Predicting if ecosystems will mitigate or exacerbate rising CO₂ requires understanding how elevated CO₂ will interact with coincident changes in diversity and nitrogen (N) availability to affect ...ecosystem carbon (C) storage. Yet achieving such understanding has been hampered by the difficulty of quantifying belowground C pools and fluxes. Thus, we used mass balance calculations to quantify the effects of diversity, CO₂, and N on both the total amount of C allocated belowground by plants (total belowground C allocation, TBCA) and ecosystem C storage in a periodically burned, 8-year Minnesota grassland biodiversity, CO₂, and N experiment (BioCON). Annual TBCA increased in response to elevated CO₂, enriched N, and increasing diversity. TBCA was positively related to standing root biomass. After removing the influence of root biomass, the effect of elevated CO₂ remained positive, suggesting additional drivers of TBCA apart from those that maintain high root biomass. Removing root biomass effects resulted in the effects of N and diversity becoming neutral or negative (depending on year), suggesting that the positive effects of diversity and N on TBCA were related to treatment-driven differences in root biomass. Greater litter production in high diversity, elevated CO₂, and enhanced N treatments increased annual ecosystem C loss in fire years and C gain in non-fire years, resulting in overall neutral C storage rates. Our results suggest that frequently burned grasslands are unlikely to exhibit enhanced C sequestration with increasing atmospheric CO₂ levels or N deposition.
We investigated the interaction of elevated CO2 and/or (Ozone) O3 on the occurrence and severity of aspen leaf rust (Melampsora medusae Thuem. f. sp. tremuloidae) on trembling aspen (Populus ...tremuloides Michx.). Furthermore, we examined the role of changes in leaf surface properties induced by elevated CO2 and/or O3 in this host–pathogen interaction. Three‐ to five‐fold increases in levels of rust infection index were found in 2 consecutive years following growing‐season‐long exposures with either O3 alone or CO2 + O3 depending on aspen clone. Examination of leaf surface properties (wax appearance, wax amount, wax chemical composition, leaf surface and wettability) suggested significant effects by O3 and CO2 + O3. We conclude that elevated O3 is altering aspen leaf surfaces in such a way that it is likely predisposing the plants to increased infection by aspen leaf rust.
The general lack of significant changes in mineral soil C stocks during CO2‐enrichment experiments has cast doubt on predictions that increased soil C can partially offset rising atmospheric CO2 ...concentrations. Here, we show, through meta‐analysis techniques, that these experiments collectively exhibited a 5.6% increase in soil C over 2–9 years, at a median rate of 19 g C m−2 yr−1. We also measured C accrual in deciduous forest and grassland soils, at rates exceeding 40 g C m−2 yr−1 for 5–8 years, because both systems responded to CO2 enrichment with large increases in root production. Even though native C stocks were relatively large, over half of the accrued C at both sites was incorporated into microaggregates, which protect C and increase its longevity. Our data, in combination with the meta‐analysis, demonstrate the potential for mineral soils in diverse temperate ecosystems to store additional C in response to CO2 enrichment.
We used natural and tracer nitrogen (N) isotopes in a Pinus taeda free air CO2 enrichment (FACE) experiment to investigate functioning of ectomycorrhizal and saprotrophic fungi in N cycling.
Fungal ...sporocarps were sampled in 2004 (natural abundance and 15N tracer) and 2010 (tracer) and δ15N patterns were compared against litter and soil pools.
Ectomycorrhizal fungi with hydrophobic ectomycorrhizas (e.g. Cortinarius and Tricholoma) acquired N from the Oea horizon or deeper. Taxa with hydrophilic ectomycorrhizas acquired N from the Oi horizon (Russula and Lactarius) or deeper (Laccaria, Inocybe, and Amanita). 15N enrichment patterns for Cortinarius and Amanita in 2010 did not correspond to any measured bulk pool, suggesting that a persistent pool of active organic N supplied these two taxa. Saprotrophic fungi could be separated into those colonizing pine cones (Baeospora), wood, litter (Oi), and soil (Ramariopsis), with δ15N of taxa reflecting substrate differences. 15N enrichment between sources and sporocarps varied across taxa and contributed to δ15N patterns.
