Flashcards in Water-limited ecosystems Deck (19):
Act as valves, opening and closing in response to moisture. This needs to be balanced with demands for opening to let CO2 in.
Across stomata, humidity gradient determines how much water is lost
The ratio of carbon assimilated (GPP) to the amount of water that has transpired (TR)
WUE = GPP / TR (in mmol C/mol H2O
In water-limited ecosystems, we would expect WUE to be
Water stress occurs because of...
1. High demand
- high PS rates or low humidity
2. Insufficient supply
- low soil moisture (poor retention or low PPT)
How does photosynthesis change throughout the day to increase WUE?
High PS in the morning and closed stomata in the afternoon to reduce water loss when huimidity is low
What are the four drought survival strategies?
- Includes annuals and ephemerals
- Sufficient rainfall means they can germinate, grow, flower and seed over 6-8 weeks
- Seeds enclosed in chemicals which inhibit germination
- >25 mm rainfall required to remove
-Most of life as seed
- (hemi)cryptophytes or geophytes
- Most of year spent as bulbs, food stored below ground
- Growth during moist, cool part of year
- Plant becomes dormant above grount in drier climate
- Shed leaves and put smaller ones out or shed entire limbs
-Most of the plant underground
-Often extensive root system (inc. tap roots)
-Reduced leaf size
-Chlorophyll not as present in leaves but in stem instead
-Pubescence - boundary layer
-Increased volume:area ratio to reduce ET
-Reduced leaf permeability
-Growth continues through drought
-Common in deserts, salt marshes and dunes
-When water arrives, uptakes quickly (shallow roots)
-Stored in stem and leaves or below ground
-high volume:surface ratio - minimise ET
-CAM photosynthetic pathway
What does CAM stand for?
Crassulacean Acid Metabolism
What is CAM?
Staggered photosynthesis - the reduction occurs at night. Stomata open at night to let CO2 in whilst low ET occurs. CO2 stored as Malic acid.
Where do desert plans occur?
a) 10-30 degrees N and S of Equator - global circulation drives moisture into the stmosphere
b) shadow of a mountain
What proportion of the land-surface consists of arid environments?
What are the two biggest arid environments?
Sahara and Australia
What is the climate like in the desert?
Hot and dry during the day time due to semi-permanent high-pressure systems.
Cold at night (lack of cloud cover).
<1 mm rainfall per year
Rainfall patterns in the desert
<1 mm per year in the driest deserts. Some areas receive no rainfall, only moisture from coastal fogs and dew. Some receive sudden downpours - erosion.