Native or Exotic? Double or Single? Evaluating Plants for Pollinator-friendly Gardens

In a series of dawn-to-dusk studies, we examined the natureand accessibility of nectar rewards for pollinating insectsby monitoring insect visits and the secretion rate and standingcrop of nectar in the British native plant species Salvia pratensis,Stachys palustris, S. officinalis, Lythrum salicaria, Linariavulgaris, the non-native Calendula officinalis, Petunia x hybrida,Salvia splendens, and the possibly introduced Saponaria officinalis.We also compared single with double variants ofLotus corniculatus, Saponaria officinalis, Petunia x hybrida andCalendula officinalis. All the British species studied are nectar-rich and are recommendedfor pollinator-friendly gardens. They showed maximal secretionrates of about 10–90 µg sugar per flower h^-1, andmost had mean standing crops of about 5–60 µg sugarper flower. In all British species studied, the corolla wasdeep enough for the relatively long-tongued bumblebee Bombuspascuorum, but the shallower flowers of Lythrum salicaria werealso much visited by shorter-tongued bees and hoverflies, aswell as by butterflies. The exotic Salvia splendens, presumablycoevolved with hummingbirds in the Neotropics, has such deepflowers that British bees cannot reach the nectar except bycrawling down the corolla. With a secretion rate approaching300 µg sugar per flower h^-1and little depletion by insects,S. splendens accumulated high standing crops of nectar. S. splendens,and single and double flowers of the two probably moth-pollinatedspecies Petunia x hybrida and Saponaria officinalis, receivedfew daytime visits despite abundant nectar but Calendula waswell visited by hoverflies and bees. We compared single anddouble variants of Lotus corniculatus,Petunia x hybrida andCalendula officinalis, and also Saponaria officinalis, the lastbeing probably introduced in Britain (Stace, 1997 New floraof the British Isles. 2nd edn. Cambridge: Cambridge UniversityPress). In Petunia, Saponaria and Lotus, double flowers secretedlittle or no nectar. In Calendula, where doubling involved achange in the proportion of disc and ray florets rather thanmodification of individual flower structure, double and singlecapitula had similar standing crops of nectar. Except inCalendula, exotic or double flowers were little exploited by insect visitors.In the exotics, this was probably due to the absence or scarcityof coevolved pollinators, coupled, in double flowers, with theabsence of nectar. Copyright 2001 Annals of Botany Company Salvia pratensis, Salvia splendens, Stachys palustris, Stachys officinalis,Lythrum salicaria , Linaria vulgaris, Lotus corniculatus, Saponaria officinalis,Petunia x hybrida, Calendula officinalis, wild flowers, double flowers, gardens, nectar, secretion rate, standing crop, pollinators, bumblebees, Bombus, honeybees, Apis, hoverflies, butterflies,Anthidium manicatum     

Influence of elevated CO2 and ultraviolet-B radiation levels on floral nectar production: a nectary-morphological perspective

 Investigations of the effects of two global events – elevated CO₂ levels and enhanced ultraviolet-B (UV-B) radiation – on floral nectar production are reviewed from twelve dicotyledonous families. Furthermore, to allow comparisons between nectary morphology and nectar production in treated plants of these fifteen species, new data on floral nectary structure are provided for Malcolmia maritima (L.) R. Br. (Brassicaceae) and Scabiosa columbaria L. (Dipsacaceae). All but the last taxon possessed mesenchymatic floral nectaries with surface stomata. Few clear relationships existed between nectary morphology and various physiological responses to CO₂ or UV-B enrichment, indicating that species responded notwithstanding nectary structure itself. Overall, nectar-solute concentration was least affected by elevated CO₂ or UV-B radiation; consequently, changes in nectar volume were responsible for differences in nectar-sugar production per flower. Three species of Fabaceae experienced no change in floral nectar production upon exposure to elevated CO₂. To date, no study of enhanced UV-B radiation reported a consistent reduction in floral nectar production; three species of Brassicaceae responded differently, but various levels of ozone depletion were simulated. Experimentation with more taxa – including those possessing nectary types such as septal (gynopleural) nectaries (e.g. many monocotyledons) or aggregations of glandular trichomes – and expanding such physiological studies to species possessing extrafloral nectaries, are recommended. Received August 8, 2002; accepted November 23, 2002 Published online: June 2, 2003     

Elevated carbon dioxide increases nectar production in <Emphasis Type="Italic">Epilobium angustifolium</Emphasis> L.

