Arid Ecohydrology
Ecohydrology
Fall 2013
Ecosystem RUE
• All terrestrial ecosystems
use MAP to create NPP,
and are therefore, at
some level, water
dependent
– Differential sensitivity to
variance in MAP
– NPP:MAP defines rain use
efficiency
– Sensitivity effectively
measures the strength of
water limitation vis-à-vis
nutrient or evolutionary
constraints
Huxman et al. (2004) - Nature
Convergent RUEmax?
• RUEmax generally occurs at low water availability
• There is a convergence of RUEmax across all biomes
– RUEmax is close to RUEmean for arid sites
– RUEmax is really low compared to RUEmean for humid sites
Two Predictions
• 1 – NPP will be
affected a lot if
PPT is driven
below the historic
minimum
• 2 – Removal of
other resource
limitations will
allow RUE to
approach RUEmax
Ergo
• Water limitation imposes a common
constraint on NPP across biomes
– Ecosystems have the same RUEmax despite large
differences in ppt, physiology, phytogenetic origin
and climate history
– Altering resource limitation underscores the
relevance of biogeochemistry on NPP (i.e.,
compared with species)
Inhibited Deep Drainage Over Millenia
• In dry areas (<500 mm)
there is a nearly
ubiquitous pore-water Clpeak at 5-15 m below
grade
• Mass balance of Cl yields
10,000-15,000 years of
accumulation (without
major downward flux
event)
– Consistent with ~ 1 mm/yr
deep drainage
Seyfried et al. (2005) - Ecology
But…
• Upward water potential
gradient
– Water is moving out of
the soil
– Ergo, no downward
movement
Seyfried et al. (2005) - Ecology
Strong Climatic Control
• Cl inventory suggests 7,000-8,000 years of
accumulation at a site with 360 mm MAP
• Inventory is 1,000 years at 400 mm MAP at
nearby site
Ecohydrologic Mechanism
• Walvoord et al. (2002) propose a
conceptual model to reconcile
these attributes
– Low water potentials at the base of
the root zone were established at
the end of the Pleistocene
(dramatically increased dryness)
– Potentials (< -1 MPa) were
maintained continuously during the
Holocene
• Any deep wetting event would reset
the chloride concentrations
– Slow upward movement of water
above ca. 20 m
– Drainage below 20 m of Pleistocene
aged water
– Simultaneous recharge and upward
flux
Vegetation is Essential
• Plants are required to impose and maintain the low
potentials
– Two key attributes:
• Deep roots (2-3 m) of sufficient density to capture all downward
percolating water
• Able to maintain low water potentials at depth continuously
• The demand for continuous maintenance has
enormous implications for contemporary vegetation
management
– Clearing vegetation can result in huge fluxes of Cl (and
nitrate!) to the deep groundwater
Deep Roots
• Root density decreases exponentially
with depth, though slower in arid
biomes
• Max rooting depths ~ 5 m, almost all
co-dominant shrubs ~ 1.8 m
• Root depths increased with MAP and
soil coarseness
• Maintaining root potentials demands
cavitation avoidance
– Geometry of xylem
– Soil hydraulic conductance goes to 0
• SW USA co-dominants can withstand
cavitation (embolism) from -4 to -9
Mpa (extremely low)
– The conceptual model requires sustained
-1 MPa at the root zone
Sperry et al. (2002) – Functional Ecology
Hydrologic Buffering
• Arid lands are hydrologically variable
• Model requires constancy
• As such, moisture conditions at 2-3 m below grade must be buffered from
the surface variability
• Water budget suggests that Dd (deep drainage) occurs only when I > (AET +
ΔS)
– ΔSmax is ~ 350 mm (over 3 m root depth)
– Dd occurs only when I is 350 mm > AET
• (basically never)
• Arid land plants use excess water in wet years
• Multiple wet years create rapid vegetation adjustments
– Expansion of deep rooted shrubs
– Explosion of annuals
• As such, the base of the root zone basically are continuously water starved
• So why invest in deep roots?
