Transcript DRYLANDS AND GLOBAL CHANGE: RAINFALL VARIABILITY …

DRYLANDS AND GLOBAL CHANGE:
RAINFALL VARIABILITY AND SUSTAINABLE
RANGELAND PRODUCTION
James F. Reynolds,1 Roberto J. Fernández, 1,2 & Paul R. Kemp 3
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2
Department of Botany, Duke University, Durham, NC 27708, USA
Permanent address: IFEVA-Facultad de Agronomía, Universidad de Buenos Aires, Buenos Aires 1417,
Argentina
3 Department of Biology, University of San Diego, San Diego, CA 92110, USA
ABSTRACT
Over a third of the land surface of the Earth is composed of drylands and much of this (about 65-70%) is seriously
degraded or desertified. The two major desertification drivers—climate change and human activities—have ecological
and social impacts at various temporal and spatial scales. These impacts include potential alterations of carbon, water,
and trace gas budgets, loss of vegetation cover, and increased wind-borne dust, all of which may affect global
biogeochemistry, radiation balance, and climate. The societal consequences of land degradation are also serious since
the fate of rural people in drylands is dependent on the effective use of natural resources, e.g., water, soils, plants,
livestock and wildlife. In this poster, we discuss how ecosystem-level predictions may be used to address issues relevant
to sustainable development of these semi-arid regions. We argue that the next step in assessing sustainability is to
incorporate ecological impacts into higher level models that consider direct and other human impacts on these systems.
This will require further testing and evaluation of ecosystem-level models in the context of different management and
land-use alternatives. We propose the incorporation of both "natural" and human factors into a spatially explicit model
of landscape elements and human land-use patterns in order to develop predictive tools capable of dynamic, integrated
assessments of impacts of global climate change on human-dominated ecosystems.
BACKGROUND: Land degradation and desertification. Arid and semiarid drylands compose 1/3 of the land
surface of the world and are home to about 20% of the human population. The vast majority of these drylands consist
of rangelands (ca. 88%), whereas the rest are classified as rainfed (3%) and irrigated croplands (9%). The rapid
growth of populations in many of these regions, often in conjunction with imprudent land management, has led to
increased social vulnerability and to rapid land degradation. Soils in drylands are especially vulnerable to wind and
water erosion, loss of organic matter, decline in fertility, salinization, and compaction. This is particularly evident
when the natural vegetative cover is reduced through human activities such as intensive livestock grazing, excessive
cultivation, urbanization, and other land uses.
Global Drylands (5,159.8 Million ha)
While global rates of land
70% degraded
S. America
degradation and the total areas
Degraded
Total
affected are difficult to estimate,
N. America
there is ample evidence that
Europe
extensive areas of the world’s
Australia
drylands have experienced some
Asia
form of chronic degradation
Africa
during the last century. It is
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2000
estimated that ca. 80% of the
world’s rangelands, 60% of
rainfed croplands, and 30% of
irrigated croplands are threatened
by various types of degradation,
generally referred to as
"desertification." Thus,
desertified drylands make up
about 65-70% of the total
dryland area of the globe (arid,
semiarid, and dry subhumid
regions, but excluding hyperarid
regions). All three of the
developing regions of the
world—Africa, Asia, and Latin
Irrigated Cropland
(145.5 Million ha)
S. America
N. America
Europe
Australia
Asia
Africa
30% degraded
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100 0
Rainfed Cropland
(457.9 Million ha)
Rangelands
(4,556.4 Million ha)
47% degraded
73% degraded
100
Million ha
200
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1000
2000
Two major desertification drivers—climate change and human activities—have ecological and social
impacts at various temporal and spatial scales, ranging from local (and short-term) to global (and longterm) (see figure). Carbon, water, and trace gas budgets may be significantly altered; losses of vegetation
may modify regional albedo, raise air temperatures, and increase wind-borne dust; and all of these
changes have the potential to act in concert to affect global biogeochemistry, radiation balance, and
climate. The societal consequences of land degradation are equally serious. The fate of rural people in
drylands is dependent on the effective use of natural resources, e.g., water, soils, plants, livestock and
wildlife. In spite of this, over large areas, natural vegetation continues to be degraded, soils are eroding,
and the capacity of the land to support livestock and wild herbivores is being reduced. Combined with
complex political, social, and economic factors, which often tend to have equally important roles, adverse
human impacts are inevitable.
