Measuring short distance dispersal ofAlliaria petiolataand determining potential long distance dispersal mechanisms

Abstract

IntroductionAlliaria petiolata, an herbaceous plant, has invaded woodlands in North America. Its ecology has been thoroughly studied, but an overlooked aspect of its biology is seed dispersal distances and mechanisms. We measured seed dispersal distances in the field and tested if epizoochory is a potential mechanism for long-distance seed dispersal.MethodsDispersal distances were measured by placing seed traps in a sector design around three seed point sources, which consisted of 15 second-year plants transplanted within a 0.25 m radius circle. Traps were placed at intervals ranging from 0.25–3.25 m from the point source. Traps remained in the field until a majority of seeds were dispersed. Eight probability density functions were fitted to seed trap counts via maximum likelihood. Epizoochory was tested as a potential seed dispersal mechanism for A. petiolata through a combination of field and laboratory experiments. To test if small mammals transport A. petiolata seeds in their fur, experimental blocks were placed around dense A. petiolata patches. Each block contained a mammal inclusion treatment (MIT) and control. The MIT consisted of a wood-frame (31 × 61× 31 cm) covered in wire mesh, except for the two 31 × 31 cm ends, placed over a germination tray filled with potting soil. A pan filled with bait was placed in the center of the tray. The control frame (11 × 31 × 61 cm) was placed over a germination tray and completely covered in wire mesh to exclude animal activity. Treatments were in the field for peak seed dispersal. In March, trays were moved to a greenhouse and A. petiolata seedlings were counted and then compared between treatments. To determine if A. petiolata seeds attach to raccoon (Procyon lotor) and white-tailed deer (Odocoileus virginianus) fur, wet and dry seeds were dropped onto wet and dry fur. Furs were rotated 180 degrees and the seeds that remained attached were counted. To measure seed retention, seeds were dropped on furs and rotated as before, then the furs were agitated for one hour. The seeds retained in the fur were counted.ResultsFor the seed dispersal experiment, the 2Dt function provided the best fit and was the most biologically meaningful. It predicted that seed density rapidly declined with distance from the point source. Mean dispersal distance was 0.52 m and 95% of seeds dispersed within 1.14 m. The epizoochory field experiment showed increased mammal activity and A. petiolata seedlings in germination trays of the MIT compared to control. Laboratory studies showed 3–26% of seeds were attached and retained by raccoon and deer fur. Retention significantly increased if either seed or fur were wet (57–98%).DiscussionWithout animal seed vectors, most seeds fall within a short distance of the seed source; however, long distance dispersal may be accomplished by epizoochory. Our data are consistent with A. petiolata’s widespread distribution and development of dense clusters of the species in invaded areas.

Spatially explicit ecological modeling improves empirical characterization of dispersal

“…Therefore, simple rules of thumb stating that to be considered as a point, the size of the source should be smaller than 1 % of the length of the gradient (Zadoks and Schein, 1979;McCartney et al, 2006), can be quite misleading, and result in inaccurate estimates of dispersal parameters. This emphasizes the importance of the spatially-explicit modeling of the source as we have done it here, considering that it is often the case that most measurements are conducted close to the source even when the overall gradient is long (Werth et al, 2006;Skarpaas and Shea, 2007;Loebach and Anderson, 2018).…”

“…cm diameter(Loebach and Anderson, 2018), 4 m 2 square(Emsweller et al, 2018), route of a single sampling dive(D'Aloia et al, 2015), even an agricultural eld as a…”

Effects of maternal source and progeny microhabitat on natural selection and population dynamics in Alliaria petiolata

“…Rather, a complex matrix of micro‐environmental conditions may impede forest incursion in these populations, making management a higher priority in edge microhabitats. To fully understand the potential for populations to establish in new locations, critical features of this species’ seed dispersal still need to be explored (e.g., Loebach and Anderson, ), especially as they relate to dispersal and gene flow within versus across microhabitat types. This would improve our ability to assess populations’ potential for local adaptation to different microhabitats, which is necessary to ultimately predict the spread of this invasive species across heterogeneous landscapes.…”

“…Alliaria petoliata , or garlic mustard, is a biennial forb that produces siliques containing five to more than 100 seeds that are passively dispersed (Anderson et al., ) and/or can sometimes be transported on animal fur, with 95% of seeds estimated to be dispersed within 1.14 m of the maternal plant (Loebach and Anderson, ). The present study populations are located at a ~1200‐hectare tract at the Harvard Forest Long Term Ecological Research Site (latitude: 42.531612, longitude −72.189963) in Petersham, Massachusetts, USA, dominated by a canopy of mature Acer saccharum Marshall , Acer rubrum L ., Quercus rubra L. , Pinus strobus L. , and Fraxinus americana L. trees.…”

Comparison of the non‐native herbAlliaria petiolatawith dominant native herbs in microhabitats of a Midwestern forest

“…Allocation to seed production by Alliaria is correspondingly high: Population averages are commonly well over 100 seeds per plant (Byers andQuinn 1998, Nuzzo 1999), and large plants are capable of producing thousands of seeds (Rodgers et al 2008a, Phillips-Mao et al 2014. Seeds disperse by explosive dehiscence, mostly falling 1-2 m from the parent plant, with occasional longer-distance dispersal possible via running water and animal coats (Nuzzo 1993, 1999, Biswas and Wagner 2015, Loebach and Anderson 2018. The propagule pressure resulting from this high individual fecundity and concentrated seed rain is thought to be an important contributor to the ability of Alliaria populations to naturalize in temperate forests Battles 2009, Dornbush andHahn 2013), particularly in communities in which the resident species are dispersal-limited (Dornbush and Hahn 2013), or in which disturbance has recently enhanced availability of resources such as light (Eschtruth and Battles 2009).…”