The Growth of Sphagnum: Experiments on, and Simulation of, Some Effects of Light Flux and Water-Table Depth Author(s): P. M. Hayward and R. S. Clymo Reviewed work(s): Source: Journal of Ecology, Vol. 71, No. 3 (Nov., 1983), pp. 845-863 Published by: British Ecological Society Stable URL: http://www.jstor.org/stable/2259597 . Accessed: 20/07/2012 08:50 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp . JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. British Ecological Society is collaborating with JSTOR to digitize, preserve and extend access to Journal of Ecology. http://www.jstor.org Journalof Ecology (1983), 71, 845-863 THE GROWTH OF SPHA GNUM: EXPERIMENTS ON, AND SIMULATION OF, SOME EFFECTS OF LIGHT FLUX AND WATER-TABLE DEPTH P. M. HAYWARD* AND R. S. CLYMO Departmentof Botany, Westfield College, London NW3 7ST SUMMARY (1) Experimentswere made to determinethe effects of bothshading and of water-table depthon the growthof threespecies of Sphagnum.The species(and theirusual habitats) were: S. capillifolium(hummocks); S. papillosum(lawns); and S. recurvum(pools and flushedlawns). (2) Water-tabledepth had littleeffect on growthmeasured as increaseof drymatter; shadingreduced growthand therewere specificdifferences associated withplant size. Therewere no significantinteractions between water-table depth and shading. (3) For growth measured as growth in length, there were highly significant interactions,individual species behavingdifferently in responseto shade and, to a lesser extent,in responseto water-tabledepth. (4) In Sphagnum lawns in two natural habitatsthere was a negativecorrelation betweendepth of the water-tableand surface-roughness.In experimentalconditions surface-roughnessincreased both as thewater-table was raisedand as shade increased. (5) A computersimulation of growthof Sphagnumin a lawn was able to reproduce theobserved variations in surfaceroughness. In mixedlawns oftwo species,the one in its 'natural'habitat out-grew the other. INTRODUCTION Most attemptsto measurethe growthof Sphagnumhave been concernedwith obtaining values of productivity(e.g. Leisman 1953; Overbeck & Happach 1956; Pearsall & Gorham 1956; Bellamy & Rieley 1967; Clymo & Reddaway 1974; Ilomets 1974; Pakarinen1978). Relativelyfew attempts have been made to measuregrowth in relationto the environment.Some of these (Chapman 1965; Clymo & Reddaway 1971; Boatman 1977) have entailedtransplanting Sphagnum from one positionto anotherin the field. Other studies,by Green (1968) and Clymo (1970, 1973) have includedlaboratory or gardenexperiments in whichthe environment was moreprecisely controlled, but only in a limitedway. The aim ofthe work presented here was to obtainmore precise details about the growth of Sphagnumin relationto itsenvironment. From this information, a quantitative model of thegrowth of a Sphagnumcarpet was developed. The earlierwork indicatedthat two environmentalvariables-light fluxand water supply-have an overridinginfluence on growth.Total lightflux is important,but so is the variationof incidentflux on a small scale, betweenone individualSphagnum plant and anotherin a singlelawn. These effectsmay be due to 'external'shading by larger,vascular plantsor to 'self-shading'by neighbouringSphagnum plants. Watersupply to the capitulum(the apical tuftof expandingbranches) has been shown to depend,in a complexway, on thestructure of the Sphagnum plant (which differs among * Presentaddress: Department of Botany, University of Leicester,Leicester LE 1 7RH. 0022-0477/83/1100-0845$02.00 (? 1983 BritishEcological Society 845 846 Effectsof light and wateron Sphagnumgrowth species)and on water-tabledepth (Hayward & Clymo 1982). In general,however, water supplyis inverselyrelated to thedepth of thewater-table and, for a givenwater content in thecapitula, the water-table will be at a greaterdepth below hummock species than it will be below lawn species.Thus, water-tabledepth can be used as a convenient,and easily measurableway, of expressingwater supply. Bogs have a water-tablewhich is, comparedwith most terrestrial habitats, relatively close to the surface.The distributionof speciesappears to be relatedto thedepth of the water-table(e.g. Ratcliffe& Walker 1958; Clymo & Hayward 1982). This impliesthat changesin the depthof the water-tablewill affect plants of differentspecies in different ways, complicatedby the fact that the water-tableis not a plane surfacebeneath the hummocks,lawns and pools ofthe microtopography (Popp 1962; Goode 1970; Boatman, Goode & Hulme 1981). Because thewater-table is close to thesurface, small changes in