Effects of elevated CO₂ and nutrient availability on nectar production and onset of flowering in five different seed families (genotypes) of Epilobium angustifolium were investigated in a greenhouse experiment. Elevated CO₂ significantly increased nectar production per day (+51%, p < 0.01), total sugar per flower (+41%, p < 0.05), amino acid concentration (+65%, p < 0.05) and total amino acids per flower (+192%, p < 0.001). All other parameters tested, i.e., nectar sugar concentration, proportion of glucose/fructose and proportion of sucrose/(glucose + fructose), were not significantly affected by elevated CO₂ and/or fertilization. However, elevated CO₂ caused a marginally significant trend for earlier flowering in highly fertilized plants. No significant family × CO₂ interaction was found in any of the tested parameters, but the response in nectar production varied considerably among seed families (+10 to +104%) and was significantly positive in two of the five seed families investigated. Our results are not consistent with earlier studies on effects of elevated CO₂ on nectar production and flowering phenology in other plant species. It seems, on the other hand, that CO₂ effects on nectar production are specific to species and genotype. Hence, no general conclusions about effects of elevated CO₂ on these floral traits can be drawn at present, but it must be cautioned that elevated CO₂ might not only increase floral rewards as in E. angustifolium, but might also lead to shifts or even disruptions in fine-tuned plant–pollinator interactions.     

How long to stay on a plant: the response of bumblebees to encountered nectar levels

This field study shows that the number of flowers visited per bee per plant (Anchusa officinalis) increases with the instantaneous nectar level at the plant. Observations during the season showed that a bee visits more flowers per plant of given nectar level, the lower the overall mean nectar level in the study area. These results agree with predictions from a model based on the ‘marginal value theorem’, but with assumptions and constraints adapted for nectar-foraging bees. It suggests that bumblebees assess the nectar level at a plant by sampling one or a few flowers, which is possible because within-plant nectar volumes are correlated. The bees compare encountered gains to an optimal plant switching threshold equal to the overall mean nectar level and leave an unrewarding plant as soon as possible, but continue to visit the flowers on a rewarding plant. However, the bees leave before having visited all flowers due to a searching constraint. The bees’ response to plant nectar levels results in systematic flower visitation, because visitation to recently depleted flowers is reduced, which reduces the variation of the inter-visit time per flower. Systematic flower visitation implies that the overall mean encountered gain per flower is higher than the overall mean standing crop, as predicted by a model of systematic foraging. However, the sampling and searching constraints on the bees’ response to plant nectar levels increase the variation of the inter-visit time per flower, and thereby limit the degree of systematic flower visitation and the effect on the mean encountered gain.     

Preference of cabbage white butterflies and honey bees for nectar that contains amino acids

Summary Amino acids occur in most floral nectars but their role in pollinator attraction is relatively unstudied. Nectars of butterfly-pollinated flower tend to have higher concentrations of amino acids than do flowers pollinated by bees and many other animals, suggesting that amino acids are important attractants of butterflies to flowers. In order to determine whether amino acids are important in attracting butterflies and bees, we tested the preference of cabbage white butterflies (Pieris rapae) and honey bees (Apis mellifera) by allowing them to feed from artificial flowers containing sugar-only or sugar-amino acid mimics ofLantana camara nectar. Honey bees and female cabbage white butterflies consumed more sugar-amino acid nectar than sugar-only nectar. In addition, female cabbage white butterflies visited artificial flowers containing sugar-amino acid nectars more frequently than flowers containing sugar-only nectars; honey bees spent more time consuming the sugar-amino acid nectar. Male cabbage white butterflies did not discriminate between the two nectars. These results support the hypothesis that the amino acids of nectar contribute to pollinator attraction and/or feeding.     

