Graduation Date
Fall 2024
Document Type
Thesis
Program
Master of Science degree with a major in Natural Resources, option Forestry, Watershed, & Wildland Sciences
Committee Chair Name
Andrew Stubblefield
Committee Chair Affiliation
HSU Faculty or Staff
Second Committee Member Name
David Roon
Second Committee Member Affiliation
Community Member or Outside Professional
Third Committee Member Name
John-Pascal Berrill
Third Committee Member Affiliation
HSU Faculty or Staff
Keywords
Watershed, Forestry, water temperature, riparian, thinning, riparian shade, forest recovery, Northern California
Subject Categories
Watershed Management
Abstract
Historical forest harvest methods such as clear-cut harvesting in the Pacific Northwest have led to dense, even-aged stands, particularly in riparian forests. In an effort to promote the recovery of late-successional conditions, land managers have turned to silvicultural approaches such as thinning, though few studies have evaluated its long-term effects in riparian areas. Due to stream-riparian linkages, changes in riparian forests can directly influence stream ecosystems, particularly thermal regimes, by altering shade and solar radiation influx. Thus, understanding the temporal and spatial impacts of riparian thinning is crucial for land managers interested in thinning as a potential restoration tool for young, second-growth riparian forests.
In this study I applied a before-after-control-impact (BACI) design to assess changes in tree growth, shade, light, and stream temperature over seven years after an initial thinning treatment of 50% canopy reduction along a Class I channel across 8 locations in the Tectah Creek watersheds in northern coastal California. Tree growth data showed significant changes in thinned reaches over this time period: redwoods and Douglas-fir increased in size and tree density relative to pre-treatment stand conditions whereas red alder decreased in size and density. Riparian canopy closure declined from ~96% in 2016 to 76% immediately after thinning in 2017, then recovered back to ~90% in 2023. Below-canopy light increased from ~5% in 2016 to 39.4% immediately after thinning treatments in 2017 and decreased to 21.1% by 2023 as canopy recovered. Stream temperature responses reflected the post-thinning temporal patterns of shade and light suggesting partial recovery over time, but responses varied between thermal regime metrics. BACI analyses revealed that the most pronounced changes were in maximum weekly average temperatures (MWMT, magnitude), daily range (variability), and days > 16°C and (duration), with the largest differences in 2018(+2.7 °C, +2.4 °C, +42.9 days respectively). However, by 2023, I observed partial evidence of recovery and these differences had decreased in magnitude over time (+1.2 °C, +0.9 °C, +12.3 days respectively). Furthermore, longitudinal profiles showed evidence of temperature recovery further downstream at most sites, with no further downstream propagation of elevated temperatures in four of eight sites. Collectively, my results suggest that thinning effects of riparian forest and stream thermal conditions can persist for over 6 years, but recovery is underway. As a result, more monitoring is needed to fully capture the temporal persistence and recovery of thinning effects. In the meantime, more time is likely needed before further thinning is advised in Tectah Creek's riparian forests to minimize effects on stream temperatures.
Citation Style
APA
Recommended Citation
Selby, Preston J., "RECOVERY OF RIPARIAN SHADE AND STREAM TEMPERATURE AFTER THINNING IN COASTAL REDWOOD RIPARIN FORESTS" (2024). Cal Poly Humboldt theses and projects. 794.
https://digitalcommons.humboldt.edu/etd/794