Autumn growth of three perennial weeds at high latitude benefits from climate change

In autumn, agricultural perennial weeds prepare for winter and can store reserves into creeping roots or rhizomes. Little is known about influence of climate change in this period. We tested the effect of simulated climate change in autumn on three widespread and noxious perennial weeds, Elymus repens (L.) Gould, Cirsium arvense (L.) Scop. and Sonchus arvensis L. We divided and combined simulated climate change components into elevated CO₂ concentration (525 ppm), elevated temperatures (+2–2.5°C), treatments in open‐top chambers. In addition, a control in the open‐top chamber without any increase in CO₂ and temperature, and a field control outside the chambers were included. Two geographically different origins and three pre‐growth periods prior to the exposure to climate change factors were included for each species. All species increased leaf area under elevated temperature, close to doubling in E. repens and quadrupling in the dicot species. E. repens kept leaves green later in autumn. C. arvense did not benefit in below‐ground growth from more leaf area or leaf dry mass. S. arvensis had low levels of leaf area throughout the experiment and withered earlier than the two other species. Below‐ground plant parts of S. arvensis were significantly increased by elevated temperature. Except for root:shoot ratio of C. arvense, the effects of pure elevated CO₂ were not significant for any variables compared to the open‐top chamber control. There was an additive, but no synergistic, effect of enhanced temperature and CO₂. The length of pre‐growth period was highly important for autumn plant growth, while origin had minor effect. We conclude that the small transfer of enhanced above‐ground growth into below‐ground growth under climate change in autumn does not favour creeping perennial plants per se, but more leaf area may offer more plant biomass to be tackled by chemical or physical weed control.     

A Direct Confirmation of the Standard Method of Estimating Intercellular Partial Pressure of CO(2)

The partial pressure of CO₂ inside leaves of several species was measured directly. Small gas exchange chambers were clamped above and below the same section of an amphistomatous leaf. A flowing gas stream through one chamber allowed normal CO₂ and water vapor exchange. The other chamber was in a closed circuit consisting of the chamber, an infrared gas analyzer, and a peristaltic pump. The CO₂ in the closed system rapidly reached a steady pressure which it is believed was identical to the CO₂ pressure inside the leaf, because there was no flux of CO₂ across the epidermis. This measured partial pressure was in close agreement with that estimated from a consideration of the fluxes of CO₂ and vapor at the other surface.     

Overestimation of respiration rates in commercially available clamp-on leaf chambers. Complications with measurement of net photosynthesis

The possible interference when measuring gas exchange with respiratory CO₂ produced under the gasket of commercially available clamp‐on leaf chambers was investigated. Two of these chambers were compared with a leaf chamber that accommodated an entire leaf without clamping it under a gasket. An overestimation of dark respiration rate (R_D) by 55% was found with Plantago major leaves, a species with homobaric leaves that have high resistance for lateral gaseous transport. The percentage was similar in the heterobaric Ficus benjamina, but was 32% in the highly porous homobaric Nicotiana tabacum. Net photosynthetic rate at low photon flux density was underestimated by 35% in the clamp‐on chamber. However, the gasket effect was not detectable at light saturation because the error was small in comparison with the high photosynthetic rates. Estimation of respiration in the light (R_L) in Nicotiana as derived from CO₂ exchange at low CO₂ concentrations was complicated by three factors. The inward diffusion of respiratory CO₂ from under the gasket was added to a diffusion of CO₂ from outside through the gasket material and through the leaf, which produced an even larger error in R_L in comparison with R_D at ambient CO₂. These errors are significant for estimations of carbon gain at whole plant and canopy level and also at the leaf level when photosynthetic rates are low. Possible improvements in gasket design and corrections of CO₂ exchange measurements for the gasket effect are discussed.     