Natural abundance and 15N tracers proved useful for tracking N from different depths into fungal taxa, generally corresponded to literature estimates of fungal activity within soil profiles, and provided new insights into interpreting natural abundance δ15N patterns.
The impact of climate change on herbivorous insects can have far‐reaching consequences for ecosystem processes. However, experiments investigating the combined effects of multiple climate change ...drivers on herbivorous insects are scarce. We independently manipulated three climate change drivers (CO2, warming, drought) in a Danish heathland ecosystem. The experiment was established in 2005 as a full factorial split‐plot with 6 blocks × 2 levels of CO2 × 2 levels of warming × 2 levels of drought = 48 plots. In 2008, we exposed 432 larvae (n = 9 per plot) of the heather beetle (Lochmaea suturalis Thomson), an important herbivore on heather, to ambient versus elevated drought, temperature, and CO2 (plus all combinations) for 5 weeks. Larval weight and survival were highest under ambient conditions and decreased significantly with the number of climate change drivers. Weight was lowest under the drought treatment, and there was a three‐way interaction between time, CO2, and drought. Survival was lowest when drought, warming, and elevated CO2 were combined. Effects of climate change drivers depended on other co‐acting factors and were mediated by changes in plant secondary compounds, nitrogen, and water content. Overall, drought was the most important factor for this insect herbivore. Our study shows that weight and survival of insect herbivores may decline under future climate. The complexity of insect herbivore responses increases with the number of combined climate change drivers.
In a multi‐factor climate change experiment, we tested effects of three independent global change drivers on insect herbivore performance. We found that most drivers adversely affected herbivore performance. The most surprising result of our study, however, was that the number of global change drivers (0, 1, 2 or 3) additively affected herbivore performance.
Free‐air CO2 enrichment (FACE) experiments provide a remarkable wealth of data which can be used to evaluate and improve terrestrial ecosystem models (TEMs). In the FACE model‐data synthesis project, ...11 TEMs were applied to two decadelong FACE experiments in temperate forests of the southeastern U.S.—the evergreen Duke Forest and the deciduous Oak Ridge Forest. In this baseline paper, we demonstrate our approach to model‐data synthesis by evaluating the models' ability to reproduce observed net primary productivity (NPP), transpiration, and leaf area index (LAI) in ambient CO2 treatments. Model outputs were compared against observations using a range of goodness‐of‐fit statistics. Many models simulated annual NPP and transpiration within observed uncertainty. We demonstrate, however, that high goodness‐of‐fit values do not necessarily indicate a successful model, because simulation accuracy may be achieved through compensating biases in component variables. For example, transpiration accuracy was sometimes achieved with compensating biases in leaf area index and transpiration per unit leaf area. Our approach to model‐data synthesis therefore goes beyond goodness‐of‐fit to investigate the success of alternative representations of component processes. Here we demonstrate this approach by comparing competing model hypotheses determining peak LAI. Of three alternative hypotheses—(1) optimization to maximize carbon export, (2) increasing specific leaf area with canopy depth, and (3) the pipe model—the pipe model produced peak LAI closest to the observations. This example illustrates how data sets from intensive field experiments such as FACE can be used to reduce model uncertainty despite compensating biases by evaluating individual model assumptions.
