California Central Valley Streamside Forest

This ecological system occurs in the floodplains of rivers of California's Central Valley. Alluvial soils and late winter/early spring flooding (usually every year) from snowmelt typify this system. Communities are predominantly floodplain woodlands, but also include shrublands, wet meadows and gravel/s and flats. Important trees and shrubs include Populus fremontii, Platanus racemosa, Quercus lobata, Salix gooddingii, Acer negundo, Cephalanthus occidentalis, and Vitis californica. Juglans nigra hybrids and Ailanthus altissima are problem invasive trees. Tamarix spp. extend as far north as Shasta County. Herbaceous components can include Carex barbarae, Artemisia douglasiana, and various marsh species along riverbanks and backwater (Schoenoplectus californicus, Typha spp.). Arundo donax is another common invasive and introduced forage species that often invades degraded areas within the floodplains. Periodic flooding and associated sediment scour are necessary to maintain growth and reproduction of vegetation. Flooding regimes have been significantly altered in all but a few tributaries that support this system.

Communities are predominantly floodplain woodlands, but also include shrublands, wet meadows and gravel/s and flats. Important trees and shrubs include Populus fremontii, Platanus racemosa, Quercus lobata, Salix gooddingii, Acer negundo, Cephalanthus occidentalis, and Vitis californica. Juglans nigra hybrids and Ailanthus altissima are problem invasive trees. Tamarix spp. extend as far north as Shasta County. Herbaceous components can include Carex barbarae, Artemisia douglasiana, and various marsh species along riverbanks and backwater (Schoenoplectus californicus (= Scirpus californicus), Typha spp.). Arundo donax is another common invasive and introduced forage species that often invades degraded areas within the floodplains.

This system is found on alluvial soils adjacent to perennial rivers and streams and their associated floodplains and riverbanks below approximately 550 m (1800 feet).

Periodic flooding and associated sediment scour are necessary to maintain growth and reproduction of vegetation (Sawyer et al. 2009). Major flood events and consequent flood scour, overbank deposition of water and sediments, and stream meandering are the key fluvial processes that provide new substrates, remove old banks and stimulate renewed growth of cottonwood and willow species (Sawyer et al. 2009). Natural fire-return interval was long or moderate with low-intensity surface fires.

Conversion of this type has commonly come from agricultural conversion and development for urban and housing, loss of the floodplain through levee development, and complete inundation by creation of reservoirs. Flooding regimes have been significantly altered through the reduction of peak spring and winter flows, less frequent and lower magnitude high flows, that result in very rare overbank flooding as well as increased low flows in all but a few tributaries that support this system (Sawyer et al. 2009). Invasive species alter composition and support different guilds of insects and reptiles and can change the fire regime. Arundo donax is a common invasive and introduced forage species that often invades degraded areas within the floodplains. Juglans nigra hybrids and Ailanthus altissima are problem invasive trees. Tamarix spp. extend as far north as Shasta County. Changes in fire regime caused by Tamarix which shortens the fire-return interval (10-20 years) in Arizona (Ohmart and Anderson 1986) and California (Brooks and Minnich 2006). Surface fuels provided by Tamarix also increase fire intensity and the probability of crown fires that result in high mortality rates in Populus fremontii (Sawyer et al. 2009). Arundo donax has the same effect of increased fuel load and fire intensity and reduced fire interval resulting in mortality of native woody species and increased post-fire dominance of Arundo (Coffman 2007).

In the Central Valley, regional climate models project mean annual temperature increases of 1.4-2.0°C (1.8-3.6°F) by 2070. The projected impacts will be warmer winter temperatures; earlier warming in spring and increased summer temperatures. Regional models project a decrease in mean annual rainfall of 47-175 mm (1-7 inches) by 2070. While there is greater uncertainty about the precipitation projections than for temperature, some projections call for a slightly drier future climate relative to current conditions. Projections include a decrease in total annual streamflows and earlier snowmelt, with streamflows increasing slightly in January and February but decreasing in all other months. Annual streamflows statewide are projected to decrease by 27%, with inflows from surrounding mountains to the Sacramento Valley projected to decrease by 22%. Today, the flow of the Sacramento River is heavily managed through a series of dams and diversions. As a result, it is likely that flows on the Sacramento River will be more influenced by management decisions than by climate change effects. However, even though the timing of flows may be mediated by hydrological infrastructure, the ability to deal with extreme flow events will likely remain limited. Accidental levee breaks in the Sacramento-San Joaquin River system have occurred in 25% of years during the 20th century. Historical flood control efforts have not reduced the occurrence or frequency of levee breaks. Current climate-change projections suggest that storm patterns and fluvial responses are expected to aggravate future risks of levee breaks (summarized from PRBO Conservation Science 2011).

In the long term, sea-level rise will greatly increase levee breaches in the Sacramento - San Joaquin Delta. Since many of the delta islands and riparian areas lie below sea level currently flooding will ironically reduce areas for riparian and replace them with standing brackish water too deep to support the natural riparian system(T. Keeler-Wolf pers. comm. 2013). Other potential climate change effects could include: further reduction in high flows; perennial streams may become intermittent; phreatophytic species under greater stress and death; drop in groundwater table; increased fire frequency due to warmer temperatures resulting in drier fuels; increased invasive species due to lack of competition from native species whose vigor is reduced by drought stress, and increased fire intervals favor certain invasive species (Brooks and Minnich 2006, Coffman 2007); and increased competition for water from all users, stresses the already overtaxed water allocation of California agricultural system (PRBO Conservation Science 2011).

Occurs in the floodplains of rivers of California's Central Valley.