bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of elevated CO2 and temperature‐grown red and sugar maple on gypsy moth performance
Authors: Williams Ray S. , Norby Richard J. , Lincoln David E.
Published: March 2003

Few studies have investigated how tree species grown under elevated CO2 and elevated temperature alter the performance of leaf‐feeding insects. The indirect effects of an elevated CO2 concentration and temperature on leaf phytochemistry, along with potential direct effects on insect growth and consumption, may independently or interactively affect insects. To investigate this, we bagged larvae of the gypsy moth on leaves of red and sugar maple growing in open‐top chambers in four CO2/temperature treatment combinations: (i) ambient temperature, ambient CO2; (ii) ambient temperature, elevated CO2 (+ 300 μL L−1 CO2); (iii) elevated temperature (+ 3.5°C), ambient CO2; and (iv) elevated temperature, elevated CO2. For both tree species, leaves grown at elevated CO2 concentration were significantly reduced in leaf nitrogen concentration and increased in C: N ratio, while neither temperature nor its interaction with CO2 concentration had any effect. Depending on the tree species, leaf water content declined (red maple) and carbon‐based phenolics increased (sugar maple) on plants grown in an enriched CO2 atmosphere. The only observed effect of elevated temperature on leaf phytochemistry was a reduction in leaf water content of sugar maple leaves. Gypsy moth larval responses were dependent on tree species. Larvae feeding on elevated CO2‐grown red maple leaves had reduced growth, while temperature had no effect on the growth or consumption of larvae. No significant effects of either temperature or CO2 concentration were observed for larvae feeding on sugar maple leaves. Our data demonstrate strong effects of CO2 enrichment on leaf phytochemical constituents important to folivorous insects, while an elevated temperature largely has little effect. We conclude that alterations in leaf chemistry due to an elevated CO2 atmosphere are more important in this plant–folivorous insect system than either the direct short‐term effects of temperature on insect performance or its indirect effects on leaf chemistry.
Direct evidence that symbiotic N2 fixation in fertile grassland is an important trait for a strong response of plants to elevated atmospheric CO2
Authors: Lüscher Andreas , Hartwig Ueli A. , Suter Daniel , Nösberger Josef
Published: March 2003

Although legumes showed a clearly superior yield response to elevated atmospheric pCO2 compared to nonlegumes in a variety of field experiments, the extent to which this is due to symbiotic N2 fixation per se has yet to be determined. Thus, effectively and ineffectively nodulating lucerne (Medicago sativa L.) plants with a very similar genetic background were grown in competition with each other on fertile soil in the Swiss FACE experiment in order to monitor their CO2 response. Under elevated atmospheric pCO2, effectively nodulating lucerne, thus capable of symbiotically fixing N2, strongly increased the harvestable biomass and the N yield, independent of N fertilization. In contrast, the harvestable biomass and N yield of ineffectively nodulating plants were affected negatively by elevated atmospheric pCO2 when N fertilization was low. Large amounts of N fertilizer enabled the plants to respond more favourably to elevated atmospheric pCO2, although not as strongly as effectively nodulating plants. The CO2‐induced increase in N yield of the effectively nodulating plants was attributed solely to an increase in symbiotic N2 fixation of 50–175%, depending on the N fertilization treatment. N yield derived from the uptake of mineral N from the soil was, however, not affected by elevated pCO2. This result demonstrates that, in fertile soil and under temperate climatic conditions, symbiotic N2 fixation per se is responsible for the considerably greater amount of above‐ground biomass and the higher N yield under elevated atmospheric pCO2. This supports the assumption that symbiotic N2 fixation plays a key role in maintaining the C/N balance in terrestrial ecosystems in a CO2‐rich world.
Leaf senescence of Quercus myrtifolia as affected by long‐term CO2 enrichment in its native environment
Authors: Li J. ‐H. , Dijkstra P. , Hymus G. J. , Wheeler R. M. , Piastuch W. C. , Hinkle C. R. , Drake B. G.
Published: March 2003

The long‐term effects of elevated (ambient plus 350 μmol mol−1) atmospheric CO2 concentration (Ca) on the leaf senescence of Quercus myrtifolia Willd was studied in a scrub‐oak community during the transition from autumn (December 1997) to spring (April 1998). Plants were grown in large open‐top chambers at the Smithsonian CO2 Research Site, Merritt Island Wildlife Refuge, Cape Canaveral, Florida. Chlorophyll (a + b) concentration, Rubisco activity and N concentration decreased by 75%, 82%, and 52%, respectively, from December (1997) to April (1998) in the leaves grown at ambient Ca. In contrast, the leaves of plants grown at elevated Ca showed no significant decrease in chlorophyll (a + b) concentration or Rubisco activity, and only a 25% reduction in nitrogen. These results indicate that leaf senescence was delayed during this period at elevated Ca. Delayed leaf senescence in elevated Ca had important consequences for leaf photosynthesis. In elevated Ca the net photosynthetic rate of leaves that flushed in Spring 1997 (last year's leaves) and were 13 months old was not different from fully‐expanded leaves that flushed in 1998, and were approximately 1 month old (current year's leaves). In ambient Ca the net photosynthetic rate of last year's leaves was 54% lower than for current year's leaves. When leaves were fully senesced, nitrogen concentration decreased to about 40% of the concentration in non‐senesced leaves, in both CO2 treatments. In April, net photosynthesis was 97% greater in leaves grown in elevated Ca than in those grown at ambient. During the period when elevated Ca delayed leaf senescence, more leaves operating at higher photosynthetic rate would allow the ecosystem dominated by Q. myrtifolia to gain more carbon at elevated Ca than at ambient Ca.
Optimum experimental design for Free‐Air Carbon dioxide Enrichment (FACE) studies
Authors: Filion Mathieu , Dutilleul Pierre , Potvin Catherine
Published: March 2003