Enhanced root system C-sink activity,water relations and aspects of nutrient acquisition in mycotrophic Bouteloua gracilis subjected to CO2 enrichment

In order to better elucidate fixed-C partitioning, nutrient acquisition and water relations of prairie grasses under elevated [CO₂], we grew the C₄ grass Bouteloua gracilis (H.B.K.) lag ex Steud. from seed in soil-packed, column-lysimeters in two growth chambers maintained at current ambient [CO₂] (350 μL L⁻¹) and twice enriched [CO₂] (700 μL L⁻¹). Once established, plants were deficit irrigated; growth chamber conditions were maintained at day/night temperatures of 25/16°C, relative humidities of 35%/90% and a 14-hour photoperiod to simulate summer conditions on the shortgrass steppe in eastern Colorado. After 11 weeks of growth, plants grown under CO₂ enrichment had produced 35% and 65% greater total and root biomass, respectively, and had twice the level of vesicular-arbuscular mycorrhizal (VAM) infection (19.8% versus 10.8%) as plants grown under current ambient [CO₂]. The CO₂-enriched plants also exhibited greater leaf water potentials and higher plant water use efficiencies. Plant N uptake was reduced by CO₂ enrichment, while P uptake appeared little influenced by CO₂ regime. Under the conditions of the experiment, CO₂ enrichment increased root biomass and VAM infection via stimulated growth and adjustments in C partitioning below-ground. The U.S. Government right to retain a non-exclusive, royalty free licence in and to any copyright is acknowledged. The U.S. Government right to retain a non-exclusive, royalty free licence in and to any copyright is acknowledged.     

In vitro growth and shoot multiplication of Wrightia tomentosa Roem et Schult in a controlled carbon dioxide environment

In vitro growth and multiplication of shoots of a woody tree species Wrightia tomentosa in a controlled carbon dioxide environment was studied. The cultures were grown on BA supplemented MS medium with or without 3% sucrose. A range of CO₂ concentrations (0.0, 0.6, 10.0 and 40.0 g m^–3) was controlled in small chambers by using solutions of NaHCO₃, Na₂CO₃, KHCO₃ and K₂CO₃. To obtain a CO₂-free environment, a saturated solution of KOH was kept in the chambers. It was concluded that the growing shoot cultures required either sucrose in the medium as a carbon source or an ambient CO₂ environment. Complete absence of a carbon source caused severe browning of the shoots and death within 30 days. The cultures grew better with 10.0 g m^–3 carbon dioxide in the environment than with 3.0% sucrose in the medium. With both CO₂ and sucrose being available, the best response was obtained at 0.6 g m^–3 CO₂ in the chamber. At this concentration the rate of shoot multiplication was nearly double the standard rate obtained when exposed to the natural CO₂ level and sucrose-supplemented medium. Total fresh and dry weight, leaf number and area per cluster also showed the best response under this condition.     

Comparative responses of model C3 and C4 plants to drought in low and elevated CO2

Interactive effects of CO₂ and water availability have been predicted to alter the competitive relationships between C3 and C4 species over geological and contemporary time scales. We tested the effects of drought and CO₂ partial pressures (pCO₂) ranging from values of the Pleistocene to those predicted for the future on the physiology and growth of model C3 and C4 species. We grew co-occurring Abutilon theophrasti (C3) and Amaranthus retroflexus (C4) in monoculture at 18 (Pleistocene), 27 (preindustrial), 35 (current), and 70 (future) Pa CO₂ under conditions of high light and nutrient availability. After 27 days of growth, water was withheld from randomly chosen plants of each species until visible wilting occurred. Under well-watered conditions, low pCO₂ that occurred during the Pleistocene was highly limiting to C3 photosynthesis and growth, and C3 plants showed increased photosynthesis and growth with increasing pCO₂ between the Pleistocene and future CO₂ values. Well-watered C4 plants exhibited increased photosynthesis in response to increasing pCO₂, but total mass and leaf area were unaffected by pCO₂. In response to drought, C3 plants dropped a large amount of leaf area and maintained relatively high leaf water potential in remaining leaves, whereas C4 plants retained greater leaf area, but at a lower leaf water potential. Furthermore, drought-treated C3 plants grown at 18 Pa CO₂ retained relatively greater leaf area than C3 plants grown at higher pCO₂ and exhibited a delay in the reduction of stomatal conductance that may have occurred in response to severe carbon limitations. The C4 plants grown at 70 Pa CO₂ showed lower relative reductions in net photosynthesis by the end of the drought compared to plants at lower pCO₂, indicating that CO₂ enrichment may alleviate drought effects in C4 plants. At the Pleistocene pCO₂, C3 and C4 plants showed similar relative recovery from drought for leaf area and biomass production, whereas C4 plants showed higher recovery than C3 plants at current and elevated pCO₂. Based on these model systems, we conclude that C3 species may not have been at a disadvantage relative to C4 species in response to low CO₂ and severe drought during the Pleistocene. Furthermore, C4 species may have an advantage over C3 species in response to increasing atmospheric CO₂ and more frequent and severe droughts.     