A field study of the effects of elevated CO2 on plant biomass and community structure in a calcareous grassland

The effects of elevated CO₂ on plant biomass and community structure have been studied for four seasons in a calcareous grassland in northwest Switzerland. This highly diverse, semi-natural plant community is dominated by the perennial grass Bromus erectus and is mown twice a year to maintain species composition. Plots of 1.3 m² were exposed to ambient or elevated CO₂ concentrations (n = 8) using a novel CO₂ exposure technique, screen-aided CO₂ control (SACC) starting in March 1994. In the 1st year of treatment, the annual harvested biomass (sum of aboveground biomass from mowings in June and October) was not significantly affected by elevated CO₂. However, biomass increased significantly at elevated CO₂ in the 2nd (+20%, P = 0.05), 3rd (+21%, P = 0.02) and 4th years (+29%, P = 0.02). There were no detectable differences in root biomass in the top 8 cm of soil between CO₂ treatments on eight out of nine sampling dates. There were significant differences in CO₂ responsiveness between functional groups (legumes, non-leguminous forbs, graminoids) in the 2nd (P = 0.07) and 3rd (P < 0.001) years of the study. The order of CO₂ responsiveness among functional groups changed substantially from the 2nd to the 3rd year; for example, non-leguminous forbs had the smallest relative response in the 2nd year and the largest in the 3rd year. By the 3rd year of CO₂ exposure, large species-specific differences in CO₂ response had developed. For five important species or genera the order of responsiveness was Lotus corniculatus (+271%), Carex flacca (+249%), Bromus erectus (+33%), Sanguisorba minor (no significant CO₂ effect), and six Trifolium species (a negative response that was not significant). The positive CO₂ responses in Bromus and Carex were most closely related to increases in tiller number. Species richness was not affected by CO₂ treatment, but species evenness increased under elevated CO₂ (modified Hill ratio; P = 0.03) in June of the 3rd year, resulting in a marginally significant increase in species diversity (Simpson's index; P = 0.09). This and other experiments with calcareous grassland plants show that elevated atmospheric CO₂ concentrations can substantially alter the structure of calcareous grassland communities and may increase plant community biomass. Received: 12 July 1997 / Accepted: 14 September 1998     

Honeybees and the nectar of Echium plantagineum L. in southeastern Australia

An account is given of the flower of Echium plantagineum in south-eastern Australia, including stages and timing of flowering, behaviour of raindrops in the flower and aspects of floral microclimate. The concentration of nectar solutes varied with time and site, with means varying from 2 to 62% (as g sucrose/100 g solution). There was a significant negative correlation between nectar solute concentration and ambient relative humidity: the drier the air, the more concentrated the nectar. Rates of nectar secretion per flower varied with the bagging method, with long-term bagging reducing net secretion rates, possibly because of re-absorption. Rates varied with time, day and site, with a temporal pattern of change suggesting a link between rates of photosynthesis and secretion. Maximum nectar secretion rates in short-term bagging experiments were ca. 300 μg sugar/flower/hr (equivalent to > 2 mglflower/24 hr). Secretion rate was correlated with flower density. As flower density increased, secretion rate per flower decreased; rate of sugar production per unit area increased relatively more slowly than flower density. E. plantagineum could produce > 500 mg sugar/m²/day. Honeybees foraged on E. plantagineum only at ambient air temperatures above ca. 17°C unless irradiance exceeded ca. 750 W m^-2. Foragers collected nectar or pollen alone, or both, with the type of visit significantly correlated with nectar solute concentration. Below 35% (as g sucrose/100 g solution) most bees took pollen only; above 40%, most took nectar. Mean standing crop of nectar was generally < 100 μg/flower when most bees were taking nectar, but could exceed 1000 μg/flower when bees were absent or foraging mainly for pollen. Honeybees did not always remove all nectar from flowers they probed. Reabsorption of residual nectar may augment the following day's secretion.     

Can nectar properties explain sex‐specific flower preferences in the Adonis Blue butterfly Lysandra bellargus?

