Download A3 : TD`s vegetation (essay outline : simplified version)

Ref. 92#4 & 97#3
Describe and explain the physical constraints faced by vegetation in tropical deserts. Using appropriate
examples, explain how they adapt to the harsh environment.
Des. & Exp. 6
Dis.
4+6+4
Eg.
5
A.
Physical constraints
1. Precipitation
a. low annual precipitation (< 250mm annually)
- Subtropical High
- sinking limb of the Hadley Cell
b. intense heat
- absence of cloud high pressure
intense insolation
potential evaporation exceeds ppt.
low effective ppt.
c. torrential downpour
- rainfall is unreliable
- short growing season
plants have to depend on torrential downpour for survival
2. thin, infertile and saline soil
- thin  slow rate of weathering  aridity
- infertile  sparse vegetation to supply nutrients
- saline  strong capillary rise
B.
Adaptation
1. drought evaders
- lie dormant as seeds in dry period
- germination occurs after sudden torrential downpours
- die down within a few weeks
- complete life cycle within 2-3 weeks
- ephemerals
- E.g. Tribulus in NE Khartoum
gama grass germinates seeds within 4 weeks
- seeds may be equipped with dispersal units to aid their distribution
2. drought resisters
- shed leaves to minimize water loss by transpiration
E.g. Ocotillo in N. American desert
- xerophytes : need little water for survival
- halophytes : can survive in salty environment
E.g. saltbrush
- leaves : small, rolled, spiny, waxy / with thick cuticle, hairy to reduce transpiration surface
- stomata closes in day time and open at night
- low in height / low in stature / low shoot to root ratio, around 1:35 to 1:20,
to avoid desiccation by wind
- succulent, swollen stem
E.g. cacti
the small ‘bi’ plants in Kalahari have tuberous root to hold 2 gallons of water
- extensive root system
Vertically extensive to tap ground water
E.g. N. American mesquite trees (30m)
Horizontally extensive root system to draw water from a wider area  avoid competition for water
C.
Spatial characteristics
- low NPP : 90g/m2/yr. compared with TRF (2200g/m2/yr.) and tundra (140g/m2/yr.)
- low density
- discontinuous
- scattered
- mainly found near water sources