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Riparian Ecosystems • Outline – 1) Riparian – definitions, distribution, and extent – 2) Characteristics of riparian zones -- Patterns – 3) Functions of riparian zones -- Processes – 4) Threats to riparian zones 1) Riparian – Definitions, distribution, and extent • “riparius” – belonging to the bank of a stream • Riparian – adjacent to flowing water – Riparian ‘wetlands’ are those areas adjacent to flowing water and in which ecosystem processes (i.e., soils, moisture, nutrient flux, etc.) are influenced by an adjacent stream or river at least periodically; found wherever streams or rivers at least occasionally cause flooding beyond their channel confines and where alluvial groundwater is seasonally within the rooting depth of perennial vegetation. • Riparian areas are the link between aquatic and terrestrial. An ecotone. – Exchanges of energy, nutrients, and species. – Think of riparian-stream linkages (Nakano, Baxter, etc.) Distribution and extent of riparian zones • 23.3 million hectares (89,800 mi2) in continental U.S. (Mitsch and Gosselink 1993) • 35.3 million hectares (136,160 mi2) of riparian wetlands in United States, including AK • Most extensive riparian forests in the U.S. are the mesic bottomland hardwoods of the southeast U.S. (tupelo, sweet gum, red maple, bald cypress) these are broad, flat, and expansive. Most extensive riparian forests in the western U.S. are the cottonwood gallery forests (Bosques) along the Rio Grande in NM. • Riparian areas are the only places that support deciduous forests in the arid regions of the world (arid defined as evaporation >> precipitation) – Great Plains, Colorado Plateau, Great Basin, deserts of the Southwest, etc. • 3 scales of control on riparian systems: (1) Continental/Regional • Climate (precipitation & temperature) – Controls growing season, evapotranspiration • Geology – Sediment and channel morphology • Biogeography – Which species are historically present? (2) River Systems/Watershed • Elevation • Hydrograph • Channel morphology (3) Local/Reach • Sediment sorting (floodplain texture) • Elevation - floodplain height above channel • Factors exerting significant selective pressure on species – A) soil oxygenation (aeration) » Plant roots need oxygen » Reflects soil structure, inundation regime – B) nutrients deposited by floodwaters » particulate organic matter » dissolved • Zonation of vegetation from stream channel to the upland (outside of the riparian zone) reflects moisture and disturbance gradients. • Elevation differences key! – Differences in many species’ distributions (Figure 4.8, Table 4.2) – Inundation regime varies among years (Fig. 4) At-a-site scale 2) Characteristics of riparian zones -- Patterns • Riparian ecosystems are the most biologically diverse terrestrial ecosystems on earth (Naiman and Décamps 1997). • Compared to surrounding uplands, they have disproportionately higher ... – – – – – structural heterogeneity species richness biomass productivity (true for desert, temperate and humid, tropical, coastal, and boreal regions) however riparian areas occupy a very small proportion (1-5%) of the landscape. • Riparian zones have inherent properties (spatial and temporal heterogeneity) that accommodate more species of vascular plants, insects, amphibians, birds and mammals • Landscape integrators: status of riparian zones can serve as an indicator of the “health” of a watershed 3) Functions of riparian zones – Processes • Several things distinguish riparian wetlands from other wetland types: – A) Linear form – B) High energy and material transfer – C) Functionally connected – D) Dependent on mechanical disturbance • A) Linear form – Proximity to streams or rivers – Serve as corridors through the larger landscape: may enable insects, birds, and mammals to move more freely • facilitating interactions between populations, genetic exchange, etc. – This connectivity may be beneficial or may provide a corridor for the movement of exotic species into landscapes that they might not have had access to otherwise. • South Platte riparian now continuous, has allowed westward migration of fox squirrels, blue jays, white-tailed deer to Colorado Front Range. • Tamarix, salt cedar, has moved into landscapes that it would not otherwise have had access to via seed dispersal. This access to the landscape has enabled it to colonize non-riparian habitats including natural springs and spring-fed lakes. Through evapotranspiration losses and groundwater depletion, Tamarix destroyed many of these desert oases. – Multi-million dollar eradication program ? • B) Energy and material – converge and pass through riparian ecosystems in greater amounts than in surrounding ecosystems. (“Open systems”) – Rivers transport a great mass (volume) of dissolved nutrients and organic matter, much of which is stored in riparian floodplain areas. • Riparian zones serve as a filter buffering aquatic from upland systems. • Very productive soils (agriculture!) • C) Functionally connected – upstream and downstream ecosystems • River accumulator hypothesis of Nilsson et al. (1994) • River channels coalesce delivering seeds from sub-basins of a watershed so rivers may maintain their own levels of species richness even when moving from a species rich to poor region. Mountainous region f(watershed area) Species richness Inland region Whole watershed scale Coastal region Mountainous region Inland region Coastal region Long-distance connections to