Oral Paper

         Paleobotany

The CuticleTrace workflow: An ImageJ toolset for capturing cell outlines of leaf cuticle — implications for paleoecology and paleoclimatology

Presenting Author
Ben Lloyd
Description
Fossilized leaf cuticle - the waxy surface of leaves common to geologic sediments from the last 400 million years - remains an underutilized data source for reconstructing paleoclimate and paleovegetation assemblages. The morphology of a leaf’s upper epidermal pavement cells reflects the environmental conditions of its growth. Traditionally, cell morphology was determined by hand-tracing cell walls using digital tools. This is a slow and tedious process, with cell-outline quality highly dependent upon both the tracing setup used and the experience level and attention to detail of the tracer. To avoid the issues associated with hand-tracing cell outlines, we developed the CuticleTrace workflow: a user-friendly automated image analysis workflow using the freeware programs ‘ImageJ’ and ‘R’. Our easily tunable code allows for the generation of large cell-shape datasets from images of leaf epidermis, with options for different measurement parameters and levels of automation. Use of this method results in a higher degree of cell-tracing consistency, while allowing for increased sample sizes by drastically decreasing the time required to generate these data.  We applied this method to an image set of 130 vouchered herbarium specimens extracted from The Cuticle Database (cuticledb.eesi.psu.edu), representing 127 species in 87 genera from 36 families. Images were chosen to maximize taxonomic breadth and span the range of cell shapes and sizes, utilizing the specimens with the highest quality image of the upper cuticle. For each image, our workflow produced a set of vectorized cell outlines, with associated shape parameters. 50 of these images were additionally traced by an expert in the field and by undergraduate researchers. Across this subset, CuticleTrace measurements are statistically similar to expert measurements >85% of the time (average p-value of 0.63), while student measurements are only statistically similar to expert measurements ~66% of the time (average p-value of 0.43).This workflow can now be used to reconstruct parameters such as Leaf Area Index, and thus the canopy structure of forests in deep time, more effectively utilizing the vast repository of leaf cuticle in the fossil record.