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     

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     

[CO2]- and density-dependent competition between grassland species

The predicted ongoing increase of atmospheric carbon dioxide levels is considered to be one of the main threats to biodiversity due to potential changes in biotic interactions. We tested whether effects of intra‐ and interspecific planting density of the calcareous grassland perennials Bromus erectus and Carex flacca change in response to elevated [CO₂] (600 ppm) by using factorial combinations of seven densities (0, 1, 2, 4, 8, 16, 24 tillers per 8 × 8 cm² cell) of both species in plots with and without CO₂ enrichment. Although aboveground biomass of C. flacca was increased by 54% under elevated [CO₂], the combined aboveground biomass of the whole stand was not significantly increased. C. flacca tended to produce more tillers under elevated [CO₂] while B. erectus produced less tillers. The positive effect of [CO₂] on the number of tillers of C. flacca was strongest at high intraspecific densities. On the other hand, the negative effect of [CO₂] on the number of tillers of B. erectus was not present at intermediate intraspecific planting densities. Seed production of C. flacca was more than doubled under elevated [CO₂], while seed production of B. erectus was not affected. Moreover, the mass per seed of C. flacca was increased by elevated [CO₂] at intermediate interspecific planting densities while the mass per seed of B. erectus was decreased by elevated [CO₂] at high interspecific planting densities. Our results show that the responses of C. flacca and B. erectus to elevated [CO₂] depend in a complex way on initial planting densities of both species. In other words, competition between these two model species is both [CO₂]‐ and density dependent. On average, however, the effects of [CO₂] on the individual species indicate that the composition of calcareous grasslands is likely to change under elevated [CO₂] in favor of C. flacca.     

Seed production and seed quality in a calcareous grassland in elevated CO2

In diverse plant communities the relative contribution of species to community biomass may change considerably in response to elevated CO₂. Along with species‐specific biomass responses, reproduction is likely to change as well with increasing CO₂ and might further accelerate shifts in species composition. Here, we ask if, after 5 years of CO₂ exposure, seed production and seed quality in natural nutrient‐poor calcareous grassland are affected by elevated CO₂ (650 μ L L^?1 vs 360 μ L L^?1) and how this might affect long‐term community dynamics. The effect of elevated CO₂ on the number of flowering shoots (+ 24%, P < 0.01) and seeds (+ 29%, P = 0.06) at the community level was similar to above ground biomass responses in this year, suggesting that the overall allocation to sexual reproduction remained unchanged. Compared among functional groups of species we found a 42% increase in seed number (P < 0.01) of graminoids, a 33% increase (P = 0.07) in forbs, and no significant change in legumes (? 38%, n.s.) under elevated CO₂. Large responses particularly of two graminoid species and smaller responses of many forb species summed up to the significant or marginally significant increase in seed number of graminoids and forbs, respectively. In several species the increase in seed number resulted both from an increase in flowering shoots and an increase in inflorescence size. In most species, seeds tended to be heavier (+ 12%, P < 0.01), and N‐concentration of seeds was significantly reduced in eight out of 13 species. The fraction of germinating seeds did not differ between seeds produced in ambient and elevated CO₂, but time to germination was significantly shortened in two species and prolonged in one species when seeds had been produced in elevated CO₂. Results suggest that species specific increases in seed number and changes in seed quality will exert substantial cumulative effects on community composition in the long run.     

The impact of elevated CO2 on plant-herbivore interactions: experimental evidence of moderating effects at the community level

Surprisingly little research has been published on the responses to elevated [CO₂] at the community level, where herbivores can select their preferred food. We investigated the combined effects of atmospheric [CO₂] and herbivory on synthesised plant communities growing on soils of different fertility. Factorial combinations of two [CO₂] (350 or 700 l l⁻¹), two fertility (fertilised or non-fertilised), and two herbivory (herbivores present or absent) treatments were applied to a standard mixture of seven fast- and eight slow-growing plants in outdoor microcosms. The herbivores used were the grain aphid (Sitobion avenae) and the garden snail (Helix aspersa). We measured plant biomass, foliar nitrogen and soluble tannin concentration, aphid fecundity, and snail growth, fecundity, and feeding preferences over one growing season. Elevated [CO₂] did not have a significant impact on (1) the combined biomass of fast-growing or slow-growing plants, (2) herbivore feeding preferences, or (3) herbivore fitness. There was, however, a significant biomass increase of Carex flacca (which represented in all cases less than 5% of total live biomass), and some chemical changes in unpalatable plants under elevated [CO₂]. The herbivory treatment significantly increased the biomass of slow-growing plants over fast-growing plants, whereas fertilisation significantly increased the abundance of fast-growing plants over slow-growing plants. Predictions on the effects of elevated [CO₂] based on published single-species experiments were not supported by the results of this microcosm study. Received: 30 November 1997 / Accepted: 24 July 1998     

