Abstracts

Presented by Dan J. Smith - Email: djssmith@ucdavis.edu

In 1986 the North American Waterfowl Management Plan called for the protection and enhancement of habitat for wintering waterfowl, specifically citing the need to increase the waterfowl carrying capacity of these habitats. In response, numerous studies across North America have assessed the abundance of food resources, typically seeds and tubers, and the management practices aimed at producing these essential food items. Many of these studies sampled foraging habitats using soil cores which can be effectively employed in a variety of wetlands. Previous studies have addressed potential sources of error associated with this sampling method, but surprisingly, no study has examined the potential variation in sample volume of soil cores, often assuming the volume to be constant. To determine if soil core volume varied and thereby impacted estimates of seed abundance, we collected soil cores from managed (N = 494) and tidal wetlands (N = 96) in the Suisun Marsh California in fall and winter 2017-18. We measured sample volume via displacement; to control for water content in the soil cores, each sample was pressed with a 5.5lb weight in a 250 m sieve until no water passed through the sieve. Although soil cores were taken with a diameter of 6cm and depth of 5cm, we found that soil core volume varied dramatically (55 to 558 ml). We regressed soil core volume against seed abundance and found relationships. We then assessed regression and density-based correction factors to standardize soil core volumes. Applying these correction factors had a large impact on seed estimates (typically lowering the values) and we discuss the merits and pitfalls of such corrections. We urge researchers to consider carefully the potential variation of soil core volumes, and to be explicit in their assumptions when extrapolating from soil cores to estimates of food availability (lbs/acre) at the landscape scale.