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We investigated the daily exchange of CO 2 between undisturbed Larix gmelinii (Rupr.) Rupr. forest and the atmosphere at a remote Siberian site during July and August of 1993. Our goal was to measure and partition total CO 2 exchanges into aboveground and belowground components by measuring forest and understory eddy and storage¯uxes and then to determine the relationships between the environmental factors and these observations of ecosystem metabolism. Maximum net CO 2 uptake of the forest ecosystem was extremely low compared to the forests elsewhere, reaching a peak of only $5 mmol m À2 s À1 late in the morning. Net ecosystem CO 2 uptake increased with increasing photosynthetically active photon ux density (PPFD) and decreased as the atmospheric water vapor saturation de®cit (D) increased. Daytime ecosystem CO 2 uptake increased immediately after rain and declined sharply after about six days of drought. Ecosystem respiration at night averaged $2.4 mmol m À2 s À1 with about 40% of this coming from the forest¯oor (roots and heterotrophs). The relationship between the understory eddy¯ux and soil temperature at 5 cm followed an Arrhenius model, increasing exponentially with temperature (Q 10 $2.3) so that on hot summer afternoons the ecosystem became a source of CO 2 . Tree canopy CO 2 exchange was calculated as the difference between above and below canopy eddy¯ux. Canopy uptake saturated at $6 mmol CO 2 m À2 s À1 for a PPFD above 500 mmol m À2 s À1 and decreased with increasing D. The optimal stomatal control model of Ma Èkela È et al. (1996) was used as a`big leaf' canopy model with parameter values determined by the non-linear least squares. The model accurately simulated the response of the forest to light, saturation de®cit and drought. The precision of the model was such that the daily pattern of residuals between modeled and measured forest exchange reproduced the component storagē ux. The model and independent leaf-level measurements suggest that the marginal water cost of plant C gain in Larix gmelinii is more similar to values from deciduous or desert species than other boreal forests. During the middle of the summer, the L. gmelinii forest ecosystem is generally a net sink for CO 2 , storing $0.75 g C m À2 d À1. Published by Elsevier Science B.V.

Summary Short‐term, intense heat waves (hamsins) are common in the eastern Mediterranean region and provide an opportunity to study the resilience of forests to such events that are predicted to increase in frequency and intensity. The response of a 50‐yr‐old Aleppo pine (Pinus halepensis) forest to hamsin events lasting 1–7 d was studied using 10 yr of eddy covariance and sap flow measurements. The highest frequency of heat waves was c. four per month, coinciding with the peak productivity period (March–April). During these events, net ecosystem carbon exchange (NEE) and canopy conductance (gc) decreased by c. 60%, but evapotranspiration (ET) showed little change. Fast recovery was also observed with fluxes reaching pre‐stress values within a day following the event. NEE and gc showed a strong response to vapor pressure deficit that weakened as soil moisture decreased, while sap flow was primarily responding to changes in soil moisture. On an annual scale, heat waves reduced NEE and gross primary productivity by c. 15% and 4%, respectively. Forest resilience to short‐term extreme events such as heat waves is probably a key to its survival and must be accounted for to better predict the increasing impact on productivity and survival of such events in future climates.

Knowledge of the relationship between soil water dynamics and tree water use is critical to understanding forest response to environmental change in water‐limited ecosystems. However, the dynamics in soil water availability for tree transpiration (Tt) cannot be easily deduced from conventional measurements of soil water content (SWC), notably because Tt is influenced by soil water potential (Ψs) that, in turn, depends on soil characteristics. Using tree sap flow and water potential and deriving depth‐dependent soil water retention curves, we quantified the ‘transpirable soil water content’ (tSWC) and its seasonal and inter‐annual variations in a semi‐arid Pinus halepensis forest. The results indicated that tSWC varied in time and with soil depth. Over one growing season Tt was 57% of rain and 72% of the infiltrated SWC. In early winter, Tt was exclusively supported by soil moisture at the top 10 cm (tSWC = 11 mm), whereas in spring (tSWC > 18 mm) and throughout the dry season, source water for Tt shifted to 20–40 cm, where the maximum fine root density occurs. Simulation with the soil–plant–atmosphere water and energy transport model MuSICA supported the idea that consistent tSWC at the 20–40 cm soil layer critically depended on limited water infiltration below 40 cm, because of high water retention below this depth. Quantifying tSWC is critical to the precise estimation of the onset and termination of the growing season (when tSWC > 0) in this semi‐arid ecosystem. Copyright © 2013 John Wiley & Sons, Ltd.

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.