This article presents the logical reasoning underlying the optimal design of an experiment. We used Free‐Air Carbon dioxide Enrichment (FACE) experiments to illustrate this trade‐off as such experiments are particularly costly. On a theoretical basis, two‐way nested designs and split‐plot designs have similar power in testing carbon dioxide (CO2) main effects. If researchers have the choice of adding two replicate rings or two control plots to their experiment, our results show that both options provide a substantial gain in statistical power, with a slightly greater gain in the former case and at reduced financial cost in the latter. The former option, however, provides an insurance against possible ring failure.

On an empirical basis, we analysed a preliminary FACE photosynthesis dataset collected at Duke University. The experiment was designed as a split‐plot design to test the effects of growth environment (GROWTH) and measurement CO2 concentration (MEAS) on photosynthetic rates of loblolly pine. Although a significant effect of MEAS was observed, we failed to detect a significant main effect of GROWTH. Power analysis was used to understand why the GROWTH main effect was not significant. The minimum detectable difference between treatment means that we calculated for GROWTH in this experiment was 4.04 μmol CO2 m−2 s−1 for a statistical power of 0.90, whereas the observed difference was 0.16 μmol CO2 m−2 s−1.

Our recommendations for the design of FACE experiments are: (i) consider a second treatment factor with many levels within each ring in order to obtain a split‐plot design that provides a powerful test of interaction between treatment factors; (ii) add control plots, unless financial constrictions disallow for necessary personnel; (3) pool the data of FACE experiments conducted in comparable ecosystems (e.g. forests or grasslands), with two rings per treatment level at each site.
Allocation responses to CO2 enrichment and defoliation by a native annual plant Heterotheca subaxillaris
Authors: Johnson Sandra L. , Lincoln David E.
Published: March 2003

Among plants grown under enriched atmospheric CO2, root:shoot balance (RSB) theory predicts a proportionately greater allocation of assimilate to roots than among ambient‐grown plants. Conversely, defoliation, which decreases the plant's capacity to assimilate carbon, is predicted to increase allocation to shoot. We tested these RSB predictions, and whether responses to CO2 enrichment were modified by defoliation, using Heterotheca subaxillaris, an annual plant native to south‐eastern USA. Plants were grown under near‐ambient (400 μmol mol−1) and enriched (700 μmol mol−1) levels of atmospheric CO2. Defoliation consisted of the weekly removal of 25% of each new fully expanded, but not previously defoliated, leaf from either rosette or bolted plants. In addition to dry mass measurements of leaves, stems, and roots, Kjeldahl N, protein, starch and soluble sugars were analysed in these plant components to test the hypothesis that changes in C:N uptake ratio drive shifts in root:shoot ratio. Young, rapidly growing CO2‐enriched plants conformed to the predictions of RSB, with higher root:shoot ratio than ambient‐grown plants (P < 0.02), whereas older, slower growing plants did not show a CO2 effect on root:shoot ratio. Defoliation resulted in smaller plants, among which both root and shoot biomass were reduced, irrespective of CO2 treatment (P < 0.03). However, H. subaxillaris plants were able to compensate for leaf area removal through flexible shoot allocation to more leaves vs. stem (P < 0.01). Increased carbon availability through CO2 enrichment did not enhance the response to defoliation, apparently because of complete growth compensation for defoliation, even under ambient conditions. CO2‐enriched plants had higher rates of photosynthesis (P < 0.0001), but this did not translate into increased final biomass accumulation. On the other hand, earlier and more abundant yield of flower biomass was an important consequence of growth under CO2 enrichment.
Consequences of CO2 and light interactions for leaf phenology, growth, and senescence in Quercus rubra
Authors: Cavender‐Bares J. , Potts M. , Zacharias E. , Bazzaz F. A.
Published: March 2003

We investigated how light and CO2 levels interact to influence growth, phenology, and the physiological processes involved in leaf senescence in red oak (Quercus rubra) seedlings. We grew plants in high and low light and in elevated and ambient CO2. At the end of three years of growth, shade plants showed greater biomass enhancement under elevated CO2 than sun plants. We attribute this difference to an increase in leaf area ratio (LAR) in shade plants relative to sun plants, as well as to an ontogenetic effect: as plants increased in size, the LAR declined concomitant with a decline in biomass enhancement under elevated CO2

