Water-limited ecosystems

Question 1

Stomata…

Answer 1

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

Question 2

WUE

Answer 2

Water-use efficiency

The ratio of carbon assimilated (GPP) to the amount of water that has transpired (TR)

Question 3

Calculate WUE

Answer 3

WUE = GPP / TR (in mmol C/mol H2O

Question 4

In water-limited ecosystems, we would expect WUE to be

Answer 4

High

Question 5

Water stress occurs because of…

Answer 5

1. High demand
   - high PS rates or low humidity
2. Insufficient supply
   - low soil moisture (poor retention or low PPT)

Question 6

How does photosynthesis change throughout the day to increase WUE?

Answer 6

High PS in the morning and closed stomata in the afternoon to reduce water loss when huimidity is low

Question 7

What are the four drought survival strategies?

Answer 7

Drought-evading
Drought-escaping
Drought-enduring
Drought-resisting

Question 8

Describe drought-escaping

Answer 8

• 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
• Shallow-rooted
• Most of life as seed

Question 9

Describe drought-evading

Answer 9

• Perennials
• (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

Question 10

Describe drought-enduring

Answer 10

• Slower PS
• Most of the plant underground
• Often extensive root system (inc. tap roots)
• Erectophile orientation
• 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

Question 11

Describe drought-resisting

Answer 11

• Growth continues through drought
• Shallow-rooted succulent
• 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

Question 12

What does CAM stand for?

Answer 12

Crassulacean Acid Metabolism

Question 13

What is CAM?

Answer 13

Staggered photosynthesis - the reduction occurs at night. Stomata open at night to let CO2 in whilst low ET occurs. CO2 stored as Malic acid.

Question 14

Where do desert plans occur?

Answer 14

a) 10-30 degrees N and S of Equator - global circulation drives moisture into the stmosphere
b) shadow of a mountain

Question 15

What proportion of the land-surface consists of arid environments?

Answer 15

1/3

Question 16

What are the two biggest arid environments?

Answer 16

Sahara and Australia

Question 17

What is the climate like in the desert?

Answer 17

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

Question 18

Rainfall patterns in the desert

Answer 18

<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.

Question 19

Temperature in the desert

Answer 19

Ranges greatly over diurnal and annual cycles