No evidence that elevated CO2 gives tropical lianas an advantage over tropical trees

Recent studies indicate that lianas are increasing in size and abundance relative to trees in neotropical forests. As a result, forest dynamics and carbon balance may be altered through liana‐induced suppression of tree growth and increases in tree mortality. Increasing atmospheric CO₂ is hypothesized to be responsible for the increase in neotropical lianas, yet no study has directly compared the relative response of tropical lianas and trees to elevated CO₂. We explicitly tested whether tropical lianas had a larger response to elevated CO₂ than co‐occurring tropical trees and whether seasonal drought alters the response of either growth form. In two experiments conducted in central Panama, one spanning both wet and dry seasons and one restricted to the dry season, we grew liana (n = 12) and tree (n = 10) species in open‐top growth chambers maintained at ambient or twice‐ambient CO₂ levels. Seedlings of eight individuals (four lianas, four trees) were grown in the ground in each chamber for at least 3 months during each season. We found that both liana and tree seedlings had a significant and positive response to elevated CO₂ (in biomass, leaf area, leaf mass per area, and photosynthesis), but that the relative response to elevated CO₂ for all variables was not significantly greater for lianas than trees regardless of the season. The lack of differences in the relative response between growth forms does not support the hypothesis that elevated CO₂ is responsible for increasing liana size and abundance across the neotropics.     

CARBOHYDRATE MOBILIZATION AND MOVEMENT IN ALPINE PLANTS

Starch retention and disappearance from leaves and carbon movement under various temperatures were studied in two alpine species, Oxyria digyna (L.) Hill and Phleum alpinum L., and in two low-elevation species, Helianthus annuus L. and Elymus canadensis L. The alpine species exhibited starch disappearance from the leaf following cool night temperatures, whereas starch retention was noted under similar conditions for the low-elevation species. The alpine species, Oxyria, exhibited the highest rates of starch disappearance from the leaf under cool temperatures as well as the highest carbohydrate translocation under cool temperatures. The low-elevation species had low rates of starch disappearance and carbohydrate translocation under low temperatures, but exhibited relatively higher rates with an increase in temperature. Such a mechanism whereby alpine species can maintain relatively high rates of translocation under cold temperature represents a major form of physiological adaptation to the short, cool, growing season in the alpine tundra.     

Multi-objective environment chamber system for studying plant responses to climate change

This paper describes the technical information and performance of a new multi-objective chamber system enabling the control of environmental variables (e.g., temperature, CO₂, air humidity, wind speed, and UV-B radiation) for understanding plant responses to climate change. Over a whole growing season, four different climate scenarios were evenly programmed into the system’s 16 chambers as ambient environment (AMB), elevated temperature (ET), elevated CO₂ concentration (EC) and elevated temperature and CO₂ concentration (ETC). Simultaneously, the chamber effects were assessed regarding the physiological responses and growth of a boreal perennial grass (reed canary grass, Phalaris arundinacea L.). During the growing season, the chamber system provided a wide variety of climatic conditions for air temperature (T _a), relative humidity (RH) and CO₂ concentration (C _a) in the AMB chambers following outside conditions. The target temperature (+3.5°C) was achieved to a good degree in the ET and ETC chambers, being on average 3.3°C and 3.7°C higher than ambient conditions, respectively. The target concentration of CO₂ (700 ppm) was also well achieved in the EC and ETC chambers, being on average 704 ppm and 703 ppm, respectively. The stable airflow condition inside all of the chambers provided a homogeneous distribution of gases and temperature. The decreases in RH and increases in vapour pressure deficit (VPD) in the elevated temperature chambers were also maintained at a low level. Chamber effects were observed, with some physiological and growth parameters of plants being significantly lower in the AMB chambers, compared to outside conditions. The plant growth was negatively affected by the reduced radiation inside the chambers.     