Elevated [CO2] and increased N supply reduce leaf disease and related photosynthetic impacts on Solidago rigida

To evaluate whether leaf spot disease and related effects on photosynthesis are influenced by increased nitrogen (N) input and elevated atmospheric CO₂ concentration ([CO₂]), we examined disease incidence and photosynthetic rate of Solidago rigida grown in monoculture under ambient or elevated (560 μmol mol⁻¹) [CO₂] and ambient or elevated (+4 g N m⁻² year⁻¹) N conditions in a field experiment in Minnesota, USA. Disease incidence was lower in plots with either elevated [CO₂] or enriched N (−57 and −37%, respectively) than in plots with ambient conditions. Elevated [CO₂] had no significant effect on total plant biomass, or on photosynthetic rate, but reduced tissue%N by 13%. In contrast, N fertilization increased both biomass and total plant N by 70%, and as a consequence tissue%N was unaffected and photosynthetic rate was lower on N fertilized plants than on unfertilized plants. Regardless of treatment, photosynthetic rate was reduced on leaves with disease symptoms. On average across all treatments, asymptomatic leaf tissue on diseased leaves had 53% lower photosynthetic rate than non-diseased leaves, indicating that the negative effect from the disease extended beyond the visual lesion area. Our results show that, in this instance, indirect effects from elevated [CO₂], i.e., lower disease incidence, had a stronger effect on realized photosynthetic rate than the direct effect of higher [CO₂].     

Activity of surface-casting earthworms in a calcareous grassland under elevated atmospheric CO2

Earthworms make up the dominant fraction of the biomass of soil animals in most temperate grasslands and have important effects on the structure and function of these ecosystems. We hypothesized that the effects of elevated atmospheric CO₂ on soil moisture and plant biomass production would increase earthworm activity, expressed as surface cast production. Using a screen-aided CO₂ control facility (open top and open bottom rings), eight 1.2-m² grassland plots in Switzerland have been maintained since March 1994 at ambient CO₂ concentrations (350 μl CO₂ l⁻¹) and eight at elevated CO₂ (610 μl CO₂ l⁻¹). Cumulative earthworm surface cast production measured 40 times over 1 year (April 1995–April 1996) in plots treated with elevated CO₂ (2206 g dry mass m⁻² year⁻¹) was 35% greater (P<0.05) than that measured in plant communities maintained at ambient CO₂ (1633 g dry mass m⁻² year⁻¹). At these rates of surface cast production, worms would require about 100 years to egest the equivalent of the amount of soil now found in the Ah horizon (top 15 cm) under current ambient CO₂ concentrations, and 75 years under elevated CO₂. Elevated atmospheric CO₂ had no influence on the seasonality of earthworm activity. Cumulative surface cast production measured over the 7-week period immediately following the 6-week summer dry period in 1995 (no surface casting) was positively correlated (P<0.05) with the mean soil water content calculated over this dry and subsequent wetter period, when viewed across all treatments. However, no correlations were observed with soil temperature or with annual aboveground plant biomass productivity. No CO₂-related differences were observed in total nitrogen (N_tot) and organic carbon (C_org) concentration of surface casts, although concentrations of both elements varied seasonally. The CO₂-induced increase in earthworm surface casting activity corresponded to a 30% increase of the amount of N_tot (8.9 mg N m⁻² vs. 6.9 mg N m⁻²) and C_org (126 mg C m⁻² vs. 94 mg C m⁻²) egested by the worms in one year. Thus, our results demonstrate an important indirect stimulatory effect of elevated atmospheric CO₂ on earthworm activity which may have profound effects on ecosystem function and plant community structure in the long term. Received: 3 November 1996 / Accepted: 11 January 1997     