Elevated CO2 prolonged the carbon gain capacity of shade‐grown plants during autumnal senescence, thus increasing their functional leaf lifespan. The prolongation of carbon assimilation, however, did not account for the increased growth enhancement in shade plants under elevated CO2. Elevated CO2 did not significantly alter leaf phenology. Nitrogen concentrations in both green and senesced leaves were lower under elevated CO2 and declined more rapidly in sun leaves than in shade leaves. Similar to nitrogen concentration, the initial slope of A/Ci curves indicated that Rubisco activity declined more rapidly in sun plants than in shade plants, particularly under elevated CO2. Absolute levels of chlorophyll were affected by the interaction of CO2 and light, and chlorophyll content declined to a minimal level in sun plants sooner than in shade plants. These declines in N concentration, in the initial slope of A/Ci curves, and in chlorophyll content were consistent with declining photosynthesis, such that elevated CO2 accelerated senescence in sun plants and prolonged leaf function in shade plants. These results have implications for the carbon economy of seedlings and the regeneration of red oak under global change conditions.
Scaling phenology from the local to the regional level: advances from species‐specific phenological models
Authors: Chuine I. , Cambon G. , Comtois P.
Published: March 2003

Plant phenology, the study of seasonal plant activity driven by environmental factors, has found a renewal in the context of global climate change. Phenological events, such as leaf unfolding, exert strong control over seasonal exchanges of matter and energy between the land surface and the atmosphere. Phenological models that simulate the start of the growing season should be efficient tools to predict vegetation responses to climatic changes and related changes in energy balance. Species‐specific phenological models developed in the eighties have not been used for global‐scale predictions because their predictions were inaccurate in external conditions. Recent advances in phenology modelling at the species level suggest that prediction at a large scale may now be possible. In the present study, we tested the performance of species‐specific phenological models in time and space, looking at their ability (i) to predict regional phenology when previously fitted at a local scale, and (ii) to predict phenological trends, linked to climate changes, observed over a long‐term. For that task we used an historical phenological dataset from Ohio from the late ninetieth century and an airborne pollen dataset from Ontario, Québec and Maryland from the late twentieth century. The results show that the species‐specific phenological models used in this study were able to predict regional phenology even though they were fitted locally. The reconstruction of a phenological time series over the twentieth century showed a significant advancement of 0.2 days per year in the date of flowering of Ulmus americana, but very weak trends for Fraxinus americana and Quercus velutina.
Impacts of extreme climatic events on competition during grassland invasions
Authors: White TodD. A. , Campbell Bruce D. , Kemp Peter D. , Hunt Chris L.
Published: March 2003

Phytometers of five C3 and five C4 species were transplanted into three different grasslands to study the effects of extreme climatic events on community invasibility and competition. Single extreme heating (eight hours at 52.5 °C) and rainfall (the equivalent of 100 mm) events in factorial combinations were superimposed on the grassland communities. A novel technique involving portable computer‐controlled chambers was used to create the heating events. In order to generate predictions of response to the extreme climatic events, the 10 phytometer species were categorized on the basis of 12 key plant functional traits. Using principal component analysis, two functional types (FTs) were identified as most likely to be advantaged (FT1, fast‐growing C4 annuals) and disadvantaged (FT2, slower‐growing C3 perennials) by an extreme climatic event. Competition between the resident vegetation and FT1 plus other C4 phytometers was consistently more intense within the exclusively C3 community compared to the dry C3/C4 community or moist C3/C4 community. The single extreme heating event had the greatest impact on competition, lowering the intensity of competition between the phytometers and resident vegetation. Our results indicate that competition is highly important in limiting the invasion of C3 grasslands by C4 species. The FT1 and FT2 responses confirmed predictions based on plant functional traits, whether growing as phytometers or as part of the resident vegetation. Future increases in climatic variability and the incidence of extreme climatic events are expected to suppress C3 competitive dominance and promote invasion of C4 species, in particular, the FT1 species.
Canopy radiation‐ and water‐use efficiencies as affected by elevated [CO2]
Authors: Hui Dafeng , Luo Yiqi , Cheng Weixin , Coleman J. S. , Johnson Dale W. , Sims Daniel A.
Published: March 2003

This study used an environmentally controlled plant growth facility, EcoCELLs, to measure canopy gas exchanges directly and to examine the effects of elevated [CO2] on canopy radiation‐ and water‐use efficiencies. Sunflowers (Helianthus annus var. Mammoth) were grown at ambient (399 μmol mol−1) and elevated [CO2] (746 μmol mol−1) for 53 days in EcoCELLs. Whole canopy carbon‐ and water‐fluxes were measured continuously during the period of the experiment. The results indicated that elevated [CO2] enhanced daily total canopy carbon‐ and water‐fluxes by 53% and 11%, respectively, on a ground‐area basis, resulting in a 54% increase in radiation‐use efficiency (RUE) based on intercepted photosynthetic active radiation and a 26% increase in water‐use efficiency (WUE) by the end of the experiment. Canopy carbon‐ and water‐fluxes at both CO2 treatments varied with canopy development. They were small at 22 days after planting (DAP) and gradually increased to the maxima at 46 DAP. When canopy carbon‐ and water‐fluxes were expressed on a leaf‐area basis, no effect of CO2 was found for canopy water‐flux while elevated [CO2] still enhanced canopy carbon‐flux by 29%, on average. Night‐time canopy carbon‐flux was 32% higher at elevated than at ambient [CO2]. In addition, RUE and WUE displayed strong diurnal variations, high at noon and low in the morning or afternoon for WUE but opposite for RUE. This study provided direct evidence that plant canopy may consume more, instead of less, water but utilize both water and radiation more efficiently at elevated than at ambient [CO2], at least during the exponential growth period as illustrated in this experiment.
The influence of elevated O3 and CO2 concentrations on secondary metabolites of Scots pine (Pinus sylvestris L. ) seedlings
Authors: Sallas Leena , Kainulainen Pirjo , Utriainen Jarkko , Holopainen Toini , Holopainen Jarmo K.
Published: March 2003

