Pioneer riparian tree seedling establishment in floodplain refugia is driven by interspecific competition in addition to water availability

Background/Question/Methods

Abandoned channels in meandering river floodplains provide critical habitat for a host of riparian species. In the lowland floodplains of California, abandoned channels serve as spatial refugia for disturbance dependent pioneer trees such as Fremont cottonwood (Populus fremontii). These refugia are increasingly important to cottonwood persistence given lack of recruitment along the active channel due to river regulation. While abiotic factors overwhelmingly drive seedling establishment on the active channel, dynamics within these refugia are less understood and are complicated by temporal changes. We used controlled community mesocosms to quantify the shift from abiotic to biotic constraints on cottonwood recruitment into abandoned channels as they terrestrialize through time. We tested the effects of water availability and interspecific competition on 1^st year cottonwood seedling growth and survival. Environmental gradients reflected field conditions sampled from the middle Sacramento River, California. Survival of 2,082 cottonwood seedlings was tracked every 3 days during a 60-day water table recession. We used survival analysis, logistic regression, and an information-theoretic approach to quantify the strength of evidence for alternative models of the influence of competition and water availability factors on cottonwood seedling survival. Predictors were selected a priori based on cottonwood ecology and evidence from the literature.

Results/Conclusions

Final survivorship was 11% (95% CI: 9.9–12.7) overall. There was a 10-fold increase in final survival from lowest to highest water availability, and a three-fold decrease when seedlings were grown with interspecific competitors. The negative effect of competition was five times stronger in conditions of lower water availability. The best model included maximum cover as a metric of competition magnitude, days to maximum cover as a competition timing metric, and days to a water stress threshold as a soil moisture timing metric, plus pairwise interactions between competition and moisture metrics. There was much stronger support for models that combined at least one competition and one soil moisture metric, along with their interaction term, which differs from recruitment dynamics on active channels. Coupled with our field observations of temporal changes in abandoned channels, our results suggest that pioneer riparian trees have a window of time to successfully colonize newly abandoned channels before conditions support the widespread establishment of better resource competitors. Results of a follow-up study will examine the relative importance of competition for above- vs. below-ground resources. From a population perspective, our results highlight the importance of maintaining the underlying hydrogeomorphic processes that lead to periodic channel abandonment.