Simplified model of various factors
(natural, human-caused) that play a
role in dryland desertification. Note
that drought and desertification
operate at different time scales
(short- vs. long-term, respectively).
Because nearly all drylands are
characterized by extreme year-toyear weather fluctuations that may
result in changes in ecosystem
appearance, it is often difficult to
distinguish between short- and
long-term trends, as well as
between temporary and permanent
changes. Shifts in vegetation (grass
-shrub) may or may not be
reversible, depending on the
interactions of numerous climatic,
Environmental sustainability ("ES")
ES means maintaining natural capital, akin to the definition of EcS. Although ES is needed by humans and originated because of social
concerns, ES itself seeks to improve human welfare and SS by protecting the sources of raw materials used for human needs and ensuring
that the sinks for human wastes are not exceeded, in order to prevent harm to humans. Humanity must learn to live within the limitations of
the biological and physical environment ("sources") and as a "sink" for wastes. This translates into holding waste emissions within the
assimilative capacity of the environment without impairing it. It also means keeping harvest rates of renewables to within regeneration rates.
Quasi-ES can be approached by holding depletion rates equal to the rate at which renewable substitutes can be created.
Sustainability. Natural, semi-natural, and intensively managed dryland ecosystems of the world offer a wide range of
different goods and services vital to human populations: food production (humans and livestock), construction materials,
climate regulation, soil maintenance, nutrient recycling, wildlife habitat, erosion control, tourism/recreation, and aesthetic
enjoyment. When weighing the advantages and disadvantages of some particular course of action that affects these
ecosystems—and hence their "sustainability"—decision-makers need quantitative assessments in order to consider these
goods and services. Understanding the interplay between ecosystem services and ecosystem functioning and structure has
urgent application to land use planning and management in dryland areas. Goodland & Daly (Environmental sustainability:
universal and non-negotiable. Ecol Appl 1996;6(4):1002-1017) argue that the true meanings ofsustainability—social,
economic and environmental—are obvious only when considered separately. In the context of global change and its
potential impacts on drylands, it is inconceivable that one type of sustainability could be realized in the
absence of the others since the inter-dependencies are so strong.
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Jornada
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2
Sahel Average
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S
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N
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rm
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(±
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F
ro
m
It is difficult to distinguish between shortterm variability and long-term changes in
ecosystem appearance, as well as between
temporary and permanent changes. Shortterm variability tends to affect the range and
frequency of "shocks," where long-term
change alters the resource base. Shifts in
vegetation may or may not be reversible,
depending on the interactions of numerous
climatic, edaphic, and biological factors. In
the long run, however, global climate change
may further exacerbate the already high
natural variability of drylands, leading to
permanent degradation of their productive
potentials, particularly since there is a lack of
"buffering" by large reserves of organic
matter in the soils or in woody vegetation.
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Long term precipitation patterns from the Jornada
Experimental Range in southern New Mexico and the
Sahel in West Africa. Variability is the norm, NOT the
average.
D
e
p
a
rt
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re
CLIMATE VARIABILITY: Long- vs. shortterm impacts. Drylands are particularly
vulnerable to climate variability, of which
precipitation is the most important
component. For example, a slight shift in
seasonal precipitation and/or frequency of
extreme rain events could potentially lead to
the over-exploitation of the meager resources
of drylands and contribute to the further
degradation of the very resource base on
which human populations are so dependant.
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-2
1900
Sahel data from Sharon E. Nicholson, Florida
1925
1950
Ye a r
1975
2000
Rainfall variability and plant production
Ranchers and farmers in arid and semi-arid regions of the world have long recognized the importance of shortterm rainfall variability on farm and livestock production, and rainfall variability continues to be the principal
source of fluctuations in global food production, particularly in developing countries. Given the extreme
variability of rainfall in drylands and the low primary production, we might ask: is there a relationship
between rainfall variability and above-ground primary production? Le Houérou et al.examined this
relationship for a variety of vegetation types around the world and concluded that dryland ecosystems are highly
variable in response to water inputs. They reported that variability in annual production was 50% greater than
the corresponding variability in annual rainfall on sites receiving less than 600 mm).