Terpene, resin acid and total phenolic concentrations in five‐year‐old Scots pine (Pinus sylvestris L.) seedlings were analysed after exposure to ambient and realistically elevated (2 × ambient) O3 and CO2 concentrations and their combination in open‐top chambers during two growing seasons. Under O3 exposure, limonene concentration in needles and isopimaric concentration in stems decreased significantly. As a response to elevated CO2, α‐pinene and total phenolic concentrations in needles increased significantly, while bornyl acetate concentration in needles and palustric + levopimaric and neoabietic acid concentrations in stems decreased significantly. Some terpenes and resin acids were found at lower concentrations in the combined O3 and CO2 treatment than in O3 exposure or elevated CO2. A negative chamber effect was found: seedlings growing inside the chambers with ambient air had significantly lower concentrations of some terpenes and resin acids than seedlings growing outside the chambers. There was a lot of between‐tree variation in terpene and resin acid concentrations, which is typical of open‐pollinated populations. The results of this study suggest that, at least in short‐term experiments, Scots pine secondary metabolites are relatively insensitive to climate change factors. Total phenolics in the needles were the most responsive group showing about 25% increase in elevated CO2, and O3 exposure did not mitigate this CO2 effect. Terpenes and resin acids were less responsive, although some individual compounds showed notable responses, e.g. α‐pinene in needles, which increased about 50% in response to elevated CO2. As a consequence, although there were only slight effects on total pools of needle secondary metabolites, considerable O3 and CO2 effects on certain individual compounds might have ecological significance via trophic amplification, e.g. in decomposing processes of needle litter.
Stress synergy between drought and a common environmental contaminant: studies with the collembolanFolsomia candida
Authors: Højer Rikke , Bayley Mark , And Christian F. Damgaard , Holmstrup Martin
Published: March 2003

The term global change is used predominantly in connection with the global temperature increase and associated changes in weather patterns over the next century. In a broader sense it also covers other anthropogenic impacts on the environment such as habitat fragmentation and pollution. The individual effects of each of these stress types have been extensively studied in the biota. However, organisms will frequently encounter these stress types in combination rather than alone and there is little information available on the effects of stress combinations.

Here an examination is made of the interaction between realistic levels of summer drought and a common contaminant of agricultural soil (4‐nonylphenol, NP), on a widespread soil invertebrate, the collembolanFolsomia candida.These stress factors were tested individually and in combination using a full factorial design. This approach revealed the existence of highly significant Bliss type synergistic interaction between the two stress types. Thus, exposure to NP significantly reduced the drought tolerance of this organism and, reciprocally, the toxicity of NP (LC50) during realistic summer drought was more than doubled in comparison to the value obtained under optimal soil moisture conditions. Furthermore, it is shown that NP has a detrimental effect on the physiological mechanisms underlying this animal's drought tolerance, thus providing some explanation for the mechanisms involved in the synergy. It is argued that this type of synergy is unlikely to be confined to this particular combination of stresses and thus there is a need to study the interactions between dominant natural stresses and pollution. The most important implication of these results is that some of the effects of global climate changes can be predicted to be most severe in polluted areas.
Herbivory in global climate change research: direct effects of rising temperature on insect herbivores
Authors: Bale Jeffery S. , Masters Gregory J. , Hodkinson Ian D. , Awmack Caroline , Bezemer T. Martijn , Brown Valerie K. , Butterfield Jennifer , Buse Alan , Coulson John C. , Farrar John , Good John E. G. , Harrington Richard , Hartley Susane , Jones T. Hefin , Lindroth Richard L. , Press Malcolm C. , Symrnioudis Ilias , Watt Allan D. , Whittaker John B.
Published: March 2003

This review examines the direct effects of climate change on insect herbivores. Temperature is identified as the dominant abiotic factor directly affecting herbivorous insects. There is little evidence of any direct effects of CO2 or UVB. Direct impacts of precipitation have been largely neglected in current research on climate change. Temperature directly affects development, survival, range and abundance. Species with a large geographical range will tend to be less affected. The main effect of temperature in temperate regions is to influence winter survival; at more northerly latitudes, higher temperatures extend the summer season, increasing the available thermal budget for growth and reproduction. Photoperiod is the dominant cue for the seasonal synchrony of temperate insects, but their thermal requirements may differ at different times of year. Interactions between photoperiod and temperature determine phenology; the two factors do not necessarily operate in tandem. Insect herbivores show a number of distinct life‐history strategies to exploit plants with different growth forms and strategies, which will be differentially affected by climate warming. There are still many challenges facing biologists in predicting and monitoring the impacts of climate change. Future research needs to consider insect herbivore phenotypic and genotypic flexibility, their responses to global change parameters operating in concert, and awareness that some patterns may only become apparent in the longer term.
Climatic regime shifts and decadal‐scale variability in calanoid copepod populations off southern California
Authors: Rebstock Ginger A.
Published: March 2003

Decadal‐scale climatic regimes and the shifts between them have important impacts on marine ecosystems. Climatic regime shifts have been observed or hypothesized in the North Pacific basin in 1976–77 and 1989. This paper examines long‐term (1951–99) trends in calanoid copepod populations off southern California, and the evidence for responses to regime shifts.