Shrub establishment under experimental global changes in a California grassland

Accelerating invasion of grasslands by woody species is a widespread global phenomenon. The native shrub Baccharis pilularis has recently increased in abundance in some California grasslands, with large local community and ecosystem effects. I investigated potential contributions of (1) future global climate and atmospheric changes and (2) variation in moisture and nutrient availability to increased Baccharis germination and early establishment rates. I examined responses of Baccharis seeds and seedlings to simulated warming (+ 1−2 °C) and elevated CO₂ (+ 300 ppm) in a 2-year field experiment. Warming and CO₂ treatments were applied at ambient and increased water and nitrogen levels chosen to simulate future increases in precipitation (+ 50%) and N deposition (+ 7 gN m⁻² y⁻¹). Elevated CO₂ and water addition each increased or accelerated germination. Herbivory strongly reduced seedling populations during the winter wet season; drought further reduced seedling survival in the spring. Overall Baccharis survivorship was extremely low (<0.1%) across all treatments, complicating the interpretation of global change effects.     

Biomass allocation and canopy development in spruce model ecosystems under elevated CO2 and increased N deposition

Ecosystem-level experiments on the effects of atmospheric CO₂ enrichment and N deposition on forest trees are urgently needed. Here we present data for nine model ecosystems of spruce (Picea abies) on natural nutrient-poor montane forest soil (0.7 m² of ground and 350 kg weight). Each system was composed of six 7-year-old (at harvest) trees each representing a different genotype, and a herbaceous understory layer (three species). The model ecosystems were exposed to three different CO₂ concentrations (280, 420, 560 μl l⁻¹) and three different rates of wet N deposition (0, 30, 90 kg ha⁻¹ year⁻¹) in a simulated annual course of Swiss montane climate for 3 years. The total ecosystem biomass was not affected by CO₂ concentration, but increased with increasing N deposition. However, biomass allocation to roots increased with increasing CO₂ leading to significantly lower leaf mass ratios (LMRs) and leaf area ratios (LARs) in trees grown at elevated CO₂. In contrast to CO₂ enrichment, N deposition increased biomass allocation to the aboveground plant parts, and thus LMR and LAR were higher with increasing N deposition. We observed no CO₂ ×  N interactions on growth, biomass production, or allocation, and there were also no genotype × treatment interactions. The final leaf area index (LAI) of the spruce canopies was 19% smaller at 420 and 27% smaller at 560 than that measured at 280 μl CO₂ l⁻¹, but was not significantly altered by increasing N deposition. Lower LAIs at elevated CO₂ largely resulted from shorter branches (less needles per individual tree) and partially from increased needle litterfall. Independently of N deposition, total aboveground N content in the spruce communities declined with increasing CO₂ (−18% at 420 and −31% at 560 compared to 280 μl CO₂ l⁻¹). N deposition had the opposite effect on total above ground N content (+18% at 30 and +52% at 90 compared to 0 kg N ha⁻¹ year⁻¹). Our results suggest that under competitive conditions on natural forest soil, atmospheric CO₂ enrichment may not lead to higher ecosystem biomass production, but N deposition is likely to do so. The reduction in LAI under elevated CO₂ suggests allometric down-regulation of photosynthetic carbon uptake at the canopy level. The strong decline in the tree nitrogen mass per unit ground area in response to elevated CO₂ may indicate CO₂-induced reductions of soil N availability. Received: 11 May 1997 / Accepted: 4 August 1997     

Ponderosa Pine Responses to Elevated CO2and Nitrogen Fertilization

The effects of elevated CO₂ (ambient, +175, and +350 μl l⁻¹) and nitrogen fertilization (0, 100, and 200 kg N ha⁻¹ yr⁻¹ as ammonium sulfate) on C and N accumulations in biomass and soils planted with ponderosa pine (Pinus ponderosa Laws) over a 6-year study period are reported. Both nitrogen fertilization and elevated CO₂ caused increases in C and N contents of vegetation over the study period. The pattern of responses varied over time. Responses to CO₂ decreased in the +175 μl l⁻¹ and increased in the +350 μl l⁻¹ after the first year, whereas responses to N decreased after the first year and became non-significant by year six. Foliar N concentrations were lower and tree C:N ratios were higher with elevated CO₂ in the early years, but this was offset by the increases in biomass, resulting in substantial increases in N uptake with elevated CO₂. Nitrogen budget estimates showed that the major source of the N for unfertilized trees, with or without elevated CO₂, was likely the soil organic N pool. There were no effects of elevated CO₂ on soil C, but a significant decrease in soil N and an increase in soil C:N ratio in year six. Nitrogen fertilization had no significant effect on tree C:N ratios, foliar N concentrations, soil C content, soil N content, or soil C:N ratios. There were no significant interactions between CO₂ and N treatments, indicating that N fertilization had no effect on responses to CO₂ and that CO₂ treatments had no effect on responses to N fertilization. These results illustrate the importance of long-term studies involving more than one level of treatment to assess the effects of elevated CO₂.     

