OOS 44-1
Riparian forest dynamics along the Sacramento River, CA: Pre- versus post-dam patterns in tree species succession
The riparian ecosystem along the Sacramento River corridor in California’s Central Valley has been greatly impacted by human activities, particularly land conversion and flow alteration. Though the construction of Shasta dam in 1945 has dampened peak flows, the middle reach between Red Bluff and Colusa remains active. This reach hosts floodplain forests dominated by Fremont cottonwoods (Populus fremontii), pioneer species which establish on bare soil surfaces following flood disturbance and channel migration. Relative floodplain elevation increases over time, facilitating establishment of shade-tolerant species such as boxelder (Acer negundo), walnut (Juglans hindisii) and Valley oak (Quercus lobata). Our objectives are to use dendroecological methods to: 1. Confirm whether pulses of cottonwood establishment coincide with peak flow events, channel migration, and the expansion of floodplain surfaces, and 2. Document the colonization timing of each species (i.e., how soon each species establishes after floodplain creation). We compared colonization patterns between pre-dam and post-dam floodplain surfaces to evaluate how changes to the natural flow regime may have impacted the forest’s successional trajectory. Non-pioneers are expected to have shorter colonization times on younger floodplain surfaces. Colonization timing was calculated using tree cores collected through vegetation surveys on floodplain surfaces across a range of known ages.
Results/Conclusions
Cottonwoods colonized floodplains within 10 years of major flood events, and recruited heavily during the periods of most extensive floodplain expansion. The largest extant cottonwood cohort (50% larger than others) established in the 1980’s during the greatest expansion of floodplain area. Larger floods during the pre-Shasta dam period may have resulted in greater forest expansion, but extensive reworking of these floodplains occurred in the post-dam period. After its 1988 peak, cottonwood recruitment fluctuated and declined by 17% through 2007, concurrent with a 66% decrease in floodplain creation rate. Changes to the natural flow regime likely influenced the riparian forest’s successional pattern, with late-successional species appearing in the community sooner after disturbance than in the past. F. latifolia, J. hindisii, and Q. lobata tree ages, which lagged floodplain creation for ≥50 years in the pre-dam period, are more similar to those of early successional species P. fremontii and Salix goodingii on post-dam floodplains. Notably, A. negundo and J. hindisii colonized up to 20 years sooner. Increased competition with non-pioneers and recent declines in establishment threaten to reduce early-successional habitat areas in the future, and may be remedied with flow augmentation to re-establish channel migration and floodplain creation on a larger scale.