    Most of the species of calanoid copepod that were analysed underwent one or more step changes during the 49 years covered by the study. All but one of these changes occurred in five periods: the late 1950s, late 1960s, mid‐1970s, early 1980s and around 1990. The late 1960s changes are considered to be artifacts of an increase in sampling depth. Strong El Niño conditions affected California waters during the late 1950s and early 1980s. The step changes of the mid‐1970s and late 1980s to early 1990s may have been responses to regime shifts or other climatic events. 28% of the species and subspecies responded to the 1976–77 event, all increasing in abundance. Another 28% of the copepod categories underwent step changes around 1990, most decreasing.

    Evidence for regime shifts in the hydrographic variables that were examined is mixed. The 10‐m temperature increased in the mid‐1970s. Abrupt changes in variables around 1990 were short‐lived. However, the population responses around 1990 and to the El Niños of the late 1950s and early 1980s indicate that some species of calanoid copepods may respond on longer time scales to environmental conditions that persist only a few years.
Changed plant and animal life cycles from 1952 to 2000 in the Mediterranean region
Authors: Peñuelas Josep , Filella Iolanda , Comas PerE.
Published: March 2003

The available data on climate over the past century indicate that the earth is warming. Important biological effects, including changes of plant and animal life cycle events, have already been reported. However, evidence of such effects is still scarce and has been mostly limited to northern latitudes. Here we provide the first long‐term (1952–2000) evidence of altered life cycles for some of the most abundant Mediterranean plants and birds, and one butterfly species. Average annual temperatures in the study area (Cardedeu, NE Spain) have increased by 1.4 °C over the observation period while precipitation remained unchanged. A conservative linear treatment of the data shows that leaves unfold on average 16 days earlier, leaves fall on average 13 days later, and plants flower on average 6 days earlier than in 1952. Fruiting occurs on average 9 days earlier than in 1974. Butterflies appear 11 days earlier, but spring migratory birds arrive 15 days later than in 1952. The stronger changes both in temperature and in phenophases timing occurred in the last 25 years. There are no significant relationships among changes in phenophases and the average date for each phenophase and species. There are not either significant differences among species with different Raunkiaer life‐forms or different origin (native, exotic or agricultural). However, there is a wide range of phenological alterations among the different species, which may alter their competitive ability, and thus, their ecology and conservation, and the structure and functioning of ecosystems. Moreover, the lengthening of plant growing season in this and other northern hemisphere regions may contribute to a global increase in biospheric activity.
The influence of plants grown under elevated CO2and N fertilization on soil nitrogen dynamics
Authors: Martín‐Olmedo Piedad , Rees Robert M. , Grace John
Published: March 2003

We investigated the effects of spring barley growth on nitrogen (N) transformations and rhizosphere microbial processes in a controlled system under elevated carbon dioxide (CO2) at two levels of N fertilization (applied with15N labelling). After 25 d, elevated CO2(twice ambient) increased plant growth (dry weight, DW) by 141% at low‐N fertilization and by 60% at high‐N fertilization, but its positive effect on the root‐to‐shoot ratio was only significant at low‐N input. As a result of this plant response, elevated CO2caused a greater soil CO2efflux, rhizosphere soil DW, and soil microbial biomass under N‐limiting conditions than under high N availability. Elevated CO2also caused a significant (P < 0.001) increase in the N recovered by the plant from both the labelled (Nf) and unlabelled (Ns + Nuf) N pools. The dynamics of N in the system as affected by elevated CO2were driven principally by mineralization–immobilization turnover, with little loss by denitrification. Under N‐limiting conditions, there is evidence to suggest enhanced nutrient release from soil organic matter (SOM) pools—a process which could be defined as priming. The results of our experiment did not indicate a direct plant‐mediated effect of elevated CO2on nitrous oxide (N2O) fluxes or denitrification activity.
Hydrologic balance in an intact temperate forest ecosystem under ambient and elevated atmospheric CO2concentration
Authors: Schäfer Karina V. R. , Oren Ram , Lai Chun‐Ta , Katul Gabriel G.
Published: March 2003

AbstractIncreasing atmospheric CO2concentration decreases stomatal conductance in many species, but the savings of water from reduced transpiration may permit the forest to retain greater leaf area index (L). Therefore, the net effect on water use in forest ecosystems under a higher CO2atmosphere is difficult to predict. The free air CO2enrichment (FACE) facility (n = 3) in a 14‐m tall (in 1996)Pinus taedaL. stand was designed to reduce uncertainties in predicting such responses. Continuous measurements of precipitation, throughfall precipitation, sap flux, and soil moisture were made over 3.5 years under ambient (CO2a) and elevated (CO2e) ambient + 200 µmol mol−1). Annual stand transpiration under ambient CO2conditions accounted for 84–96% of latent heat flux measured with the eddy‐covariance technique above the canopy. Under CO2e,P. taedatranspired less per unit of leaf area only when soil drought was severe.Liquidambar styraciflua, the other major species in the forest, used progressively less water, settling at 25% reduction in sap flux density after 3.5 years under CO2e. BecauseP. taedadominated the stand, and severe drought periods were of relatively short duration, the direct impact of CO2eon water savings in the stand was undetectable. Moreover, the forest used progressively more water under CO2e, probably because soil moisture availability progressively increased, probably owing to a reduction in soil evaporation caused by more litter buildup in the CO2eplots. The results suggest that, in this forest, the effect of CO2eon transpiration was greater indirectly through enhanced litter production than directly through reduced stomatal conductance. In forests composed of species more similar toL. styraciflua, water savings from stomatal closure may dominate the response to CO2e.
Effects of nutrition and soil warming on stemwood production in a boreal Norway spruce stand
Authors: STRÖMGREN MONIKA , LINDER SUNE
Published: March 2003