Consequences of growth at two carbon dioxide concentrations and two temperatures for leaf gas exchange in Pascopyrum smithii (C3) and Bouteloua gracilis (C4)*

Continually rising atmospheric CO₂ concentrations and possible climatic change may cause significant changes in plant communities. This study was undertaken to investigate gas exchange in two important grass species of the short-grass steppe, Pascopyrum smithii (western wheat-grass), C₃, and Bouteloua gracilis (blue grama), C4, grown at different CO₂ concentrations and temperatures. Intact soil cores containing each species were extracted from grasslands in north-eastern Colorado, USA, placed in growth chambers, and grown at combinations of two CO₂ concentrations (350 and 700 μmol mol⁻¹) and two temperature regimes (field average and elevated by 4°C). Leaf gas exchange was measured during the second, third and fourth growth seasons. All plants exhibited higher leaf CO₂ assimilation rates (A) with increasing measurement CO₂ concentration, with greater responses being observed in the cool-season C₃ species P. smithii. Changes in the shape of intercellular CO₂ response curves of A for both species indicated photosynthetic acclimation to the different growth environments. The photosynthetic capacity of P. smithii leaves tended to be reduced in plants grown at high CO₂ concentrations, although A for plants grown and measured at 700μmol mol⁻¹ CO₂ was 41% greater than that in plants grown and measured at 350 μmol mol⁻¹ CO₂. Low leaf N concentration may have contributed to photosynthetic acclimation to CO₂. A severe reduction in photosynthetic capacity was exhibited in P. smithii plants grown long-term at elevated temperatures. As a result, the potential response of photosynthesis to CO₂ enrichment was reduced in P. smithii plants grown long-term at the higher temperature.     

An improved hexagon open‐top chamber system for stable diurnal and nocturnal warming and atmospheric carbon dioxide enrichment

The use of solar passive hexagonal open‐top chambers (POTCs) is a viable method for experimentally manipulating daytime air temperatures in low‐stature plant communities at high latitudes. Here we describe a new hexagon POTC‐based system that uses thermal inertia to increase overnight temperatures and variable chamber height to reduce overheating in summer. Field data collected in tall temperate grasslands show that the presence of thermal mass raised minimum and mean nighttime air temperatures by up to 1.5 °C while lowering chamber height, along with thermal mass, limited the development of extreme daytime chamber temperatures in summer. We also demonstrate that, by using a simple, inexpensive twin carbon dioxide (CO₂) injection system regulated by an infrared gas monitor, it is possible to generate targeted and stable atmospheric CO₂ enrichment within these chambers. These innovations significantly improve the conventional hexagon POTC design and represent a low‐cost method for assessing the effects of warming and CO₂ enrichment on low‐stature vegetation in low latitude environments.     

Effects of atmospheric CO2 enrichment and root restriction on leaf gas exchange and growth of banana (Musa)

The effects of atmospheric CO₂ enrichment and root restriction on photosynthetic characteristics and growth of banana (Musa sp. AAA cv. Gros Michel) plants were investigated. Plants were grown aeroponically in root chambers in controlled environment glasshouse rooms at CO₂ concentrations of 350 or 1 000 μmol CO₂ mol^-1. At each CO₂ concentration, plants were grown in large (2001) root chambers that did not restrict root growth or in small (20 1) root chambers that restricted root growth. Plants grown at 350 μmol CO₂ mol^-1 generally had a higher carboxylation efficiency than plants grown at 1 000 μmol CO₂ mol^-1 although actual net CO₂ assimilation (A) was higher at the higher ambient CO₂ concentration due to increased intercellular CO₂ concentrations (C_i resulting from CO₂ enrichment. Thus, plants grown at 1 000 μmol CO₂ mol^-1 accumulated more leaf area and dry weight than plants grown at 350 μmol CO₂ mol^-1. Plants grown in the large root chambers were more photosynthetically efficient than plants grown in the small root chambers. At 350 μmol CO₂ mol^-1, leaf area and dry weights of plant organs were generally greater for plants in the large root chambers compared to those in the small root chambers. Atmospheric CO₂ enrichment may have compensated for the effects of root restriction on plant growth since at 1 000 μmol CO₂ mol^-1 there was generally no effect of root chamber size on plant dry weight.     

Low Temperature Effects on Early Vegetative Growth, Leaf Gas Exchange and Water Potential of Chilling-sensitive and Chilling-tolerant Crop Species

Two chilling-sensitive (Phaseolus vulgaris L., Zea mays L.)and two chilling-tolerant (Pisum sativum L., Spinacia oleraceaL.) species were raised in growth chambers under warm (28/18°Cday/night cycle) and cool (18/12°C) temperature regimes.Growth analysis techniques were used to evaluate leaf area andbiomass partitioning during early autotrophic growth. Plantsacclimated to both temperatures were measured for leaf gas exchangeand water potential (     

Responses of communities of tropical tree species to elevated CO2 in a forest clearing

