Assessing effects of water quality and aquatic ecosystem responses across gradients of forest stand age

Forest successional stage can affect aquatic ecosystems through bottom-up food web processes by altering light, organic matter, and nutrient inputs, but it can also affect physical habitat via recruitment of large woody debris or altered flow and thermal regimes. Most importantly, stand age can inform us about time since last disturbance and forest condition within a catchment. In Oregon, approximately 30% of federal lands have an area-weighted stand age > 120 years old while on private lands ≤ 5% carry these same characteristics. Accordingly, landscape composition differs substantially from historic conditions when stands >160 years old dominated catchments, and the consequences of these changes on aquatic species and their habitat are not well understood. Consequently, investigating catchments that vary by ownership patterns can inform relationships between stand age and freshwater biodiversity across watersheds with complex site history. We sampled 24 watersheds across a gradient of stand age in coastal Oregon in 2019 and 2020 to promote a unified understanding of the consequences of intensive stand management for water quality, habitat, ecosystem function, and biological diversity. Preliminary analyses indicate mean watershed stand age was not strongly correlated with abiotic or biotic variables in 2019. Background stream chemistry across all 24 study sites indicate few trends in nutrient (as inorganic and total nitrogen and phosphorus) or dissolved organic carbon concentrations with mean watershed stand age. Similarly, periphyton biomass (as chlorophyll a and ash free dry mass) did not differ across this gradient. Ongoing monitoring across these watersheds will provide additional information on macroinvertebrate, fish and amphibian assemblages and will aid in our understanding of the role of legacy and ongoing forest management effects on aquatic ecosystems.