ecosystem dynamics and carbon sinks (terrestrial lecture 3)

Question 1

What is the leaf economics spectrum (growth-survival trade-offs in tropical forests?)

Answer 1

Leaf toughness increases longevity but decreases photosynthesis

• thicker leaves are more resistant to herbivores/damage
• thicker leaves have lower allocation of N to photosynthetic machinery & have thicker cell walls and thus more resistance to CO2 diffusion to chloroplasts (Zhang et al 2017)

Less tough leaves have a higher photosynthetic rate and can exploit gaps when they form. Trade off between rapid growth and longevity

Question 2

Neutral theory vs niche differentiation theory

Answer 2

Neutral theory: ecological equivalence of species, chance events determine success

Niche differentiation: ecological differences between species, growth-survival tradeoffs

• leaf & wood functional traits underpin plant growth and survival
• different niches: e.g. pioneer, understory specialist, primary forest species

Question 3

What are the tropical forest niches and their traits?

Answer 3

Pioneers (large gap niche): shade intolerant, colonise large gaps, fast growing but short lived, w short lived leaves and low wood density. poor competitors

Understorey specialists: shade tolerant, confined to forest floor, slow-growing, long lived, don’t need gaps

Primary forest species (small gap niche): majority of canopy forming species. suppressed seedlingbank regenerates in shade, matures in small gaps, long lived. slow growing w tough leaves and high wood density. strong competitors + tolerate competition

Question 4

What is a carbon sink?

Answer 4

an ecosystem that:

• fixes more C in photosynthesis than is released through plant + soil respiration
• does so every year (interannual variability)
• sequesters fixed C in long-term stores (e.g. wood/soil)

Question 5

How to monitor forest growth/dynamics long term?

Answer 5

Survey sites every 20-40 years in old-growth forest

• resample structure, biomass & biodiversity
• look for evidence of change over time

Question 6

What are the extent, mechanisms and strength of the Amazonian C sink?

Answer 6

Amazon = ~1/3 of the global C sink
0.8 petagrams of C per year out of 2.6

• growth increase 10% decade
• mortality increase 5% decade
• C pool is growing as growth increase > mortality increase
  BUT
• composition shift to species that are faster growing with lower C density (-30%)
• faster C turnover

Amazonian C sink is weakening

• net biomass change is decreasing
• productivity is levelling
• mortality is increasing

Question 7

Why is tropical forest production increasing?

Answer 7

• more solar radiation increases photosynthesis

- rising atmospheric C stimulates photosynthesis

Question 8

What is the effect of drought on C sinks?

How sensitive to drought is the Amazon?

Answer 8

Droughts weaken tropical carbon sinks
2005 drought nullified Amazon C sink (Phillips et al, 2005)
- stopped growth, killed trees
- light wooded trees more likely to die than dense wooded

Question 9

Summary:

Where is the major long-term tropical C sink?
Main drivers?
Dependent on?
What has accelerated?
What is linked to weakening C sink?

Answer 9

• Amazon rainforest
• more light and increasing CO2 drive the C sink
• dependent on productivity and C turnover
• tree growth and mortality have accelerated
• weakening sink linked to increased tree mortality