No.
52, November/December 2002 Special issue: Selected papers from the IALC Conference: Assessing Capabilities of Soil and Water Resources in Drylands: The Role of Information Retrieval and Dissemination Technologies |
Mesquite (Prosopis velutina) has roots from 10 to 30 m long that
enable the plant to tap into underground lenses of fresh water. This morphological
adaptation allows mesquite to occupy flood plains and other sites with relatively
shallow groundwater and, in doing so, avoid stresses that other plants undergo
during drought. The evergreen creosote bush (Larrea divaricata) has
a wide-reaching root system and specialized adaptations that reduce competition
for soil moisture by surrounding plants. The evenly-spaced pattern of creosote
bush that is evident from above is due largely to excretion of toxic substances
that kill other plants. Spacing of creosote bush is related to rainfall
- the less the rainfall, the wider the spacing. Greater amounts of rainfall
seem to leach the "poisons" from the soil.
Annual plants also have characteristics that enable them to survive. Seeds of some annuals remain dormant in the soil for many years and only germinate under special conditions. Many of the annuals in the southwestern United States have no morphological adaptations to withstand drought conditions, but they require only 10 to 20 mm of rainfall before they will germinate. Seeds lie dormant in the upper layers of the soil until this threshold amount of rainfall occurs. Apparently, the mechanism by which seeds discriminate between rainfall of more than the threshold amount is the amount of leaching required to remove inhibitors to germination.
(Went 1955)