Two Methods of Quantifying Plant Cover for Evaluating Species for Extensive Green Roof Culture

This research examined plant area covered for six plant species on an existing modular extensive green roof in semi-arid Colorado. Species evaluated were Antennaria parvifolia Nutt., Bouteloua gracilis (Kunth) Lag., Delosperma cooperi (Hook. f.) L. Bol., Eriogonum umbellatum Torr. aureum 'Psdowns', Opuntia fragilis Nutt. and Sedum lanceolatum Torr. Most reported methods for measuring plant area covered (plant cover) are subjective and not based on quantitative measurements. This study compared digital image analysis data (DIA) to manually collected converted two-dimensional data (C2D) for plants grown on an extensive green roof. For each plant in the study, digital images and manual two-dimensional measurements were taken on four dates (at six week intervals) in 2008 and on four dates (at six week intervals) in 2009. Using SigmaScan Pro 5.0 image analysis software, DIA was performed on these images. Additionally, comparisons between DIA data and final biomass, and C2D and final biomass, were performed. Plant cover increased for all six species during the 2008 growing season. However, E. umbellatum aureum ‘Psdowns’ had a low overwintering rate (12.5%) and was removed from analysis in 2009. In the spring of 2009, four of the five remaining species exhibited decreased plant cover due to winter dieback; the one exception was O. fragilis. In terms of plant cover, both quantification methods (C2D and DIA) revealved that B. gracilis and D. cooperi out performed A. parvifolia, O. fragilis and S. lanceolatum. Thus, five of the six species evaluated in this study are appropriate for use in extensive green roof applications. High levels of correlation were found between the DIA and C2D data sets (r = 0.77) averaged over the five species on all eight data collection dates. The groundcover species (A. parvifolia, D. cooperi and S. lanceolatum) had a higher correlation on average (r = 0.83) than the upright (B. gracilis, r = 0.70) and decumbent (O. fragilis, r = 0.65) species. Additionally, DIA and final biomass correlations showed parallel trends with groundcovers averaging r = 0.83, upright r = 0.64 and decumbent r = 0.41. Therefore, using DIA to evaluate plant cover and biomass accumulation is especially appropriate for groundcover species.