its depthhave a proportionallygreater effect on bog plantsthan they would if the water-table wereat a greaterdepth. Bothlight flux and water-tabledepth were therefore controlled in theseexperiments and were includedin the model of Sphagnumgrowth. The model had severalquantitative consequences.Of these,the one whichcan most easily be measuredin the fieldis the surfaceroughness of a Sphagnumcarpet. There is someindication (Clymo 1973) thatthis is negativelycorrelated with depth of the water-table.Here we reportmeasurements of surfaceroughness both in the field and in experiments. THE SPECIES USED IN EXPERIMENTS Plants of threespecies, S. capillifolium*,S. papillosumand S. recurvum,were used in experiments.These species are typicallyassociated with differentbog microhabitats. Sphagnumcapillifolium is naturallya species of drierareas such as hummocks,S. papillosumforms lawns at a heightintermediate between bog pool and hummock,at or just above thewater-table, and S. recurvumis foundin pools and wetflushed lawns. METHODS Growthin relationto lightfluxand water-tabledepth An experimentwas made withplants of the above threespecies collected from Moor House N.N.R., Cumbria,from the areas aroundBurnt Hill and Bog End (NationalGrid referencesNY 753329 and NY 765329 respectively)growing in pots in a gardenin London.It includedtreatments, for each species,of factorial combinations of five depths to the water-table(0, 3, 6, 10 and 14 cm below the surface)and fivedegrees of shade (absorbanceof 0, 0.54, 0.80, 0.91 and 0.96 of incidentlight). We choseto use shade,with the consequentfluctuations in irradiance,rather than constantartificial light partly for convenience,and partlybecause we wanted to be able to relate the resultsto field behaviour. Plantswere cut initially to (known)length, sufficient to allowall cutends to be at least 1 cm underwater at thestart of the experiment (Table 2). The lowest1 cm ofeach plantwas stainedwith a cationicdye (crystalviolet) so thatany loss duringthe experiment could be detected.There were no losses. All plantsof the same size and taxon werewell mixed * Nomenclatureof Sphagnumfollows that of M. 0. Hill (1978); thatof vascularplants follows Clapham, Tutin& Warburg(1981). P. M. HAYWARD AND R. S. CLYMO 847 beforebeing allocated, forty plants to each treatment.The fortyplants were tied into loose bundles,so thatthey could be recoveredeasily, surrounded by othercut but unmarked plants(to reduceedge effects)and packed at naturaldensity into pots made fromsquare section2-litre plastic bottles by cuttingoff the top partleaving a 10-cmsquare, 20-cm tall containerwith straight sides. The shorterplants were placed on an underlyingbed of choppedSphagnum to bringthe capitula of the plants in all treatmentsto thesame level in thepot. The apparatusshown at thetop of Fig. 1 was used to pack theplants in thepots. The leftcontainer was a slidingfit inside the right one. The plantswere packed as shown and theleft container plus plantsslid into the right container. The leftcontainer was then withdrawnwith the string while the piston held the plants in positionin theright container. The filledpots wereclose packedon a flatlevel surface and weresurrounded by a 'guard ring' of otherpots filledwith Sphagnum on whichno measurementswere made. The experimentlasted for 77 days from17 June.During this time the orientation and position ofthe pots was changedrandomly each weekaccording to a Latinsquare arrangement. The waterlevel in each pot was maintainedthrough a tubeinserted into its base (Fig. 1). The tubewas attachedto a devicewhich kept the water level constant either by allowing excess rainto runaway or by addingdistilled water. With this arrangement there was no obvious accumulationof evaporitesat the surfaceof the plants.As the plantsgrew, the pots werelowered to maintainthe water-table at a constantdepth below the surface of the Sphagnumlawn in each pot. The potswere individually shaded by coveringthem with 0, 1, 2, 3 or 4 layersof black polyestergauze. The absorbanceof lightby this materialwas measuredon a recording String Ofherplants 40 plants Homogenized cut to length dead plants Sphagnum surface Water-table 4l-Air FIG. 1. Methodused to maintaina constantwater level in pots duringthe growthexperiment. Onlytwo of thefive levels are shown.Water was cycledby raisingit on a streamof air bubblesin a verticalglass tube.The 25-litreMariotte bottle was refilledwith distilled water as required.The upperdiagram shows the apparatus used to pack plantsin containers. 848 Effectsof light and wateron Sphagnumgrowth spectrophotometer.Between