1. Field observations in the Swiss Jura mountains showed that males and females of the bivoltine Adonis Blue butterfly Lysandra bellargus Rott. differed significantly in their flower visitation patterns. 2. In both generations, females visited a broader range of available nectar plants than did males. The specific flower visitation patterns of males and females were not affected by the general availability and abundance of potential nectar plant species during both flight periods, indicating high selectivity for nectar plants by both males and females. 3. In addition, the sexes differed in their nectar foraging behaviours: distances between successively visited flowers were significantly longer in males than in females, indicating that male and female butterflies have different foraging strategies. 4. Investigations of nectar characteristics showed that the sexes preferred flowers with different nectar compositions. Males of both generations preferred flowers with high proportions of sucrose and high amounts of total sugar, whereas females preferred flowers with high portions of glucose in their nectar, and, in the spring generation, flowers rich in amino acids. 5. Flowers visited exclusively by males or females in spring differed significantly in their amino acid composition. 6. This clear‐cut pattern did not hold for the autumn generation, most probably due to the limited availability of flowers. 7. The observed nectar foraging patterns underline the importance of adult feeding for longevity and reproduction in butterflies. The findings are particularly relevant for conservation, because L. bellargus is an increasingly threatened species in many European countries.     

Populus tremuloides photosynthesis and crown architecture in response to elevated CO2 and soil N availability

We tested the hypothesis that elevated CO₂ would stimulate proportionally higher photosynthesis in the lower crown of Populus trees due to less N retranslocation, compared to tree crowns in ambient CO₂. Such a response could increase belowground C allocation, particularly in trees with an indeterminate growth pattern such as Populus tremuloides. Rooted cuttings of P. tremuloides were grown in ambient and twice ambient (elevated) CO₂ and in low and high soil N availability (89 ± 7 and 333 ± 16 ng N g⁻¹ day⁻¹ net mineralization, respectively) for 95 days using open-top chambers and open-bottom root boxes. Elevated CO₂ resulted in significantly higher maximum leaf photosynthesis (A _max) at both soil N levels. A _max was higher at high N than at low N soil in elevated, but not ambient CO₂. Photosynthetic N use efficiency was higher at elevated than ambient CO₂ in both soil types. Elevated CO₂ resulted in proportionally higher whole leaf A in the lower three-quarters to one-half of the crown for both soil types. At elevated CO₂ and high N availability, lower crown leaves had significantly lower ratios of carboxylation capacity to electron transport capacity (V _cmax/J _max) than at ambient CO₂ and/or low N availability. From the top to the bottom of the tree crowns, V _cmax/J _max increased in ambient CO₂, but it decreased in elevated CO₂ indicating a greater relative investment of N into light harvesting for the lower crown. Only the mid-crown leaves at both N levels exhibited photosynthetic down regulation to elevated CO₂. Stem biomass segments (consisting of three nodes and internodes) were compared to the total A _leaf for each segment. This analysis indicated that increased A _leaf at elevated CO₂ did not result in a proportional increase in local stem segment mass, suggesting that C allocation to sinks other than the local stem segment increased disproportionally. Since C allocated to roots in young Populus trees is primarily assimilated by leaves in the lower crown, the results of this study suggest a mechanism by which C allocation to roots in young trees may increase in elevated CO₂. Received: 12 August 1996 / Accepted: 12 November 1996     

The fraction of expanding to expanded leaves determines the biomass response of Populus to elevated CO2

We examined whether the effects of elevated CO₂ on growth of 1-year old Populus deltoides saplings was a function of the assimilation responses of particular leaf developmental stages. Saplings were grown for 100 days at ambient (approximately 350 ppm) and elevated (ambient + 200 ppm) CO₂ in forced-air greenhouses. Biomass, biomass distribution, growth rates, and leaf initiation and expansion rates were unaffected by elevated CO₂. Leaf nitrogen (N), the leaf C:N ratio, and leaf lignin concentrations were also unaffected. Carbon gain was significantly greater in expanding leaves of saplings grown at elevated compared to ambient CO₂. The Rubisco content in expanding leaves was not affected by CO₂ concentration. Carbon gain and Rubisco content were significantly lower in fully expanded leaves of saplings grown at elevated compared to ambient CO₂, indicating CO₂-induced down-regulation in fully expanded leaves. Elevated CO₂ likely had no overall effect on biomass accumulation due to the more rapid decline in carbon gain as leaves matured in saplings grown at elevated compared to ambient CO₂. This decline in carbon gain has been documented in other species and shown to be related to a balance between sink/source balance and acclimation. Our data suggest that variation in growth responses to elevated CO₂ can result from differences in leaf assimilation responses in expanding versus expanded leaves as they develop under elevated CO₂. Received: 28 September 1998 / Accepted: 23 June 1999     