Communities of ten species of tropical forest tree seedlings from three successional classes were grown at ambient and elevated CO₂ in large open-top chambers on the edge of a forest in Panamá. Communities grew from 20?cm to approximately 2?m in height in 6 months. No enhancements in plant biomass accumulation occurred under elevated CO₂ either in the whole communities or in growth of individual species. Reductions in leaf area index under elevated CO₂ were observed, as were decreases in leaf nitrogen concentrations and increases in the C:N ratio of leaf tissue. Species tended to respond individualistically to elevated CO₂, but some generalizations of how successional groupings responded could be made. Early and mid-successional species generally showed greater responses to elevated CO₂ than late-successional species, particularly with respect to increases in photosynthetic rates and leaf starch concentrations, and reductions in leaf area ratio. Late-successional species showed greater increases in C:N ratios in response to elevated CO₂ than did other species. Our results indicate that there may not be an increase in the growth of regenerating tropical forest under elevated CO₂, but that there could be changes in soil nutrient availability because of reductions in leaf tissue quality, particularly in late-successional species.     

Effects of carbon dioxide concentration on the interactive effects of temperature and water vapour on stomatal conductance in soybean

Soybeans were grown at three CO₂ concentrations in outdoor growth chambers and at two concentrations in controlled-environment growth chambers to investigate the interactive effects of CO₂, temperature and leaf-to-air vapour pressure difference (LAVPD) on stomatal conductance. The decline in stomatal conductance with CO₂ was a function of both leaf temperature and LAVPD. In the field measurements, stomatal conductance was more sensitive to LAVPD at low CO₂ at 30 °C but not at 35 °C. There was also a direct increase in conductance with temperature, which was greater at the two elevated carbon dioxide concentrations. Environmental growth chamber results showed that the relative stomatal sensitivity to LAVPD decreased with both leaf temperature and CO₂. Measurements in the environmental growth chamber were also performed at the opposing CO₂, and these experiments indicate that the stomatal sensitivity to LAVPD was determined more by growth CO₂ than by measurement CO₂. Two models that describe stomatal responses to LAVPD were compared with the outdoor data to evaluate whether these models described adequately the interactive effects of CO₂, LAVPD and temperature.     

Winners always win: growth of a wide range of plant species from low to future high CO2

Evolutionary adaptation to variation in resource supply has resulted in plant strategies that are based on trade‐offs in functional traits. Here, we investigate, for the first time across multiple species, whether such trade‐offs are also apparent in growth and morphology responses to past low, current ambient, and future high CO₂ concentrations. We grew freshly germinated seedlings of up to 28 C₃ species (16 forbs, 6 woody, and 6 grasses) in climate chambers at 160 ppm, 450 ppm, and 750 ppm CO₂. We determined biomass, allocation, SLA (specific leaf area), LAR (leaf area ratio), and RGR (relative growth rate), thereby doubling the available data on these plant responses to low CO₂. High CO₂ increased RGR by 8%; low CO₂ decreased RGR by 23%. Fast growers at ambient CO₂ had the greatest reduction in RGR at low CO₂ as they lost the benefits of a fast‐growth morphology (decoupling of RGR and LAR [leaf area ratio]). Despite these shifts species ranking on biomass and RGR was unaffected by CO₂, winners continued to win, regardless of CO_2. Unlike for other plant resources we found no trade‐offs in morphological and growth responses to CO₂ variation, changes in morphological traits were unrelated to changes in growth at low or high CO₂. Thus, changes in physiology may be more important than morphological changes in response to CO₂ variation.     

Elevated CO2 enhances water relations and productivity and affects gas exchange in C3 and C4 grasses of the Colorado shortgrass steppe.

