Initial benthic macroinvertebrate community response during the third experimental stand density reduction in the Caspar Creek Experimental Watersheds, 2016-2021 (draft)

The Caspar Creek Experimental Watersheds (a collaboration between the USDA Forest Service and the California Department of Forestry and Fire Protection, CAL FIRE) in Mendocino County, CA, have been the subject of a wide variety of timber harvest monitoring studies since they were established in 1961. These studies have largely been associated with three experimental harvests: the First Experiment removed approximately two thirds of the timber in the South Fork watershed in the early 1970s using selection harvest, the Second Experiment removed about 50% of the timber in the North Fork watershed between 1985 and 1992 (Dymond et al. 2021) using clearcut harvest. The Third Experiment included a mixed harvest intensities of the South Fork watershed between 2017 and 2019; timber was harvested at different rates in each of nine different sub-reaches and followed current California Forest Practice Rules (Dymond et al. 2021). Numerous ancillary studies were conducted within the framework of the Third Experiment.

This report describes the results of a bioassessment study that was conducted at nine locations within the South Fork watershed between 2016 and 2021. Aquatic bioassessments use information about the resident biological assemblages of a waterbody to assess its ecological condition. Over the last two decades, California’s natural resource agencies are increasingly prioritizing ecological performance measures like bioassessment indices because they provide direct evidence of the status and trends of key ecological resources. Accordingly, they are a key component of effective resource management strategies and provide an objective framework for supporting regulatory policies and actions.

To assess the ecological integrity of streams, California’s natural resource agencies use standardized bioassessment protocols (Ode et al. 2016a), to measure biological, physical, and chemical conditions at stream reaches. These procedures were developed by the California Department of Fish and Wildlife’s Aquatic Bioassessment Laboratory (CDFW-ABL) and the State Water Quality Control Board’s Surface Water Ambient Monitoring Program (SWAMP), based on protocols developed for the U.S. EPA’s National Aquatic Resource Surveys (US EPA 2017) and have been the primary method for evaluating the general ecological condition of streams in California since the early 2000s (Ode et al. 2011).

The effects of timber harvest activities on resident benthic macroinvertebrate assemblages have been the subject of interest in forestry management for many decades (Hawkins 1982, Stone and Wallace 1998, see also summaries in Herlihy et al. 2005, Gravelle et al. 2009, and Gerth et al. 2022). Accordingly, investigators have incorporated bioassessment surveys into the Caspar Creek studies since at least the Second Experiment. Bottorff and Knight described benthic invertebrate responses to the Second Experiment in the North Fork (Bottorff and Knight 1996), Cummins and Malkauskas (2008) described benthic invertebrate community structure in samples collected at five locations distributed throughout the Caspar Creek watershed in 2008. Danehy and Arismendi (2018) reanalyzed the previous work along with the first two years (2016 and 2017) of data from this survey.

In 2015, CDFW’s Aquatic Bioassessment Lab (ABL) collaborated with CAL FIRE and the USDA Forest Service/Jackson State Forest Caspar Creek Research Team to develop a study that would evaluate the response of benthic macroinvertebrate communities in the South Fork Caspar Creek stream network to stand density reductions made during the Third Experiment. The project had three initial objectives:

1. Establish current conditions of biological integrity for streams draining the South Fork Caspar Creek watershed.
2. Evaluate the variability of bioassessment metrics in these streams before, during and after the experimental harvest period, and potential associations with harvest intensity.
3. Develop recommendations for future bioassessment monitoring in the Caspar Creek Experimental Watersheds.

This report is intended to provide an overview of the initial results, data to support additional analyses, and suggestions for improving the efficiency and effectiveness of future bioassessments in the Caspar Creek system.