A Green Ash by Any Other Name Would be White Ash
Carolynne Wang, Thomas Whitlow and Wayne Zipperer
What began as a case of suspected identity theft at the community level led to a study of mistaken species identity. At the BES Spring Science meeting, 2000, Zipperer reported that frequency of green ash increased in riparian woodlands along the rural-urban gradient in the Gwynns Falls watershed. This shift in the floristic signature of the forest community led to speculation that street tree progeny (green ash clones are common as street trees) were invading native stands. We set out to test this hypothesis using microsatellite markers to distinguish between domesticated and native genotypes.
During reconnaissance of sample stands, it became clear that white ash co-occurred with green and that we needed a simple method to separate the species before we collected samples for DNA analysis. The most reliable character difference between green and white ash is the presence of papillae on the abaxial surface of white ash leaves. The accepted method for detecting papillae is to prepare an epidermal peel and examine it under a compound microscope, which is arguably a clumsy method for field use. However, we were aware that papillose surfaces are hydrophopic, leading us to develop a simple field method using inexpensive, readily available technology that is easily grasped by non-taxonomists. We will conduct a live demonstration of the technique.
In addition to this identification aid, we developed a quantitative method for characterizing leaf surface topography. By applying water droplets of varying volume to leaves, we can portray leaf surface behavior in 3-dimensions along axes of drop volume, contact angle and critical angle of inclination. When leaves from many species are presented together, the resulting response surface locates species (and even opposing sides of the same leaf) in discrete zones in a functional space that is tempting to view as an adaptive landscape (sensu Wright.). Viewing leaf surfaces from this perspective is likely to increase our understanding of the kinetics of plant-aerosol interactions. The potential relevance to biogeochemistry and biocomplexity has not escaped our notice.
Regarding street tree invasions, we are still examining the DNA evidence and the jury is still out.
Tom Whitlow: Participant, NSF Research Experience for Undergraduates, Dept. of Agricultural and Biological and Engineering, Cornell University
Carolynne Wang: Associate Professor, Plant Physiological Ecology, Urban Horticulture Institute, Cornell University
Wayne Zipperer: Senior Scientist, NE Forest Experiment Station, SUNY ESF, Syracuse.
green ash, white ash