Graduate Thesis Or Dissertation
 

Fall directions and breakage of riparian trees along streams in the Pacific Northwest

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/rj430664g

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  • In the Pacific Northwest, regulatory agencies have recently implemented management strategies for restoration and maintenance of wood recruitment to streams over time. This allochthonous organic material is a critical component in the geomorphic and biological structure of forested streams. Mathematical models are commonly used to evaluate long-term wood dynamics in stream ecosystems. Accuracy and applicability of these models in a wide range of forest, geomorphic and regional conditions are limited by current understanding of tree fall patterns. I quantified fall directions and breakage of riparian trees at 21 second- to fourth-order stream sites in the Pacific Northwest, USA. These patterns influence rates, amounts and spatial distribution of wood delivered to streams. Riparian tree fall directions strongly differed by species, rooting position away from the stream and side slope steepness adjacent to the channel. Though average tree fall directions oriented towards stream regardiess species, rooting position, and side slope, variance of fall directions significantly declined between valley bottoms and hillslopes and between gentle and steep side slopes. Standard deviation of fall directions, as calculated from circular statistical techniques, was two times greater on gentle slopes (0-10% steepness) than on extremely steep slopes (> 90% steepness) (± 80° versus ± 40°). A similar pattern occurred based on rooting position away from the stream (valley bottom versus hillslopes). Valley constraint, forest structure, forest age and region (west of the Cascades Mountains crest versus the Interior Columbia Basin) were not associated with variation in fall directions (p>0.10 in all cases). Several sites with relatively thin trees (height/dbh > 50 rn/rn) had overall upstream fall directions. Over half (58%) of the riparian trees measured in this study did not break when they fell. Of riparian trees that were broken, 62% of breaks could not be attributed to physical features of the stream or riparian zone. Percent of trees with breaks slightly increased with forest age (p=0.08). More significantly, percent of trees with fall breaks increased with tree height (p<0.0001). Models indicated slight differences among species except for western larch (Larix occidental/s Nutt.), which had a significantly higher rate of breakage than other species. Trees that broke when they fell typically produced 2 to 3 pieces (geometric mean of 2.7 pieces with 42% coefficient of variation) and the first break typically occurred at half height (average of 0.50 with ± 0.24 standard deviation; proportion of height from base to top). Effects of species type were insignificant on pieces produced or location of first break. These results have several important implications for riparian zone management of forested streams in the Pacific Northwest. Topographic influences on tree growth patterns and rooting conditions could affect stream wood loading rates and patterns where riparian forest dynamics dominate input pathways. Stream sections with side slopes > 90% on both banks potentially receive 1 .5 times more falling trees (by numbers) than sections with slopes 0-10% and 2.5 times more than what is expected from a random fall pattern. Breakage by falling trees on impact needs to be considered in simulations of wood recruitment from riparian forests. Large channel-spanning wood pieces that have potential to form pools and jams also have the highest probability to break when initially falling into the channel. A simple model of wood recruitment based on field data and riparian forest dynamics of an oldgrowth forest in the West Cascades of Oregon indicated that fall breakage might reduce numbers of channel spanning wood pieces entering a 10-m wide stream by 7 to 79% over a 100-year time period. At the same time, breakage may amplify absolute number of wood pieces entering a stream channel by 1.1 to 1 .8 times. Overall, results presented in this thesis may improve accuracy and calibration of stream wood recruitment models for use in riparian management in the Pacific Northwest.
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