Fremont cottonwood (Populus fremontii) is an early successional species which serves as a foundation for riparian ecosystems in the North American Southwest. Along rivers the upper limit of the seedling establishment zone depends on proximity of seedling roots to the declining water table. The lower limit is a function of the maximum elevation of inundation or scour. Under both natural and human-influenced hydrologic regimes, Fremont cottonwood seedlings are likely to experience short-term (one to five week) inundation during their first year of growth. Previous studies show that inundation can account for more than 70% of seedling mortality during this time. Our study aimed to characterize inundation survival of seedlings during their first year of growth under various hydrologic regimes. We used controlled inundation experiments to assess the tolerance of two different ages of Fremont cottonwood seedlings to inundations at different depths and temperatures and for different durations.
Results/Conclusions
We found that seedlings of Fremont cottonwood have high tolerance of inundation to the soil surface and a reasonable tolerance of complete shoot submergence for a duration of one or two weeks (22 and 50% mortality respectively). Mortality increased linearly with days of complete submergence (mortality percentage = 4.6 + (2.5 × days of submergence)). Warm water temperature (25/18°C day/night) during complete submergence adversely affected seedling biomass and survival, resulting in 64% mortality versus 39% with cooler water temperatures (18/11°C day/night). Our results indicate that establishment of new Fremont cottonwood populations in the riparian corridor will be more successful when flows do not completely cover the shoots of seedlings for more than two weeks and if water temperatures during inundation are cool. From the perspective of the management of river flows for cottonwood recruitment, deep, prolonged, late season (warm water) inundations are the most detrimental.