Global Change Factors Research Articles

Attribution of spatial and temporal variations in terrestrial methane flux over North America

Abstract. The attribution of spatial and temporal variations in terrestrial methane (CH4) flux is essential for assessing and mitigating CH4 emission from terrestrial ecosystems. In this study, we used a process-based model, the Dynamic Land Ecosystem Model (DLEM), in conjunction with spatial data of six major environmental factors to attribute the spatial and temporal variations in the terrestrial methane (CH4) flux over North America from 1979 to 2008 to six individual driving factors and their interaction. Over the past three decades, our simulations indicate that global change factors accumulatively contributed 23.51 ± 9.61 T g CH4-C (1 Tg = 1012 g) emission over North America, among which ozone (O3) pollution led to a reduced CH4 emission by 2.30 ± 0.49 T g CH4-C. All other factors including climate variability, nitrogen (N) deposition, elevated atmospheric carbon dioxide (CO2), N fertilizer application, and land conversion enhanced terrestrial CH4 emissions by 19.80 ± 12.42 T g CH4-C, 0.09 ± 0.02 T g CH4-C, 6.80 ± 0.86 T g CH4-C, 0.01 ± 0.001 T g CH4-C, and 3.95 ± 0.38 T g CH4-C, respectively, and interaction between/among these global change factors led to a decline of CH4 emission by 4.84 ± 7.74 T g CH4-C. Climate variability and O3 pollution suppressed, while other factors stimulated CH4 emission over the USA; climate variability significantly enhanced, while all the other factors exerted minor effects, positive or negative, on CH4 emission in Canada; Mexico functioned as a sink for atmospheric CH4 with a major contribution from climate change. Climatic variability dominated the inter-annual variations in terrestrial CH4 flux at both continental and country levels. Precipitation played an important role in the climate-induced changes in terrestrial CH4 flux at both continental and country-levels. The relative importance of each environmental factor in determining the magnitude of CH4 flux showed substantially spatial variation across North America. This factorial attribution of CH4 flux in North America might benefit policy makers who would like to curb climate warming by reducing CH4 emission.

Carbon partitioning in a wet and a semiwet subarctic mire ecosystem based on in situ 14C pulse-labelling

In this study we quantify the partitioning of recent assimilates to above- and below-ground carbon (C) pools in two subarctic mire ecosystems - wet minerotrophic and semiwet ombrotrophic mire - using in situ C-14 pulse-labelling. Ecosystem C partitioning to rhizomes, coarse roots, fine roots, dissolved organic carbon (DOC) and microbes were quantified twice during the growing season at three different soil depths. Finally the C-14-partitioning data from this and a previous study were combined to estimate the overall C partitioning of the three main vegetation types of a Scandinavian subarctic mire in early and late summer. The semiwet ombrotrophic ecosystem hosted a much larger root biomass on an area basis compared to the wet minerotrophic ecosystem which might be due to differences in the soil nutrient level. Microbial C was found to be the largest C-pool in both ecosystems. Ecosystem C-14 partitioning was poorly related to plant biomass for the semiwet and the wet ecosystem. Overall a higher partitioning of recent assimilates to below-ground compartments was apparent in August-September compared to June-July, while the opposite was found for the above-ground C-pools. In the semiwet ecosystem twice as much C-14 was found in DOC compared to the wet ecosystem, where root density, litter and above-ground biomass were important controls of the C-14-recovery in DOC. Plant-derived DOC was estimated to be 15.4 versus 12.9 mg C m(-2) d(-1) in the semiwet and wet ecosystem, respectively. Graminoid dominated and dwarf shrub dominated vegetation types of the subarctic mire Stordalen differ with respect to the relative amount of recently assimilated C partitioned to C-pools with slow versus fast decomposition rate. The capacity for sequestration of recently fixed C within slow C-pools might affect the ecosystem C balance (NEE) and C-storage. The potential for vegetation changes might therefore be an important factor to consider in studies of response of ecosystem C-dynamics to global change factors in subarctic mires. (C) 2010 Elsevier Ltd. All rights reserved. (Less)

Climate warming increases soil erosion, carbon and nitrogen loss with biofuel feedstock harvest in tallgrass prairie

