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 Spartinaas opposed to a proportion <strong>of</strong> 0.34 <strong>of</strong> <strong>the</strong> meadow plants’seeds germinating. There was no obvious effect <strong>of</strong> proximity<strong>of</strong> neighboring clones on isolated individuals. While somelow density areas were more crowded than o<strong>the</strong>rs, <strong>the</strong>re wasno obvious trend towards relatively higher or lower seed setas long as <strong>the</strong> clones were not touching one ano<strong>the</strong>r. Thissuggests that <strong>the</strong> clones need to merge toge<strong>the</strong>r before <strong>the</strong>step-wise increase in seed set is achieved by meadowindividuals. This work is reported in full in Davis et al.(2004a).We <strong>the</strong>n investigated whe<strong>the</strong>r pollen limitation could be<strong>the</strong> mechanism behind <strong>the</strong> Allee effect in Willapa BaySpartina. While this may appear to be an intuitivehypo<strong>the</strong>sis, pollen limitation is generally assumed not tooccur in wind-pollinated plants, such as grasses, conifers andoak trees. There are many studies <strong>of</strong> pollen limitation inanimal-pollinated plants (Burd 1994; Larson & Barrett2000), very few <strong>of</strong> wind-pollinated plants, and none beforethis one <strong>of</strong> wind-pollinated invasive plants. Pollen limitationin animal vectored plants can diminish reproduction andconstrain invasion success (Bar<strong>the</strong>ll et al. 2001; Parker1997). Using simulation and analytic models, we askedwhe<strong>the</strong>r <strong>the</strong> Allee effect in Willapa Bay Spartina could haveslowed <strong>the</strong> rate <strong>of</strong> invasion and found it did produce a lagtime (Taylor et al. <strong>the</strong>se proceedings; Taylor et al. 2004).We conducted an observational and manipulative studyto assess <strong>the</strong> role <strong>of</strong> pollen limitation in <strong>the</strong> ability <strong>of</strong>Willapa Bay S. alterniflora to set seed. To see if <strong>the</strong>re is adifference in pollen deposition rates on stigmas, we collectedstigmas from isolated clones and high density meadows overthree different sites (Cedar River n = 20; Bone River n = 40;Palix River n = 94) and screened one lobe (half) <strong>of</strong> stigmasfor pollen load. We found that meadow plants’ stigmascaptured nine times <strong>the</strong> pollen that <strong>the</strong> isolated clones’ did.Isolated plants on average had less than one pollen grain perstigma lobe while meadow plants had more than six.Although plants at individual sites differed in <strong>the</strong>ir pollenloads, in every case, <strong>the</strong> meadow plants always had more(Davis et al. 2004b). Interestingly, <strong>the</strong> Cedar Rivercollections had very little pollen with even meadow plantshaving on average less than one pollen grain per stigma lobe,probably because it rained on <strong>the</strong> day <strong>of</strong> collection and <strong>the</strong>previous two days suggesting pollen flow is inhibited on wetdays and possibly rainy years.To find how pollen deposition changes over awindward-to-leeward gradient and to see if <strong>the</strong> amount <strong>of</strong>pollen in <strong>the</strong> air is correlated with <strong>the</strong> amount <strong>of</strong> pollendeposited on stigmas, we set out pollen traps and collectedadjacent stigmas. We set 10 traps along each <strong>of</strong> six transects.Oriented along <strong>the</strong> direction <strong>of</strong> <strong>the</strong> prevailing wind: two inwindward isolated clones (I & II), one along <strong>the</strong> windwardmeadow edge (III), one in <strong>the</strong> windward meadow (IV), onein <strong>the</strong> leeward meadow (V) and one 100 m from <strong>the</strong> leewardside <strong>of</strong> <strong>the</strong> meadow across an unvegetated channel (VI)(Davis et al. 2004). We found extremely high correlationbetween <strong>the</strong> pollen loads on <strong>the</strong> traps and stigmas (r = 0.99)supporting <strong>the</strong> hypo<strong>the</strong>sis that <strong>the</strong> amount <strong>of</strong> pollen onstigmas is controlled by pollen abundance on <strong>the</strong> wind. On<strong>the</strong> pollen traps, we found very little pollen anywhere in <strong>the</strong>isolated clones or along <strong>the</strong> meadow edge. Pollen loadsincreased inside <strong>the</strong> windward end <strong>of</strong> <strong>the</strong> meadow andincreased yet more dramatically at <strong>the</strong> leeward end <strong>of</strong> <strong>the</strong>meadow. However, <strong>the</strong> amount <strong>of</strong> pollen droppedprecipitously across <strong>the</strong> unvegetated channel suggesting that<strong>the</strong> gap was too far for nearly all <strong>of</strong> <strong>the</strong> airborne pollen tocross (Davis et al. 2004).To conclusively determine whe<strong>the</strong>r isolated clones aremaking fewer seeds than <strong>the</strong> meadow plants because <strong>of</strong>pollen scarcity, we performed a manipulative experiment bysupplementing and excluding pollen received byinflorescences and leaving control inflorescences open toambient pollen availability. Twenty-four plants received alltreatments and a fur<strong>the</strong>r 20 plants received pollen additionand control treatments only. We found <strong>the</strong> pollen exclusiontreatment reduced seed set in <strong>the</strong> meadow plants by morethan sixfold, but caused no reduction in <strong>the</strong> isolated clones.The pollen addition treatment had no effect on <strong>the</strong> seed set<strong>of</strong> <strong>the</strong> meadow plants, but did raise that <strong>of</strong> <strong>the</strong> isolatedclones by more than threefold. We were not able to saturate<strong>the</strong> plants’ inflorescences with pollen, as an inflorescencehas receptive stigmas for about 10 days (~3 days per stigma)and we were able to supplement on only three consecutivedays. This is likely <strong>the</strong> reason why <strong>the</strong> isolated clones’pollen addition seed set was not as high as <strong>the</strong> meadowplants’ ambient control (Davis et al. 2004). These resultsindicate that isolated clones, colonists at low density, areextremely pollen-limited whereas seed set in <strong>the</strong> highdensity meadow is not limited by pollen. This work isreported in full in Davis et al. (2004b). We found that anAllee effect contributed to <strong>the</strong> lag time in spatial spread <strong>of</strong> S.alterniflora in Willapa Bay. Fur<strong>the</strong>rmore, this Allee effectappears to be <strong>the</strong> result <strong>of</strong> pollen limitation <strong>of</strong> <strong>the</strong> colonizingclones at <strong>the</strong> leading edge <strong>of</strong> <strong>the</strong> invasion. In <strong>the</strong> studiesoutlined here, and also in two years <strong>of</strong> unpublished data, weconsistently found that isolated individuals set very fewseeds. However, <strong>the</strong>se studies were done in <strong>the</strong> singlelocation <strong>of</strong> Willapa Bay, so similar results may or may notbe duplicated in o<strong>the</strong>r locations. We found a wide range inself-compatibility <strong>of</strong> Spartina alterniflora from <strong>the</strong> nativerange (Davis 2005), so <strong>the</strong> ability <strong>of</strong> isolated plants to use<strong>the</strong>ir own seeds in different locations may be largelyinfluenced by <strong>the</strong> source <strong>of</strong> <strong>the</strong> colonizing individuals. Itwould be advisable for managers <strong>of</strong> o<strong>the</strong>r invaded areas toconfirm <strong>the</strong> lack <strong>of</strong> seeds in isolated Spartina alterniflorabefore discounting <strong>the</strong>ir potential as significant seed sources.-92-