Contact Information
Mary A. McKenna, PhD
Associate Professor - Department of Biology
Office Location: EE Just Hall Room 214B
Office Telephone: 202-806-6103
Laboratory Location: EE Just Hall Room 410C
Laboratory Telephone: 202-806-4865

B.S., Boston College (1976)
Ph.D., State University of New York, Stony Brook (1987)

Chair, Science and Education Coordination Committee, National Ecological  
Observatory Network (NEON), Mid-Atlantic Domain (2009-present)
Ecological Society of America Representative on AIBS Council (1998-present)
Faculty Sponsor, HU SEEDS Chapter, Ecological Society of America (2006-present)
Board of Directors, American Institute of Biological Sciences (AIBS) 1999-2006
IBRCS Working Group, Blueprint and Rationale for NSF NEON (2002-2004)
Chair, Meeting Activities, Botanical Society of America, SE Chapter (1996-1999)
Chair, Chair-Elect, Ecological Society of America, Metro DC Chapter (1994-1996)
Teaching Award, College of Arts and Sciences Honors Association (2006)
CETLA Featured Teacher Award (2005)
Faculty of the Year Award, HU Biology Graduate Students Association (1997)

Ecology (BIOL230)
Plant Ecology (BIOL432)
Topics in Ecology and Evolution (BIOL502)

Research Interests
Plant population biology
Ecology and Evolution of Angiosperm Reproduction
Genetic &environmental factors that influence pollen growth

Grants and Funding Awards
 NSF URM Environmental Biology Scholars 2010-2015
 NSF REU Blandy REU Program 2005-2012
 NSF UMEB Environmental Biology Scholars 2004-2009
 USDA CREES Reproduction in Hyperaccumulators 2001-2002
 NSF RIMI Plant Science Research 1996-2001
 USDA FS Acid Rain Impacts on Forest Understory 1993-1995
 USDA FS Howard U/USDA FS Partnership Program 1994-1995
 USDA FS Rocky Mt Field Courses for HU students 1990, 1992, 1995
 USDA FS Global Change in Alpine Plant Populations 1988-1995

Research Detailed
We are exploring ecological factors that contribute to endemism and invasibility in nickel hyperaccumulators in the genus Alyssum (Brassicaceae). Several species of nickel hyperaccumulators in the genus Alyssum are found on serpentine (ultramafic) soils throughout southern and eastern Europe and Asia, and some are endemic to serpentine substrates. We are studying the role of nickel in the life history of these plants in an effort to determine the importance of soil nickel in vegetative growth, reproduction and overall fitness.

Students working with Dr. McKenna conduct field and greenhouse research at University of Virginia’s Blandy Research Station (a future NEON Mid-Atlantic satellite site), greenhouse, growth chamber and lab studies at Howard University, and field studies in local serpentine sites (Soldier’s Delight Environmental Area in MD, State Line Serpentine Barrens in MD and PA). We are exploring ecological factors that contribute to endemism and invasibility in nickel hyperaccumulators in the genus Alyssum.

We are studying the ecological risks of introducing nickel hyperaccumulators such as Alyssum for phytoremediation, since the weedy characteristics of these species suggest that they might invade natural plant communities adjacent to phytoremediation sites. Nickel-hyperaccumulator plants hold promise for phytoremediation of soil contaminated by nickel as a result of smelting and mining operations because they remove nickel from soil and accumulate it in plant tissues at levels up to 2% of plant dry weight. Evaluating the potential invasiveness of Alyssum is particularly important since natural serpentine plant communities represent unique “islands” of biodiversity due to a high proportion of endemic species. Serpentine endemics are particularly vulnerable to invasion because serpentine soil can provide a refuge for specialist species that would be out-competed in other habitats.

Since 2004, McKenna mentored 17 undergraduate researchers (Environmental Biology Scholars, Blandy REU students, Howard U Honors Program students) studying the role of nickel in the life history of Alyssum. We found that soil nickel has a significant positive effect on vegetative growth in Alyssum. Seedlings grown on soil with Ni are larger (Sylvain 2007). Under field conditions, plants grown in nickel soil experienced greater survivorship (Pendergrass 2006). Alyssum seedlings are not strong competitors on soil without nickel (Scantlebury 2005, Archibald 2006, Smith 2006, Volpe 2006), but Alyssum is likely to persist on these soils. A recent study using an additive design investigated the competitive ability of A. murale seedlings on soils with and without nickel (Ervin, 2010). Alyssum was grown in pots with different densities of Vulpia microstachys, a dominant grass in serpentine regions of the western US. Alyssum grew better on soils with nickel, although its biomass decreased significantly as the density of Vulpia increased. These growth responses to soil nickel suggest that although Alyssum murale is not a strong competitor, it may pose a real ecological risk to conservation of native serpentine communities.

McKenna and her students also conducted a series of studies to examine the role of nickel in reproduction of Alyssum. Alyssum demonstrates some key traits of successful weedy invaders such as prolific flowering, generalist pollinators (hoverflies and sweat bees), and abundant seed production (Ahern 2006, Arienzo 2006). Hoverflies do not discriminate between Alyssum plants grown on soil with and without nickel, but sweat bees preferentially visit plants grown on soil without nickel (Arienzo 2006). A. murale produces abundant small seeds with wind dispersal, so it is likely that propagules could travel from management sites unless plants are monitored very closely (and harvested prior to flowering). Seeds also germinate readily on soils with and without nickel (Pendergrass 2006, Sylvain 2007). A remarkable finding is that seeds produced by plants grown on soil with nickel are significantly heavier and are twice as likely to germinate compared to seeds produced by plants grown on soils without nickel (Sylvain 2007). This is the first study to demonstrate a positive effect of nickel on reproductive processes in a Ni-hyperaccumulating plant species.

