Ecological dynamics

Community Dynamics Section

This ecological site is a complex tidally-influenced riverine complex of marshes, seasonal wetlands and emergent wetland vegetation types of the Suisun Marsh. It is also the most extensive ecological site within the land resource unit making up approximately 87% of identified soils but comprising just 12% of the MLRA. In the most general sense, this historically this site would be a series of islands subject to inundation by runoff and/or tidal waters several times each year.

This ecological site has been subject to some of the most extensive and intensive modification of any lands in the western US. In the late 1800s, agricultural interests initiated the construction of levees along some islands with most of these islands leveed by 1917. Following the leveeing, draining and burning of the hallmark organic soils (histosols) are legendary in the West for food production and having peat surface often exceeding 50’ thickness.

The soils of this ecological site are relatively new arrivals to the landscape with some of the oldest and deepest histosols having been radiocarbon dated to approximately 6,500 years BP (USGS, 2001) and corresponding to increased sea levels at the tail end of the post-Ice Age melt-back. This period of increasing sea level transformed the underlying braided river networks of the current Delta into something more akin to a marine estuary. As a result of river waters meeting marine waters inland, fine sediment loads of the upper watersheds were increasingly deposited over the top of the old riverine system and these histosols developed as sediment and organic matter accumulated.

Under natural hydrology, this ecological site grades upslope into the Salt-Affected, Stratified, Fluventic Sites (R016XB002CA) ecological site which occupies a slightly higher elevation than the flanking organic soils of this site. Unlike the similar Tidally-Influenced, Freshwater Sites (R016XA001), this site is more influenced by salt and less immediately by riverine hydrology. This salt influence is due in large part to the LRU positioned as something of a backwater to the inland watersheds of the Sacramento and San Joaquin rivers but is also by virtue of being closer to the Pacific Ocean on the western front of the constricting narrows between Antioch and Collinsville.

Organic soils support the emergent wetland vegetation (the buildup of organic material occurs through anoxic conditions created by saturated soils). The importance of salt in this context is that decomposition (mineralization) rates of organic materials are increased in the presence of salt water (Weston et. al., 2006), thus likely reducing the rate at which organic soils can build and ultimately limiting the stability of coarse organic materials as found in peaty soils. Unlike with the peaty soils of inland LRU A, the ecological site’s hallmark Joice and Suisun series muck soils display comparatively highly decomposed organic material; these soils may develop recalcitrant cracks upon drying thus complicating both water management and production demands under cultivated conditions.

Soils of this ecological site are subject to subsidence due to oxidation of soil organic matter under the influence of soil aeration. In such cases, unrepaired levee breaches may result in some extents of subsided areas being permanently inundated as in the cases of Franks Tract, Big Break, Mildred Island, and western Sherman Island (Sherman Lake). Complications of salt water influence, water management, economic considerations, and conservation priorities have led much of this ecological site to be managed as waterfowl habitat.

Where constructed levees have been installed across the range of this ecological site, most of the preexisting natural channels and levees have been muted by land levelling but many are still apparent on LIDAR imagery. This is due in part to differential shrinkage of organic soils relative to the underlying material. Within channels where organic material failed to historically accumulate, oxidation and shrinkage apparently occurs more rapidly than in adjacent areas with deep (>30’) peat substrates, thus exposing the underlying topography.

Narrow low elevation natural levees included in this ecological site were historically extensive within this portion of the MLRA. Occurrence of long-lived upland species such as walnut and oak was presumably rare while dominance by single aged stands of cottonwood and willow were more typical where those species could compete with cattail and tule. Such levees were typically a mixture of fine and coarse sediment but unlike the higher elevation levees associated with CA016XA002, these levees were typically subject to regular inundation by tidal waters and consolidation of sediment was largely localized and/or largely unpronounced due to frequent “washing” of the surface sediments and breaching of these levees. As a result of this tidal action, the height of many of these natural levees above the adjacent soils might have been as slight as a few inches or even be apparent only seasonally where tidal action was most influential.

The importance of natural levee height and channel sinuosity relative to vegetative pattern is not fully understood for this ecological site and the range of historic reports mainly focuses on dominant vegetation. However, it is conceivable that a great degree of variability in vegetation across the ecological site could be tied to island and channel hydrology as modified by position and elevation within the watershed. The likely pattern is that the closer to the headwaters an island occurred, the more likely that wider and more stable levees would develop. Considering an east to west cross section of the MLRA, this would imply a likely decreasing potential for significant oak, sycamore and walnut presence as well as increased turnover of cottonwood in particular as large shallow-rooted trees in such soils eventually topple under their own weight if not by windthrow or force of floating debris.

Vegetation assemblages vary depending on physical drivers. For instance, Schoenoplectus californicus was likely more domininant in the western Delta and along channels given its wind and wave reisistent structure, while the taller S. acutus grows in more protected areas like those in the north Delta flood basins (Keeler-Wolf pers. comm.). Particularly in the western-central Delta, this habitat type includes woody shrubs such as willow (Salix spp., primarily S. lucida lasiandra) and ferns (Athyrium felix-femina) to make up unique plant community, perhaps related to maritime influences (Atwater 1980, Keeler-Wolf pers. comm.). The wetland species are not precluded by seasonally dry conditions.

Freshwater emergent wetlands can be either tidal or non-tidal. Tidal freshwater emergent wetlands include those areas wetted at mean higher high water during low river stage and comprise what historical records often refer to as tidelands. Non-tidal freshwater emergent wetlands are not directly and predominantly affected by tidal action. However, tides may indirectly affect water table levels in freshwater emergent wetland and hydrological connectivity across landscapes during floods.

State 3 conceivably produces the most vegetative biomass due to agricultural inputs whereas the two preceding states are similar in overall productivity over long periods of time.