Anthropogenic soil erosion severely affects land ecosystems by reducing plant productivity and stimulating horizontal carbon and nitrogen movement at the surface. Climate warming may accelerate soil erosion by altering soil temperature, moisture, and vegetation coverage. However, no experiments have been carried out to quantify soil erosion with warming. In a long-term field experiment, we explored how annual clipping for biofuel feedstock production and warming caused soil erosion and accompanying carbon and nitrogen losses in tallgrass prairie in Oklahoma, USA. We measured relative changes in soil surface elevation between clipped and unclipped plots with or without experimental warming. Our results show that average relative erosion depth caused by clipping was 1.65±0.09 and 0.54±0.08 mm yr−1, respectively, in warmed and control plots from November 21, 1999 to April 21, 2009. The soil erosion rate was 2148±121 g m−2 yr−1 in the warmed plots and 693±113 g m−2 yr−1 in the control plots. Soil organic carbon was lost at a rate of 69.6±5.6 g m−2 yr−1 in the warmed plots and 22.5±2.7 g m−2 yr−1 in the control plots. Total nitrogen was lost at a rate of 4.6±0.4 g m−2 yr−1 in the warmed plots and 1.4±0.1 g m−2 yr−2 in the control plots. The amount of carbon and nitrogen loss caused by clipping is equivalent to or even larger than changes caused by global change factors such as warming and rising atmospheric CO2 concentration. In addition, soil erosion rates were significantly correlated with clipping-induced changes in soil moisture. Our results suggest that clipping for biofuel harvest results in significant soil erosion and accompanying losses of soil carbon and nitrogen, which is aggravated by warming.

Does an extreme drought event alter the response of grassland communities to a changing climate?

Global temperatures and atmospheric CO 2 concentrations are expected to both increase, but their combined effect on plant communities has been far less investigated than the single factors of global change. Moreover, drought events are expected to become more frequent and intense in the near future what might alter plant responses to the changing climate. In this study synthesised grassland communities in a current or future climate were subjected to several drought levels (0, 15, 22 and 35 days of drought). The grassland communities were grown in six sunlit, climate-controlled chambers. Three of the chambers were exposed to ambient temperature and CO 2 (current climate), while the other three were continuously warmed 3 °C above ambient temperature at 620 ppm of CO 2 (future climate). The aim of this study was to investigate the effect of drought on the response of grassland communities to a future climate. Therefore, the response to future climate was observed (1) in the absence of drought and (2) in the occurrence of an extreme drought event, both early and late in the growing season. (1) In the absence of drought, plant productivity was positively affected by future climate early in the growing season. Later in the growing season this effect tended to turn negative, resulting in a disappearance of the overall effect of climate at the end of the growing season. (2) During drought there was a stronger decrease in net CO 2 assimilation rate ( A sat) in future than in current climate due to stronger stomatal closure. Consistently, the beneficial biomass response to future climate stagnated during drought. At the end of the season, after a period of recovery, there was no effect of climate on plant productivity. As in the absence of drought, plant productivity was not affected by climate at the end of the growing season. Hence, the occurrence of an extreme drought event during the growing season did not alter the overall response of plant productivity to a future climate.

Plant responses to low [CO2] of the past

During the Last Glacial Maximum (LGM; 18,000-20,000 yr ago) and previous glacial periods, atmospheric [CO(2)] dropped to 180-190 ppm, which is among the lowest concentrations that occurred during the evolution of land plants. Modern atmospheric CO(2) concentrations ([CO(2)]) are more than twice those of the LGM and 45% higher than pre-industrial concentrations. Since CO(2) is the carbon source for photosynthesis, lower carbon availability during glacial periods likely had a major impact on plant productivity and evolution. From the studies highlighted here, it is clear that the influence of low [CO(2)] transcends several scales, ranging from physiological effects on individual plants to changes in ecosystem functioning, and may have even influenced the development of early human cultures (via the timing of agriculture). Through low-[CO(2)] studies, we have determined a baseline for plant response to minimal [CO(2)] that occurred during the evolution of land plants. Moreover, an increased understanding of plant responses to low [CO(2)] contributes to our knowledge of how natural global change factors in the past may continue to influence plant responses to future anthropogenic changes. Future work, however, should focus more on the evolutionary responses of plants to changing [CO(2)] in order to account for the potentially large effects of genetic change.