Deep Roots
• C expenditure that needs to be net
benefit
• Existing profile data suggest very little
plant available water below 60 cm
– 31 yr data record at 2 wk interval
• Living roots at 180 cm
• Low potentials (< -1.5 Mpa)
• Net energy benefit?
– Plants may actually be hydrologically
subsidizing deep roots to maintain their
viability during wet periods
– Confers benefits during periods of deep
drought
Seyfried et al. (2001) – WRR
Implications
• Natural deep rooted perennial communities are most
frequently replaced with shallow rooted annuals
– Grazing, planting
– Changes episodic downward fluxes of water and therefore
salt, nutrients, hazardous materials
• Extremely difficult to reestablish deep rooted
vegetation
• Adaptive significance of this “ecological inheritance”
– Low water potentials, high Cl peaks
Alternative Stable States
What is a “State”
• A state is a configuration of a community of
organisms that can be described by a set of
(dynamic) “state” variables
– Biomass, density, species (ages, guilds,
abundances), nutrients/organic matter
• A “stable state” is a particular configuration
that possesses self-reinforcing feedbacks that
confer some resilience in the presence of
perturbations
Origins of the Idea
• Stability of populations
(constant environment)
– “If the system of equations
describing the transformation of
state is nonlinear…there may be
multiple stable points with all
species present so that local
stability does not imply global
stability” (Lewontin 1969)
– Overfishing, invasions, predator
removal
• Stability of ecosystems
(variable environment)
– (Lord) Robert May (1974)
Stable and Non-Stable Equilibria
dP/dt = B(P) – D(P)
D = a*P + c
B = q + K/(1+e-rP)
Solve for when dP/dt = 0
or B(P) = D(P)
The Demographic Transition
• Introduces ideas:
– Equilibria
– Disequilibrated
systems
– Time lags between
states
– Path dependency?
Demographic Stability – Allee Effects
• Per capita birth rates
decline at low
density
– Mate finding,
dispersal
• Creates two
population equilibria
• Which is stable?
Unstable?
Balls-in-cups
• Ball represents an ecosystem
state
• Cup represents the domain (or
basin) of attraction
– Remember that there are
feedbacks that create
“resilience”
• Regime shifts occur by:
– Perturbation to the variables
(i.e., move the ball)
– Perturbation to the “fitness
landscape” (i.e., change the
basin)
Conceptual Model
– Stability
Landscapes
Ecosystem Response to Change
a. One equilibrium for each environmental condition
b. One equilibrium, but rapid rates of change over short
span of conditions
c. Three equilibria can persist for one state (two stable,
one unstable), strong hysteresis (path dependency,
temporal lags) in the transition between states
Resilience – A Related Concept
• Two aspects of the basin of attraction matter
– Width of stability domain [Over what range of
conditions/perturbations will the community recover?]
– Steepness of the stability domain [How quickly will the
system recover?]
• Resilience can be used to define both properties
– The former is dubbed “ecological resilience” the latter
“engineering resilience”
• Basically some measure of a systems capacity to
buffer the effects of exogenous changes
Hysteresis – A Key Emergent Property
• A systems trajectory over
time may depend on its
current and historical
position
• Changes in system
configuration may delay or
even prevent recovery
Suding et al. (2005)
Trends in Ecology and Evolution
Why Should We Care?
• Costly surprises (catastrophic shifts, weak ecosystem
responses to management)
–
–
–
–
Collapse of fishery stocks
Outbreaks of disease
Exotic species invasions
Ecosystem shifts and loss of services
• Underscore the complex, contingent (dependent of
antecedent conditions) nature of environmental
systems
– Thresholds imply the need for caution when dealing with
complex systems
– Climate change, biodiversity loss, stock markets, land use
change
Tidal Marsh Alternative Equilibria
Example: Shallow Lakes
• Submerged Aquatics
– Rooted plants little
phytoplankton
– Low turbidity due to:
• Plant filtration
• High sediment cohesion
limits resuspension due
to wind action
• Algal-dominated
– Phytoplankton shades
SAV
– High turbidity
• Reduced filtration
• Loose, flocculent
sediments that can be
easily wind-entrained
Empirical Evidence
• Enrichment of P leads to
increased production
• Eventually there is
sufficient water column
P to start to erode the
controls exerted by SAV
• Catastrophic flip (short
time) to phytoplankton
dominance
• Reductions in P
necessary to reverse the
process are much larger
Incipient Shifts – Where are the Tipping
Points?