Effects of elevated atmospheric CO2 and tropospheric O3 on nutrient dynamics: decomposition of leaf litter in trembling aspen and paper birch communities

Atmospheric changes could strongly influence how terrestrial ecosystems function by altering nutrient cycling. We examined how the dynamics of nutrient release from leaf litter responded to two important atmospheric changes: rising atmospheric CO₂ and tropospheric O₃. We evaluated the independent and combined effects of these gases on foliar litter nutrient dynamics in aspen (Populus tremuloides Michx) and birch (Betula papyrifera Marsh)/aspen communities at the Aspen FACE Project in Rhinelander, WI. Naturally senesced leaf litter was incubated in litter bags in the field for 735 days. Decomposing litter was sampled six times during incubation and was analyzed for carbon, and both macro (N, P, K, S, Ca, and Mg) and micro (Mn, B, Zn and Cu) nutrient concentrations. Elevated CO₂ significantly decreased the initial litter concentrations of N (−10.7%) and B (−14.4%), and increased the concentrations of K (+23.7%) and P (+19.7%), with no change in the other elements. Elevated O₃ significantly decreased the initial litter concentrations of P (−11.2%), S (−8.1%), Ca (−12.1%), and Zn (−19.5%), with no change in the other elements. Pairing concentration data with litterfall data, we estimated that elevated CO₂ significantly increased the fluxes to soil of all nutrients: N (+12.5%), P (+61.0%), K (+67.1%), S (+28.0%), and Mg (+40.7%), Ca (+44.0%), Cu (+38.9%), Mn (+62.8%), and Zn (+33.1%). Elevated O₃ had the opposite effect: N (−22.4%), P (−25.4%), K (−27.2%), S (−23.6%), Ca (−27.6%), Mg (−21.7%), B (−16.2%), Cu (−20.8%), and Zn (−31.6%). The relative release rates of the nine elements during the incubation was: K ≥ P ≥ mass ≥ Mg ≥ B ≥ Ca ≥ S ≥ N ≥ Mn ≥ Cu ≥ Zn. Atmospheric changes had little effect on nutrient release rates, except for decreasing Ca and B release under elevated CO₂ and decreasing N and Ca release under elevated O₃. We conclude that elevated CO₂ and elevated O₃ will alter nutrient cycling more through effects on litter production, rather than litter nutrient concentrations or release rates.     

Effects of elevated CO2 and cutting frequency on plant community structure in a temperate grassland

Monoliths of a fertile, N limited, C3 grassland community were subjected (or not) to an atmospheric CO₂ enrichment (600 µmol mol^‐‐1) using a Mini‐FACE system, from August 1998 to June 2001 and were subjected to two contrasting cutting frequencies (3 and 6 cuts per year). We report here the effects of the CO₂ and cutting frequency factors on the plant community structure and its diversity. Species‐specific responses to elevated CO₂ and cutting frequency were observed, which resulted in significant changes in the botanical composition of the grassland monoliths. Elevated CO₂ significantly increased the proportion of dicotyledones (forbs + legumes) and reduced that of the monocotyledones (grasses). Management differentiated this response as elevated CO₂ increased the proportion of forbs when infrequently and of legumes when frequently defoliated. However, among the two dominant forbs species only one was significantly enhanced by elevated CO₂. Moreover, not all grass species responded negatively to high CO₂. At a low cutting frequency, the observed decline under ambient CO₂ in species diversity (Shannon‐Weaver index) and in forb species number was partly alleviated by elevated CO₂. This experiment shows that the botanical composition of temperate grasslands is likely to be affected by the current rise (+ 0.5% per year) in the atmospheric CO₂ concentration, and that grassland management guidelines may need to be adapted to a future high CO₂ world.     

