UBC Theses and Dissertations

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UBC Theses and Dissertations

Cottonwood ecology and restoration in the context of river diking and channelization in the semi-arid Okanagan-Similkameen Region of B.C. Moran, Kasey


Threats to riparian forests caused by floodplain disconnection are a global issue, but effective restoration plans require species-specific knowledge and local context concerning current conditions and accompanying causes of degradation. This thesis supports the recovery of black cottonwood ecosystems in Syilx Okanagan territory, Canada, by documenting current conditions, investigating mechanisms driving degradation, and evaluating restoration activities. We began with an observational study of current forest structure in relation to diking and channelization, two river training methods suspected of driving poor recruitment. We found that floodplain connectivity, disturbance regime, and dike maintenance policies explained recruitment density better than engineering categories did. Next, we asked whether dikes exacerbate water stress in cottonwoods by blocking subsurface connectivity between the river and alluvial aquifer. We compared canopy senescence and branch dieback in trees growing on the landward side of dikes to trees growing on the riverward side and along un-diked reaches, and found no evidence that dikes increase water stress in cottonwoods. However, we found that trees on the riverward side of dikes tended to be taller than elsewhere, hinting at a recruitment issue in that setting. Finally, we monitored four cottonwood planting projects, used those data to develop geographically appropriate and species-specific guidance for restoration practitioners, and assisted in the adaptive management of a floodplain reconnection project. We discovered that the conditions associated with natural cottonwood recruitment described in our first data chapter were not necessarily conducive to the survival of plantings, especially with respect to planting elevation relative to the water table, canopy cover, and flood disturbance. We also found that herbivore protection greatly enhanced survival, and we suggest that irrigation and/or deep pole planting might substantially increase the area over which planting could be successful in the future.

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