Effects of tree species and N additions on forest floor microbial communities and extracellular enzyme activities

Forest nitrogen (N) retention and soil carbon (C) storage are influenced by tree species and their associated soil microbial communities. As global change factors alter forest composition, predicting long-term C and N dynamics will require understanding microbial community structure and function at the tree species level. Because atmospheric N deposition is increasing N inputs to forested ecosystems across the globe, including the northeastern US, it is also important to understand how microbial communities respond to added N. While prior studies have examined these topics in mixed-species stands, we focused on the responses of different tree species and their associated microbial communities within a single forest type – a northern hardwood forest in the Catskills Mountains, NY. Based on prior studies, we hypothesized that N additions would stimulate extracellular enzyme activities in relatively labile litters, but suppress oxidative enzyme activities in recalcitrant litters, and tested for independent tree species effects within this context. During the 2007 growing season (May–June), we measured enzyme activities and microbial community composition (using phospholipid fatty acid analysis - PLFA) of the forest floor in single-species plots dominated by sugar maple ( Acer saccharum ), yellow birch ( Betula alleghaniensis ), red oak ( Quercus rubra ), American beech ( Fagus grandifolia ) and eastern hemlock ( Tsuga canadensis ), species whose litters range from relatively labile to recalcitrant. Half the plots were fertilized with N by adding NH 4 NO 3 (50 kg ha −1 y −1 ) from 1997 to 2009. Non-metric multidimensional scaling (NMS) and multi-response permutation procedures (MRPP) were used to examine microbial community structure and relationship to enzyme activities. We found that in response to N additions, both microbial community composition and enzyme activities changed; however the strength of the changes were tree species-specific and the direction of these changes was and not readily predictable from prior studies conducted in mixed-species stands. For example, in contrast to other studies, we found that N additions caused a significant overall increase in fungal biomass that was strongest for yellow birch (24% increase) and weakest for sugar maple (1% increase). Contrary to our initial hypotheses and current conceptual models, N additions reduced hydrolytic enzyme activities in hemlock plots and reduced oxidative enzyme activity in birch plots, a species with relatively labile litter. These responses suggest that our understanding of the interactions between microbial community composition, enzyme activity, substrate chemistry, and nutrient availability as influenced by tree species composition is incomplete. NMS ordination showed that patterns in microbial community structure (PLFA) and function (enzyme activity) were more strongly influenced by tree species than by fertilization, and only partially agreed with the structure–function relationships found in other studies. This finding suggests that tree species-specific responses are likely to be important in determining the structure and function of northeastern hardwood forests in the future. Enhanced understanding of microbial responses to added N in single and mixed-species substrates with varying amounts of lignin and phenols may be needed for accurate predictions of future soil C and N dynamics.

The response of heterotrophic activity and carbon cycling to nitrogen additions and warming in two tropical soils

AbstractNitrogen (N) deposition is projected to increase significantly in tropical regions in the coming decades, where changes in climate are also expected. Additional N and warming each have the potential to alter soil carbon (C) storage via changes in microbial activity and decomposition, but little is known about the combined effects of these global change factors in tropical ecosystems. In this study, we used controlled laboratory incubations of soils from a long‐term N fertilization experiment to explore the sensitivity of soil C to increased N in two N‐rich tropical forests. We found that fertilization corresponded to significant increases in bulk soil C concentrations, and decreases in C loss via heterotrophic respiration (P< 0.05). The increase in soil C was not uniform among C pools, however. The active soil C pool decomposed faster with fertilization, while slowly cycling C pools had longer turnover times. These changes in soil C cycling with N additions corresponded to the responses of two groups of microbial extracellular enzymes. Smaller active C pools corresponded to increased hydrolytic enzyme activities; longer turnover times of the slowly cycling C pool corresponded to reduced activity of oxidative enzymes, which degrade more complex C compounds, in fertilized soils. Warming increased soil respiration overall, and N fertilization significantly increased the temperature sensitivity of slowly cycling C pools in both forests. In the lower elevation forest, respired CO2 from fertilized cores had significantly higher Δ14C values than control soils, indicating losses of relatively older soil C. These results indicate that soil C storage is sensitive to both N deposition and warming in N‐rich tropical soils, with interacting effects of these two global change factors. N deposition has the potential to increase total soil C stocks in tropical forests, but the long‐term stability of this added C will likely depend on future changes in temperature.