Many studies have shown that adult metal hyperaccumulator plant species can defend against herbivores due to the high concentration of heavy metals in their shoot tissue. Very few studies have focused on seedling herbivory, even though the seedling is usually the most vulnerable stage in a plant’s life history. McKenna and her students tested the ability of Alyssum seedlings to defend themselves against herbivores (Kissell 2008). Seedlings were grown in soil without nickel and soil with 2000 ppm nickel; herbivore damage was assessed with a field study and laboratory studies using “choice” and “no-choice” designs. Overall, the results suggest that nickel does little to protect against damage and mortality due to generalist herbivores under field conditions. Laboratory studies with the specialist herbivore Pieris rapae suggest that nickel can be an effective defense against a specialist herbivore, but toxicity is more important than deterrence. Physical defenses (trichomes) may also play an important role in defense. Current projects examine the importance of trichomes in defense of Alyssum at different life stages and the interaction between soil nickel levels and trichome production. Studies are also underway to examine the root and shoot growth responses of Alyssum across a range of soil nickel concentrations. Additional competition studies include native species mixtures under long- term field conditions that allow assessment of lifetime fitness impacts. Detailed studies are underway to explore relationships between Alyssum and its pollinators and herbivores. Other research projects available for students working in McKenna’s lab include field and lab studies of reproduction processes in native North American “boden vag plants” that colonize serpentine and non-serpentine soils. Controlled hand crosses by a graduate student in McKenna’s lab demonstrated that gene flow between serpentine and non-serpentine populations can be restricted.

We are also studying the role of essential oils in stimulating growth and root nodule formation in legumes grown in association with plants in the mint family (Lamiaceae). Thyme essential oil inhibits soil pathogens and stimulates nodulation of white clover and alfalfa under growth chamber and field conditions (Summers 1993, Stewart and McKenna 2002, Nicholson 2007, Rodriguez 2010). A greenhouse study (Hamilton 2007) also found an unusual compatibility in interspecies mixtures of white clover and a variety of mints (Mentha spicata, Nepeta cataria, Prunella vulgaris). These interactions between mints and nodule-forming legumes have important implications for nutrient cycling and soil fertility in natural and agricultural ecosystems.


Selected Publications

  Stewart, JC and MA McKenna. 2002. Nodulation and growth of white clover is enhanced by a monoterpene from wild thyme. In Galloway, J. N., Cowling, E. B., Erisman, J. W., Wisniewski, J., and Jordan, C. (eds). 2002. Optimizing; Nitrogen Management in Food and Energy Production and Environmental Protection: Contributed papers of the Second International Nitrogen Conference, Potomac, MD, 14-18 October 2001. Publisher: A.A. Balkema The Scientific World. ISBN: 90 265 1927 3. 1033pp.
  McKenna, MA., R.L. Chaney and E. Brewer. 2002. Endemism vs invasiblility in nickel hyperaccumulators. 9th New Phytologist Symposium.,U. Pennsylvania, Philadelphia, PA.
  McKenna, M.A. 1998. Pollen response to environmental stress in natural populations. p. 38.  In  Pollen and Spores 1998: Proceedings of an International Conference. The Royal Botanic Gardens, Kew and The Natural History Museum, London.
  McKenna, M.A., N.V. Summers and M. Bond. 1994. Effects of pH on pollen germination, reproduction and growth in alpine plants. In: Stephenson, A.G. and T.H. Kao (eds.), Pollen-Pistil Interactions and Pollen Tube Growth.  American Society of Plant Physiologists, Rockville, MD.
  McKenna, M.A. 1994. Genotypic and phenotypic components of alpine plant response to acid rain.  Report to Forest Service Rocky Mountain Station, Fort Collins, Colorado.
  McKenna, M.A. 1993. Atmospheric effects on plant reproduction.  Final Report to Forest Service Rocky Mountain Station, Fort Collins, Colorado.
  McKenna, M.A., N.V. Summers and M. Bond. 1994. Effects of pH on pollen germination, reproduction and growth in alpine plants. In: Stephenson, A.G. and T.H. Kao (eds.), Pollen-Pistil Interactions and Pollen Tube Growth. American Society of Plant Physiologists, Rockville, MD.
  McKenna, M.A. 1992. Pollen competition and heterostyly. pp. 225-246 in S. Barrett (ed) Evolution and Function of Heterostyly, Monographs in Theoretical and Applied Genetics. Springer-Verlag, Zurich.
  Thomson, J.D., M.A. McKenna, and M. Cruzan. 1989. Temporal patterns of nectar and pollen production in Aralia hispida: Implications for reproductive success. Ecology 70:1061-1068.
  McKenna, M.A. and J.D. Thomson. 1988. A technique for sampling and measuring sugar content of small amounts of floral nectar. Ecology 69: 1306-1307.

McKenna, M.A. 1986.  Heterostyly and microgametophytic selection: The effect of pollen competition on sporophytic vigor in two distylous species. pp. 443-448 in D.L. Mulcahy, G.B. Mulcahy and E. Ottaviano eds. Biotechnology and Ecology of Pollen, Springer-Verlag, New York.