Sun-induced fluorescence and gross primary productivity during a heat wave.

Remote sensing of sun-induced chlorophyll fluorescence (SIF) has been suggested as a promising approach for probing changes in global terrestrial gross primary productivity (GPP). To date, however, most studies were conducted in situations when/where changes in both SIF and GPP were driven by large changes in the absorbed photosynthetically active radiation (APAR) and phenology. Here we quantified SIF and GPP during a short-term intense heat wave at a Mediterranean pine forest, during which changes in APAR were negligible. GPP decreased linearly during the course of the heat wave, while SIF declined slightly initially and then dropped dramatically during the peak of the heat wave, temporally coinciding with a biochemical impairment of photosynthesis inferred from the increase in the uptake ratio of carbonyl sulfide to carbon dioxide. SIF thus accounted for less than 35% of the variability in GPP and, even though it responded to the impairment of photosynthesis, appears to offer limited potential for quantitatively monitoring GPP during heat waves in the absence of large changes in APAR.

Leaf conductance and CO(2) assimilation of Larix gmelinii growing in an eastern Siberian boreal forest.

In July 1993, we measured leaf conductance, carbon dioxide (CO(2)) assimilation, and transpiration in a Larix gmelinii (Rupr.) Rupr. ex Kuzen forest in eastern Siberia. At the CO(2) concentration of ambient air, maximum values (mean of 10 highest measured values) for CO(2) assimilation, transpiration and leaf conductance for water vapor were 10.1 micro mol m(-2) s(-1), 3.9 mmol m(-2) s(-1) and 365 mmol m(-2) s(-1), respectively. The corresponding mean values, which were much lower than the maximum values, were 2.7 micro mol m(-2) s(-1), 1.0 mmol m(-2) s(-1) and 56 mmol m(-2) s(-1). The mean values were similar to those of Vaccinium species in the herb layer. The large differences between maximum and actual performance were the result of structural and physiological variations within the tree crowns and between trees that reduced maximum assimilation and leaf conductance by about 40 and 60%, respectively. Thus, maximum assimilation and conductance values averaged over the canopy were 6.1 micro mol m(-2) s(-1) and 146 mmol m(-2) s(-1), respectively. Dry air caused stomatal closure, which reduced assimilation by an additional 26%. Low irradiances in the morning and evening had a minor effect (-6%). Daily canopy transpiration was estimated to be 1.45 mm day(-1), which is higher than the value of 0.94 mm day(-1) measured by eddy covariance, but similar to the value of 1.45 mm day(-1) calculated from the energy balance and soil evaporation, and less than the value of 2.1 mm day(-1) measured by xylem flux. Daytime canopy carbon assimilation, expressed on a ground area basis, was 0.217 mol m(-2) day(-1), which is higher than the value measured by eddy flux (0.162 mol m(-2) day(-1) including soil respiration). We discuss the regulation of leaf gas exchange in Larix under the extreme climatic conditions of eastern Siberia (temperature > 35 degrees C and vapor pressure deficit > 5.0 kPa).