Ecosystem response to elevated CO2 levels limited by nitrogen-induced plant species shift

Terrestrial ecosystems gain carbon through photosynthesis and lose it mostly in the form of carbon dioxide (CO(2)). The extent to which the biosphere can act as a buffer against rising atmospheric CO(2) concentration in global climate change projections remains uncertain at the present stage. Biogeochemical theory predicts that soil nitrogen (N) scarcity may limit natural ecosystem response to elevated CO(2) concentration, diminishing the CO(2)-fertilization effect on terrestrial plant productivity in unmanaged ecosystems. Recent models have incorporated such carbon-nitrogen interactions and suggest that anthropogenic N sources could help sustain the future CO(2)-fertilization effect. However, conclusive demonstration that added N enhances plant productivity in response to CO(2)-fertilization in natural ecosystems remains elusive. Here we manipulated atmospheric CO(2) concentration and soil N availability in a herbaceous brackish wetland where plant community composition is dominated by a C(3) sedge and C(4) grasses, and is capable of responding rapidly to environmental change. We found that N addition enhanced the CO(2)-stimulation of plant productivity in the first year of a multi-year experiment, indicating N-limitation of the CO(2) response. But we also found that N addition strongly promotes the encroachment of C(4) plant species that respond less strongly to elevated CO(2) concentrations. Overall, we found that the observed shift in the plant community composition ultimately suppresses the CO(2)-stimulation of plant productivity by the third and fourth years. Although extensive research has shown that global change factors such as elevated CO(2) concentrations and N pollution affect plant species differently and that they may drive plant community changes, we demonstrate that plant community shifts can act as a feedback effect that alters the whole ecosystem response to elevated CO(2) concentrations. Moreover, we suggest that trade-offs between the abilities of plant taxa to respond positively to different perturbations may constrain natural ecosystem response to global change.

TOMORROW'S CHALLENGES IN FRUIT NUTRITION RESEARCH: QUO VADIS?

Historically, nutritional research in fruit crops has emphasized the development of strategies to overcome soil and plant limitations resulting in nutrient deficiencies. Recently the emphasis has been placed on N and Ca, the nutrients most closely associated with fruit quality. Major global change factors are likely to affect the future conduct of nutritional research. These issues include the possible consequences of climate change, effects of high energy costs, efforts to maintain environmental quality, and also a secure supply of high quality fruit. Possible responses include precision horticulture with more targeted nutrient management. This in turn will require improved understanding of application efficiencies and the timing and magnitude of nutrient demand in order to synchronize fertilization more closely with plant requirements. Reductions in spatial and temporal variation in plant nutritional status should be investigated with respect to the potential to improve overall fruit yield and quality. Improvements in application technology to allow differential fertilization of individual plants would improve the flexibility of fertigation. Efforts to adopt more environmentally benign low input and organic production systems will require improved understanding of nutrient availability from organic and biological amendments. Improved genetic and molecular understanding of the complex processes comprising nutrient uptake, assimilation and recycling will need to be paralleled by similar efforts to understand nutrient cycling in ecosystems.

Responses of fungal root colonization, plant cover and leaf nutrients to long‐term exposure to elevated atmospheric CO2 and warming in a subarctic birch forest understory

