L12 - Plants in a changing climate

Question 1

How are drylands defined?

How is PET measured?

Answer 1

• Areas where average rainfall < potential moisture losses through evaporation and transpiration annually.
• P/PET < 0.65 (annual precipitation/potential evapotranspiration)
• PET measured using a pan evaporimeter

Question 2

Give the four sub-categories of dryland and their characteristics.

Answer 2

Hyper-arid
- No growth without irrigation
- P/PET < 0.05

Arid
- Short crop growing season
- 0.05< P/PET < 0.20

Semi-Arid
- 0.20 < P/PET < 0.50
- Short growing season

Note: droughts of 1-2 years common in arid regions

Dry sub-humid
- 0.50 < P/PET < 0.65

Question 3

Give two of the impacts of aridity on vegetation communities.

Answer 3

1) Risk of overheating
- Plants cool leaves via transpiration but lack of water inhibits that.
- Actual leaf temp. closest to critical temp. for photosynthesis in arid zones.

2) C4 plants are dominant in drylands
- C4 photosynthesis increases [CO2] around RuBisCO, reducing photorespiration, increasing photosynthesis rates at high temp.
- Photorespiration occurs when O2, not CO2, fixed by RuBisCO. More common in drylands as [O2] builds up as stomata close to avoid water loss.
- Photorespiration causes loss of carbon + energy

Question 4

How does canopy structure change due to water limitation?

Answer 4

• More open canopies: trees become shorter + more spaced apart to reduce water competition
• Trees become multi-stemmed to reduce cavitation impact. Damaged stem can be shed
• Smaller leaves, higher LMA (reduce photosynthesis rate to conserve water), more negative Ψs.
• Trees can become deciduous in extended dry season

Question 5

Explain how savannah vegetation varies with annual precipitation habits.

What can open canopy lead to?

What have these arid conditions caused evolutionarily?

Answer 5

• Higher annual precipitation + PREDICTABLE dry season allows trees (open canopy)
• As annual precipitation decreases as does dry season predictability = no more trees. Multi-stemmed shrubs and grasses dominate

(exceptions e.g Karoo in SA where landform gives predictability, CAM succulents dominate).

• Open canopy allows grasses = fire prone ecosystem
• Diversification of C4 grasses + savannah vegetation 5-10 Mya in response to seasonal aridity.

Question 6

What method do some plants use in semi-arid habitats to collect water?

Outline this method and give two example plants

Answer 6

• Phreatophytes use long roots to tap into groundwater - more permanent source - via hydraulic lift. E.g. Acacia sp. - Africa/Australia
• Soil becomes drier nearer to the surface, water can move down potential gradient from roots to soil in hydraulic shift
• Lift + shift mainly occurs at night during rehydration
• 30% of daily water requirement from lift in Acer saccharum

Question 7

How different seasonal rains be distinguished?

Answer 7

• Analysing stable isotope signals δ18O and δ2H using Standard Mean Oceanic Water (SMOW) as the reference
• Evaporation favours lighter isotope, decreasing both signals.
• Condensation for rain favours heavier isotopes, increasing both signals.
• Summer rains heavy isotope deficient (negative)
• Winter rains/snow even more heavy isotope deficient (more negative)

Question 8

How can the isotopic difference in seasonal rains give information on the source of water in a plant?

Give an example case study

Answer 8

• Groundwater normally replenished by winter rain = more isotopically depleted (more negative) signal
• Upper soil recharged by summer rains with shallow penetration = less depleted isotopic signal

Todd Dawson (1996):
- Analysed stable isotopes in plants growing at base of an Acer saccharum
- Tree used mainly ground water
- Area closest to tree replenished by ground water
- Xylem sap showed herbaceous plants closest to tree were accessing lifted groundwater (e.g. P. peltatum)
- Greater growth close to tree base
- 4-5m away from base P. peltatum dependent on summer rainfall = more water stressed seen from more negative Ψw

Question 9

How do phreatophytes establish themselves?

Answer 9

• Transient rains wet soil surface causing reverse Ψw gradient in roots.
• Water travels down to growing apices, moistening surrounding soil + causing growth down
• Phreatophyte seedlings start as “savers” (High WUE, only grow in rainy season)
• Once roots reach ground water they “spend” ( low WUE, grow during all seasons)

Question 10

How do succulent plant such as cacti and agave survive in arid conditions?

Answer 10

• They AVOID drought rather then TOLERATE it
• As soon as soil dries, Ψw = -0.3, outer root cortices shrink, isolating tissues from soil water deficit. This prevents water returning to soil.
• In next rain their chunky roots rehydrate and restore water storage
• So succulents never have water potentials more negative than -1 MPa
• Can protect mature roots by cavitation at junction w/ new roots - if new growth dies back the rest of the plant is protected