Plant physiological responses to precipitation in the Amazon forest, an isotopic approach Universidade de São Paulo: Jean Pierre Ometto; Luiz Martinelli; Françoise Yoko Ishida; Edmar Mazzi Universidade Federal do Pará: Sebastião Lopes, H. Jackson Silva University of Utah: Jim Ehleringer, Tomas Domingues Carnegie Institution : Joseph Berry University of Lethbridge: Larry Flanagan

How Amazonian ecological process reflect elsewhere in the world?

Carbon cycle Tropical forests are a key biome in the modern carbon cycle, not only because their extent or the large amount of carbon stored, but also because a net carbon gain or loss in these regions would have a significant impact for this cycle.

The global carbon cycle

Forest complexity –Number of species - high with few individuals coexisting per area. –Nutrient cycling - most of the elements, like Ca and P, for instance, have a closed cycle in these systems, while several evidences suggested that these forests have an open cycle of nitrogen

Plant ecological studies The core of these studies is the notion of plant environment interactions physicalbiological

Carbon Isotopes Trace CO 2 processes

 In C 3 plants (dominate photosynthetic pathway in forest vegetation), discrimination against 13 C by the carboxylating enzyme, Rubisco (~27‰) is linked to photosynthesis via ci/ca ratio (intercellular to atmospheric CO 2 concentrations);  This ratio reflects the relative magnitudes of net assimilation (A) and stomatal conductance (g) Stable isotopes are a powerful tool to address questions in plant and ecosystem ecology demand supply CO 2

As we know, c i is affected by environmental factors such water and light availability, temperature, nitrogen content, among others, so changes in the environmental conditions will be recorded in the stable carbon composition of plant tissues Farquhar et al. (1982), developed a model where (  13 C) in C 3 plants is basically controlled by environmental and physiological variables. Environmental variable ~ atmospheric [CO 2 ] and  13 C Physiological variable ~ CO 2 concentration inside the leaf intercellular space (c i )

The carbon isotope composition of plant tissues depends on  13 C a, atmospheric source a, 13 CO 2 diffusion rates relative to 12 CO 2 b, enzymatic discrimination during carboxylation c i /c a, ratio of internal to ambient CO 2  13 C leaf =  13 C a - a - (b - a)c i /c a 4.4 ‰-8 ‰27 ‰  = a + (b – a) x ci/ca  =  13 CO 2_canopy -  13 C leaf

Objective Investigate the variability of carbon isotope (in CHO and CO 2 ) as a proxy of carbon cycle in Amazonian rainforest, along a gradient of precipitation. Where we worked… ZF2 – Manaus FLONA TAPAJÓS – Santarém REBIO JARÚ – Rondonia State

SiteP (mm/yr) MonthsN (ind./ha)* Biomass (t/ha)* Manaus – ZF Santarém– Flona *Source:Vieira (2003)

Leaves sampling for δ 13 C CHO

Air Sampling δ 13 C CO 2

Stable isotope facility LEI-CENA/USP

Results Vertical profiles - canopy height x δ 13 C leaves

“Canopy Effect” Explain large proportion of the variance found in the carbon isotopic composition of tropical tree leaves Causes Light exposure CO 2 available Water use efficiency (ratio of carbon assimilation to water vapor loss – transpiration)

Canopy height (m)  13 C- leaf (‰) c i /c a Manaus_ZF2  13 C leaf =  13 C a - a - (b - a)c i /c a Canopy height (m)  13 C-leaf (‰) c i /c a Santarém_Flona

Canopy height (m)  13 C-leaf (‰)  (‰) Manaus_ZF2 Canopy height (m)  13 C-leaf (‰)  (‰) Santarém_Flona  = a + (b – a) x ci/ca

Precipitation; δ 13 C leaves ; intercept δ 13 C x height Manaus - ZF2

Precipitation; δ 13 C leaves ; intercept δ 13 C x height Santarém - Flona km 67

C forest = C atm + C biog.

Ometto et al, 2002

The very negative δ 13 C values characterizes tropical forests as open systems in relation to the balance between stomatal conductance and photosynthetic rate –confirmed by the high  and high ci /ca ratios found in these forests As stomatal conductance decreases, with less water available, the average ci /ca ratio at Flona was lower than the average ci/ca ratio observed in the ZF2 site. Though increases in water availability determine a more positive δ 13 C value at Flona. – lag in ppt amount and carbon isotopic values The difference among organic δ 13 C and δ 13 C CO2 can be related to a more recent fixed carbon being respired in comparison to a longer history in the leaves. Final remarks

Cheia em Mamirauá – foto: Luiz Claudio Marigo Understanding how extreme events drives adaptation is crucial to understand general functioning of tropical regions