what controls global biome distributions/why dont palms grow in sheffield (terrestrial lecture 7)

Question 1

Why don’t palm trees grow in Sheffield?

• three hypotheses

Answer 1

• palms can’t survive cold climate of sheffield winters
• palms could grow in sheffield but are outcompeted by better adapted native species
• palms evolved in the tropics & could grow in sheffield but haven’t dispersed that far north yet

Question 2

Factors determining global distributions:

Environmental filters

Answer 2

Environmental filters:

• survival of cold or dry environmental extremes need specialist adaptations.
• poleward extent of plant life forms controlled by minimum temperatures/physiological ability to survive low temperatures (Woodward, 1987

Question 3

What is the response of species from hotter climates to low temperatures?

• tropical + subtropical species
• olive & bay trees, cork oak: min winter temps in sheffield botanical gardens

Answer 3

tropical and sub tropical species:

• chilling injury: less than 10 - 12oC
• freezing injury + death below 0oC

UK min winter temperatures
1981-82 = -16.1 °C
2011-12 = -17.8 °C :

– Olea europea, Laurus noblis: killed these mediterranean species

– Quercus suber: defoliated this evergreen broadleaf species (from fireprone environment)

Question 4

What are the mechanisms of freezing damage?

Answer 4

• damage from ice formation within cells
• ice formation between cells causes cell dessication and collapse
• freeze-thaw cycles cause xylem embolism

Question 5

What are mechanisms of freezing resistance?

Answer 5

• accumulation of solutes (e.g. sucrose,
  proline) raises osmotic potential, depresses freezing point & opposes desiccation.
• anti-freeze proteins depress freezing point,
  prevent large ice crystals, stabilize
  membranes to minimise ice damage to cells.
• short, narrow xylem cells less vulnerable
  to embolism

Question 6

Does competition exclude species that are more tolerant of extremes?

Answer 6

• temperate species more resistant to freezing than tropical
• tropical species faster growth rate

—– trade-off between freezing resistance and growth rate

Question 7

Factors determining global distributions:

Competition

Answer 7

Competition:
- stronger competitors exclude specialists from warm and/or wet environments
- resistance of freezing or drought
carries metabolic costs.
- cold- or dry-adapted species are excluded from the wet tropics by competition

Question 8

Factors determining global distributions:

Phyleogeographic history

Answer 8

Phylogeographic history:

• for some groups migration may be prevented
• geographic/environmental/ecological barriers