Controls on tropical forest CO 2 and energy exchange Michael 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: H1Trying to get bigger and bigger? Do they allocate resources to maximize further growth? AH1Trying to maintain their position of dominance? Do they allocate resources to hold on to what they already have?

Theory King “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

Predictions If canopy trees: H1Are 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. AH1Are 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 A n (  mol m -2 s -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? Whole Ecosystem CO 2 Exchange (  mol m -2 s -1 ) PPFD (  mol m -2 s -1 )

Litterfall begins before drought. Soil water (cm 3 cm –3 ) Leaf Litter fall (kg C ha -1 day -1 ) Are trees trying to maximize their photosynthesis seasonally?

The early litterfall costs the trees carbon. Less whole forest Photosynthesis 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? Soil water (cm 3 cm –3 ) Residual CO 2 Exchange (  mol m -2 s -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 Production30 TC/hayr Net Primary Production8 TC/hayr Litterfall3 TC/hayr Gross wood increment 2 TC/hayr Root growth 3 TC/hayr (Similar results in Chambers et al) Are trees trying to maximize their growth over a year?

Predictions If canopy trees: H1Are 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. AH1Are trying to maintain their position of dominance They should not worry about maximizing growth, but should emphasize 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

Relative Torque 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

Implications If canopy trees are: H1 Trying to get bigger and bigger Then favorable changes in the environment such as elevated CO2 or more diffuse light should cause increased growth and carbon storage. AH1Trying to maintain dominance Then 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.