Long-term responses of the green-algal lichen Parmelia caperata to natural CO2 enrichment

Acclimation to elevated CO₂ was investigated in Parmelia caperata originating from the vicinity of a natural CO₂ spring, where the average daytime CO₂ concentration was 729 ± 39 μmol mol⁻¹ dry air. Thalli showed no evidence of a down-regulation in photosynthetic capacity following long-term exposure to CO₂ enrichment in the field; carboxylation efficiency, total Ribulose bisphosphate carboxylase/oxygenase (Rubisco) content, apparent quantum yield of CO₂ assimilation, and the light-saturated rate of CO₂ assimilation (measured under ambient and saturating CO₂ concentrations) were similar in thalli from the naturally CO₂ enriched site and an adjacent control site where the average long-term CO₂ concentration was about 355 μmol mol⁻¹. Thalli from both CO₂ environments exhibited low CO₂ compensation points and early saturation of CO₂ uptake kinetics in response to increasing external CO₂ concentrations, suggesting the presence of an active carbon-concentrating mechanism. Consistent with the lack of significant effects on photosynthetic metabolism, no changes were found in the nitrogen content of thalli following prolonged exposure to elevated CO₂. Detailed intrathalline analysis revealed a decreased investment of nitrogen in Rubisco in the pyrenoid of algae located in the elongation zone of thalli originating from elevated CO₂, an effect associated with a reduction in the percentage of the cell volume occupied by lipid bodies and starch grains. Although these differences did not affect the photosynthetic capacity of thalli, there was evidence of enhanced limitations to CO₂ assimilation in lichens originating from the CO₂-enriched site. The light-saturated rate of CO₂ assimilation measured at the average growth CO₂ concentration was found to be significantly lower in thalli originating from a CO₂-enriched atmosphere compared with that of thalli originating and measured at ambient CO₂. At lower photosynthetic photon flux densities, the light compensation point of net CO₂ assimilation was significantly higher in thalli originating from elevated CO₂, and this effect was associated with higher usnic acid content. Received: 8 May 1998 / Accepted: 22 January 1999     

Relationships among nectar‐dwelling yeasts,flowers and ants: patterns and incidence on nectar traits

Nectar‐dwelling yeasts are emerging as widely distributed organisms playing a potentially significant and barely unexplored ecological role in plant pollinator mutualisms. Previous efforts at understanding nectar–pollinator–yeast interactions have focused on bee‐pollinated plants, while the importance of nectarivorous ants as vectors for yeast dispersal remains unexplored so far. Here we assess the abundance and composition of the nectar fungal microbiota of the ant‐pollinated plant Cytinus hypocistis, study whether yeast transmission is coupled with ant visitation, and discern whether ant‐ transported yeasts promote changes in nectar characteristics. Our results show that a high percentage of flowers (77%) and plants (94%) contained yeasts, with yeast cell density in nectar reaching up to 6.2 × 10⁴ cells mm^?3, being the highest densities associated with the presence of the nectar‐specialist yeast Metschnikowia reukaufii. The establishment of fungal microbiota in nectar required flower visitation by ants, with 70% of yeast species transported by them being also detected in nectar. Ant‐vectored yeasts diminished the nutritional quality of nectar, with flowers exposed to pollinators and yeasts containing significantly lower nectar sugar concentration than virgin flowers (13.4% and 22.8%, respectively). Nectar of flowers that harbored M. reukaufii showed the lowest quality, with nectar concentration declining significantly with increasing yeast density. Additionally, yeasts modified patterns of interpopulation variation in nectar traits, homo genizing differences between populations in some nectar attributes. We show for the first time that the outcome of the tripartite pollinator–flower–yeast interaction is highly dependent on the identity and inherent properties of the participants, even to the extent of influencing the species composition of this ternary system, and can be mediated by ecological characteristics of plant populations. Through their influence on plant functional traits, yeasts have the potential to alter nectar consumption, pollinator foraging behavior and ultimately plant reproduction.     

