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We examined the interactive effects of elevated concentration ([CO2]) and water stress on growth and physiology of 1-year-old peach (Prunus persica L.) seedlings grown in 10 dm3 pots in open top chambers with ambient (350 mmol mol-1) or elevated (700 mmol mol-1) [CO2]. Seedlings were supplied weekly with a non-limiting nutrient solution. Water was withheld from half of the plants from each treatment for a four week drying cycle, to simulate a sudden and severe water stress during the phase of rapid plant growth. Throughout the growing season, seedlings in elevated [CO2] had higher assimilation rates, measured at the growth [CO2], than seedlings in ambient [CO2], and this caused an increase in total dry mass of about 35%. Stomatal conductance, total water uptake, leaf area, and leaf number were unaffected by [CO2] treatment. Because, seedlings in the two CO2 treatments had similar transpiration despite large differences in total dry mass, water-use efficiency (WUE) of well-watered and water-stressed seedlings grown in elevated [CO2] was an average of 51% and 63% higher, respectively, than WUE of comparable seedlings grown in ambient [CO2]. Elevated [CO2] enhanced total biomass of water-stressed seedlings by 31%, and thus ameliorated the effects of water limitation. However, the percentage increases in total dry mass between well-watered and water-stressed seedlings were similar in ambient (53%) and elevated (58%) [CO2], demonstrating that there was no interaction between elevated [CO2] and water stress. This finding should be considered when predicting responses of trees to global climate change in hot and dry environments, where predicted temperature increases will raise the evaporative demands and exacerbate the effects of drought on tree growth.

The observation of acclimation in leaf photosynthetic capacity to differences in growth irradiance has been widely used as support for a hypothesis that enables a simplification of some soil-vegetation-atmosphere transfer (SVAT) photosynthesis models. The acclimation hypothesis requires that relative leaf nitrogen concentration declines with relative irradiance from the top of a canopy to the bottom, in 1 : 1 proportion. In combination with a light transmission model it enables a simple estimate of the vertical profile in leaf nitrogen concentration (which is assumed to determine maximum carboxylation capacity), and in combination with estimates of the fraction of absorbed radiation it also leads to simple 'big-leaf' analytical solutions for canopyphotosynthesis. We tested how forests deviate from this condition in five tree canopies, including four broadleaf stands, and one needle-leaf stand: a mixed-species tropical rain forest, oak (Quercus petraea (Matt.) Liebl), birch (Betula pendula Roth), beech (Fagus sylvatica L.) and Sitka spruce (Picea sitchensis (Bong.) Carr). Each canopy was studied when fully developed (mid-to-late summer for temperate stands). Irradiance (Q, ~S^~mol m-2 s-1) was measured for 20 d using quantum sensors placed throughoutthe vertical canopy profile. Measurements were made to obtain parameters from leaves adjacent to the radiation sensors: maximum carboxylation and electron transfer capacity (Va, Ja, ~S^~mol m-2 s-1), day respiration (Rda, ~S^~mol m-2 s-1), leaf nitrogen concentration (Nm, mg g-1) and leaf mass per unit area (La, g m-2). Relative to upper-canopy values, Va declined linearly in 1 : 1 proportion with Na. Relative Va also declined linearly with relative Q, but with a significant intercept at zero irradiance (P < 0.01). This intercept was strongly related to La of the lowest leaves in each canopy (P < 0.01, rr = 0.98, n = 5). For each canopy, daily ln Q was also linearly related with ln Va (P < 0.05), and the intercept was correlated with the value for photosynthetic capacity per unit nitrogen (PUN: Va/Na, ~S^~mol g-1s-1) of the lowest leaves in each canopy (P < 0.05). Va was linearly related with La and Na (P < 0.01), but the slope of the Va : Na relationship varied widely among sites. Hence, whilst there was a unique Va : Na ratio in each stand, acclimation in Va to Q varied predictably with La of the lowest leaves in each canopy. The specific leaf area, Lm (cmr g-1), of the canopy-bottom foliage was also found to predict carboxylation capacity (expressed on a mass basis; Vm, ~S^~mol g-1 s-1) at all sites (P < 0.01). These results invalidate the hypothesis of full acclimation to irradiance, but suggest that La and Lm of the most light-limited leaves in a canopy are widely applicable indicators of the distribution of photosynthetic capacity with height in forests.

International audience

This paper responds to the Forum contribution by Piovesan & Adams (2000) who criticized the results obtained by the EUROFLUX network on carbon fluxes of several European forests. The major point of criticism was that the data provided by EUROFLUX are inconsistent with current scientific understanding. It is argued that understanding the terrestrial global carbon cycle requires more than simply restating what was known previously, and that Piovesan & Adams have not been able to show any major conflicts between our findings and ecosystem or atmospheric-transport theories.

International audience; To investigate the extent to which the energy balance of a globally important ecosystem is controlled by biological and environmental processes, measurements of water vapor flux were made on individual black spruce (Picea mariana [Mill.] B.S.P.) shoots, branches, and a whole canopy at the BOREAS Southern Study Area Old Black Spruce (SSA OBS) site. These measurements were used to estimate stomatal, branch boundary layer and canopy boundary layer conductances to water vapor. On a projected needle area basis, stomatal conductances varied between 14 and 92 mmol m(-2) s(-1), and total branch conductance varied seasonally between zero and about 35 mmol m(-2) s(-1). On a ground area basis, total canopy conductance varied between 24 and 105 mmol m(-2) s(-1) Total canopy conductance was partitioned into aerodynamic and physiological components by using shoot-scale measurements scaled by leaf area index. Good agreement was found with an independent estimate of aerodynamic conductance measured when the canopy was wet. Compared with most coniferous forests, the canopy was relatively uncoupled from the atmosphere, and at the ecosystem scale, the control of water vapor flux was approximately equally divided between physiological and abiotic conductances. Two widely used steady-state models of stomatal conductance were parameterized from the shoot and branch measurements. Parameters varied considerably throughout the growing season. A time-constant term was added to these static models to construct dynamic models of stomatal conductance under naturally varying environmental conditions. The dynamic versions of the models outperformed the static versions in explaining stomatal response to rapidly changing environmental conditions. The length of the time-constant term, derived using the dynamic models, suggested that stomata were slow to respond to changing environmental conditions, and that the speed of the response was strongly temperature-dependent.

International audience

This ecophysiological primer serves as an introductory chapter on the concepts applied in subsequent chapters reviewing the state of knowledge concerning the effects of climate change on ecological processes that affect impacts on terrestrial ecosystems and some socio-economic systems. Because many of the responses of terrestrial plants in the diverse ecosystems are similar, this primer discusses: the effects of important climatic driving forces that are likely to change; soil fertility as a modifying effect with a brief discussion on soil biological factors; and ecological interactions between the different organisms in each ecosystem.