Controls on tropical forest CO2 and

energy exchangeMichael L Goulden, Scott D Miller, Humberto da Rocha, Chris Doughty, Helber

Freitas, Adelaine Michela Silva e Figueira, Cleilim Albert Dias de Sousa  

Tapajos km 83 tower site

Measurements –micrometeorology, physiology, ecology, biometry

A big question – Are old growth forests large carbon sinks?

In an evolutionary sense, are canopy trees:

H1 Trying to get bigger and bigger? Do they allocate resources to maximize further growth?

AH1 Trying to maintain their position of dominance? Do they allocate resources to hold on to what they already have?

TheoryKing 1990 - “An important conclusion is that the growth pattern maximizing individual survival may differ greatly from that maximizing collective wood production.”

Grime – Disturbance opens things up

Early succession:Rapidly growing plants that are good at competing to reach resources first

Old growth:Slow growing plants that are good at holding onto resources

PredictionsIf canopy trees:

H1 Are trying to get bigger and bigger

They should maximize carbon gain by carrying out photosynthesis at all times of the day and in all seasons of the year when it is possible. They should turn the sugars fixed during photosynthesis into new growth as efficiently as possible.

AH1 Are trying to maintain their position of dominance

They should not worry about maximizing growth, but should emphasize a strategy of anticipating and avoiding risk.

Leaves exposed to constant light for 36 hours reduce photosynthesis in the afternoon.

Leaf in chamber exposed to constant light

-0.5

0

0.5

1

1.5

2

2.5

6 12 18 24 30 36 42

An (m

ol m

-2s-1

)Outside Day 1 Outside Night Outside Day 2

Hours

Are trees trying to maximize photosynthesis over a day?

The same pattern is observed at the whole forest level using eddy covariance.

Photosynthesis is reduced at a given light level in the early morning and late afternoon compared to the middle of the day.

Why not take advantage of all the light? This is a cautious approach – trees are not trying to maximize growth.

Are trees trying to maximize photosynthesis over a day?W

hole

Eco

syst

em C

O2

Exc

hang

e (

mol

m-2s-1

)

PPFD (mol m-2s-1)

1315-1515

0515-0715

1515-1800

715-915

915-1115

1115-1315

0 500 1000 1500 2000 2500

-10

0

10

20

-10

0

10

20

0 500 1000 1500 2000 2500

Litterfall begins before drought.

So

il w

ater

(c

m3

cm –3

)

0.250.300.350.400.450.500.55

July 1 Oct 1 Jan 1 Apr 1 July 1

4

6

8

10

12

14

Lea

f L

itte

r fa

ll (k

g C

ha-1

day

-1)

Are trees trying to maximize their photosynthesis seasonally?

The early litterfall costs the trees carbon.

Less whole forestPhotosynthesis at a given light level

Why drop your leaves before you have to? Why not take advantage of moisture from May to July?

This is a cautious approach – trees are not trying to maximize growth.

Are trees trying to maximize their photosynthesis seasonally?

-20

-10

0

10

20

So

il w

ater

(c

m3

cm –3

)

0.250.300.350.400.450.500.55

July 1 Oct 1 Jan 1 Apr 1 July 1

Res

idu

al C

O2

Exc

han

ge

(m

ol m

-2s-1

)

Only 25 to 30% of annual total photosynthesis (GPP) is used for plant growth (NPP).

Rapidly growing crops and herbs use 75%; many trees use 50%.

This is a cautious approach – trees are not trying to maximize growth.

Gross Primary Production 30 TC/hayr

Net Primary Production 8 TC/hayr

Litterfall 3 TC/hayrGross wood increment 2 TC/hayrRoot growth 3 TC/hayr

(Similar results in Chambers et al)

Are trees trying to maximize their growth over a year?

PredictionsIf canopy trees:

H1 Are trying to get bigger and bigger

They should maximize carbon gain by carrying out photosynthesis at all times of the day and in all seasons of the year when it is possible. They should turn the sugars fixed during photosynthesis into new growth as efficiently as possible.

AH1 Are trying to maintain their position of dominance

They should not worry about maximizing growth, but shouldemphasize a strategy of anticipating and avoiding risk.

Why should big trees want to get bigger?

Overstorey plants have won - they have the light

Understorey plants don’t have enough light to grow rapidly – they don’t pose much risk

There’s not much advantage in getting bigger once a tree has overtopped its competitors

0

1

2

3

4

5

6

7

20 30 40 50 60

Rel

ativ

e T

orq

ue

Tree Height (m)

Why not get bigger?

One of several possible costs is the increased risk of damage/toppling

The force from wind increases rapidly once the tree rises above the canopy, increasing the risk of damage.

Why risk it if you’ve already won?

Is wind really a risk to big trees?

Three major tree falls over 7 years of observation at the Tapajos National Forest (2 at km 83, 1 at km 67).

~ 0.2 to 1 large tree fall/ha yr

Is wind really a risk to big trees?

Both tree falls at km 83 occurred during extraordinarily strong winds

ImplicationsIf canopy trees are:

H1 Trying to get bigger and biggerThen favorable changes in the environment such as elevated CO2 or more diffuse light should cause

increased growth and carbon storage.

AH1 Trying to maintain dominanceThen favorable changes in the environment should

not cause a large increase in growth and carbon storage. The trees already have plenty of carbon – growth is limited by something else.

Conclusions

We need to think critically about the assumption that plants “want” to maximize their primary production and carbon gain.

Some plants, especially old-growth tropical trees, have probably evolved under pressure to optimize things other than growth.