Unusual disturbance: forest change following a catastrophic debris flow in the Canadian Rocky Mountains

Trees are often well adapted to periodic physical disturbances such as fires or floods. However, I investigated forest response to an extremely unusual disturbance event. Following heavy rain in June 1995 a catastrophic debris flow from Vimy Peak in the Canadian Rocky Mountains terminated as an alluvial debris fan that plowed through a trembling aspen (Populus tremuloides Michx.) grove. I analyzed the site over a decade to monitor forest response and determine whether there would be recolonization to the prior forest type. In contrast to my expectation, aspen recolonization did not occur; instead, black cottonwoods (Populus trichocarpa Torr. & Gray) colonized the site. These originated from seedlings and not through clonal propagation, and by 2004, black cottonwoods composed 99% of the saplings and were typically 0.6–1.4 m tall with a density of about 1/m2. The debris fan dramatically changed the physical environment, which partly resembled a floodplain depositional zone and was colonized by the regionally dominant riparian tree. I propose the concept of foreign disturbance to recognize an unusual disturbance that an organism would very rarely experience and thus to which it is unlikely to be adapted. In this example the disturbance produced an abrupt transition to an alternative forest type and this response may provide insight into forest response to other unusual disturbances, such as extreme weather events, that might increase with climate change.