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 Spartinathose in Scheffer’s photo can be seen in <strong>the</strong>se photos atwidely separated locations around <strong>the</strong> bay (Civille et al.2005).A time series analysis <strong>of</strong> Spartina spread across <strong>the</strong>Willapa tidal flats was conducted using 55 years <strong>of</strong> synopticaerial photography, spanning from 1945 to 2000. Themethodology used for <strong>the</strong> study was a combination <strong>of</strong>photogrammetry, image analysis and geographic informationsystem (GIS) techniques (Wilkie 1996). Over 2,000 photos(black and white, true color and color infra-red) weredigitized, examined for Spartina plants, <strong>the</strong>n geo- ororthorectified to fit <strong>the</strong> curvature <strong>of</strong> <strong>the</strong> earth and assignedgeographic coordinates. The resulting corrected images were<strong>the</strong>n classified into <strong>the</strong>matic representations <strong>of</strong> Spartinameadows and clones. The final year <strong>of</strong> <strong>the</strong> series, comprised<strong>of</strong> color infrared images acquired in 2000, was orthorectifiedto remove radial and scale distortions, <strong>the</strong>nclassified through hierarchical supervised and unsupervisednon-parametric methods into 330 <strong>the</strong>matic raster files, withless than one meter positional error and a 0.3 m minimummapping unit (Civille 2005). All o<strong>the</strong>r spatial data weregeorectified to this baseline layer. These tasks wereaccomplished with a large format scanner (28 x43 centimeter [cm] platform), Photoshop (Adobe), andImagine (ERDAS), a robust digital photogrammetrys<strong>of</strong>tware package. The resulting raster files were <strong>the</strong>nconverted into GIS vector coverages in ArcInfo (ESRI), andcombined with a bathymetry digital elevation model (DEM)to obtain z (elevation) values for each polygon.A vector-based coverage model was chosen for ouranalysis, ra<strong>the</strong>r than raster-based, because polygons areautomatically assigned a unique identifier, area andperimeter by ArcInfo s<strong>of</strong>tware (Flynn and Pitts 2000). Thisallowed us to extract demographic history for each plant(greater than 300,000 individuals in 1997 alone) that wedetected on <strong>the</strong> photos. These data were used toparameterize ma<strong>the</strong>matical population models (Taylor et al.2004), and establish <strong>the</strong> ages <strong>of</strong> plants used in genetic andreproductive fitness experiments (Davis et al. 2004).Regions, which are a higher order organizational object inArcInfo, were applied to <strong>the</strong> coverages, allowing us toanalyze clonal growth and meadow formation year-to-year.Each contributing polygon (clone) within a region (largemeadow), maintains its own unique identifier and also has aregion identifier, which are maintained through dataextraction and statistical analyses (Civille 2005).Spartina elevation data were analyzed with twoseparate bathymetry (elevation) layers; one that wascollected by <strong>the</strong> National Oceanic and AtmosphericAdministration (NOAA) with depth soundings in 1953(channels updated in 1986); and ano<strong>the</strong>r that was acquired in<strong>the</strong> spring <strong>of</strong> 2002. The new bathymetry data was collectedwith LiDAR (Light Detection And Ranging) techniques, andis <strong>the</strong> first <strong>of</strong> its kind to be ga<strong>the</strong>red for an entire estuarineintertidal zone with tidal control (Cracknell 1999; Irish andLillycrop 1999; Flowers 2002). Tidal exposure times werecalculated for <strong>the</strong> Willapa estuary during winter monthswhen plant aboveground growth has died back, leavingmudflat elevations as <strong>the</strong> primary targets reflected by <strong>the</strong>laser. Data collection was scheduled for predicted tidalelevations lower than 30 cm Mean Low Water (MLW),although <strong>the</strong> actual tidal elevations at <strong>the</strong> time <strong>of</strong> acquisitionvaried from this target (Civille 2005). The LiDAR data wascollected through a collaborative effort with <strong>the</strong> NOAACoastal Services Center in Charleston, South Carolina, anddata was acquired and compiled by photogrammetrists atSpencer B. Gross in Portland, Oregon.To determine whe<strong>the</strong>r individual Spartina plants weregrowing more rapidly in different abiotic conditions, weextracted areas from polygons that overlapped one ano<strong>the</strong>rin separate years. These were assumed to be <strong>the</strong> sameindividual increasing (or diminishing) in area through time.A stratified random selection <strong>of</strong> 408 <strong>of</strong> <strong>the</strong>se co-occurringclones was extracted from <strong>the</strong> 1994 and 1997 layers, <strong>the</strong>nstatistically analyzed according to a simple relative arealgrowth index [(Area1997-Area1994)/Area1994]. Thisindex relates <strong>the</strong> amount <strong>of</strong> areal growth to <strong>the</strong> initial size <strong>of</strong><strong>the</strong> plant. An ANOVA was performed on <strong>the</strong> 1994-1997data set and significant differences in relative growth rateswere observed between mud and sand substrates and tidalelevation <strong>of</strong> <strong>the</strong> clones (Fig. 1). Clones in s<strong>of</strong>t mudsubstrates with proximity to fresh water sources exhibitedsignificantly higher areal growth rates than those on sand.Clones at higher elevations in general are growing morequickly than those at lower elevations, although differenceswere not significant in all elevational classes. Spartinaplants are noticeably absent from sand substrates lower thanFig. 1: Differences in relative lateral growth rate <strong>of</strong> Spartina comparingtidal elevation and substrate type, 1994-1997.-84-