Response of 12 weedy species to elevated CO2 in low-phosphorus-availability soil

We conducted an experiment on responses of weedy species from an orchard ecosystem to elevated CO₂ (700–800 μmol mol⁻¹) under low phosphorus (P) soil in an environment-controlled growth chamber. Twelve local weedy species, Poa annua L., Lolium perenne L., Avena fatua L., Vicia cracca L., Medicago lupulina L., Kummerowia striata (Thunb.) Schindl., Veronica didyma Ten., Plantago virginica L., Gnaphalium affine D.Don., Echinochloa crusgalli var. mitis (L.) Beauv., Eleusine indica (L.) Gaertn. and Setaria glauca (L.) P. Beauv., grouped into four functional groups (C₃ grass, C₃ forb, legume and C₄ grass), were used in the experiment. The total plant biomass, P uptake, and mycorrhizal colonization were measured. The results showed that the total biomass of the 12 weedy species tended to increase under elevated CO₂. But changes in the total biomass under elevated CO₂ significantly differed among functional groups: legumes showed the greatest increase in the total biomass of all functional groups, following the order C₃ forbs > C₄ grasses > C₃ grasses. Elevated CO₂ significantly increased mycorrhizal colonization and P uptake of legumes, C₃ forbs and C₄ grasses but did not change C₃ grasses. Positive correlations between mycorrhizal colonization and shoot P concentration, and between total P uptake and total biomass were found under elevated CO₂. The results suggested that the interspecific difference in CO₂ response at low P availability was caused by the difference in CO₂ response in mycorrhizae and P uptake. These differences among species imply that plant interaction in orchard ecosystems may change under future CO₂ enrichment.     

Responses of Vegetation and Ecosystem CO2 Exchange to 9 Years of Nutrient Addition at Mer Bleue Bog

Anthropogenic nitrogen (N) loading has the potential to affect plant community structure and function, and the carbon dioxide (CO₂) sink of peatlands. Our aim is to study how vegetation changes, induced by nutrient input, affect the CO₂ exchange of a nutrient-limited bog. We conducted 9- and 4-year fertilization experiments at Mer Bleue bog, where we applied N addition levels of 1.6, 3.2, and 6.4 g N m⁻² a⁻¹, upon a background deposition of about 0.8 g N m⁻² a⁻¹, with or without phosphorus and potassium (PK). Only the treatments 3.2 and 6.4 g N m⁻² a⁻¹ with PK significantly affected CO₂ fluxes. These treatments shifted the Sphagnum moss and dwarf shrub community to taller dwarf shrub thickets without moss, and the CO₂ responses depended on the phase of vegetation transition. Overall, compared to the large observed changes in the vegetation, the changes in CO₂ fluxes were small. Following Sphagnum loss after 5 years, maximum ecosystem photosynthesis (Pg_max) and net CO₂ exchange (NEE_max) were lowered (−19 and −46%, respectively) in the highest NPK treatment. In the following years, while shrub height increased, the vascular foliar biomass did not fully compensate for the loss of moss biomass; yet, by year 8 there were no significant differences in Pg_max and NEE_max between the nutrient and the control treatments. At the same time, an increase (24–32%) in ecosystem respiration (ER) became evident. Trends in the N-only experiment resembled those in the older NPK experiment by the fourth year. The increasing ER with increasing vascular plant and decreasing Sphagnum moss biomass across the experimental plots suggest that high N deposition may lessen the CO₂ sink of a bog.     

Stimulated N2O flux from intact grassland monoliths after two growing seasons under elevated atmospheric CO2

Long-term exposure of native vegetation to elevated atmospheric CO₂ concentrations is expected to increase C inputs to the soil and, in ecosystems with seasonally dry periods, to increase soil moisture. We tested the hypothesis that these indirect effects of elevated CO₂ (600 μl l⁻¹ vs 350 μl l⁻¹) would improve conditions for microbial activity and stimulate emissions of nitrous oxide (N₂O), a very potent and long-lived greenhouse gas. After two growing seasons, the mean N₂O efflux from monoliths of calcareous grassland maintained at elevated CO₂ was twice as high as that measured from monoliths maintained at current ambient CO₂ (70 ± 9 vs 37 ± 4 μg N₂O m⁻² h⁻¹ in October, 27 ± 5 vs 13 ± 3 μg N₂O m⁻² h⁻¹ in November after aboveground harvest). The higher N₂O emission rates at elevated CO₂ were associated with increases in soil moisture, soil heterotrophic respiration, and plant biomass production, but appear to be mainly attributable to higher soil moisture. Our results suggest that rising atmospheric CO₂ may contribute more to the total greenhouse effect than is currently estimated because of its plant-mediated effects on soil processes which may ultimately lead to increased N₂O emissions from native grasslands. Received: 11 September 1997 / Accepted: 20 March 1998     

