A free-air CO2 enrichment (FACE) experiment was conducted at Maricopa, Arizona, on wheat from December 1992 through May 1993. The FACE apparatus maintained the COj concentration, [CO2], at 550 jimol mor' across four replicate 25-m-diameter circular plots under natural conditions in an open field. Four matching Control plots at ambient [CO2](about 370 jimol mol"^) were also installed in the field. In addition to the two levels of [COjI, there were ample (Wet) and limiting (Dry) levels of water supplied through a subsurface drip irrigation system in a strip, split-plot design.Measurements were made of net radiation, R^; soil heat flux, GQ,-soil temperature; foliage or surface temperature; air dry and wet bulb temperatures; and wind speed. Sensible heat flux, H, was calculated from the wind and temperature measurements. Latent heat flux, XET, and evapotranspiration, ET, were determined as the residual in the energy balance. The FACE < treatment reduced daily total R^ by an average 4%. Daily FACE sensible heat flux, H, was higher in the FACE plots. Daily latent heat flux, XET, and evapotranspiration, ET, were consistently lower in the FACE plots than in the Control plots for most of the growing season, about 8% on the average. Net canopy photosynthesis was stimulated by an average 19 and 44% in the Wet and Dry plots, respectively, by elevated [CO2] for most of the growing season. No significant acclimation or down regulation was observed. There was little above-ground growth response to elevated [CO2] early in the season when temperatures were cool. Then, as temperatures warmed into spring, the FACE plants grew about 20% more than the Control plants at ambient [CO2], as shown by above-ground biomass accumulation. Root biomass accumulation was also stimulated about 20%. In May the FACE plants matured and senesced about a week earlier than the Controls in the Wet plots. The FACE plants averaged 0.6 °C wanner than the Controls from February through April in the well-watered plots, and we speculate that this temperature rise contributed to the earlier maturity. Because of the acceleration of senescence, there was a shortening of the duration of grain filling, and consequently, there was a narrowing of the final biomass and yield differences. The 20% mid-season growth advantage of FACE shrunk to about an 8% yield advantage in the Wet plots, while the yield differences between FACE and Control remained at about 20% in the Dry plots.

Abstract:In this paper the ecohydrological model SWIM developed for regional impact assessment is presented, and examples of approaches to climate and land use change impact studies are described. SWIM is a continuous-time semidistributed ecohydrological model, integrating hydrological processes, vegetation, nutrients (nitrogen and phosphorus) and sediment transport at the river basin scale. Its spatial disaggregation scheme has three levels: (1) basin, (2) subbasins and (3) hydrotopes within sub-basins. The model was extensively tested and validated for hydrological processes, nitrogen dynamics, crop yield and erosion (mainly in mesoscale sub-basins of the German part of the Elbe River basin). After appropriate validation in representative sub-basins, the model can be applied at the regional scale for impact studies. Particular interest in the global change impact studies is given to effects of expected changes in climate and land use on hydrological processes and agro-ecosystems, including water balance components, water quality and crop yield. This paper (a) introduces the reader to the class of process-based ecohydrological catchment scale models, (b) introduces SWIM as one such model, and (c) presents two examples of impact studies performed with SWIM for the federal state of Brandenburg (Germany), which overlaps with the lowland part of the Elbe drainage area. The impact studies provide a better understanding of the complex interactions between climate, hydrological processes and vegetation, and improve our potential adaptation to the expected changes.

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Atmospheric CO 2 concentration (C a ) continues to rise. An imperative exists, therefore, to elucidate the interactive effects of elevated C a and drought on plant water relations of wheat (Triticum aestivum L.). A spring wheat (cv. Yecora Rojo) crop was exposed to ambient (Control: 370 mmol mol 21 ) and free-air CO 2 enrichment (FACE: ambient 1 180 mmol mol 21 ) under ample (Wet), and reduced (Dry), water supplies (100 and 50% replacement of evapotranspiration, respectively) over a 2-yr study. Our objective was to characterize and quantify the responses of 26 edaphic, gas exchange, water relations, carbohydrate pool dynamics, growth, and development parameters to rising C a and drought. Increasing C a minimized the deleterious effects of soil-water depletion by increasing drought avoidance (i.e., lower stomatal conductance and transpiration rate, and growth and development of a more robust root system) and drought tolerance (i.e., enhanced osmoregulation and adaptation of tissue) mechanisms, resulting in a 30% reduction in water stress-induced midafternoon depressions in net assimilation rate. An elevated C a -based increase in daily and seasonal carbon gain resulted in a positive feedback between source capacity (shoots) and sink demand (roots). Devoid of a concomitant rise in global temperature resulting from the rise in C a , improved water relations for a herbaceous, cool-season, annual, C 3 cereal monocot grass (i.e., wheat) are anticipated in a future high-CO 2 world. These findings are applicable to other graminaceous species of a similar function-type as wheat common to temperate zone grassland prairies and savannas, especially under dryland conditions.

Spring wheat was grown from emergence to grain maturity in two partial pressures of CO 2 (pCO 2 ): ambient air of nominally 37 Pa and air enriched with CO 2 to 55 Pa using a free-air CO 2 enrichment (FACE) apparatus. This experiment was the first of its kind to be conducted within a cereal field without the modifications or disturbance of microclimate and rooting environment that accompanied previous studies. It provided a unique opportunity to examine the hypothesis that continuous exposure of wheat to elevated pCO 2 will lead to acclimatory loss of photosynthetic capacity. The diurnal courses of photosynthesis and conductance for upper canopy leaves were followed throughout the development of the crop and compared to model-predicted rates of photosynthesis. The seasonal average of midday photosynthesis rates was 28% greater in plants exposed to elevated pCO 2 than in contols and the seasonal average of the daily integrals of photosynthesis was 21% greater in elevated pCO 2 than in ambient air. The mean conductance at midday was reduced by 36%. The observed enhancement of photosynthesis in elevated pCO 2 agreed closely with that predicted from a mechanistic biochemical model that assumed no acclimation of photosynthetic capacity. Measured values fell below predicted only in the flag leaves in the mid afternoon before the onset of grain-filling and over the whole diurnal course at the end of grain-filling. The loss of enhancement at this final stage was attributed to the earlier senescence of flag leaves in elevated pCO 2 . In contrast to some controlled-environment and field-enclosure studies, this field-scale study of wheat using free-air CO 2 enrichment found little evidence of acclimatory loss of photosynthetic capacity with growth in elevated pCO 2 and a significant and substantial increase in leaf photosynthesis throughout the life of the crop.

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We argue that differences in the perception and governance of adaptation to climate change and extreme weather events are related to sets of beliefs and concepts through which people understand the environment and which are used to solve the problems they face (mental models). Using data gathered in 31 in-depth interviews with adaptation experts in Europe, we identify five basic stakeholder groups whose divergent aims and logic can be related to different mental models they use: advocacy groups, administration, politicians, researchers, and media and the public. Each of these groups uses specific interpretations of climate change and specifies how to deal with climate change impacts. We suggest that a deeper understanding and follow-up of the identified mental models might be useful for the design of any stakeholder involvement in future climate impact research processes. It might also foster consensus building about adequate adaptation measures against climate threats in a society.