Proceedings of the Third International Conference on Invasive ...

Chapter 2: Spartina Distribution and Spread<strong>Proceedings</strong> <strong>of</strong> <strong>the</strong> <strong>Third</strong> <strong>International</strong> <strong>Conference</strong> on Invasive SpartinaTHE CURRENT STATUS OF S. ANGLICA IN EUROPEInvasions <strong>of</strong> European mudflats by S. anglica (hereafterrefered to simply as ‘Spartina’) has been characterised by adifferent sequence <strong>of</strong> events at different latitudes. In <strong>the</strong>south <strong>of</strong> England, and from <strong>the</strong> southwest Ne<strong>the</strong>rlandssouthwards, large monospecific stands developed rapidlyfollowing initial introduction and have in many places beenfollowed in <strong>the</strong> early years <strong>of</strong> <strong>the</strong> last century by ‘dieback,’<strong>the</strong> swards breaking up and retreating in area. By contrast innor<strong>the</strong>rn England north <strong>of</strong> about 54° N and in <strong>the</strong> Dutch andGerman Waddensea, invading Spartina is replacedsuccessionally by o<strong>the</strong>r species, most commonly <strong>the</strong> grassPuccinellia maritima (hereafter ‘Puccinellia’). For examplein Morecambe Bay a 100-ha marsh formerly dominated bySpartina is now a Puccinellia-dominated sward from whichSpartina has almost disappeared (Gray and Raybould 1997).Although confounded in some cases by latitudinal variationin sediment type (<strong>the</strong> nor<strong>the</strong>rn marshes tend to be moresandy) this contrast between nor<strong>the</strong>rn and sou<strong>the</strong>rn marshesmay be related to <strong>the</strong> effects <strong>of</strong> differences in climate anddaylength on <strong>the</strong> two species (see below).Although <strong>the</strong> Spartina invasion has not completelyhalted it has certainly slowed, and with dieback widespreadin <strong>the</strong> south and a slow rate <strong>of</strong> colonization in <strong>the</strong> north,<strong>the</strong>re is probably a net loss <strong>of</strong> area <strong>of</strong> Spartina marsh at <strong>the</strong>present day. Small isolated foci <strong>of</strong> recolonization may befound in <strong>the</strong> south where, in response to rising relative sealevels, sea defences have been removed to create newmudflats and salt marsh, a process termed ‘managedrealignment’. Active invasion by Spartina <strong>of</strong> such newlycreated intertidal mudflats can be observed at Tollesbury inEssex, a county which has lost more than 25% <strong>of</strong> its saltmarsh in <strong>the</strong> last thirty years. For <strong>the</strong> moment it seems <strong>the</strong>Spartina invasion is ‘on hold.’INTERACTIONS BETWEEN SPARTINA AND PUCCINELLIAThe most parsimonious explanation for Spartina’ssuccess as an invader is that it has been able to grow onintertidal mudflats at lower elevations than <strong>the</strong> existingperennial saltmarsh vegetation. This is undoubtedly due to arange <strong>of</strong> adaptive morphological and physiological features<strong>the</strong> net effect <strong>of</strong> which enable Spartina to withstand higherfrequencies <strong>of</strong> tidal submergence. The ‘elevational niche’ <strong>of</strong>Spartina in Britain has been measured by surveyingtransects across salt marshes and recording <strong>the</strong> highest andlowest levels <strong>of</strong> <strong>the</strong> sward (in meters above <strong>the</strong> standard UKdatum height) and relating this to tidal constants (Gray et al.1991, 1995). Comparison <strong>of</strong> this niche with that <strong>of</strong> o<strong>the</strong>rsaltmarsh species confirms that Spartina extends on average68 centimeters (cm) below Puccinellia, <strong>the</strong> species with <strong>the</strong>next lowest elevational limit. There is also a niche overlapbetween <strong>the</strong> two species by 20 cm. Within this overlap zone<strong>the</strong> distribution <strong>of</strong> <strong>the</strong> two species is likely to be principallydetermined by interspecific competition, and Scholten andRozema (1990) provide clear evidence <strong>of</strong> this using anelegant removal experiment. The outcome <strong>of</strong> interspecificcompetition was shown to be critically dependent onvariation in local marsh elevation.Measurement <strong>of</strong> Spartina’s elevational niche alsoindicated that its upper limit varies with latitude. Theregression equation describing this limit (Gray et al. 1991)was:Upper Limit = 4.74 + 0.483(R) + 0.068(F) – 0.099(L)where R = spring tide range (m), F = fetch in <strong>the</strong> direction <strong>of</strong><strong>the</strong> transect (kilometers) and L = latitude (decimal degreesN). This equation, which accounted for 90.2% <strong>of</strong> <strong>the</strong>variation in <strong>the</strong> species’ upper limit, shows that Spartinadoes not extend so far upshore in more nor<strong>the</strong>rly latitudes. Apossible explanation for this is that <strong>the</strong> competitiveinteraction between Spartina and Puccinellia increasinglyfavours <strong>the</strong> latter as one goes northwards, enabling it toinvade <strong>the</strong> Spartina zone at lower elevations.The increasing competitive advantage <strong>of</strong> Puccinellia athigher latitudes may be linked to <strong>the</strong> different effects <strong>of</strong>temperature on Spartina and Puccinellia, which are revealedby differences in <strong>the</strong>ir seasonal growth patterns. Fieldmeasurements on a salt marsh at 52° N showed thatPuccinellia shoot weight increased in March when airtemperatures rose above 5° C, with growth peaking in Juneand July, whereas Spartina did not begin to grow until May,when temperatures reached 9° C, and reached maximumgrowth in October (Dunn et al. 1981). This laterdevelopment and growth <strong>of</strong> Spartina can be related to <strong>the</strong>species’ utilization <strong>of</strong> <strong>the</strong> C4 photosyn<strong>the</strong>tic pathway. One<strong>of</strong> only eight C4 species in <strong>the</strong> UK flora, Spartina is onlypartially adapted to cooler climates (C4 photosyn<strong>the</strong>sis, inwhich <strong>the</strong> first product <strong>of</strong> carbon dioxide fixation isoxaloacetate instead <strong>of</strong> phosphoglycerate as in C3 species, ismost common in semi-arid tropical and subtropical regions)(Long 1983,1990).THE EFFECTS OF CLIMATE CHANGEThe studies outlined above suggest that <strong>the</strong> northwardinvasion <strong>of</strong> Spartina is being prevented, or slowed, by <strong>the</strong>species inability to grow at low temperatures, and that, at itsnor<strong>the</strong>rn limits it is replaced by Puccinellia (a C3 specieswith a circumpolar distribution from 70° N southwards).These two pioneer saltmarsh species bioengineer vast tracts<strong>of</strong> intertidal mudflats and must be regarded as keystonespecies in saltmarsh development. The obvious andintriguing question is: will increases in air temperaturespredicted under various climate change scenarios enableSpartina to invade northwards into marshes currentlydominated by Puccinellia? Higher temperatures will bothincrease growth rate and extend <strong>the</strong> period <strong>of</strong> growth during<strong>the</strong> early months <strong>of</strong> <strong>the</strong> year. Spartina is unlikely to beaffected by increased atmospheric carbon dioxide as, where- 104 -