Rising atmospheric [CO2 ], ca , is expected to affect stomatal regulation of leaf gas-exchange of woody plants, thus influencing energy fluxes as well as carbon (C), water, and nutrient cycling of forests. Researchers have proposed various strategies for stomatal regulation of leaf gas-exchange that include maintaining a constant leaf internal [CO2 ], ci , a constant drawdown in CO2 (ca  - ci ), and a constant ci /ca . These strategies can result in drastically different consequences for leaf gas-exchange. The accuracy of Earth systems models depends in part on assumptions about generalizable patterns in leaf gas-exchange responses to varying ca . The concept of optimal stomatal behavior, exemplified by woody plants shifting along a continuum of these strategies, provides a unifying framework for understanding leaf gas-exchange responses to ca . To assess leaf gas-exchange regulation strategies, we analyzed patterns in ci inferred from studies reporting C stable isotope ratios (δ(13) C) or photosynthetic discrimination (∆) in woody angiosperms and gymnosperms that grew across a range of ca spanning at least 100 ppm. Our results suggest that much of the ca -induced changes in ci /ca occurred across ca spanning 200 to 400 ppm. These patterns imply that ca  - ci will eventually approach a constant level at high ca because assimilation rates will reach a maximum and stomatal conductance of each species should be constrained to some minimum level. These analyses are not consistent with canalization toward any single strategy, particularly maintaining a constant ci . Rather, the results are consistent with the existence of a broadly conserved pattern of stomatal optimization in woody angiosperms and gymnosperms. This results in trees being profligate water users at low ca , when additional water loss is small for each unit of C gain, and increasingly water-conservative at high ca , when photosystems are saturated and water loss is large for each unit C gain.

The effect of temperature and light conditions on sexual reproduction (sporophyte formation) of in vitro cultures of the moss Physcomitrella patens was analysed. All parameters tested, i.e., temperature, light intensity and day length had a strong impact on the number of sporophytes formed. The highest number of sporophytes, 559 g fresh weight, developed at 15 °C, 8 h light/day with an intensity of 20 μmol/m2/s. In contrast, at 25 °C, as well as with a day length of 16 h per day, the number of sporophytes was drastically reduced. Vegetative growth, determined as fresh weight per petri dish, was impeded under conditions favouring sporophyte formation, probably due to nutrient transfer to the sporophytes. Microscopic documentation of the developing sporophytes revealed that, although archegonia were arranged in bundles at the gametophore apices, usually only one archegonium per gametophore apex developed into a mature sporophyte. From an EST database six novel MADS‐box genes were identified which, in phylogenetic analyses, did not cluster with the known groups of higher plant MADS‐box genes. One of these genes was represented only as a singleton in a cDNA library specifically derived from gametophore apices and developing sporophytes, and, therefore, designated PpMADS‐S. RNA amounts of PpMADS‐S were two to three times higher under conditions that stimulate sporophyte development (15 °C, 8 h light per day) when compared to conditions favouring vegetative growth (25 °C, 16 h light per day), indicating a possible function in sexual reproduction of this moss. Thus, an efficient experimental system was established to study sex organ formation, fertilization and embryo development in Physcomitrella.

Summary Are mature forests carbon limited? To explore this question, we exposed ca. 110‐year‐old, 40‐m tall Picea abies trees to a 550‐ppm CO2 concentration in a mixed lowland forest in NW Switzerland. The site receives substantial soluble nitrogen (N) via atmospheric deposition, and thus, trees are unlikely N‐limited. We used a construction crane to operate the free‐air CO2 release system and for canopy access. Here, we summarize the major results for growth and carbon (C) fluxes. Tissue 13C signals confirmed the effectiveness of the CO2 enrichment system and permitted tracing the continuous flow of new C in trees. Tree responses were individually standardized by pre‐treatment signals. Over the five experimental years, needles retained their photosynthetic capacity and absorbed up to 37% more CO2 under elevated (E) compared to ambient (A) conditions. However, we did not detect an effect on stem radial growth, branch apical growth and needle litter production. Neither stem nor soil CO2 efflux was stimulated under elevated CO2. The rate at which fine roots filled soil ingrowth cores did not significantly differ between A‐ and E‐trees. Since trees showed no stomatal responses to elevated CO2, sap flow remained unresponsive, both in the long run as well as during short‐term CO2 on–off experiments. As a consequence, soil moisture remained unaffected. We trapped significantly more nitrate in the root sphere of E‐trees suggesting a CO2‐stimulated breakdown of soil organic matter, presumably induced by extra carbohydrate exudation (‘priming’). Synthesis. The lack of a single enhanced C sink to match the increased C uptake meant a missing C sink. Increased C transport to below‐ground sinks was indicated by C transfer to ectomycorrhiza and on to neighbouring trees and by increased C export to soil. We conclude that these tall Picea abies trees are not C limited at current CO2 concentrations and further atmospheric CO2 enrichment will have at most subtle effects on growth, despite enhanced N availability.

2.5 years. The isotopic signals in soil cO 2 arrived 12 days after the onset of Face, yet it contained only ca. 15 % new c thereafter. We conclude that new c first feeds into fast turnover c pools in the canopy and becomes increasingly mixed with older c sources as one moves away (downward) from the crown. We speculate that enhanced c turnover (its metabolic cost) along the phloem path, as evidenced by basipetal isotope signal depletion, explains part of the 'missing carbon' in trees that assimilated more c under elevated cO 2. Keywords carbon isotopes • elevated cO 2 • Face • Forest • respiration communicated by rowan sage.

With their dominant share in global plant biomass carbon (C), forests and their responses to atmospheric CO 2 enrichment are key to the global C balance. In this free air CO 2 enrichment (FACE) study, we assessed respiratory losses from stems and soil, and fine root growth of ca. 110-year-old Picea abies growing in a near-natural forest in NW Switzerland. We anticipated a stimulation of all three variables in response to a ca. 150 ppm higher CO 2 concentration in the tree canopies. During the first 2.5 years of the experiment, stem CO 2 efflux (R stem ) remained unresponsive to CO 2 enrichment. This indicates that there is no enhancement of metabolic activity in phloem and xylem of these mature trees. Soil CO 2 efflux (R soil ) beneath trees experiencing elevated CO 2 (eCO 2 ) showed a slight but significant reduction compared to R soil under control trees. High CO 2 trees did not increase their fine root biomass in ingrowth cores after 20 months under FACE relative to the fine root fractions collected in undisturbed soil. Tree growth (stem radial increment, not shown here) remained completely unchanged although earlier experiments showed largest responses (if any) during the early years after a step increase in atmospheric CO 2 concentration. The data presented here suggest C saturation of the study trees at the current close to 400 ppm CO 2 ambient concentrations. Together with the high local atmospheric N-deposition rates (ca. 20 kg N ha -1 a -1 ), our findings imply that factors other that C and N supply appear to constrain growth and metabolism of these mature P. abies trees under eCO 2 .