The boreal forest is expected to experience the greatest warming of all forest biomes. The extent of the boreal forest, the large amount of carbon contained in the soil, and the expected climate warming, make the boreal forest a key biome to understand and represent correctly in global carbon models. It has been suggested that an increase in temperature could stimulate the release of CO2 caused by an increased decomposition rate, more than biomass production, which could convert current carbon sinks into carbon sources. Most boreal forests are currently carbon sinks, but it is unclear for how long in the future the carbon sink capacity of the boreal forest is likely to be maintained. The impact of soil warming on stem volume growth was studied during 6 years, in irrigated (I) and irrigated‐fertilized (IL) stands of 40‐year‐old Norway spruce in Northern Sweden. From May to October heating cables were used to maintain the soil temperature on heated‐irrigated plots (Ih and ILh) 5 °C above that on unheated control plots (Ic and ILc). After six seasons' warming, stem volume production (m3 ha−1 a−1) was 115% higher on Ih than on unheated (Ic) plots, and on heated and irrigated‐fertilized plots (ILh) it was 57% higher than on unheated plots (ILc). The results indicate that in a future warmer climate, an increased availability of nitrogen, combined with a longer growing season, may increase biomass production substantially, on both low‐ and high‐fertility sites. It is, however, too early to decide whether the observed responses are transitory or long lasting. It is therefore crucial to gain a better understanding of the responses of boreal forest ecosystems to climate change, and to provide data to test and validate models used in predicting the impact of climate change.
Acute seasonal drought does not permanently alter mass loss and nitrogen dynamics during decomposition of red maple (Acer rubrumL.) litter
Authors: O'Neill E. G. , Johnson D. W. , Ledford J. , Todd D. E.
Published: March 2003

Litter decomposition is a major component of nutrient cycling in forest ecosystems and is known to be sensitive to both temperature and moisture. A drought during periods of the year when moisture is normally abundant may impact decomposition more than moisture limitation during seasons that are routinely dry. We excluded precipitation for a 3‐month period during each of four seasons and measured mass loss and nitrogen (N) dynamics inAcer rubrumleaf litter by using a litterbag approach. We hypothesized that rainfall exclusion would retard mass loss during the exclusion period, with the maximum effect occurring during the spring and summer months when moisture was normally adequate and temperatures optimum for decomposition, and that exclusion during these two periods would have the greatest impact on subsequent timing of decomposition events, particularly N mineralization. Mass loss in litterbags was initially reduced by precipitation exclusion in the spring treatment only. For each season, a set of bags exposed to drying in the same way and allowed to remain in the field under ambient conditions for an additional 9 months no longer showed a difference in mass loss relative to control bags. Nitrogen accumulation was immediately and significantly reduced in the exclusion treatments in all seasons; however, like mass loss, there was no difference 9 months later. We conclude that short‐term effects of drought do occur, especially with regard to N accumulation, and that these effects do not persist and are unlikely to have any substantial effect on nutrient cycling.
A global change‐induced biome shift in the Montseny mountains (NE Spain)
Authors: Peñuelas Josep , Boada Martí
Published: March 2003

Shifts in plant species and biome distribution in response to warming have been described in past climate changes. However, reported evidence of such shifts under current climate change is still scarce. By comparing current and 1945 vegetation distribution in the Montseny mountains (Catalonia, NE Spain), we report here a progressive replacement of cold‐temperate ecosystems by Mediterranean ecosystems. Beech (Fagus sylvatica) forest has shifted altitudinally upwards by ca. 70 m at the highest altitudes (1600–1700 m). Both the beech forests and the heather (Calluna vulgaris) heathlands are being replaced by holm oak (Quercus ilex) forest at medium altitudes (800–1400 m). This beech replacement has been observed to occur through a progressive isolation and degradation of beech stands. In ‘isolated’ (small and surrounded by holm oaks) beech stands, beech trees are 30% more defoliated, beech recruitment is 41% lower, and holm oak recruitment is three times higher than in ‘continental’ (large and continuous) beech stands. The progressively warmer conditions, complemented by the land use changes (mainly the cessation of traditional land management) are the apparent causes, providing a paradigmatic example of global change affecting distributions of plant species and biomes.
Sensitivity of white clover to UV‐B radiation depends on water availability, plant productivity and duration of stress
Authors: HOFMANN RAINER W. , CAMPBELL BRUCE D. , FOUNTAIN DAVID F.
Published: March 2003

