Showing papers on "Productivity (ecology) published in 2016"

Plant diversity effects on grassland productivity are robust to both nutrient enrichment and drought

Abstract: Global change drivers are rapidly altering resource availability and biodiversity. While there is consensus that greater biodiversity increases the functioning of ecosystems, the extent to which biodiversity buffers ecosystem productivity in response to changes in resource availability remains unclear. We use data from 16 grassland experiments across North America and Europe that manipulated plant species richness and one of two essential resources—soil nutrients or water—to assess the direction and strength of the interaction between plant diversity and resource alteration on above-ground productivity and net biodiversity, complementarity, and selection effects. Despite strong increases in productivity with nutrient addition and decreases in productivity with drought, we found that resource alterations did not alter biodiversity–ecosystem functioning relationships. Our results suggest that these relationships are largely determined by increases in complementarity effects along plant species richness gradients. Although nutrient addition reduced complementarity effects at high diversity, this appears to be due to high biomass in monocultures under nutrient enrichment. Our results indicate that diversity and the complementarity of species are important regulators of grassland ecosystem productivity, regardless of changes in other drivers of ecosystem function.

Effects of species diversity on fine root productivity in diverse ecosystems: a global meta‐analysis

Abstract: Aim Positive relationships between plant species diversity and above-ground productivity have been observed across a wide range of terrestrial ecosystems. Despite a critical contribution of below-ground productivity to overall terrestrial productivity, no consensus exists about the nature of the relationship between species diversity and below-ground productivity. Location Global. Methods We collected data from published studies conducted in natural and planted forests and experimental grassland, crop and pot systems that were purposely implemented to isolate the effects of plant species diversity from other factors, such as soil conditions and topographic features. We conducted meta-analyses of 170 observations for root biomass and 23 observations for root production, derived from 48 published studies, using weighted linear modelling with bootstrap procedures to reconcile the effects of diversity on fine root productivity. Results We found that species mixtures had, on average 28.4% higher fine root biomass and 44.8% higher annual production than monocultures. Higher fine root biomass in species mixtures than in monocultures was consistent across natural forests, planted grasslands, croplands and pot systems, except for young planted forests. Transgressive overyielding was only evident for planted grasslands. The log response ratio of fine root biomass in species mixtures to that in respective monocultures increased with species richness across all ecosystem types, and also increased with experiment age in grasslands. Main conclusions Our meta-analysis reveals positive effects of species diversity on below-ground productivity. Despite profound differences in environments among terrestrial ecosystems, our analysis demonstrated that below-ground productivity responds similarly to variations in species richness. Furthermore, our study also reveals shifts in the effects of diversity over time in both forests and grasslands. Future efforts are needed to further understand below-ground productivity–diversity relationships.

Large-Scale Production of Algal Biomass: Raceway Ponds

Abstract: Raceway ponds are widely used in commercial production of algal biomass. They are effective and inexpensive, but suffer from a relatively low productivity and vagaries of weather. This chapter discusses design and operation of raceways for large-scale production of algal biomass.

Climate seasonality limits leaf carbon assimilation and wood productivity in tropical forests

Abstract: The seasonal climate drivers of the carbon cycle in tropical forests remain poorly known, although these forests account for more carbon assimilation and storage than any other terrestrial ecosystem. Based on a unique combination of seasonal pan-tropical data sets from 89 experimental sites (68 include aboveground wood productivity measurements and 35 litter productivity measurements), their associated canopy photosynthetic capacity (enhanced vegetation index, EVI) and climate, we ask how carbon assimilation and aboveground allocation are related to climate seasonality in tropical forests and how they interact in the seasonal carbon cycle. We found that canopy photosynthetic capacity seasonality responds positively to precipitation when rainfall is < 2000 mm yr(-1) (water-limited forests) and to radiation otherwise (light-limited forests). On the other hand, independent of climate limitations, wood productivity and litterfall are driven by seasonal variation in precipitation and evapotranspiration, respectively. Consequently, light-limited forests present an asynchronism between canopy photosynthetic capacity and wood productivity. First-order control by precipitation likely indicates a decrease in tropical forest productivity in a drier climate in water-limited forest, and in current light-limited forest with future rainfall < 2000 mm yr(-1).

