The Effects of Experimental Canopy Thinning on the Population Viability of a Rare Wetland Understory Herb
Todd Bittner, Cornell Botanic Gardens
Improving our understanding of how abiotic factors affect rare wetland plant species is necessary to inform conservation and management efforts. The decline of some rare fen perennials is believed to be related to the transition from open to closed canopy wetlands. To assess the role that light levels has on the rare perennial herb, Trollius laxus (American globeflower), we undertook an experimental study to measure the effect of canopy thinning on population numbers at one of the Cornell Botanic Gardens’ natural areas in upstate New York. Light levels (lum/ft2) were collected using HOBO data loggers from 2018 through 2020 during portions of the growing season. In 2019 we removed 45 adjacent Tsuga canadensis (eastern hemlock) trees, totaling 14.1 ft2 basal area. Results show that canopy thinning significantly increased light levels across the wetland from 13.9% available light pre-clearing to 21.8% available two years post-clearing. Data on the numbers of individual sterile and flowering T. laxus remain preliminary, but have declined 55% over the study period. To better assess individual responses to the clearing, we intend to shift to a demographic monitoring effort starting in 2021.