Abstract Nine populations of white clover (Trifolium repens L.) were grown for 12 weeks with supplemental application of 13.3 kJ m−2 d−1 ultraviolet‐B (UV‐B) radiation under controlled environmental conditions. Drought was applied during the last four weeks of the experiment. Under well‐watered conditions, UV‐B decreased white clover growth on average by 20%. Cultivars bred for agricultural performance were sensitive to UV‐B, while slow‐growing ecotypes were UV‐B‐tolerant. After four weeks of water stress, there were no significant population differences in UV‐B responsiveness. UV‐B sensitivity decreased with increasing exposure to drought and with longer duration of UV‐B irradiation, suggesting that the direction and extent of the UV‐B 3× drought interaction depends on the duration of stress. The population comparisons indicate that low constitutive growth rate and adaptation to other forms of stress may be related to UV‐B tolerance under well‐watered conditions, but not during extended periods of drought.
Seasonal characteristics of nitric oxide emission from a typical Chinese rice–wheat rotation during the non‐waterlogged period
Authors: Zheng Xunhua , Huang Yao , Wang Yuesi , Wang Mingxing
Published: March 2003

Nitric oxide emissions from a typical rice–wheat rotation system in southeastern China were continuously measured with an automatic system in 1996–1997. The seasonal pattern of the NO emissions was characterized for the non‐waterlogged period of a rotation cycle. Nitric oxide emissions during the period from March through June were 3.9–6.3 folds for the fertilized plots and 1.6 folds for the unfertilized plot larger than those from November through December. Nitric oxide emissions were not detectable during the winter period from January through February. Amendment of synthetic fertilizer N significantly enhanced the NO emission by a factor of 6.5, but the enhancement was significantly mitigated by 25% through substituting ca. 16% of the synthetic fertilizer N with organic N from fermented crop residues or by 21% through deep tillage. The NO–N emission factor, defined as the amount of NO–N released per unit of synthetic fertilizer N input, was determined to be 0.025 kg NO–N kg−1 of N applied for the non‐waterlogged period, which was reduced by 32% through substituting part of the synthetic N fertilizer with fermented crop residues or by 24% through deep tillage. In addition, the NO emission factor, defined as the amount of NO–N emitted from unit unfertilized area per day, was observed to be ca. 3.8 g N ha−1 d−1. Approximately 0.55 Tg N yr−1 was likely released as NO from Chinese cultivated lands.
Effects of elevated CO2, nitrogen and fungal endophyte‐infection on tall fescue: growth, photosynthesis, chemical composition and digestibility
Authors: NEWMAN J. A. , ABNER M. L. , DADO R. G. , GIBSON D. J. , BROOKINGS A. , PARSONS A. J.
Published: March 2003

Rising global carbon dioxide levels may lead to profound changes in plant composition, regardless of the degree of global warming that may result from the accumulation of this greenhouse gas. We studied the interaction of a CO2 doubling and two levels of nitrogen fertilizer on the growth and chemical composition of tall fescue (Festuca arundinacea Schreber cv. KY‐31) when infected and uninfected with the mutualistic fungal endophyte Neotyphodium coenophialum Morgan‐Jones and Gams. Two‐year‐old plants were harvested to 5 cm every 4 weeks, and after 12 weeks of growth plants grown in twice ambient CO2 concentrations: photosynthesized 15% more; produced tillers at a faster rate; produced 53% more dry matter (DM) yield under low N conditions and 61% more DM under high N conditions; the % organic matter (OM) was little changed except under elevated CO2 and high N when %OM increased by 3%; lignin decreased by 14%; crude protein (CP) concentrations (as %DM) declined by 21%; the soluble CP fraction (as %CP) increased by 13%; the acid detergent insoluble CP fraction (as %CP) increased by 12%, and in vitro neutral detergent fiber digestibility declined by 5% under high N conditions but not under low N. Plants infected with the endophytic fungus: photosynthesized 16% faster in high N compared with under low N; flowered earlier than uninfected plants; had 28% less lignin in high N compared with under low N; and had much smaller reductions in CP concentration (as %DM) and smaller increases in the soluble CP fraction (as %CP) and the acid detergent insoluble CP fraction (as %CP) under elevated CO2. Such large and varied changes in plant quality are likely to have large and significant effects on the herbivore populations that feed from these plants.
Effects of elevated CO2, nitrogen deposition, and decreased species diversity on foliar fungal plant disease
Authors: MITCHELL CHARLES E. , REICH PETER B. , TILMAN DAVID , GROTH JAMES V.
Published: March 2003

Three components of global change, elevated CO2, nitrogen addition, and decreased plant species richness (‘diversity’), increased the percent leaf area infected by fungi (pathogen load) for much to all of the plant community in one year of a factorial grassland experiment. Decreased plant diversity had the broadest effect, increasing pathogen load across the plant community. Decreased diversity increased pathogen load primarily by allowing remaining plant species to increase in abundance, facilitating spread of foliar fungal pathogens specific to each plant species. Changes in plant species composition also strongly influenced community pathogen load, with communities that lost less disease prone plant species increasing more in pathogen load. Elevated CO2 increased pathogen load of C3 grasses, perhaps by decreasing water stress, increasing leaf longevity, and increasing photosynthetic rate, all of which can promote foliar fungal disease. Decreased plant diversity further magnified the increase in C3 grass pathogen load under elevated CO2. Nitrogen addition increased pathogen load of C4 grasses by increasing foliar nitrogen concentration, which can enhance pathogen infection, growth, and reproduction. Because changes in foliar fungal pathogen load can strongly influence grassland ecosystem processes, our study suggests that increased pathogen load can be an important mechanism by which global change affects grassland ecosystems.
The effect of drying temperature on in vitro dry‐matter digestibility and water‐soluble carbohydrate content of perennial ryegrass varieties
Authors: Davies  , Wilkins
Published: March 2003