other ecosystems Hypothesis -- salmon help riparian trees grow! Helfield and Naiman (2001, Ecology) Anadromous fish (i.e., salmon) migrate upstream hundreds of km upstream to their natal streams to spawn. Fish concentrate in sites with good spawning gravels. Bears also congregate and feed at these sites. The carcasses are dragged into the riparian areas and bears eat, generate waste, and leave the remains to rot in the riparian forest. What would you predict about tree growth rates?? - spawning vs. non-spawning streams? - distance from stream? ** Main result: Spruce grow about 3 x faster within 25 m of spawning streams than reference streams. ** Sitka spruce How can they demonstrate a salmon fertilization effect? What would you measure? [Hint: food web studies] 15N is indicator of marine-derived nitrogen Main result: Spruce have more marine N in salmon streams. Not (quite) a significant distance effect for salmon streams. ** ** • D) Mechanical disturbance • Critical to riparian functioning, productivity and diversity • Disturbance (flooding) – maintains structural heterogeneity (e.g., landform elevations) – creates steep environmental gradients of soil texture, water availability, oxygen levels – resets successional processes in riparian vegetation. Interpret --> The greater the topographic variation … ? The greater the flood frequency … ? Maximum species richness … ? • Channel Morphology & Riparian dynamics • A) Confined Channels – Stable, don’t move much – Rare, high magnitude floods remove plants, rather than more frequent channel processes • B) Meandering Channels – Channel meanders across valley floor over time, transporting sediment through erosion and deposition. – Dynamic creation of new landforms and associated successional sequences of plants on floodplain – Plants are extremely important in this process as they capture and retain sediment and nutrients, they influence and are influenced by channel morphology. – Meandering forms a variety of fluvial landforms of different elevations, sediment particle sizes, steepness of environmental gradients and duration of flooding which in turn influence biotic processes such as water availability to plants, rates of decomposition, nutrient status, carbon cycling, and plant, invertebrate and mammal and bird communities. • Point bars are areas of sedimentation on the convex sides of river curves – the point bar is colonized, aggradation of the point bar is accelerated, enhancing further channel migration. • Fluvial forms associated with meandering – Scroll topography, oxbow lakes, levees, sloughs, etc. • Plant adaptations to mechanical disturbance – Many riparian plants have adaptations to transient habitat, flooding, scouring, abrasion, and drought: • 1) Reproductive – Seed dispersal, timing of release • 2) Morphological and physiological – Cottonwoods and Willows have adventitious roots – Willows’ broken limbs can re-sprout downstream The Cottonwood Story Robust Cottonwood (Populus) Forests in western U.S. require … • Meandering, alluvial rivers • Floodplain inundation – Wet, bare spots for seed germination • Timing of floodplain inundation with seed release – Seeds are wind and water dispersed – Seeds die w/in 2 days • Suitable rate of recession of alluvial groundwater as river stage drops – Roots grow in contact with receding water table • Interannual variation in flood magnitudes – Persistence over years requires being high on floodplain to avoid big flood within few years of establishment. • Cottonwood trees can accelerate sediment deposition (and thus meandering) by increasing hydraulic roughness during floods. • Cottonwoods and Flow Regimes – Establishment Flows: • Flood … Magnitude, Timing, Duration, Rate-of-change – Survival flows: baseflow Recruitment Box Concept River Stage • Timing of seed release • Inundation of floodplain • Rate of flow recession Rate of Decline (i.e., 2.5 cm/d) Populus (seed release) MAY JUN JUL Time of Year Reproduced from Mahoney and Rood, 1998 4) Threats to riparian zones Human uses and riparian integrity Timeline of riparian alteration: 1920s - US Corps of Engineers meander straightening 1960s - dams in US -fragmentation, peak flow capture 1970s - overgrazing in West Human Effects on Riparian • Sever tie between channel and floodplain (main problem) – Levees (artificial, created to prevent floodplain inundation or to increase water depth in channels for navigation) – Dams • >75,000 dams >2 meters high in US • Reservoirs cover 3% of US land mass • 60% of streamflow is detained behind dams Effects of dams: reduced flooding: – – – – A) lowered soil moisture on downstream floodplains B) loss of sediment to build point bars, etc. C) loss of nutrient inputs from river D) reduced rates of decomposition, causes nutrients to be sequestered in deep litter layers, further reducing productivity of riparian areas. • Manuel Molles et al. (1998) showed that this was the case for extensive reaches of the middle Rio Grande and demonstrated the pulse of nutrients and increased productivity of the riparian zone that could be generated by conducting a controlled release from a dam located upstream from a wildlife refuge. • Dams – Promote exotics? • When disturbance dynamics change, so does habitat template • Salt cedar (Tamarix) – Prefers modified floodplains – Poor nesting, feeding habitat for most birds • Russian Olive Mature bosque Natural recruitment Invasive Tamarix Russian Olive Russian Olive invasion along tributary to the Colorado River in Silt, CO