A meta-analysis of elevated CO2 effects on woody plant mass, form, and physiology

Quantitative integration of the literature on the effect of elevated CO₂ on woody plants is important to aid our understanding of forest health in coming decades and to better predict terrestrial feedbacks on the global carbon cycle. We used meta-analytic methods to summarize and interpret more than 500 reports of effects of elevated CO₂ on woody plant biomass accumulation and partitioning, gas exchange, and leaf nitrogen and starch content. The CO₂ effect size metric we used was the log-transformed ratio of elevated compared to ambient response means weighted by the inverse of the variance of the log ratio. Variation in effect size among studies was partitioned according to the presence of interacting stress factors, length of CO₂ exposure, functional group status, pot size, and type of CO₂ exposure facility. Both total biomass (W _T) and net CO₂ assimilation (A) increased significantly at about twice ambient CO₂, regardless of growth conditions. Low soil nutrient availability reduced the CO₂ stimulation of W _T by half, from +31% under optimal conditions to +16%, while low light increased the response to +52%. We found no significant shifts in biomass allocation under high CO₂. Interacting stress factors had no effect on the magnitude of responses of A to CO₂, although plants grown in growth chambers had significantly lower responses (+19%) than those grown in greenhouses or in open-top chambers (+54%). We found no consistent evidence for photosynthetic acclimation to CO₂ enrichment except in trees grown in pots <0.5 l (−36%) and no significant CO₂ effect on stomatal conductance. Both leaf dark respiration and leaf nitrogen were significantly reduced under elevated CO₂ (−18% and −16% respectively, data expressed on a leaf mass basis), while leaf starch content increased significantly except in low nutrient grown gymnosperms. Our results provide robust, statistically defensible estimates of elevated CO₂ effect sizes against which new results may be compared or for use in forest and climate model parameterization. Received: 16 May 1997 / Accepted: 9 September 1997     

Long-Term Leaf Production Response to Elevated Atmospheric Carbon Dioxide and Tropospheric Ozone

Elevated concentrations of atmospheric CO₂ and tropospheric O₃ will profoundly influence future forest productivity, but our understanding of these influences over the long-term is poor. Leaves are key indicators of productivity and we measured the mass, area, and nitrogen concentration of leaves collected in litter traps from 2002 to 2008 in three young northern temperate forest communities exposed to elevated CO₂ and/or elevated O₃ since 1998. On average, the overall effect of elevated CO₂ (+CO₂ and +CO₂+O₃ versus ambient and +O₃) was to increase leaf mass by 36% whereas the overall effect of elevated O₃ was to decrease leaf mass by 13%, with similar effects on stand leaf area. However, there were important CO₂ × O₃ × year interactions wherein some treatment effects on leaf mass changed dramatically relative to ambient from 2002 to 2008. For example, stimulation by the +CO₂ treatment decreased (from +52 to +25%), whereas the deleterious effects of the +O₃ treatment increased (from −5 to −18%). In comparison, leaf mass in the +CO₂+O₃ treatment was similar to ambient throughout the study. Forest composition influenced these responses: effects of the +O₃ treatment on community-level leaf mass ranged from +2 to −19%. These findings are evidence that community composition, stand development processes, CO₂, and O₃ strongly interact. Changes in leaf nitrogen concentration were inconsistent, but leaf nitrogen mass (g m⁻²) was increased by elevated CO₂ (+30%) and reduced by elevated O₃ (−16%), consistent with observations that nitrogen cycling is accelerated by elevated CO₂ but retarded by elevated O₃.     

