Arid Ecosystems - Global Change Biology

Arid Ecosystems
….are dry.
Defined by one resource: Water
Divided into 4 categories (“zones”):
Dry Sub-Humid: Kinda Dry
Semi-Arid: Definitely Dry
Arid: Really Dry
Hyper-Arid: Pretty much like Mars
Semi-Arid Woodland
Dry Sub-Humid
2 ways to define “zones”
• Precipitation only: Areas are classified by their
mean annual precipitation.
• Aridity Index: AI = p/ETP
• P = precipitation
• ETP = potential evapotranspiration
– Basically, the amount of precipitation divided by
the ability to lose water through evaporation.
• The sum amount of water that is lost
through evaporation (from soil or water
body surfaces) and plant transpiration
(water that is cycled through plants).
• Potential Evapotranspiration is the
amount of evapotranspiration without
water limitation, so a maximum value.
• ETP takes into account abiotic factors like
humidity, insolation and wind as well as
biotic factors like plant cover, leaf area,
and stomatal resistance.
• So, it’s harder to calculate AI, but it
includes the benefit of taking into
account information about the ecosystem
itself as opposed to using precipitation
Mean precipitation
% of Earth’s Surface
Dry Sub-Humid 800-1000
These values are not exactly universally agreed upon, but represent a fairly good
representation of the amounts of rainfall and AI expected for each zone.
Also, notice that the IA’s are all <1, meaning that there is more potential evaporation
than precipitation = water limited.
Take home message: These areas are Water Limited….
…. Sometimes during rainy periods, they do become nitrogen limited. Water washes
away nitrogen.
I live in Massachusetts and get rained/snowed
on every other day, why should I care about arid
• They represent a large area of Earth’s land surface ~20-30%
depending on your definition.
• Are found on every continent.
• Are home to ~2 billion people, and rising.
• Harbor large amounts of species and genetic diversity.
Especially cool species that have adapted to live in extreme
• Are predicted to expand in current climate change models, so
they’ll probably be getting bigger.
• Arid zones form in three key areas.
– Areas where dry air from Hadley Cells descends,
or ~ 300 North and South latitude. Sahara &
– The leeward side of mountains. Atacama & Gobi.
– Polar regions. McMurdo Dry Valleys.
Where, on a map, do we find Arid ecosystems?
Arid and Hyper-Arid
Stuff actually lives there?!
High species diversity is due in part to sheer size and geographic distribution; arid
ecosystems are everywhere.
Many of these areas are isolated…
…and ancient.
High stress generates lots of selective pressure.
– Interesting defensive strategies (poison/spines or coloration)
– Metabolic pathways (C4 grasses)
– Water use strategies (Succulents/cacti or mice that satisfy water requirement
from seeds)
People live there too
Why that’s a problem
Many of the countries that contain large areas of arid land are under developed or
Many residents of these areas are subsistence farmers that rely on their crops for food.
The economics and politics of these regions prevent local governments from being able
to step in when drought or other conditions destroy crops. i.e Sudan this year.
Consequently, these residents are at the mercy of the climate. And anything that
changes the productivity of the area will directly change the amount of food in their
So, the environment creates a problem for the people, but the people are the principal
agents of the land use change that contributes to desertification, and creates a problem
for the environment. It’s a vicious cycle.
Wow, these areas really are important. But how
are they changing?
• Highly susceptible/sensitive to climate and land
use change.
• Productivity is changing because
– Precipitation events are becoming more variable in
terms of intensity, timing, and inter-annual amounts.
– CO2 concentrations in the atmosphere are
• The area of arid land is predicted to increase as
temperature increases.
• The quality of the land itself is changing.
Variability in precipitation
• Precipitation is a function of atmospheric circulation. So, as atmospheric
circulation changes so will the timing, location, and size of precipitation
• How these changes will manifest themselves is unique to the area/desert.
– The Chihuahuan desert, for example, is predicted to have the monsoon season come
earlier in the year and with increased inter-annual variability.
– Also, as we saw in the paper, changes in the timing of rainfall can favor certain species.
• In general though, the prediction is that arid ecosystems as a whole will
become drier.
• Also, periods of extreme drought are predicted by climate models to
become more frequent and severe.
Productivity and Precipitation are inextricably linked (remember: water
limited). So, as these ecosystems become drier, productivity and food
production will see a corresponding decrease.
Responses to changes in precipitation are rapid
and extreme
11–16% (cover) pre-El Niño
54–80% during El Niño
13–21% post-El Niño
Northwest Peru
As the Earth warms, the area of arid land is
predicted to get bigger
Expansion is due to three factors.
First, a general poleward
expansion of the Hadley Cells.
Second, less precipitation is
predicted in the areas near the
Third, as an area warms, its
evaporative potential increases.
This compounds the effect of
lower precipitation.
(Bigger not necessarily better)
• Defined as “land degradation in the drylands
due to various factors, including climatic
variations and human activities”
• Lots of “factors”: land cover/use change,
grazing, biomass burning, air pollution, etc…
• Pretty big deal (we’ll talk about this when we
discuss the paper)
Text citations
Factors affecting ETP:
El Nino cover data: Gutiérrez JR, Arancio G, and Jaksic FM. 2000.Variation in vegetation and seed bank in a Chilean semi-arid
community affected by ENSO 1997. J Veg Sci 11: 641–48.
Desertification definition: Sivakumar, MVK. 2007. Interactions between climate and desertification. Agricultural and Forest Meterology.
142, 143-155.
Image credits.
Sonoran Desert:
Semi arid woodland :
Dry Sub-humid:
Evapotranspiration diagram:
Evapotranspiration map:
Climate area maps: Peel et al. 2007, Updated world map of the Koppen-Geiger climate classification. Hydrol. Earth Syst. Sci., 11, 1633–
Sudan woman farming:
Egyptian farmer:,-watersecurity-UN.aspx
Northern Peru variability: Gutiérrez JR, Arancio G, and Jaksic FM. 2000.Variation in vegetation and seed bank in a Chilean semi-arid
community affected by ENSO 1997. J Veg Sci 11: 641–48.
Predicted change in Sahel: Held IM, Delworth TL, Lu J, Findell KL, Knutson TR. 2005. Simulation of Sahel drought in the 20th and
21st centuries. PNAS. 102, 17891-17896

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