Presenter/s: John Bair
Symposium Session: 2023 - 10 Charismatic Megaflora: Cottonwoods Restoration
Topics covered: adaptive management and monitoring, california, cottonwoods, dredged rivers, lessons learned, and riparian
Many large rivers in Northern and Central California were transformed through hydraulic mining, dredge mining, and streamflow regulation. Cottonwood trees are a component of the riparian forest and were once far more abundant. Alluvial valleys were turned over by mining activities, leaving coarse unsorted materials, disconnected from subsurface and surface hydrology. Remaining mine tailings are often barren, sometimes rising more than 25 ft above the river.
Physical reconstruction of tailings and vegetative planting have been used to rehabilitate dredger mined rivers with varying success. Removing and lowering dredger tailing piles below the 1-yr recurrence interval flood to increase annual inundation frequency and duration has been a primary factor in successful recovery of riparian function and increasing planting survival. Lowering dredger tailings creates better access to shallow groundwater for plantings and is more likely to restore physical and hydrologic conditions where passive vegetation recruitment can occur. Once tailings have been lowered, cottonwoods are ideal candidates to plant because they are easy to grow from cuttings and grow into a largest deciduous tree.
Cuttings can be made at any time of the year but require handling and management during creation, soaking, and planting. Cuttings are less prone to scour mortality than container stock due to their deep planting, they typically cost less to make, and do not require irrigation after the project if they are already planted into a perennial water source. Cuttings tend to be most successful if they are quickly transported and placed in a pond or wide trench with water in it, then placed into a trench or hole with 2/3 their length in water. Depending on the time of year, cuttings can start to grow roots or rot if soaked too long; therefore, cuttings need to be planted quickly after a 7-to-14-day soaking period. Our most successful summer cottonwood plantings have been installed with an excavator that digs a hole to groundwater. Excavator planting creates irregular patterns and microtopography. Six or more cottonwood cuttings are placed around the perimeter of the hole into visible groundwater and the hole backfilled. Additional water is pumped into the hole as it is backfilled to ensure good cutting contact with fine sediment washed into the hole during backfilling. If planted during the summer, sprinklers are set up and clusters of cuttings are irrigated until the rainy season or construction ends.
Cottonwood cutting handling and planting methods have had the greatest effect on survival. Initially cuttings were not soaked and had a 20 to 40% survival rate; however, when cuttings were soaked, survival rate increased to 60 to 90%. Clusters of cottonwood cuttings planted into late season shallow groundwater has been the most successful method. One or more cuttings are expected to survive in each cluster, with an average post-project survival of 60 to 70% without follow-up maintenance or irrigation. These planting methods, when combined with mechanical lowering of dredger tailings to increase hydrologic connectivity have led to the greatest revegetation survival and tailing recovery.