Will rising atmospheric CO2 affect leaf litter quality and in situ decomposition rates in native plant communities?

Though field data for naturally senesced leaf litter are rare, it is commonly assumed that rising atmospheric CO₂ concentrations will reduce leaf litter quality and decomposition rates in terrestrial ecosystems and that this will lead to decreased rates of nutrient cycling and increased carbon sequestration in native ecosystems. We generally found that the quality of␣naturally senesced leaf litter (i.e. concentrations of C, N and lignin; C:N, lignin:N) of a variety of native plant species produced in alpine, temperate and tropical communities maintained at elevated CO₂ (600–680 μl l⁻¹) was not significantly different from that produced in similar communities maintained at current ambient CO₂ concentrations (340–355 μl l⁻¹). When this litter was allowed to decompose in situ in a humid tropical forest in Panama (Cecropia peltata, Elettaria cardamomum, and Ficus benjamina, 130 days exposure) and in a lowland temperate calcareous grassland in Switzerland (Carex flacca and a graminoid species mixture; 261 days exposure), decomposition rates of litter produced under ambient and elevated CO₂ did not differ significantly. The one exception to this pattern occurred in the high alpine sedge, Carex curvula, growing in the Swiss Alps. Decomposition of litter produced in situ under elevated CO₂ was significantly slower than that of litter produced under ambient CO₂ (14% vs. 21% of the initial litter mass had decomposed over a 61-day exposure period, respectively). Overall, our results indicate that relatively little or no change in leaf litter quality can be expected in plant communities growing under soil fertilities common in many native ecosystems as atmospheric CO₂ concentrations continue to rise. Even in situations where small reductions in litter quality do occur, these may not necessarily lead to significantly slower rates of decomposition. Hence in many native species in situ litter decomposition rates, and the time course of decomposition, may remain relatively unaffected by rising CO₂. Received: 12 September 1996 / Accepted: 30 November 1996     

Increased allocation to external hyphae of arbuscular mycorrhizal fungi under CO2 enrichment

Prunella vulgaris was inoculated with different arbuscular mycorrhizal fungi (AMF) and grown at two concentrations of CO₂ (ambient, 350 μl l⁻¹, and elevated, 600 μl l⁻¹) to test whether a plants response to elevated CO₂ is dependent on the species of AMF colonizing the roots. Using compartments accessible only to AMF hyphae but not to roots, we also tested whether elevated CO₂ affects the growth of external AMF hyphae. Plant biomass was significantly greater at elevated than at ambient CO₂; the biomass of the root system, for example, increased by a factor of 2. The colonization of AMF inside the root remained constant, indicating that the total AMF inside the root system also increased by a factor of 2. The length of external AMF hyphae at elevated CO₂ was up to 5 times that at ambient CO₂, indicating that elevated CO₂ promoted allocation of AMF biomass to the external hyphae. The concentration and content of phosphorus in the stolons differed significantly between ambient and elevated CO₂ but this resulted in either an increase or a decrease, according to which AMF isolate occupied the roots. We hypothesized that an increase in external hyphal growth at elevated CO₂ would result in increased P acquistion by the plant. To test this we supplied phosphorus, in a compartment only accessible to AMF hyphae. Plants did not acquire more phosphorus at elevated CO₂ when phosphorus was added to this compartment. Large increases in AMF hyphal growth could, however, play a significant role in the movement of fixed carbon to the soil and increase soil aggregation. Received: 28 March 1998 / Accepted: 27 August 1998     

Early reproductive failure increases nectar production and pollination success of late flowers in south Andean Alstroemeria aurea