Opposing effects of elevated CO2 and N deposition on Lymantria monacha larvae feeding on spruce trees

The effects of elevated atmospheric CO₂ and increased wet N deposition on leaf quality and insect herbivory were evaluated in nine model ecosystems composed of 7-year-old spruce trees (Picea abies) and three understorey species established on natural forest soil. Each model ecosystem was grown in a simulated montane climate, and was exposed to one of three CO₂ concentrations (280, 420, and 560 μl l⁻¹), and to one of three levels of N deposition (0, 30, and 90 kg ha⁻¹ year⁻¹) for 3 years. In the 3rd year of the experiment second to third instars of the nun moth (Lymantria monacha) were allowed to feed directly on current-year needles of top canopy branches of each tree for 12 days. Specific leaf area (SLA), water content, and N concentration decreased in needles exposed to elevated CO₂, whereas the concentrations of starch, condensed tannins, and total phenolics increased. Increased N deposition had no significant effect on SLA, and water content, but the concentrations of starch, condensed tannins, and total phenolics decreased, and sugar and N concentrations increased. Despite higher relative consumption rates (RCRs) larvae consumed 33% less N per unit larval biomass and per day at the two high CO₂ treatments, compared to those feeding on 280 μl l⁻¹-needles, but they maintained similar N accumulation rates due to increased N utilization efficiencies (NUE). However, over the 12-day experimental period larvae gained less N overall and reached a 35% lower biomass in the two high-CO₂ treatments compared to those at 280 μl l⁻¹. The effects of increased N deposition on needle quality and insect performance were generally opposite to those of CO₂ enrichment, but were lower in magnitude. We conclude that altered needle quality in response to elevated CO₂ will impair the growth and development of L. monacha larvae. Increasing N deposition may mitigate these effects, which could lead to altered insect herbivore distributions depending on regional patterns of N deposition. Received: 8 June 1998 / Accepted: 27 October 1998     

Root Biomass of Individual Species, and Root Size Characteristics After Five Years of CO2 Enrichment on Native Shortgrass Steppe

Information from field studies investigating the responses of roots to increasing atmospheric CO₂ is limited and somewhat inconsistent, due partly to the difficulty in studying root systems in situ. In this report, we present standing root biomass of species and root length and diameter after five years of CO₂ enrichment (∽720 μmol mol⁻¹) in large (16 m² ground area) open-top chambers placed over a native shortgrass steppe in Colorado, USA. Total root biomass in 100 cm long×20 cm wide×75 cm depth soil monoliths and root biomass of the three dominant grass species of the site were not significantly affected by elevated CO₂. Root biomass of Stipa comata in the 0–20 cm soil depth was nearly 100% greater in elevated vs. ambient CO₂ chambers, but this was not statistically significant (P=0.14). However, there was a significant 37% increase in fine root length under elevated CO₂ in the 0–10 cm soil depth layer. Other reports from this study suggest that the increase in fine roots is primarily from improved seedling recruitment of S. comata under elevated CO₂. Few treatment differences in root length or diameter were detected in lower 10 cm depth increments, to 80 cm. These results reflect the root status integrated over two wet, two dry and one normal precipitation years and approximately one complete cycle of root turn-over on the shortgrass steppe. We conclude that increasing atmospheric CO₂ will have only small effects on standing root biomass and root length and diameter of most shortgrasss steppe species. However, the potential increased competitive ability of Stipa comata, a low forage quality species, could alter the ecosystem from the current dominant, high forage quality species, Bouteloua gracilis. B. gracilis is very well adapted to the frequent droughts of the shortgrass steppe. Increased competitive ability of less desirable plant species under increasing atmospheric CO₂ will have large implications for long-term sustainability of grassland ecosystems.     

Contribution of root to soil respiration and carbon balance in disturbed and undisturbed grassland communities, northeast China

Changes in the composition of plant species induced by grassland degradation may alter soil respiration rates and decrease carbon sequestration; however, few studies in this area have been conducted. We used net primary productivity (NPP), microbial biomass carbon (MBC), and soil organic carbon (SOC) to examine the changes in soil respiration and carbon balance in two Chinese temperate grassland communities dominated by Leymus chinensis (undisturbed community; Community 1) and Puccinellia tenuiflora (degraded community; Community 2), respectively. Soil respiration varied from 2.5 to 11.9 g CO₂ m⁻² d⁻¹ and from 1.5 to 9.3 g CO₂ m⁻² d⁻¹, and the contribution of root respiration to total soil respiration from 38% to 76% and from 25% to 72% in Communities 1 and 2, respectively. During the growing season (May–September), soil respiration, shoot biomass, live root biomass, MBC and SOC in Community 2 decreased by 28%, 39%, 45%, 55% and 29%, respectively, compared to those in Community 1. The considerably lower net ecosystem productivity in Community 2 than in Community 1 (104.56 vs. 224.73 g C m⁻² yr⁻¹) suggests that the degradation has significantly decreased carbon sequestration of the ecosystems.