UBC Theses and Dissertations
Hydrogeomorphic disturbance, landscape development and riparian vegetation dynamics of an alluvial, temperate rainforest in the Carmanah River valley, British Columbia, Canada Little, Patrick James
The alluvial forest of the Carmanah River valley on the west coast of Vancouver Island, British Columbia, was studied to examine the role of hydrogeomorphic disturbance in perpetuating the shifting-mosaic of habitats within this diverse ecosystem. Field-based research was complemented by a landscape-scale analysis that examined changes in the extent of specific forest types using a 70-year aerial photographic record. Thirty-eight plots containing 4509 trees were sampled for forest structure, composition, age, understory composition, and elevation above the contemporary channel. These field data, including a vegetation chronosequence spanning over 500 years, were used to examine vegetation dynamics. Over the past century, Carmanah River has eroded nearly 30% of the alluvial forest in this study area – 65% over the past 500 years. High magnitude floods result in diminished floodplain forest area by converting forests to channel. This results in a subsequent course of vegetation succession and geomorphic development. Fluvial deposits are colonized by a high density of Alnus rubra accompanied by a subcanopy of Picea sitchensis individuals. As Alnus die off after 60-100 years, Picea increasingly dominates the canopy while Tsuga heterophylla regenerate within the understory. The original cohort of Picea dies off after 300-500 years, which allows Tsuga to dominate old growth terrace forests. Picea or Alnus do not tend to regenerate under these dense canopies and without disturbance Tsuga may remain dominant indefinitely. Understory composition was related to landform age, however species distributions at low elevation floodplain sites were also driven by elevation above thalweg and flood frequency. Light availability was also a significant factor in driving community composition. It appears that understory dynamics were linked to overstory succession and geomorphic development processes, which alter environmental conditions at the understory level. That is, species distributions are driven by dynamic environmental filters, which change as a result of biogeomorphic succession. Mature forest patches tended to persist longer than young forests. The landscape composition reflects a balance between episodes of hydrogeomorphic disturbance and periods of successional development. Increased hydrogeomorphic disturbance rates due to climate change have the capacity to alter the landscape composition resulting in diminished mature forest area.
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