Key Points
Two temperate forest FACE experiments were simulated with 11 ecosystem models
Transpiration biases were often caused by leaf area biases
Accuracy was sometimes achieved with compensating biases in component variables
Sustained increased productivity of trees growing in elevated CO₂ depends in part on their stoichiometric flexibility, i.e., increasing their nutrient use efficiency, or on increased nutrient uptake ...from the soil. Phosphorus (P) may be a nutrient as limiting as nitrogen (N) in terrestrial ecosystems and may play a key-process in global terrestrial C storage. For this study archived litter and soil samples of two free air CO₂ enrichment (FACE) experiments were analyzed for C, N and P. Populus euramericana, nigra and alba and Betula pendula, Alnus glutinosa and Fagus sylvatica were grown in ambient and elevated CO₂ at respectively the Euro- and BangorFACE experiments. At EuroFACE, aboveground litter accumulated in L, F and H layers, while at BangorFACE almost all aboveground litter was incorporated into the mineral soil due to bioturbation. At EuroFACE, more P was lost from the F and H litter layers due to trees growing in elevated CO₂, while at BangorFACE more P was lost from the mineral soil. Results of this study imply that trees growing in elevated CO₂ were P limited at both experiments. Therefore, with increasing atmospheric CO₂, P may play a more pronounced role than previous thought in regulating secondary forest growth. Moreover, increased atmospheric CO₂ and ample N may allow a larger pool of P to become available for uptake due to, for instance, increased phosphatase activity resulting in increased organic matter turnover and biogenic weathering. Therefore, it may be postulated that under non-N-limited conditions, e.g., during regrowth, under high N deposition or in systems with high N₂-fixation, increased P availability and uptake may allow P-limited forests to sustain increased growth under increasing atmospheric CO₂.
The impact of anthropogenic CO₂ emissions on climate change may be mitigated in part by C sequestration in terrestrial ecosystems as rising atmospheric CO₂ concentrations stimulate primary ...productivity and ecosystem C storage. Carbon will be sequestered in forest soils if organic matter inputs to soil profiles increase without a matching increase in decomposition or leaching losses from the soil profile, or if the rate of decomposition decreases because of increased production of resistant humic substances or greater physical protection of organic matter in soil aggregates. To examine the response of a forest ecosystem to elevated atmospheric CO₂ concentrations, the Duke Forest Free-Air CO₂ Enrichment (FACE) experiment in North Carolina, USA, has maintained atmospheric CO₂ concentrations 200 μL L⁻¹ above ambient in an aggrading loblolly pine (Pinus taeda) plantation over a 9-year period (1996-2005). During the first 6 years of the experiment, forest-floor C and N pools increased linearly under both elevated and ambient CO₂ conditions, with significantly greater accumulations under the elevated CO₂ treatment. Between the sixth and ninth year, forest-floor organic matter accumulation stabilized and C and N pools appeared to reach their respective steady states. An additional C sink of ~30 g C m⁻² yr⁻¹ was sequestered in the forest floor of the elevated CO₂ treatment plots relative to the control plots maintained at ambient CO₂ owing to increased litterfall and root turnover during the first 9 years of the study. Because we did not detect any significant elevated CO₂ effects on the rate of decomposition or on the chemical composition of forest-floor organic matter, this additional C sink was likely related to enhanced litterfall C inputs. We also failed to detect any statistically significant treatment effects on the C and N pools of surface and deep mineral soil horizons. However, a significant widening of the C : N ratio of soil organic matter (SOM) in the upper mineral soil under both elevated and ambient CO₂ suggests that N is being transferred from soil to plants in this aggrading forest. A significant treatment x time interaction indicates that N is being transferred at a higher rate under elevated CO₂ (P=0.037), suggesting that enhanced rates of SOM decomposition are increasing mineralization and uptake to provide the extra N required to support the observed increase in primary productivity under elevated CO₂.
Gene expression responses of paper birch (Betula papyrifera) leaves to elevated concentrations of CO(2) and O(3) were studied with microarray analyses from three time points during the summer of 2004 ...at Aspen FACE. Microarray data were analyzed with clustering techniques, self-organizing maps, K-means clustering and Sammon's mappings, to detect similar gene expression patterns within sampling times and treatments. Most of the alterations in gene expression were caused by O(3), alone or in combination with CO(2). O(3) induced defensive reactions to oxidative stress and earlier leaf senescence, seen as decreased expression of photosynthesis- and carbon fixation-related genes, and increased expression of senescence-associated genes. The effects of elevated CO(2) reflected surplus of carbon that was directed to synthesis of secondary compounds. The combined CO(2)+O(3) treatment resulted in differential gene expression than with individual gas treatments or in changes similar to O(3) treatment, indicating that CO(2) cannot totally alleviate the harmful effects of O(3).
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