Proceedings of the Third International Conference on Invasive ...

<strong>Proceedings</strong> <strong>of</strong> <strong>the</strong> <strong>Third</strong> <strong>International</strong> <strong>Conference</strong> on Invasive SpartinaChapter 2: Spartina Distribution and SpreadFig. 3. Recruitment <strong>of</strong> S. alterniflora seedlings in plots varying insubstrate texture. Values are means ± 1 S.E.Seedlings/ha25002000150010005000NoneControlFig. 4. The influence <strong>of</strong> control efforts in 2003 on local seedlingrecruitment in 2004. Control methods included a variety <strong>of</strong> mechanicalmethods and herbicide application. Values are means ± 1S.E.did not receive control. The local reduction in seed productioninfluenced local (sub-region) seed recruitment <strong>the</strong> followingyear (Fig. 4). Areas such as <strong>the</strong> Palix and Cedar riversthat did not receive control in 2003 had high seed productionthat fall and correspondingly high seedling recruitment<strong>the</strong> following year. In contrast, sites that received controlin 2003 such as Tarlatt Slough and Diamond Point experiencedlow rates <strong>of</strong> seed production that fall and had correspondinglylow recruitment <strong>the</strong> following year.DISCUSSIONCauses <strong>of</strong> variabilitySpartina alterniflora recruitment in Willapa Bay ishighly variable across years and across sub-regions <strong>of</strong> <strong>the</strong>bay. The yearly variation observed in this study corroborateslonger-term data from historical aerial photographssuggesting that <strong>the</strong>re have been strong episodic bursts <strong>of</strong> S.alterniflora colonization (Civille et al. 2005). A significantcomponent <strong>of</strong> this yearly variation in recruitment appears tobe directly related to levels <strong>of</strong> seed production <strong>the</strong> previousyear. For <strong>the</strong> limited two years that we are able to makecomparisons, <strong>the</strong> average estuary-wide recruitment tracksaverage estuary-wide seed production <strong>the</strong> previous year. Inaddition, <strong>the</strong>re is evidence that S. alterniflora seed set is significantlyinfluenced by pollen availability, and that pollenavailability is significantly reduced by wet and cool conditions(Davis et al. 2004, and this volume). Yearly averagebay-wide recruitment is positively correlated with <strong>the</strong> number<strong>of</strong> August degree days (<strong>the</strong> cumulative number <strong>of</strong> growingdegree days in August) and negatively correlated withtotal August precipitation in <strong>the</strong> previous year (Lambrinos,unpublished data).Substrate characteristics appear to play an importantrole in <strong>the</strong> site-to-site variability in recruitment. Spartinaalterniflora seedlings are far less likely to establish at sandysites than at muddy sites. The mechanisms generating thispattern are still unclear. Spartina seeds may be less frequentlydeposited at more erosive sandy sites. In river systemsseed and sediment deposition are closely related(Goodson et al. 2003). In addition, seedlings may be lesslikely to establish at sandy sites because <strong>of</strong> higher erosionand physical stress. Alternatively, seedlings could performpoorly in sandy sediments because <strong>of</strong> nutritional limitations.Increasing sediment sand corresponds with decreasing sedimentnitrogen (N) content at <strong>the</strong>se sites (Tyler et al. 2007).Evidence from transplant experiments and common gardenexperiments indicate that S. alterniflora seedlings growpoorly on sand relative to mud irrespective <strong>of</strong> <strong>the</strong> ambienthydrological conditions (Lambrinos and Bando 2008).Undoubtedly, ano<strong>the</strong>r factor in <strong>the</strong> site-to-site variabilityin recruitment is seed supply. Spartina alterniflora can potentiallydisperse long distances (Howard and Sytsma 2010).However, <strong>the</strong> fact that regions that received control in 2003had correspondingly reduced recruitment levels in 2004,while no reductions in recruitment occurred in regions thatdid not receive control suggests that a sizeable portion <strong>of</strong>recruitment is derived from local seed sources.Implications for managementThe strongly episodic and spatially variable nature <strong>of</strong> S.alterniflora recruitment in Willapa Bay has important implicationsfor management. Years <strong>of</strong> poor recruitment haveprobably slowed <strong>the</strong> rate <strong>of</strong> <strong>the</strong> invasion in Willapa Bay.Both vegetative growth and recruitment by seed contribute-89-