Print PagePioneer species that rely on disturbance events for regeneration must find ways to maintain their populations during periods of relative stability. Use of spatial refugia is one common persistence strategy, particularly for mobile organisms within disturbance-prone or spatially heterogeneous systems like rivers. However, the importance of spatial refugia for riparian plant species or more generally for trees is less appreciated. Abandoned channels in meandering river floodplains serve as such refuges, but the conditions necessary for establishment of pioneer riparian trees within them are complex and ephemeral. We are testing a conceptual model linking physical processes of sedimentation with vegetation dynamics. This model suggests conditions change through time from a period where abiotic factors are the primary drivers of forest dynamics, to conditions dominated by biotic interactions, particularly competition for light and water. In a controlled community competition study, we tested the effects of water availability (mediated by substrate texture) and interspecific competition (grass, sedge, and forb species) on cottonwood (Populus fremontii) seedling growth and survival under simulated water table decline (2cm/day). Plant communities were grown outdoors in 25-cm diameter “rhizopod” growth tubes. Three substrate textures (gravel/sand, coarse sand, very fine sand) and a gradient of interspecific cover (control, 20–75%) were chosen based on field sampling of abandoned channels of varying age in the middle reach of the Sacramento River, California. A total of 3,230 individual cottonwood seedlings were monitored every 3-days for a water table drawdown duration of 60-days.
Results/Conclusions
Final survivorship was 2%, 0%, and 20% in the coarse, mid, and fine textures, respectively. We expected cottonwood survival and growth to decrease with increasing substrate size due to moisture stress (i.e., from lower capillary rise and water-holding capacity); however, they were lowest in the mid-texture. Median time-to-failure in coarse and fine textures occurred approximately 2-3 weeks after start of drawdown versus ~1 week in the mid-texture. The broader grain size distribution of this substrate likely resulted in higher porosity than the more bimodal gravel/sand mix, leading to faster moisture loss with receding water table. Growth, as measured by stem height and biomass, was highest in the fine texture control (no interspecific competitors). We are investigating if this is due to above- or below-ground resource competition. Together with an ongoing field study, the results will help us refine our process-based conceptual model and support the importance of spatial refugia in the long-term population dynamics of these riparian pioneers.
