Erik Barry Erhardt, PhD

Laurel Goode, Erik Erhardt, Louis Santiago, and Michael Allen.
δ13C of soluble sugars in Tillandsia epiphytes vary in response to shifts in habitat.
Oecologia, Physiological ecology section, 2010.

I met Laurel at SIRFER 2008 where we enjoyed a wide range of stable isotope lectures and lab experience. She first used my software, SISUS, to estimate the proportion of C3 vs CAM photosynthesis of epiphytes. Our work and friendship led to the collaboration where we thought about and developed a model for the environmental factors affecting the phothsynthetic pathways of the species studied.

Abstract

We studied carbon stable isotopic composition (δ13C) of bulk leaf tissue and extracted sugars of four epiphytic Tillandsia species to investigate flexibility in the use of crassulacean acid metabolism (CAM) and C3 photosynthetic pathways. Plants growing in two seasonally-dry tropical forest reserves in Mexico that differ in annual precipitation were measured during wet and dry seasons, and among secondary, mature, and wetland forest types within each site. Dry season sugars were more enriched in 13C than wet season sugars, but there was no seasonal difference in bulk tissues. Bulk tissue δ13C differed by species and by forest type, with values from open-canopied wetlands more enriched in 13C than mature or secondary forest types. The shifts within forest habitat were related to temporal and spatial changes in vapour pressure deficits (VPD). Modeling results estimate a possible 4% increase in the proportional contribution of the C3 pathway during the wet season, emphasizing that any seasonal or habitat-mediated variation in photosynthetic pathway appears to be quite moderate and within the range of isotopic effects caused by variation in stomatal conductance during assimilation through the C3 pathway and environmental variation in VPD. Carbon isotopic analysis of sugars together with bulk leaf tissue offer a useful approach for incorporating short- and long-term measurements of carbon isotope discrimination during photosynthesis.

Statistics, stable isotopes

Christopher P. Bickford, Nate G. Mcdowell, Erik Barry Erhardt, Heath H. Powers, David T. Hanson. (2009)
“High frequency field measurements of diurnal carbon isotope discrimination and internal conductance in a semi-arid species, Juniperus monosperma“.
Plant, Cell & Environment, In print (1/28/09).

Chris Bickford, PhD candidate UNM Biology, and I met when we attended Iso-Camp at Jim Ehleringer’s lab at U Utah Summer 2008.  On the flight home we started discussing a challenge he was facing in his first of three dissertation papers. He studies details of plant photosynthesis.  He had complicated expressions for leaf carbon isotope discrimination and internal conductance based on CO concentrations of CO isotopologues and . He needed to propigate the variation of the CO measurements into his variables of interest, and .  He also needed to compare his accurate and precise measurements using tunable diode laser spectroscopy (TDL) to predictions from three models.

There were a number of statistical issues.  One was how to make model and observation comparisons.  I suggested using RMSE since it includes both variance and bias in the single measurement.  The main issue was the incorporation of variation from the CO measurements into the quantities of interest.  The bootstrap allowed us to do this.  There were a number of programming sessions in R to write functions and scripts to do all the calculations, create plots, output spreadsheets of results, and so on.  Chris has become a convert from Excel to R over the course of this project.  These methods implemented on this paper will likely flow into later pubs for both Chris and Dave.

Chris has taken a postdoc in New Zealand, where he and his wife, Karen, will spend the next two years with their dog.  He defends his dissertation on April 13th.

stable isotopes