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 SpartinaRGR (mg mg -1 day -1 )20181614121086420accbFull competitionReduced competitionNo competitionbdIN SITU MUD SALIC SALIC DIST DISTFig. 1. Percent survival (mean±SE; n=10 plots) <strong>of</strong> Spartina seedlingsplanted within each intertidal zone. IN SITU: unmanipulated mudflatseedlings; MUD: transplanted mudflat seedlings; SALIC: Salicornia zone;DIST: Distichlis zone. Different letters indicate significant differences(p < 0.05) between treatment groups using Tukey’s multiple comparisonstest. Due to <strong>the</strong> lack <strong>of</strong> variation in <strong>the</strong> IN SITU and <strong>the</strong> Distichlis fullcompetition plots, <strong>the</strong>se treatments could not be compared using ANOVA.Instead, comparisons <strong>of</strong> <strong>the</strong>se treatments were made using 95% confidenceintervals.INCREASING RELATIVE INTERTIDAL HEIGHTFig. 2. Spartina seedling RGR (mean±SE; n=10 plots) across intertidalzones (abbreviations as in Figure 1). Different letters indicate significantdifferences (p < 0.05) between treatment groups using Tukey’s multiplecomparisons test. Due to a lack <strong>of</strong> variance, <strong>the</strong> Distichlis full competitiontreatment was omitted from <strong>the</strong> ANOVA and comparisons were made with95% confidence intervals.established in <strong>the</strong> Salicornia virginica-dominated salt marshzone immediately adjacent to <strong>the</strong> mudflat. Ten paired plotsalso were placed in <strong>the</strong> Distichlis spicata-dominated zonelocated immediately above <strong>the</strong> Salicornia zone. Plots wererandomly placed along 30 m transects in near monocultures<strong>of</strong> Salicornia and Distichlis. In <strong>the</strong> Salicornia and Distichliszones each plot consisted <strong>of</strong> a subplot with native vegetationintact (full competition) and native vegation removed(reduced competition). In June 2001, three seedlings weretransplanted into each plot or subplot within each zone.Seedlings were watered immediately after being transplantedand were tagged for identification. Reduced competitionplots were cleared <strong>of</strong> all native vegetation (ca. 0.25 mdiameter) and maintained throughout <strong>the</strong> study.In late September-early October 2002, each Spartinaplant was harvested from <strong>the</strong> experimental plots. Plants weretransported to <strong>the</strong> lab, washed repeatedly, dried for 72 hoursat 70ºC, and weighed. Biomass within each subplot wasaveraged for <strong>the</strong> three replicate seedlings in each subplot.Relative growth rates (RGR) <strong>of</strong> Spartina were calculated forall competition treatments at each intertidal level (Eqn. 1;Hunt 1978)RGR = lnW 2 - lnW 1 / T 2 -T 1 Eqn. 1where W 1 and W 2 are <strong>the</strong> plant weights at times 1 (T 1 ) and 2(T 2 ). Initial weight (W 1 ) was estimated using a regression <strong>of</strong>tiller number in relation to dried seedling biomass (Hellquist2005).Survival data were transformed using <strong>the</strong> arcsin squareroot transformation. Survival and RGR data were analyzedusing analysis <strong>of</strong> variance in <strong>the</strong> mixed model procedure <strong>of</strong>SAS 9.1 (SAS Institute 2000). Mixed models are appropriatewhen random factors (split plots) are present in anexperimental design (SAS Institute 2000). The Satterthwaiteadjustment was applied to compensate for heterogeneity <strong>of</strong>variances. Unless o<strong>the</strong>rwise noted (see Figs. 1 and 2),Tukey’s pairwise comparisons were made (p < 0.05) acrosstreatments.RESULTSAll seedlings survived over <strong>the</strong> course <strong>of</strong> <strong>the</strong> experimentwithin <strong>the</strong> unmanipulated, in situ mudflat treatment.Seedlings transplanted within <strong>the</strong> mudflat had a 40%decrease in survival (Fig.1). Percent survival was high (>90%) and not different between <strong>the</strong> full and reducedcompetition plots in <strong>the</strong> Salicornia zone. However, allseedlings died with full competition in <strong>the</strong> Distichlis zone.Fifty percent <strong>of</strong> <strong>the</strong> seedlings survived in reducedcompetition subplots within <strong>the</strong> Distichlis zone. Survival in<strong>the</strong> Salicornia zone was higher than in both Distichlistreatments (Fig. 1).The RGR <strong>of</strong> Spartina anglica generally decreasedmoving from <strong>the</strong> low Salicornia-dominated zone to <strong>the</strong>higher Distichlis-dominated zone (Fig. 2). Theunmanipulated, in situ plants had <strong>the</strong> greatest RGR. Themudflat in situ seedlings had a growth rate over two timesgreater than <strong>the</strong> seedlings transplanted within <strong>the</strong> mudflat.In <strong>the</strong> Salicornia zone <strong>the</strong>re were no differences in RGRbetween full competition and reduced competition subplots.Distichlis zone seedlings grew slower than within <strong>the</strong> in situand Salicornia treatments. The lowest RGRs were in <strong>the</strong>Distichlis zone and in <strong>the</strong> transplanted mudflat seedlings(Fig. 2).- 100 -