• Key problem:
– It would be nice to know when these are about to
happen
• Interesting theoretical wrinkles:
– Are the conditions for incipient regime shift the
same?
– Volatility? Stability?
• Of sufficient interest to be the subject of an
NPR piece
What Kinds of Incipient Shifts?
• Multiple settings
–
–
–
–
Medicine: Onset of asthma and epileptic seizures
Finance: Stock market crashes
Agriculture: Drought-society
Environment: Ocean circulation, fish stocks, rangeland
woody cover, boreal climate feedbacks
• “Canaries in the coal mine” – predicted catastrophic
bifurcations
– Volatility/variance
– Skewness
Critical Slowing
• At critical thresholds,
small changes in
state or conditions
can induce a
dramatic shift
– That is, the system
becomes increasingly
“slow” in recovering
Scheffer et al. (2009)
Nature
How Do You Measure Slowing Down?
• Response to experimental perturbations
– Measure the rate of return
– Impossible to control in large systems, but these are always perturbed
– Measure rate at which system fluctuates around the mean
• Are the fluctuations autocorrelated?
• Are they increasing in time?
• Are they changing in their statistical properties?
Scheffer et al. (2009)
Nature
Autocorrelation, Variance and Skewness
Scheffer et al. (2009)
Nature
Guttal and Jayaprakash 2008
Ecology Letters
Scheffer et al. (2009)
Nature
Confronting Theory With Data
• Detecting alternative
attractors
(observational)
– Time series shifts
– Bimodality in system
states (across
systems)
– Dual relationships to
a given control factor
Scheffer and Carpenter (2003)
TREE
Confronting Theory with Data
• Experimentally
– Different initial
conditions lead to
different final
states
– Disturbance
triggered
transitions
– Path dependency
(hysteresis)
Scheffer and Carpenter (2003)
TREE
Alt. Stable State – Desert Streams
• Wetlands used to be a major component of
desert streams (Sycamore Creek, AZ)
• Reduced to zero over the late 19th and 20th C.
• Recently become reestablished with controls
on grazing
Basic Mechanism
• Wetland plants control sediment
dynamics
– Density dependent stabilization
• Floods and drying are the major
hydrologic disturbance in deserts
• This makes areas with dense
vegetation better able to persist
through floods and dry periods,
and areas without vegetation less
able
– Two predicted states – vegetated
and gravel bed
– Region of global bi-satbility defined
by:
G – veg growth
S – vegetation mortality
Ks – channel sediment stability w/o veg
Cs – per capita stabilization of sediment
rs – scour vegetation mortality
V – vegetation
Q – flood frequency
Empirical Support
• Water and Vegetation cover over time
Major Resetting Flood
Vegetation Effects on Vegetation Loss
(the biogeomorphic feedback)
• Vegetation density
affects cover loss
• Stream sites self-organize
into 1 of 2 modes
Idle Restoration Thoughts
• Two strategies for restoration
– Change conditions (e.g., reduce P)
– Force state (e.g., massive disturbance)
• Are there other alternative stable states lurking?
Management and A.S.S.
Ecological release
Disturbance
Changes in herbivory
Nutrient enrichment
Hydrilla
Dominated
Native SAV
Dominated
Physical removal
Chemical removal
?
Blue-Green
Dominated
Should massive disturbance be part of the restoration tool-box?
Are there states less desirable than the one we’re trying to restore?
In a Spatial Domain…(Next Time)