.Wardle P. (1991). Vegetation of New Zealand. Cambridge University Press, Cambridge,U.K.Table 1 - Mean densities of N fixing shrubs on different land units (plants/ha)Land UnitSpecies Front face(sunny)SunnyslopeShadyslopeTop ofcatchmentGully floor Estimated totalplants incatchment(49.2 ha.)Nativebroomnil nil 52 26 nil 1189Matagouri nil n/a(patch<0.02 ha)4 4 plants,twolocations5 plants,onelocation95Kowhai 98 plants in defined area nil 30(estimated)nil 98-130Areas ofgeneric landunits (ha)2.21 17.73 19.56 6.62 1.94Figure 1 - Map of Tomahawk Gully showing generic land units, GPS plotted locations of individualkowhai and matagouri plants, and native broom in belt transect sampling locations (Inset showslocation of catchment area in the South Island of New Zealand).native broom plantskowhai plantsmatagouri plantstop of catchmentshady facegully floortracksunny facefront facerocktransectLegend:170 E44 SStudyAreaMACKENZIEBASIN

Mapping Nitrogen Fixing Shrubs in Dry Unimproved Tussock Grassland

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Mapping Nitrogen Fixing Shrubs in Dry Unimproved Tussock Grassland

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