THE NATURE OF DROUGHT
Agricultural drought primarily affects dryland farming and grazing
Hydrological drought affects water supplies and riverine ecosystems, agriculture dependent solely on surface-water irrigation
Ecological drought affects ecosystems dependent on meteoric water
AGRICULTURAL DROUGHT (Grazing)
Before widespread use of stockponds and wells, water supply for grazing animals was the largest problem (1891-1904 drought)
Now, forage availability is a large problem exasperated by previous grazing history
Woody plant increases in 20th century also increase drought effects
AGRICULTURAL DROUGHT (Grazing)
HYDROLOGICAL DROUGHT
Water availability is generally less (particularly winter runoff) but changes in flood frequency is less predictable
Riparian vegetation is unlikely to reproduce but may not have profound short-term impacts
HYDROLOGICAL DROUGHT
Salt River near Roosevelt, Arizona
HYDROLOGICAL DROUGHT
HYDROLOGICAL DROUGHT
Riparian vegetation is unlikely to be significantly affected along perennial streams
Plants on distal floodplain margins or along ephemeral streams will be most affected
Extended drought can kill riparian trees in sites with less dependable water (influent reaches)
Additional groundwater pumpage or surface-water diversion could significantly compound drought effects
HYDROLOGICAL DROUGHT
Extreme events arent necessarily linked to droughts
In 1896, a major Utah drought year, large floods occurred during record rainfall in September
In 1951, during the mid-century drought, record flooding occurred in central Arizona during August
ECOLOGICAL DROUGHT
Desert plants are dependent on meteoric water
Deep soil water is distinguished from shallow soil water but is actually moisture recharging to greater depths from past high rainfall years
Root structure and physiological characteristics (C3/C4/CAM) may determine survivability of desert species to drought
DYNAMICS OF UNDISTURBED VEGETATION IN THE MOJAVE DESERT
Before our study, most ecologists viewed Mojave Desert vegetation as static and unchanging
Some ecologists retain the community view of ecosystem processes; others consider species response to be individualistic
NTS CLIMATE,
1960-2000
NATIVE VERSUS
NON-NATIVE ANNUALS
POSITIVE RESPONSE OF INDIVIDUALS
LARGE, FLUCTUATING RESPONSE
HIGH DROUGHT-RELATED MORTALITY
CLIMATICALLY-INSENSITIVE SPECIES
LONG-TERM (1963-2003) CHANGE IN UNDISTURBED PERENNIAL VEGETATION
LONG-TERM STABILITY OF VEGETATION ASSOCIATIONS?
In 1979, Beatley used her data to define 16 vegetation associations in the Mojave and Transition Deserts. Of those, 8 named spiny hopsage (Grayia spinosa) as a dominant.
In 2000, Ostler et al. defined 10 vegetation associations for Mojave and Transition Deserts. One lists Grayia as dominant.
In 1999-2002, Grayia doesnt dominate any of Beatleys plots, and only 3 of 28 plots had enough Grayia to justify calling them associations.
DROUGHTS ARE ESSENTIAL TO ECOSYSTEM FUNCTION
Drought helps reset elements of Mojave Desert shrub assemblages
Drought may drive successional processes, killing short-lived species and enabling long-lived species to establish dominance
Drought ultimately may help control non-native annual species that are adapted to more predictable climates
THE PROBLEM OF DROUGHT SEVERITY AND FREQUENCY
The severity of a drought depends upon its nature (whether agricultural, hydrological, or ecological)
Magnitude and frequency are difficult to assess some ecological catastrophes may be strongly influenced by the history leading up to the drought (e.g., high, unsustainable growth periods beforehand)
Droughts do not affect all landscape elements equally