In official forage grass variety trials in the UK, herbage samples are dried at 100°C or more to facilitate quick drying of the large numbers of samples that are currently needed. When assessing herbage quality, however, 80°C has been recommended as the highest temperature that can be used during drying without serious deterioration. Herbage from six perennial ryegrass varieties, which were known to show a range of variation in in vitro concentration of digestible dry matter (DMD) and water‐soluble carbohydrate concentration (WSC) during the vegetative phase of growth, was dried in an oven with forced ventilation at 100°C and at 80°C before analysis. Mean DMD over all varieties was slightly lower [6 g kg−1 dry matter (DM)] and mean WSC over all varieties was considerably lower (24 g kg−1 DM) at 100°C. Varieties differed in mean DMD over both drying temperatures by 37 g kg−1 DM and in mean WSC by 63 g kg−1 DM. There was, however, no significant interaction between variety and drying temperature for either DMD or WSC. Although drying at the higher temperature underestimated both DMD and WSC, it did not significantly alter the ranking of varieties or the range among them.
Competition in perennial ryegrass–white clover mixtures under cutting. 2. Leaf characteristics, light interception and dry‐matter production during regrowth
Authors: Nassiri  , Elgersma
Published: March 2003

The effect of defoliation interval on growth patterns of contrasting perennial ryegrass (Lolium perenne)–white clover (Trifolium repens) mixtures was studied. The dynamics of increase in leaf area, light interception and dry‐matter (DM) production were measured within successive regrowth periods. No N fertilizer was applied. During 1995 six mixtures were cut eight (F1) or six times (F2) at a stubble height of 5 cm. The stubble composition was stable throughout the growing season: after harvest about 50 g DM m−2 (with a white clover proportion of 0·52) was present with a leaf area index (LAI) of 0·5 (0·38 white clover). The percentage of intercepted radiation after cutting was 20–30% and increased during 3 weeks to about 95%.

The relative growth rate of leaf area and DM was higher for white clover than for perennial ryegrass, with the proportion of clover in the LAI and DM increasing during each regrowth period. Mixtures with large‐leaved white clover cv. Alice had a lower initial clover content after harvest, but a more rapid increase in clover LAI and DM than mixtures with the smaller leaved cvs Gwenda or Retor. Alice had the highest total and clover LAI and DM at harvest. Cutting frequency affected the change in white clover–perennial ryegrass ratio during regrowth. This was significantly higher in mixtures with Alice than in mixtures with Gwenda, but only under less frequent cutting (F2).

In spring there was a mean white clover proportion of about 0·55 in the LAI and 0·45 in the total harvested DM. In summer the white clover proportion in the LAI and DM increased to 0·70–0·75. There was a decline during autumn, especially in F2 and in the mixtures with the small‐leaved white clover cv. Gwenda and the medium‐leaved cv. Retor. In contrast, grass DM and LAI declined from spring to summer. The decline in clover LAI in autumn was similar in Alice and Gwenda at frequent cutting (F1), but stronger in Gwenda in F2. Retor had the lowest clover specific leaf area (SLA). The SLA values of Alice and Gwenda were similar, SLA being similar between cutting treatments. No differences were found for leaf weight ratio (LWR) among the three white clover cultivars or between the grass cultivars, and LWR was not affected by cutting treatment. Defoliation interval had limited effects on the growth pattern and leaf characteristics of perennial ryegrass–white clover mixtures.
Root characteristics of temperate pasture in New South Wales after grazing at three stocking rates for 30 years
Authors: Greenwood K.L. , Hutchinson K.J.
Published: March 2003

The distribution of length, diameter, surface area and volume of roots was measured in northern New South Wales, Australia, under temperate pasture that had been previously grazed at low and high stocking rates for 30 years; these root characteristics were compared with those of roots under ungrazed pasture. The ungrazed pasture was dominated by Phalaris (Phalaris Aquatica), whereas annual grasses and dicotyledons were a large component of the pasture at low and high stocking rates. A fine‐meshed (0.250 mm) sieve was used for separating the roots from the soil, and the root characteristics were measured using image analysis techniques. With this sieve size, root length densities were many times higher than published data for astures where a larger mesh sieve had been used for sample preparation. The lengths of root per unit of soil volume (root length densities) were high for all stocking rates and averaged 91 cm cm−3 near the soil surface (0 – 5 cm) declining to 4.0 cm cm−3 at the deepest depth measured (65–75 cm). There was a greater proportion of roots near the soil surface at the higher stocking rates. A greater proportion of fine roots occurred at the higher stocking rates, which was probably due to the differences in botanical composition. Reciprocal, power and logarithmic functions best described the distribution of root length density, root surface area density and root volume density, root surface area density and root volume desnity with depth.