Download 2) Characteristics of riparian zones

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