Six open‐top chambers were installed on the shortgrass steppe in north‐eastern Colorado, USA from late March until mid‐October in 1997 and 1998 to evaluate how this grassland will be affected by rising atmospheric CO₂. Three chambers were maintained at current CO₂ concentration (ambient treatment), three at twice ambient CO₂, or approximately 720 μmol mol^?1 (elevated treatment), and three nonchambered plots served as controls. Above‐ground phytomass was measured in summer and autumn during each growing season, soil water was monitored weekly, and leaf photosynthesis, conductance and water potential were measured periodically on important C₃ and C₄ grasses. Mid‐season and seasonal above‐ground productivity were enhanced from 26 to 47% at elevated CO₂, with no differences in the relative responses of C₃/C₄ grasses or forbs. Annual above‐ground phytomass accrual was greater on plots which were defoliated once in mid‐summer compared to plots which were not defoliated during the growing season, but there was no interactive effect of defoliation and CO₂ on growth. Leaf photosynthesis was often greater in Pascopyrum smithii (C₃) and Bouteloua gracilis (C₄) plants in the elevated chambers, due in large part to higher soil water contents and leaf water potentials. Persistent downward photosynthetic acclimation in P. smithii leaves prevented large photosynthetic enhancement for elevated CO₂‐grown plants. Shoot N concentrations tended to be lower in grasses under elevated CO₂, but only Stipa comata (C₃) plants exhibited significant reductions in N under elevated compared to ambient CO₂ chambers. Despite chamber warming of 2.6 °C and apparent drier chamber conditions compared to unchambered controls, above‐ground production in all chambers was always greater than in unchambered plots. Collectively, these results suggest increased productivity of the shortgrass steppe in future warmer, CO₂ enriched environments.     

Effect of imitated global warming on Δ<Superscript>13</Superscript>C values in seven plant species growing in tibet alpine meadows

Open-top chambers were used to estimate the possible effects of global warming on δ¹³C of seven plant species grown in alpine meadow ecosystem. The δ¹³C values of plant species were lower after long-term growth in open-top chambers. In the course of experiment, temperature significantly increased inside the chambers by 4°C. Plant species grown at a lower elevation above sea level had higher δ¹³C values as compared to those grown at a higher elevation. This was in accordance with the effect of open-top chamber on δ¹³C values in plants. Greater availability of CO₂ and lower water vapor pressure at higher temperature inside the chambers, as indicated by an increase in discrimination against ¹³CO₂, probably result in more negative δ¹³C values of plants because higher stomatal conductance increases availability of CO₂ and causes greater discrimination against ¹³CO₂. The plant species studied could be the indicator species for testing global warming by the change in carbon isotope ratios at the two growth temperatures. This text was submitted by the authors in English.     

Effects of species of VA-mycorrhizal fungi on growth and mineral uptake of sorghum at different temperatures

Sorghum [Sorghum bicolor (L.) Moench] plants were grown in growth chambers at 20, 25 and 30°C in a low P Typic Argiudoll (3.65 µg P g^–1 soil, pH 8.3) inoculated with Glomus fasciculatum, Glomus intraradices, and Glomus macrocarpum to determine effects of vesicular-arbuscular mycorrhizal fungi (VAMF) species on plant growth and mineral nutrient uptake. Sorghum root colonization by VAMF and plant responses to Glomus species were temperature dependent. G. macrocarpum colonized sorghum roots best and enhanced plant growth and mineral uptake considerably more than the other VAMF species, especially at 30°C. G. fasciculatum enhanced shoot growth at 20 and 25°C, and mineral uptake only at 20°C. G. intraradices depressed shoot growth and mineral uptake at 30°C. G. macrocarpum enhanced shoot P, K, and Zn at all temperatures, and Fe at 25 and 30°C above that which could be accounted for by increased biomass. Sorghum plant growth responses to colonization by VAMF species may need to be evaluated at different temperatures to optimize beneficial effects.     

Photosynthesis in Plants of Four Tropical Species Growing under Elevated CO2

We studied the responses of leaf gas exchange and growth to an increase in atmospheric CO₂ concentration in four tropical deciduous species differing in carbon fixation metabolism: Alternanthera crucis, C3-C4; Ipomoea carnea, C3; Jatropha gossypifolia, C3; and Talinum triangulare, inducible-CAM. In the first stage, plants were grown in one open-top chamber at a CO₂ concentration of 560±40 mol mol^-1 (EC), one ambient CO₂ concentration chamber (AC), and one unenclosed plot (U). In the second stage, plants were grown in five EC chambers (CO₂ concentration = 680±30 mol mol^-1), five AC chambers, and five unenclosed plots. During the first weeks under EC in the first stage, plants of all the species had a very marked increase in their maximal net photosynthetic rates (P _max) of 3.5 times on average; this stimulatory effect was maintained for 11-15 weeks, rates dampening afterward to values still higher than controls for 37 weeks. After a suspension of CO₂ enrichment for 6 weeks, an increase in P _max of EC plants over the controls was found in plants of all the species until week 82 of the experiment. Stomatal conductance (g) showed no response to EC. Carboxylation efficiency decreased in all the species under EC