Fertile ramets of bumblebee-pollinated Alstroemeria aurea, a clonal perennial native to the temperate forests of the southern Andes, produce single terminal inflorescences that may bear two or more temporally non-overlapping whorls of flowers. While fruit set is commonly high (>80%) among early-opening flowers, it is usually low (<20%) among late-opening flowers within ramets. Using flowering ramets with two whorls of flowers, we examined experimentally the following related hypotheses. First, late flowers act as a reserve of ovaries, increasing their likelihood of setting seed when early fruits abort due to either pollen or resource limitation. Second, where early fruit abortion has occurred, plants may actively ensure pollination of late flowers by increasing their attractants and rewards. In a natural population, we simulated (1) lack of pollen deposition in early flowers, by excising their stigmas just before receptivity, and (2) resource limitation, by removing all the leaves from an experimental flowering ramet. Treatments were applied to individual ramets according to a 2 × 2 factorial design. We found that when early flowers failed to set fruit due to stigma excision, nectar secretion and particularly pollen receipt strongly increased in late flowers. Higher pollen deposition contributed significantly to the observed five-fold increase in seed output of late flowers. Fruit and seed set from early flowers were more negatively affected by defoliation than that from late flowers. Defoliation did not interfere with a ramet's capacity to increase late reproductive output when early reproduction failed. These results support the assertion that late flowers act as a reserve of ovaries helping a plant to cope with an unpredictable environment. These results also suggest that plants may actively increase pollinator visitation by opportunistically increasing flower rewards. Received: 23 June 1998 / Accepted: 3 May 1999     

Effects of elevated CO2 and phosphorus addition on productivity and community composition of intact monoliths from calcareous grassland

We investigated the effects of elevated CO₂ (600 μl l⁻¹ vs 350 μl l⁻¹) and phosphorus supply (1 g P m⁻² year⁻¹ vs unfertilized) on intact monoliths from species-rich calcareous grassland in a greenhouse. Aboveground community dry mass remained almost unaffected by elevated CO₂ in the first year (+6%, n.s.), but was significantly stimulated by CO₂ enrichment in year two (+26%, P<0.01). Among functional groups, only graminoids contributed significantly to this increase. The effect of phosphorus alone on community biomass was small in both years and marginally significant only when analyzed with MANOVA (+6% in year one, +9% in year two, 0.1 ≥P > 0.05). Belowground biomass and stubble after two seasons were not different in elevated CO₂ and when P was added. The small initial increase in aboveground community biomass under elevated CO₂ is explained by the fact that some species, in particular Carex flacca, responded very positively right from the beginning, while others, especially the dominant Bromus erectus, responded negatively to CO₂ enrichment. Shifts in community composition towards more responsive species explain the much larger CO₂ response in the second year. These shifts, i.e., a decline in xerophytic elements (B. erectus) and an increase in mesophytic grasses and legumes occurred independently of treatments in all monoliths but were accelerated significantly by elevated CO₂. The difference in average biomass production at elevated compared to ambient CO₂ was higher when P was supplied (at the community level the CO₂ response was enhanced from 20% to 33% when P was added, in graminoids from 17% to 27%, in legumes from 4% to 60%, and in C. flacca from 120% to 298% by year two). Based on observations in this and similar studies, we suggest that interactions between CO₂ concentration, species presence, and nutrient availability will govern community responses to elevated CO₂. Received: 12 July 1997 / Accepted: 28 March 1998     

Photosynthetic acclimation of maize to growth under elevated levels of carbon dioxide

The effects of elevated CO₂ concentrations on the photochemistry, biochemistry and physiology of C₄ photosynthesis were studied in maize (Zea mays L.). Plants were grown at ambient (350 μL L⁻¹) or ca. 3 times ambient (1100 μL L⁻¹) CO₂ levels under high light conditions in a greenhouse for 30 d. Relative to plants grown at ambient CO₂ levels, plants grown under elevated CO₂ accumulated ca. 20% more biomass and 23% more leaf area. When measured at the CO₂ concentration of growth, mature leaves of high-CO₂-grown plants had higher light-saturated rates of photosynthesis (ca. 15%), lower stomatal conductance (71%), higher water-use efficiency (225%) and higher dark respiration rates (100%). High-CO₂-grown plants had lower carboxylation efficiencies (23%), measured under limiting CO₂, and lower leaf protein contents (22%). Activities of a number of C₃ and C₄ cycle enzymes decreased on a leaf-area basis in the high-CO₂-grown plants by 5–30%, with NADP-malate dehydrogenase exhibiting the greatest decrease. In contrast, activities of fructose 1,6-bisphosphatase and ADP-glucose pyrophosphorylase increased significantly under elevated CO₂ condition (8% and 36%, respectively). These data show that the C₄ plant maize may benefit from elevated CO₂ through acclimation in the capacities of certain photosynthetic enzymes. The increased capacity to synthesize sucrose and starch, and to utilize these end-products of photosynthesis to produce extra energy by respiration, may contribute to the enhanced growth of maize under elevated CO₂. Received: 30 April 1999 / Accepted: 17 June 1999     

