Willows, water regime, and recent cover change in the Peace–Athabasca Delta

Abstract

:This study examines the relationships between water level changes and willow cover, one aspect of the controversy regarding ecosystem changes in the Peace–Athabasca Delta, northern Alberta, Canada. What water conditions are optimal for the delta’s willows? Does flooding cause willow dieback? Are there species-specific differences in susceptibility, and what attributes of flooding are related to dieback? How has willow cover changed in recent decades? Vegetation cover and its temporal response to flooding and drying were investigated for five common willow species in relation to water levels. Willow cover declined over the period 1993–2001, but cover changes differed among species. Willow cover decreased the most on the wetter transects, while drier transects increased in willow cover. Salix bebbiana and S. discolor appeared to be the most susceptible to flood-induced dieback, and S. petiolaris the least susceptible, while S. planifolia (the dominant willow in the delta) and Salix exigua were intermediate in flood susceptibility. Willow dieback was correlated with water depth, duration of flooding, and time since flooding. Large, old willows were more tolerant of flooding than small, young willows. A pulse of willow establishment occurred during the early 1980s that coincided with a drying period in the delta, increased regional wildfire activity, a decline in river discharge, and a decline in the level of Lake Athabasca. An increase in willow cover followed and reached a peak ca 1993. Flooding in the mid- to late 1990s resulted in a decline in willow cover. Given current climatic trends towards desiccation, willows may continue to be a dominant feature of the delta’s landscape.