However, the great diversity of sites evaluated (e.g., shrublands, grasslands, etc.) make it difficult to understand
what causes the high variability in plant production in relation to rainfall. For example, we might expect that arid
shrub communities should show less variation in production than semi-arid grasslands because the former are
deeper rooted and, therefore, less dependent on current year’s precipitation than shallow-rooted grasslands.
Le Houérou HN, et al. Relationship between the variability of primary production
and the variability of annual precipitation in world arid lands. J Arid Environ
1988;15:1-18
CVP = 0.16913 + 0.92707 *CVR
Coefficient of variation
in production (CVP)
1.2
R2 = 0.247
(P=0.0001)
We are conducting a series of field, laboratory, and
modeling studies in rangelands to elucidate the
relative importance of different climate scenarios
and management practices on ecosystem processes
in drylands. We now present a brief case study that
illustrates the importance of the question—how do
the different plant functional types respond to
variation in seasonal and annual precipitation?
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CASE STUDY: USE OF SIMULATION
MODELING
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Coefficient of variation in rainfall (CVR)
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Jornada
Experimental
Range,
southern
New Mexico
CASE STUDY: Our case study is based on work in the
Jornada Basin of south-central New Mexico. The Jornada
Basin was once dominated by warm-season perennial
grasses (e.g. Bouteloua eriopoda), but much of the area is
now dominated by shrubs (e.g., Larrea tridentata and
Prosopis glandulosa). This transition—which was
initiated in the late 1800s or early 1900s— is believed to
have been driven by overgrazing and global change. The
Jornada Basin now contains remnant grassland
communities, and most areas are dominated by shrubs or
co-dominated by shrubs, subshrubs, forbs, succulents, and
grasses. The perennial grasses provide the most valuable
forage for livestock in this region.
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Photoperiod
Precipitation
Radient E
CO2
Phenological triggers
[nutrient]
Tair
Psn
Trans
gr o
wth
Rainfall
stomatal
conductance
0
&a
lloc
a
% Nitrogen
tion
> 0.05
0.04-0.05
0.03-0.04
0.02-0.03
Infiltration &
percolation
Plant uptake
10
Soil Depth (cm)
To address the importance of rainfall variability and
rangeland production, with particular emphasis on
the relative effect on different plant functional
types, we used the Patch Arid Land Simulator
(PALS) developed for the Jornada. PALS is a
physiologically-based ecosystem model that
contains the principal components of ecosystem
carbon, water, and nutrient cycles. The results
presented here are from our phenology-based
version (PALS-FT), which includes the key plant
functional types (FT) of the northern Chihuahuan
Desert—shrubs, cool season annuals, summer
annuals, forbs, and perennial grasses.
PAL
S
H2O
20
30
40
50
.
..
100
Soil N
Soil Water
Plant
uptake
Root
turnover
1 2/3 1/3 0 1/3 2/3 1
Canopy Distance
Mineralization /
Immobilization
Tsoil
Soil respiration
Litterfall
Labile
Litter
Refractory
Litter
Active
SOM
Stable
SOM
RESULTS:
In the first series of PALS
simulations, we examined the
potential impacts of historical
variations in precipitation on
productivity, on both a year-to-year
basis and over periods associated with
decade-length climate shifts. We
selected three periods from the long
term records at the Jornada Basin
(1914-1997, average annual rainfall =
247 mm): a "normal" decade (19681977, average=250 mm); a "dry"
decade (1947-1956, 33% below
normal or 166 mm); and a "wet"
decade (1984-1993, 32% above
average rainfall or 325 mm) (see
previous figure).
