Placement of felled western juniper trees over a stream channel : Effects on water temperature, streamside willow shrubs, and redband trout Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/6395w981k

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  • All streams in Oregon that are inhabited by salmon and trout have a statemandated water temperature standard. However, temperatures of many streams, especially during summer months, exceed the seven-day average maximum temperature parameter (200 C for redband trout inhabited streams) accepted by the Oregon Department of Environmental Quality. To date, management of stream temperature has focused primarily on increasing shade by planting riparian shrubs and preventing livestock access to the riparian zone. In remote locations, construction and maintenance of riparian fencing is often physically and economically infeasible. To address this problem, we tested the feasibility of using western juniper, an invasive native tree in semi-arid ecosystems, as a resource for creating shade and reducing summer stream temperatures. Our study tested whether or not the placement of felled western juniper trees over a 2nd order stream channel in southeastern Oregon would: (1) reduce maximum summer stream temperatures, (2) exclude large herbivores from streamside willow shrubs, and (3) affect distribution and movement of native redband trout. Four 152 m felled western juniper treatments of covered and open were applied to a 1.2 km length of stream. Willows (<2 m tall) within treatment areas were censused, tagged, examined for evidence of browse, and measured for maximum height and maximum width during 2002, prior to treatment. Measurements were repeated August 2003 and October 2003 after treatment. Hourly air and stream water temperature and biweekly stream discharge data from July through August were collected. Data were collected before and after juniper treatments had been applied, during 2002 and 2003 respectively. Movement and distribution of redband trout prior to treatment (n = 100) and following treatment (n = 42) were tracked by swim-through passive integrated transponder antennae. Juniper buffered maximum stream water temperatures by (1) decreasing the amount of stream heating, (2) decreasing the number of hours the stream exceeded 20° C by an average of 1 hr a day, and (3) increasing the separation between maximum daily water temperature and air temperature by over 3° C. Willows treated with juniper cover demonstrated a 3-fold greater increase in height and 2-fold greater increase in maximum width compared to non-treated willows. By October, the year after treatment, unprotected willows had a 2 times greater likelihood of being browsed than juniper covered shrubs. Additionally, movement and distribution of redband trout throughout the study reach were affected by juniper treatment. Increases in channel cover and buffered maximum summer stream temperatures were associated with decreased dependence of trout on headwaters and increased use of downstream areas as well as increased hours of daily trout activity. Percent time spent by trout in covered versus open treatments was not different (p <0.05). The application of juniper cover of the stream channel yielded (1) improved water quality, (2) decreased herbivory of streamside willow shrubs, and (3) increased use of and activity within the study reach by redband trout. However, the results of this case study are directly linked to site-specific factors. Managers should proceed with caution before implementing juniper treatment on other streams.
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