AbstractResponses of the mycorrhizal fungal community in terrestrial ecosystems to global change factors are not well understood. However, virtually all land plants form symbiotic associations with mycorrhizal fungi, with approximately 20% of the plants' net primary production transported down to the fungal symbionts. In this study, we investigated how ericoid mycorrhiza (ErM), fine endophytes (FE) and dark septate endophytes (DSE) in roots responded to elevated atmospheric CO2 concentrations and warming in the dwarf shrub understory of a birch forest in the subarctic region of northern Sweden. To place the belowground results into an ecosystem context we also investigated how plant cover and nutrient concentrations in leaves responded to elevated atmospheric CO2 concentrations and warming. The ErM colonization in ericaceous dwarf shrubs increased under elevated atmospheric CO2 concentrations, but did not respond to warming following 6 years of treatment. This suggests that the higher ErM colonization under elevated CO2 might be due to increased transport of carbon belowground to acquire limiting resources such as N, which was diluted in leaves of ericaceous plants under enhanced CO2. The elevated CO2 did not affect total plant cover but the plant cover was increased under warming, which might be due to increased N availability in soil. FE colonization in grass roots decreased under enhanced CO2 and under warming, which might be due to increased root growth, to which the FE fungi could not keep up, resulting in proportionally lower colonization. However, no responses in aboveground cover of Deschampsia flexuosa were seen. DSE hyphal colonization in grass roots significantly increased under warmer conditions, but did not respond to elevated CO2. This complex set of responses by mycorrhizal and other root‐associated fungi to global change factors of all the fungal types studied could have broad implications for plant community structure and biogeochemistry of subarctic ecosystems.

Effects of interactive global changes on methane uptake in an annual grassland

The future size of the terrestrial methane (CH4) sink of upland soils remains uncertain, along with potential feedbacks to global warming. Much of the uncertainty lies in our lack of knowledge about potential interactive effects of multiple simultaneous global environmental changes. Field CH4 fluxes and laboratory soil CH4 consumption were measured five times during 3 consecutive years in a California annual grassland exposed to 8 years of the full factorial combination of ambient and elevated levels of precipitation, temperature, atmospheric CO2 concentration, and N deposition. Across all sampling dates and treatments, increased precipitation caused a 61% reduction in field CH4 uptake. However, this reduction depended quantitatively on other global change factors. Higher precipitation reduced CH4 uptake when temperature or N deposition (but not both) increased, and under elevated CO2 but only late in the growing season. Warming alone also decreased CH4 uptake early in the growing season, which was partly explained by a decrease in laboratory soil CH4 consumption. Atmospheric CH4 models likely need to incorporate nonadditive interactions, seasonal interactions, and interactions between methanotrophy and methanogenesis. Despite the complexity of interactions we observed in this multifactor experiment, the outcome agrees with results from single‐factor experiments: an increased terrestrial CH4 sink appears less likely than a reduced one.

Long-term dynamic modeling of global population and built-up area in a spatially explicit way: HYDE 3.1

This paper describes a tool for long-term global change studies; it is an update of the History Database of the Global Environment (HYDE) with estimates of some of the underlying demographic driving factors of global change. We estimate total and urban/rural population numbers, densities and fractions (including built-up area) for the Holocene, roughly the period 10 000 BC to AD 2000 with a spatial resolution of 5 min longitude/latitude. With a total global population increase from 2 to 6145 million people over that time span, resulting in a global population density increase of < 0.1 cap/km2 to almost 46 cap/km 2 and a urban built-up area evolving from almost zero to 0.5 million km2 (still only <0.5% of the total global land surface, but with a huge impact in terms of demands of food, services, building materials, etc.), it is clear that this must have had, and will continue to have, a profound influence on the Earth’s environment and its associated (climate) change. We hope that this data base can contribute to the Earth System Modelers community to gain better insight into long-term global change research.

Biological stoichiometry of plant production: metabolism, scaling and ecological response to global change

Biological stoichiometry theory considers the balance of multiple chemical elements in living systems, whereas metabolic scaling theory considers how size affects metabolic properties from cells to ecosystems. We review recent developments integrating biological stoichiometry and metabolic scaling theories in the context of plant ecology and global change. Although vascular plants exhibit wide variation in foliar carbon:nitrogen:phosphorus ratios, they exhibit a higher degree of 'stoichiometric homeostasis' than previously appreciated. Thus, terrestrial carbon:nitrogen:phosphorus stoichiometry will reflect the effects of adjustment to local growth conditions as well as species' replacements. Plant stoichiometry exhibits size scaling, as foliar nutrient concentration decreases with increasing plant size, especially for phosphorus. Thus, small plants have lower nitrogen:phosphorus ratios. Furthermore, foliar nutrient concentration is reflected in other tissues (root, reproductive, support), permitting the development of empirical models of production that scale from tissue to whole-plant levels. Plant stoichiometry exhibits large-scale macroecological patterns, including stronger latitudinal trends and environmental correlations for phosphorus concentration (relative to nitrogen) and a positive correlation between nutrient concentrations and geographic range size. Given this emerging knowledge of how plant nutrients respond to environmental variables and are connected to size, the effects of global change factors (such as carbon dioxide, temperature, nitrogen deposition) can be better understood.

