Are tropical forests near a high temperature threshold?

Abstract

[1] We used leaf gas exchange, sap flow, and eddy covariance measurements to investigate whether high temperature substantially limits CO 2 uptake at the LBA-ECO (Large-scale Biosphere-Atmosphere) km-83 tropical forest site in Brazil. Leaf-level temperature-photosynthesis curves, and comparisons of whole-canopy net ecosystem CO 2 exchange (NEE) with air temperature, showed that CO 2 uptake declined sharply during warm periods. Observations of ambient leaf microclimate showed that leaves oscillate between two states: a cool, dimly lit stage and a hot, brightly illuminated stage where leaf temperatures are often greater than 35°C. The leaf-level rates of photosynthesis decreased when shaded leaves ($ambient air temperature and < 500 mmol m À2 s À1 ) were transferred into a prewarmed, brightly illuminated chamber (35°to 38°C and 1000 mmol m À2 s À1 ), coincident with increased leaf temperature, increased evaporative demand, and stomatal closure. The rates of whole-canopy CO 2 uptake calculated at 5-min intervals increased initially at the onset of sunny periods that followed extended cloudy periods, but then decreased as the sunlight continued, leaf temperature and evaporative demand increased, and canopy conductance decreased. The forest at km-83 appears to be close to a high temperature threshold, above which CO 2 uptake drops sharply. This sensitivity results in part from the covariance between leaf temperature and leaf illumination; the brightly illuminated leaves that contribute disproportionately to canopy photosynthesis are warmed to the point that leaf gas exchange is curtailed.

Reviews and syntheses: Field data to benchmark the carbon cycle models for tropical forests

“…The choice can turn a very large carbon sink into a moderate one or even into a small source. (Araújo et al, 2002) When taken at short time steps during the daytime, abovecanopy measurements of the net ecosystem exchange (NEE) of CO 2 based on the eddy flux (also eddy covariance) technique have provided valuable indications of the environmental responses of tropical-forest physiology (e.g., depression of daytime NEE at high temperatures and/or high VPD; Doughty and Goulden, 2008;Vourlitis et al, 2011). No other technique provides direct field observations of the short-term climatic responses of forest-level CO 2 exchange.…”

Modelling basin-wide variations in Amazon forest productivity – Part 1: Model calibration, evaluation and upscaling functions for canopy photosynthesis

“…Lloyd et al (1995) obtained a value of 43 • C when fitting a big leaf model to eddy correlation measurements at the Jaru site. More measurements of leaf level temperature response at different rainforest sites is needed to check if temperature threshold measured at Tapajos (Doughty and Goulden, 2008b) prevails elsewhere. Finally, another possible explanation for the decline in photosynthesis and stomatal closure at high D C and temperature conditions might be related to a reduction in the activation state of Rubisco due to the high temperature being above its thermal optimum (Sage and Kubien, 2007).…”

“…Doughty and Goulden (2008b) measured leaf level canopy temperatures in parallel with canopy and leaf level photosynthesis and evaporation. From their observations, these authors suggest that the forest (neighbor to the Tapajos km 67 studied here) appears to be close to a high temperature threshold above which photosynthesis declines.…”

The covariation of Northern Hemisphere summertime CO&lt;sub&gt;2&lt;/sub&gt; with surface temperature in boreal regions

“…On these timescales, warm years tend to be associated with more rapid increases in atmospheric CO 2 , and cool years with reduced growth rates (Braswell et al, 1997). This positive relationship between temperature and atmospheric CO 2 is attributed primarily to an increase in ecosystem respiration (R e ) with increasing surface temperature, and a concurrently muted gross primary production (GPP, or photosynthesis) (Doughty and Goulden, 2008).…”