Graduate Thesis Or Dissertation
 

Stream Temperature and Autotrophic Responses to Riparian Canopy Gaps Over Forested Headwater Streams

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/x920g321j

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  • Across much of North America, legacies of historic and contemporary timber harvest have created a landscape dominated by regenerating forest stands in the early to middle stages of development. Most streamside forests are currently in the stem-exclusion phase of stand development and these closed canopies shade the forest understory and reduce light flux to streams. However, this highly shaded environment in second-growth forest streams contrasts with those in old-growth forests, which contain spatially dynamic and temporally complex light conditions created by canopy gaps. Gaps in the riparian canopy that increase local light availability can enhance primary production in streams, which can propagate up the food web leading to increased fish abundances. Although increasing light availability to streams can enhance bottom-up drivers of invertebrate, fish and salamander production in streams, this can also increase stream temperature, which can be detrimental if temperatures increase above species specific thresholds. Due to this connection between increased light and potential detrimental effects of temperature, many current riparian forest management regulations focus on maintaining shade and hence cool temperatures. Therefore, knowing whether and to what degree opening riparian canopies through gap creation (via natural or anthropogenic processes that increase light exposure to forested streams) leads to changes in temperature has important ecological and management implications. To determine the impacts of riparian canopy gaps and subsequent localized increases in light on stream temperature and on stream biofilms, we created experimental gaps in second-growth riparian forest canopies. Using a Before-After-Control-Impact design, we analyzed the following stream summer temperature metrics: the maximum seven day moving average maximum, the maximum seven day moving average mean, daily maximum and daily mean summer temperature responses. We also quantified changes in light and reach scale chlorophyll a at finer spatial scales following the implementation of riparian canopy gaps in six heavily shaded headwater streams with second-growth stands regenerating from forest harvest. We observed small but consistent increases in temperature due to the gap treatment and also increases in chlorophyll a. In addition to considerations regarding forest management, understanding temperature and aquatic ecosystem responses to riparian canopy gaps is critical to predicting stream responses to natural disturbances and stand development processes in forests recovering from past use.
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  • 2019-06-21 to 2021-07-21

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