Plant health and global change – some implications for landscape management

Global change (climate change together with other worldwide anthropogenic processes such as increasing trade, air pollution and urbanization) will affect plant health at the genetic, individual, population and landscape level. Direct effects include ecosystem stress due to natural resources shortage or imbalance. Indirect effects include (i) an increased frequency of natural detrimental phenomena, (ii) an increased pressure due to already present pests and diseases, (iii) the introduction of new invasive species either as a result of an improved suitability of the climatic conditions or as a result of increased trade, and (iv) the human response to global change. In this review, we provide an overview of recent studies on terrestrial plant health in the presence of global change factors. We summarize the links between climate change and some key issues in plant health, including tree mortality, changes in wildfire regimes, biological invasions and the role of genetic diversity for ecosystem resilience. Prediction and management of global change effects are complicated by interactions between globalization, climate and invasive plants and/or pathogens. We summarize practical guidelines for landscape management and draw general conclusions from an expanding body of literature.

Like moths to a street lamp: exaggerated animal densities in plot-level global change field experiments

Many recent field experiments have examined plant responses to global change factors such as climate warming, elevated atmospheric CO 2 , increased nitrogen addition or altered precipitation simulated at the plot level, yet the mechanisms underlying these responses can be difficult to isolate. One concern has been that the infrastructure used in these experiments can restrict the access of influential herbivores, detritivores or pollinators to the plots, and the absence of these animals is confounded with the treatment effects. However, in this paper we describe why free access by animals to experimental plots does not ensure realistic animal densities in response to global change treatments. On the contrary, much like moths swarm around streetlamps, animals that prefer the local conditions in treated plots may congregate at artificially high densities, or conversely, those that are repelled by the treatments may choose to avoid them. Therefore, animal densities or herbivore damage in the plots of global change experiments may grossly exaggerate or underestimate the contributions of animals to primary productivity or plant species composition under future environmental conditions. We describe how these potential animal congregation and avoidance artifacts may have been overlooked in the interpretation of results from many plot-level global change field experiments. We also provide suggestions for how to best interpret the results of these experiments and how to isolate the effects of animal density artifacts.

The responses of three C4 grasses to elevated temperature and CO2 in the field

A series of field experiments were carried out to assess the impact of two important global change factors (increased temperature and elevated atmospheric CO2) on the germination and growth of three C4 grass species found in New Zealand pastures: Digitaria sanguinalis (summer grass), Paspalum dilatatum (paspalum) and Pennisetum clandestinum (kikuyu).

Methane from Gas Hydrates in the Black Sea

Gas hydrates are crystalline solids that form from mixtures of water and light natural gases such as methane, carbon dioxide, ethane, propane, and butane. The Black Sea is the world's most isolated sea. Methane exists as gas hydrates or methane clathrate form in the Black Sea. Gas hydrates potential in the Black Sea is investigated as a source of methane. Methane gas hydrate is a solid combination of methane and ice. It is found under continental shelves and on land under permafrost and can contain from 80–99.9% of methane. Gas hydrate is found in sub-oceanic sediments and in continental slope sediments, where pressure and temperature conditions combine to make it stable. Natural gas hydrate contains highly concentrated methane, which is important both as an energy resource and as a factor in global climate change. The difficulty with recovering this source of energy is that the fuel is in solid form and is not amenable to conventional gas and oil recovery techniques.

