Sherri Johnson - Academia.edu (original) (raw)

Papers by Sherri Johnson

Science of The Total Environment

SIL Proceedings, 1922-2010

Environmental science & technology, Jan 7, 2018

Land-use activities can alter hydrological and biogeochemical processes that can affect the fate,... more Land-use activities can alter hydrological and biogeochemical processes that can affect the fate, transformation, and transport of mercury (Hg). Previous studies in boreal forests have shown that forestry operations can have profound but variable effects on Hg export and methylmercury (MeHg) formation. The Pacific Northwest is an important timber producing region that receives large atmospheric Hg loads, but the impact of forest harvesting on Hg mobilization has not been directly studied and was the focus of our investigation. Stream discharge was measured continuously, and Hg and MeHg concentrations were measured monthly for 1.5 years following logging in three paired harvested and unharvested (control) catchments. There was no significant difference in particulate-bound Hg concentrations or loads in the harvested and unharvested catchments which may have resulted from forestry practices aimed at minimizing erosion. However, the harvested catchments had significantly higher dischar...

Ecology, Jan 7, 2017

Studies of trophic-level material and energy transfers are central to ecology. The use of isotopi... more Studies of trophic-level material and energy transfers are central to ecology. The use of isotopic tracers has now made it possible to measure trophic transfer efficiencies of important nutrients and to better understand how these materials move through food webs. We analyzed data from thirteen (15) N-ammonium tracer addition experiments to quantify N transfer from basal resources to animals in headwater streams with varying physical, chemical, and biological features. N transfer efficiencies from primary uptake compartments (PUCs; heterotrophic microorganisms and primary producers) to primary consumers was lower (mean: 11.5%, range: <1%-43%) than N transfer efficiencies from primary consumers to predators (mean: 80%, range: 5%- >100%). Total N transferred (as a rate) was greater in streams with open compared to closed canopies and overall N transfer efficiency generally followed a similar pattern, although was not statistically significant. We used principal component analysi...

Global change biology, Aug 31, 2016

Streams and rivers are important conduits of terrestrially derived carbon (C) to atmospheric and ... more Streams and rivers are important conduits of terrestrially derived carbon (C) to atmospheric and marine reservoirs. Leaf litter breakdown rates are expected to increase as water temperatures rise in response to climate change. The magnitude of increase in breakdown rates is uncertain, given differences in litter quality and microbial and detritivore community responses to temperature, factors that can influence the apparent temperature sensitivity of breakdown and the relative proportion of C lost to the atmosphere vs. stored or transported downstream. Here, we synthesized 1025 records of litter breakdown in streams and rivers to quantify its temperature sensitivity, as measured by the activation energy (Ea , in eV). Temperature sensitivity of litter breakdown varied among twelve plant genera for which Ea could be calculated. Higher values of Ea were correlated with lower-quality litter, but these correlations were influenced by a single, N-fixing genus (Alnus). Ea values converged ...

Trans Amer Fish Soc, 1993

Journal of the North American Benthological Society, Mar 29, 2011

Science advances, 2016

Climate change is predicted to cause widespread declines in biodiversity, but these predictions a... more Climate change is predicted to cause widespread declines in biodiversity, but these predictions are derived from coarse-resolution climate models applied at global scales. Such models lack the capacity to incorporate microclimate variability, which is critical to biodiversity microrefugia. In forested montane regions, microclimate is thought to be influenced by combined effects of elevation, microtopography, and vegetation, but their relative effects at fine spatial scales are poorly known. We used boosted regression trees to model the spatial distribution of fine-scale, under-canopy air temperatures in mountainous terrain. Spatial models predicted observed independent test data well (r = 0.87). As expected, elevation strongly predicted temperatures, but vegetation and microtopography also exerted critical effects. Old-growth vegetation characteristics, measured using LiDAR (light detection and ranging), appeared to have an insulating effect; maximum spring monthly temperatures decr...

Usda Forest Service General Technical Report Pnw, 2013

Hydrological Processes, Nov 1, 2000

Forest Science

To evaluate long-term effects of forest harvest, we sampled benthic and emergent macroinvertebrat... more To evaluate long-term effects of forest harvest, we sampled benthic and emergent macroinvertebrate communities in headwater streams through young-growth and old-growth forests in the HJ Andrews Experimental Forest, Oregon, from June, 2003 to June, 2004. Thirty to forty years after ...