Acclimation of the summer annual species, Lolium temulentum, to CO2 enrichment

Lolium temulentum L. Ba 3081 was grown hydroponically in air (350 μmol mol⁻¹ CO₂) and elevated CO₂ (700 μmol mol⁻¹ CO₂) at two irradiances (150 and 500 μmol m⁻² s⁻¹) for 35 days at which point the plants were harvested. Elevated CO₂ did not modify relative growth rate or biomass at either irradiance. Foliar carbon-to-nitrogen ratios were decreased at elevated CO₂ and plants had a greater number of shorter tillers, particularly at the lower growth irradiance. Both light-limited and light-saturated rates of photosynthesis were stimulated. The amount of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) protein was increased at elevated CO₂, but maximum extractable Rubisco activities were not significantly increased. A pronounced decrease in the Rubisco activation state was found with CO₂ enrichment, particularly at the higher growth irradiance. Elevated-CO₂-induced changes in leaf carbohydrate composition were small in comparison to those caused by changes in irradiance. No CO₂-dependent effects on fructan biosynthesis were observed. Leaf respiration rates were increased by 68% in plants grown with CO₂ enrichment and low light. We conclude that high CO₂ will only result in increased biomass if total light input favourably increases the photosynthesis-to-respiration ratio. At low irradiances, biomass is more limited by increased rates of respiration than by CO₂-induced enhancement of photosynthesis. Received: 23 February 1999 / Accepted: 15 June 1999     

Nutrient relations in calcareous grassland under elevated CO2

Plant nutrient responses to 4 years of CO₂ enrichment were investigated in situ in calcareous grassland. Beginning in year 2, plant aboveground C:N ratios were increased by 9% to 22% at elevated CO₂ (P < 0.01), depending on year. Total amounts of N removed in biomass harvests during the first 4 years were not affected by elevated CO₂ (19.9 ± 1.3 and 21.1 ± 1.3 g N m⁻² at ambient and elevated CO₂), indicating that the observed plant biomass increases were solely attained by dilution of nutrients. Total aboveground P and tissue N:P ratios also were not altered by CO₂ enrichment (12.5 ± 2 g N g⁻¹ P in both treatments). In contrast to non-legumes (>98% of community aboveground biomass), legume C/N was not reduced at elevated CO₂ and legume N:P was slightly increased. We attribute the less reduced N concentration in legumes at elevated CO₂ to the fact that virtually all legume N originated from symbiotic N₂ fixation (%N_dfa ≈ 90%), and thus legume growth was not limited by soil N. While total plant N was not affected by elevated CO₂, microbial N pools increased by +18% under CO₂ enrichment (P = 0.04) and plant available soil N decreased. Hence, there was a net increase in the overall biotic N pool, largely due increases in the microbial N pool. In order to assess the effects of legumes for ecosystem CO₂ responses and to estimate the degree to which plant growth was P-limited, two greenhouse experiments were conducted, using firstly undisturbed grassland monoliths from the field site, and secondly designed `microcosm' communities on natural soil. Half the microcosms were planted with legumes and half were planted without. Both monoliths and microcosms were exposed to elevated CO₂ and P fertilization in a factored design. After two seasons, plant N pools in both unfertilized monoliths and microcosm communities were unaffected by CO₂ enrichment, similar to what was found in the field. However, when P was added total plant N pools increased at elevated CO₂. This community-level effect originated almost solely from legume stimulation. The results suggest a complex interaction between atmospheric CO₂ concentrations, N and P supply. Overall ecosystem productivity is N-limited, whereas CO₂ effects on legume growth and their N₂ fixation are limited by P. Received: 12 July 1997 / Accepted: 15 April 1998