Interactive effects of warming and increased nitrogen deposition on 15N tracer retention in a temperate old field: seasonal trends

AbstractThe extent to which increased atmospheric nitrogen (N) deposition will drive changes in plant productivity and species composition over the next century will depend on how other influential global change factors, such as climate warming, affect the N retention of ecosystems. We examined the interactive effects of simulated climate warming and N deposition on the recoveries of 15N‐labeled ammonium and 15N‐labeled nitrate tracers added as a pulse to grass‐dominated, temperate old‐field plots at spring thaw. In addition to the year‐round warming treatment, a winter‐only warming treatment was applied to a set of plots to explore the contribution of this component of climate warming to the overall warming effect. By the end of the plant growing season, there was approximately twice as much 15N enrichment in the plant roots and bulk soil from 15NH4+‐addition plots than from 15NO3−‐addition plots, but there were no effects of warming or N fertilization on 15N recovery. Over winter, approximately half of the excess 15N present in plant shoots was lost, which corresponded with large 15N losses from bulk soil in N fertilized plots and large 15N increases in bulk soil in nonfertilized plots. By the next spring, there was decreased 15N recovery in plants in response to N fertilization, which was largely offset by increases in plant 15N recovery in response to year‐round warming. However, 15N retention in bulk soil, where the major part of the 15N label was recovered, was approximately 40% higher in nonfertilized plots than in N fertilized plots. Overall, our results indicate that climate warming increases plant N sequestration in this system but this effect is overwhelmed by the overall effect of nitrogen deposition on ecosystem N losses.

Solar UVB and warming affect decomposition and earthworms in a fen ecosystem in Tierra del Fuego, Argentina

AbstractCombined effects of co‐occurring global climate changes on ecosystem responses are generally poorly understood. Here, we present results from a 2‐year field experiment in aCarexfen ecosystem on the southernmost tip of South America, where we examined the effects of solar ultraviolet B (UVB, 280–315 nm) and warming on above‐ and belowground plant production, C : N ratios, decomposition rates and earthworm population sizes. Solar UVB radiation was manipulated using transparent plastic filter films to create a near‐ambient (90% of ambient UVB) or a reduced solar UVB treatment (15% of ambient UVB). The warming treatment was imposed passively by wrapping the same filter material around the plots resulting in a mean air and soil temperature increase of about 1.2 °C. Aboveground plant production was not affected by warming, and marginally reduced at near‐ambient UVB only in the second season. Aboveground plant biomass also tended to have a lower C : N ratio under near‐ambient UVB and was differently affected at the two temperatures (marginal UVB × temperature interaction). Leaf decomposition of one dominant sedge species (Carex curta) tended to be faster at near‐ambient UVB than at reduced UVB. Leaf decomposition of a codominant species (Carex decidua) was significantly faster at near‐ambient UVB; root decomposition of this species tended to be lower at increased temperature and interacted with UVB. We found, for the first time in a field experiment that epigeic earthworm density and biomass was 36% decreased by warming but remained unaffected by UVB radiation. Our results show that present‐day solar UVB radiation and modest warming can adversely affect ecosystem functioning and engineers of this fen. However, results on plant biomass production also showed that treatment manipulations of co‐occurring global change factors can be overridden by the local climatic situation in a given study year.

Near‐surface remote sensing of spatial and temporal variation in canopy phenology

There is a need to document how plant phenology is responding to global change factors, particularly warming trends. "Near-surface" remote sensing, using radiometric instruments or imaging sensors, has great potential to improve phenological monitoring because automated observations can be made at high temporal frequency. Here we build on previous work and show how inexpensive, networked digital cameras ("webcams") can be used to document spatial and temporal variation in the spring and autumn phenology of forest canopies. We use two years of imagery from a deciduous, northern hardwood site, and one year of imagery from a coniferous, boreal transition site. A quantitative signal is obtained by splitting images into separate red, green, and blue color channels and calculating the relative brightness of each channel for "regions of interest" within each image. We put the observed phenological signal in context by relating it to seasonal patterns of gross primary productivity, inferred from eddy covariance measurements of surface-atmosphere CO2 exchange. We show that spring increases, and autumn decreases, in canopy greenness can be detected in both deciduous and coniferous stands. In deciduous stands, an autumn red peak is also observed. The timing and rate of spring development and autumn senescence varies across the canopy, with greater variability in autumn than spring. Interannual variation in phenology can be detected both visually and quantitatively; delayed spring onset in 2007 compared to 2006 is related to a prolonged cold spell from day 85 to day 110. This work lays the foundation for regional- to continental-scale camera-based monitoring of phenology at network observatory sites, e.g., National Ecological Observatory Network (NEON) or AmeriFlux.