PloS one, 2015

Land use and climate change occur simultaneously around the globe. Fully understanding their sepa... more Land use and climate change occur simultaneously around the globe. Fully understanding their separate and combined effects requires a mechanistic understanding at the local scale where their effects are ultimately realized. Here we applied an individual-based model of fish population dynamics to evaluate the role of local stream variability in modifying responses of Coastal Cutthroat Trout (Oncorhynchus clarkii clarkii) to scenarios simulating identical changes in temperature and stream flows linked to forest harvest, climate change, and their combined effects over six decades. We parameterized the model for four neighboring streams located in a forested headwater catchment in northwestern Oregon, USA with multi-year, daily measurements of stream temperature, flow, and turbidity (2007-2011), and field measurements of both instream habitat structure and three years of annual trout population estimates. Model simulations revealed that variability in habitat conditions among streams (d...

In small streams, riparian vegetation often composes the food base for many aquatic invertebrates... more In small streams, riparian vegetation often composes the food base for many aquatic invertebrates. Forest harvest can result in major changes to riparian vegetation. If abundance, richness, or community structure of invertebrate assemblages is affected by removal of riparian vegetation, are legacies of these practices evident 20-40 years post-harvest? If so, are these differences uniform through time, or are they temporally dependant? We investigated stream invertebrate assemblage dynamics between old growth and second growth forest types and across seasons in six small-basins in the H.J. Andrews Experimental Forest. Individual basins range from 500m to 1000m in elevation and 12.4ha to 98.1ha in area. Six benthic samples and four, 1-week emergence samples were collected in each basin each season (June 2003 through May 2004). Preliminary results from benthic samples suggest strong changes in community structure between seasons as additional taxa not found in summer (i.e. Agathon, Ana...

Hydrology and Earth System Sciences, 2015

As part of its plan to revise the Northwest Forest Management Plan, the Oregon Department of Fore... more As part of its plan to revise the Northwest Forest Management Plan, the Oregon Department of Forestry (ODF) assembled a Stakeholder Group which created 5 management alternatives: 70/30 Approach, Timber Harvest Optimization, Restoration & Recreation, FMP 2.1, and Land Sale; all of which are described in this report. ODF then sought the assistance of a panel of experienced scientists who were asked to advise the Department on the merits and drawbacks of each alternative. The Science Panel was comprised of one expert each in the Social Sciences and Economics domains, and four experts in the Ecological and Forest Management domains. Working alone, each panelist spent time reading, researching and writing. They then gathered for five intensive days of discussion of each panelist’s writing, critiquing and collaborating to develop refined analyses and collaborative syntheses in the following topic areas: vegetation and plant communities, terrestrial wildlife, aquatic ecosystems, terrestria...

Recent warming of terrestrial climates in most parts of the world has motivated concern about cor... more Recent warming of terrestrial climates in most parts of the world has motivated concern about corresponding increases in water temperatures. The link between terrestrial climate and water temperature is complicated in part by uncertainties regarding the roles of climate change versus changes in other factors, such as changes in land or water use and land cover. To evaluate evidence for potential climate impacts in locations minimally impacted by these factors, we analyzed long-term trends in stream temperatures in 18 least-disturbed watersheds across western North America. Based on hypothesized influences of observed trends in air temperatures and stream flows, we predicted climate impacts to be manifested as increasing magnitudes (warming) and variability of stream temperatures over time. Although we found warming trends for some sites during 1951-2009 there was an opposite cooling trend in more recent time series (1987-2009). Lack of coherence in temperature trends that we evaluat...

Canadian Journal of Fisheries and Aquatic Sciences, 2015

The importance of multiple processes and instream factors to aquatic biota has been explored exte... more The importance of multiple processes and instream factors to aquatic biota has been explored extensively, but questions remain about how local spatiotemporal variability of aquatic biota is tied to environmental regimes and the geophysical template of streams. We used an individual-based trout model to explore the relative role of the geophysical template versus environmental regimes on biomass of trout (Oncorhynchus clarkii clarkii). We parameterized the model with observed data from each of the four headwater streams (their local geophysical template and environmental regime) and then ran 12 simulations where we replaced environmental regimes (stream temperature, flow, turbidity) of a given stream with values from each neighboring stream while keeping the geophysical template fixed. We also performed single-parameter sensitivity analyses on the model results from each of the four streams. Although our modeled findings show that trout biomass is most responsive to changes in the ge...