Life stage fluctuations in garlic mustard (Alliaria petiolata) abundance: incorporating long-term ecological monitoring into an undergraduate curriculum

Brittany Verrico and Jason Kilgore, Biology Dept., Washington and Jefferson College, Washington, PA e-mail: verricobm@washjeff.edu, jkilgore@washjeff.edu

Acknowledgments This work was supported in part by Undergraduate Science Program Education Grant No. 52006323 from the Howard Hughes Medical Institute to Washington and Jefferson College and PLANTS Award from the Botanical Society of America. In addition, we would like to thank the Abernathy family for allowing us to conduct our research at AFS.

In a silique:

Methods • Abernathy Field Station (AFS) is a 57-acre biological field station consisting of a hardwood forest, a mixed species plantation, and a lowland and is located in the Pittsburgh Plateau of southwestern Pennsylvania. • Eight transects of 10 plots at 5-m intervals were established in Summer 2009 to monitor changes in garlic mustard populations. • LEM interns (9 from 2009-2010), and students from first-year biology (217 from 2009-2010), botany (41 from 2010-2011), and experimental biology (4 in 2011) courses participated in plant population and environment data collection. • Student-entered data are stored in a LEM server. • Data for this analysis included density of garlic mustard rosettes and adults from June 2009-2011 across all 8 transects; analyses were conducted in SPSS V17.0.

Introduction Garlic mustard (Alliaria petiolata) is an obligate biennial herb native to western Eurasia (Cavers et al. 1979). Although noted for high levels of phenotypic plasticity (Byers and Quinn 1998), this invasive species is often located in mesic shaded areas (Cavers et al. 1979). Invasive to eastern North American forests (Cavers et al. 1979), garlic mustard outcompetes native vegetation and negatively impacts ecosystem dynamics (Rodgers et al. 2008). Dense, core populations exhibit strong density-dependence and life-stage oscillation (Pardini et al. 2009). Adult garlic mustard may inhibit juvenile success due to increased shade which could further generate temporal and spatial segregation of life stages in this species (Winterer et al. 2005). Furthermore, seedling survival in high rosette density areas is lower than in areas of low rosette density (Meekins and McCarthy 2002).

Literature Cited Byers, D.L., and J.A. Quinn. 1998. Demographic variation in Alliaria petiolata (Brassicaceae) in four contrasting habitats. J Tor Bot Soc 125(2):138-149. Cavers P.B., M.I. Heagy, and R.F. Kokron. 1979. The biology of Canadian weeds, 35: Alliaria petiolata (M. Bieb) Cavara and Grande. Can J Pl Sci 59:217-229. Meekins, J.F., and B.C. McCarthy. 2002. Effect of population density on the demography of an invasive plant (Alliaria petiolata, Brassicaceae) population in a southeastern Ohio forest. Amer Mid Nat 147(2):256-278. Pardini, E.A., J.M. Drake, J.M. Chase, and T.M. Knight. 2009. Complex population dynamics and control of the invasive biennial Alliaria petiolata (garlic mustard). Ecol Appl 19(2):387-397. Pisula, N.L., and S.J. Meiners. 2010. Relative allelopathic potential of invasive plant species in a native young disturbed woodland. J Tor Bot Soc 137(1):81-87. Rodgers, V.L., K.A. Stinson, and A.C. Finzi. 2008. Ready or not, garlic mustard is moving in: Alliaria petiolata as a member of eastern North American forests. BioSci 85(5):426-436. Winterer, J., M.C. Walsh, M. Poddar, J.W. Brennan, and S.M. Primak. 2005. Spatial and temporal segregation of juvenile and mature garlic mustard (Alliaria petiolata) in a central Pennsylvania woodland. Amer Mid Nat 153(2):209-216. Wolfe , B.E., V.L. Rodgers, K.A. Stinson, and A. Pringle. 2008. The invasive Alliaria petiolata (garlic mustard) inhibits ectomycorrhizal fungi in its introduced range. J Ecol 96:777-783.

Garlic mustard colonization (Rodgers et al. 2008)

Allelopathic chemicals kill ectomycorrhizal fungi (Wolfe et al. 2008) and decrease germination success of native species (Pisula and Meiners 2010)

Decrease in native vegetation (Rodgers et al. 2008)

Per capita resources increase for garlic mustard

Garlic mustard density increases

Per capita resources become limited

Garlic mustard populations become stage-structured density-dependent (Pardini et al. 2009)

Conclusion

Results and Interpretation

Figure 1. Rosette (A) and adult (B) density across three consecutive Junes. Rosette and adult density significantly decreased in 3 and 2 transects, respectively, from 2009 to 2010, but increased in 2 and 1 transects, respectively, from 2010 to 2011; the other transects had overlapping 95% CI with zero. Transect 1 (mixed plantation) had significant oscillations in rosette density across the 3 years, and Transect 8 (mildly disturbed forest) had significant oscillations in adult density across the 3 years. However, the general trend shows that rosette and adult density decreased in 2010; in that year, the maximum number of adults in any given plot was 4. Precipitation was not likely an influential factor on garlic mustard density. Mean June precipitation (2002-2011) was 3.89 in, while cumulative precipitation for June for 2009, 2010, and 2011 were 3.16, 4.47, and 1.73 in, respectively (Pittsburgh; wunderground.com). Rosette survivorship to 2010 was expected to be lower due to reduced mean precipitation (19%) the previous year, which may explain the low density of adults in 2010. However, with higher than average precipitation in 2010, both rosette and adult densities were lower than in previous and subsequent years. Furthermore, June 2011 precipitation was 56% lower than the 10-year mean, yet rosette and adult densities were higher than in 2010.

Figure 2. Change in rosette density compared to change in adult density from 2010-2009 (A) and 2011-2010 (B) across 80 plots in 8 transects. The regression for 2010-2009 was significant (F=14.0, p<0.001) but explained only 15% of the variation in change in adult density. The regression for 2011-2010 was not significant (F=2.0, p=0.16) and explained only 3% of the variation in change in adult density. The relationships between each of the variables across both sets of years by transect were not significant (p=0.512 for 2010-2009 and p=0.616 for 2011-2010).

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Change in rosette density (#/m2) from 2011-2010 Change in rosette density (#/m2) from 2010-2009

Path to Stage-Structured Density Dependence in Garlic Mustard Invasions

• Three years of data were insufficient to detect life-stage density-dependence in garlic mustard at this field station, likely due to varying habitat and micro-site conditions, time since invasion, and multiple invasion events.

• Reduced densities of both rosettes and adults in 2010 are not related to June precipitation but may be due to other climatic

factors during critical life history stages.

• Marked rosettes are being tracked seasonally through next Summer to

determine sensitivity of this life stage

through time (sensu Pardini et al. 2009).

• Long-term monitoring projects provide opportunities for many students across the biology curriculum to be exposed to invasive plant population ecology.

• Monitored garlic mustard rosette and adult densities in June 2009-2011. • Both rosette and adult densities declined in 2010. • Low June precipitation cannot explain the 2010 decline. • Varying habitat and micro-site conditions, time since invasion, and multiple invasion events may be possible reasons why garlic mustard populations at this site do not exhibit stage-structured density dependence. • Over 250 biology students were exposed to invasive plant ecology through this long-term ecological monitoring program.