Grouping all of these years together,
our model simulations show that
rainfall has large impacts on
simulated annual net primary
production (ANPP), although the
absolute magnitude varies with plant
functional type. The scatter for these
simulated results is substantial, which
illustrates that while there is a general
increase in productivity with
increasing annual precipitation, there
is also considerable variation
associated with the timing of that
rainfall within individual years and
with the utilization of this moisture
by different functional types.
RESULTS: 10 series of 10-year model runs using rainfall of normal
decade (1968-1977), but in which each year's rainfall modified by a
random amount varying from -30 to +30% (daily distributions fixed). Thus
each 10-year rainfall series had a coefficient of variation (CV) varying
from a low of 0.25 to a high of about 0.45, and for the 100-year period, the
CV was 0.351 (identical to that of the natural rainfall for the period 19151997).
(1) Both shrubs & forbs in had low CVs (ca. 0.4) that were statistically
invariant with increasing variation of rainfall (p > .05). This CV is close to
the long term average CV of rainfall in the Jornada Basin (0.35). These
functional types best exemplify plants capable of utilizing moisture that
may occur during any season (note that he dominant shrub is a droughttolerant evergreen and the forbs are short-lived, herbaceous plants that can
grow in any season).
(2) both winter and summer annuals had production CVs much higher than
CV rainfall (ca. 1.45 for winter annuals and 1.25 for summer annuals) and,
again, not dependent on rainfall. This high variation reflects the fact that
productivity in these species depends on germination responses that are
highly seasonal and rainfall specific. Small rainfall events within a season
or large rainfall outside of their strict seasons result in almost no
productivity because of lack of germination. Furthermore, their roots are
located in the upper soil profile, which is subject to rapid drying. Thus,
there are a number of seasons where substantial rain may not translate into
any production; at other times, a small amount of rainfall that happens to
be precisely timed for use by a seasonal annual species, may result in
substantial productivity.
(3) Grasses only functional type with significant increase (p < .05, r2 =
0.63) in CV of production with increasing CV of precipitation (Fig. B).
Because these perennial grasses grow mainly during the summer, they
appear to be utilizing the most reliable moisture resource. However, there
is competition for this moisture from other plant functional types, and high
evaporative demand can quickly remove moisture from small rainfall
events. Many small events are not equivalent to the same amount of rain in
a single event that percolates deeper within the profile. Thus, timing and
amount of individual events becomes more important in summer, when
Le Houérou, et al.
PALS output
PALS output
CONCLUDING REMARKS
The case study we present in this poster illustrates how fundamental knowledge of environmental factors (rainfall
variability) and ecosystem structure and function (plant functional type physiology) can be integrated within a
modeling synthesis. Our results underscore the impact of short-term climate shifts and year-to-year variability
upon plant production, especially perennial grasses. Year-to-year variability in total plant productivity was found
to be roughly 50% greater than the variability in "normal" rainfall and variability in grass productivity was 3x
greater. From a management standpoint, perennial grass is one of the most important functional types because of
its contribution to forage production and ground cover. Maintaining a stable grass cover is desirable in order to be
able to project long-term land use strategies and economic return. However, our modeling results suggest that
maintaining stable grass cover may be very difficult in this semi-arid region, given the potential impacts of natural
climate variability upon vegetation production. Furthermore, our results support the point of view that dryland
agriculturists and pastoralists must be adaptive in terms of their response to climate variability.
While these ecosystem-level predictions may be used independently to address issues relevant to sustainable
development of these semi-arid regions, we suggest that the next step in assessing sustainability is the
incorporation of the ecological impacts into higher level models that incorporate direct and other human impacts
on these systems. This step will require further testing and evaluation of ecosystem-level models in the context of
different management and land-use alternatives. Finally, at the highest level of integration, we propose the
incorporation of both "natural" and human factors into a spatially explicit model of landscape elements and human
land-use patterns. Only at this level, will we have predictive tools capable of dynamic, integrated assessments of
impacts of global climate change on human-dominated ecosystems. Although we are far from being able to do
this, a number of programs around the world, such as the urban LTERs in the U.S., are striving to develop
integrated approaches to link human and natural systems. We believe this task is essential in preparing for—and
planning corrective measures to—the complex interactions between climate, human activities, and the well-being
of the Earth