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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