Shifting grassland plant community structure drives positive interactive effects of warming and diversity on aboveground net primary productivity

Abstract: Ecosystems worldwide are increasingly impacted by multiple drivers of environmental change, including climate warming and loss of biodiversity. We show, using a long-term factorial experiment, that plant diversity loss alters the effects of warming on productivity. Aboveground primary productivity was increased by both high plant diversity and warming, and, in concert, warming (≈1.5 °C average above and belowground warming over the growing season) and diversity caused a greater than additive increase in aboveground productivity. The aboveground warming effects increased over time, particularly at higher levels of diversity, perhaps because of warming-induced increases in legume and C4 bunch grass abundances, and facilitative feedbacks of these species on productivity. Moreover, higher plant diversity was associated with the amelioration of warming-induced environmental conditions. This led to cooler temperatures, decreased vapor pressure deficit, and increased surface soil moisture in higher diversity communities. Root biomass (0-30 cm) was likewise consistently greater at higher plant diversity and was greater with warming in monocultures and at intermediate diversity, but at high diversity warming had no detectable effect. This may be because warming increased the abundance of legumes, which have lower root : shoot ratios than the other types of plants. In addition, legumes increase soil nitrogen (N) supply, which could make N less limiting to other species and potentially decrease their investment in roots. The negative warming × diversity interaction on root mass led to an overall negative interactive effect of these two global change factors on the sum of above and belowground biomass, and thus likely on total plant carbon stores. In total, plant diversity increased the effect of warming on aboveground net productivity and moderated the effect on root mass. These divergent effects suggest that warming and changes in plant diversity are likely to have both interactive and divergent impacts on various aspects of ecosystem functioning.

Nonlinear responses of ecosystem carbon fluxes and water‐use efficiency to nitrogen addition in Inner Mongolia grassland

Abstract: Summary Nitrogen (N) deposition is a continuous process and likely to affect ecosystem carbon (C) and water fluxes in a nonlinear way. However, experimental evidence is still lacking because most previous studies on these impacts usually used two discrete levels of N treatment. By a 12-year, 6-level N addition experiment in Inner Mongolia grassland, we found that the responses of C fluxes, including net ecosystem carbon exchange (NEE), gross ecosystem productivity (GEP), ecosystem respiration (ER), all exhibited nonlinear patterns with increasing N addition rate while that of evapotranspiration did not significantly change. As a result, the response of ecosystem water-use efficiency (EWUE) followed a similar pattern with NEE. These N-induced changes in C fluxes were greatly affected by the distribution of precipitation among different stages of growing seasons and mainly driven by the alterations in biomass production rather than soil temperature and soil moisture. This study highlights the importance of the nonlinearity of N addition impacts on ecosystem C fluxes, which should be incorporated into the global-C-cycling models for better predicting future C balance. Our results also have implications for the use of fertilization in restoring the degraded grasslands given that N addition can promote biomass production and ecosystem C uptake without additional water evapotranspiration but also change ecosystem composition.

Soil moisture mediates alpine life form and community productivity responses to warming

Abstract: Climate change is expected to alter primary production and community composition in alpine ecosystems, but the direction and magnitude of change is debated. Warmer, wetter growing seasons may increase productivity; however, in the absence of additional precipitation, increased temperatures may decrease soil moisture, thereby diminishing any positive effect of warming. Since plant species show individual responses to environmental change, responses may depend on community composition and vary across life form or functional groups. We warmed an alpine plant community at Niwot Ridge, Colorado continuously for four years to test whether warming increases or decreases productivity of life form groups and the whole community. We provided supplemental water to a subset of plots to alleviate the drying effect of warming. We measured annual above-ground productivity and soil temperature and moisture, from which we calculated soil degree days and adequate soil moisture days. Using an information-theoretic approach, we observed that positive productivity responses to warming at the community level occur only when warming is combined with supplemental watering; otherwise we observed decreased productivity. Watering also increased community productivity in the absence of warming. Forbs accounted for the majority of the productivity at the site and drove the contingent community response to warming, while cushions drove the generally positive response to watering and graminoids muted the community response. Warming advanced snowmelt and increased soil degree days, while watering increased adequate soil moisture days. Heated and watered plots had more adequate soil moisture days than heated plots. Overall, measured changes in soil temperature and moisture in response to treatments were consistent with expected productivity responses. We found that available soil moisture largely determines the responses of this forb-dominated alpine community to simulated climate warming.

Carbon storage in seagrass soils: long-term nutrient history exceeds the effects of near-term nutrient enrichment

Abstract: . The carbon sequestration potential in coastal soils is linked to aboveground and belowground plant productivity and biomass, which in turn, is directly and indirectly influenced by nutrient input. We evaluated the influence of long-term and near-term nutrient input on aboveground and belowground carbon accumulation in seagrass beds, using a nutrient enrichment (nitrogen and phosphorus) experiment embedded within a naturally occurring, long-term gradient of phosphorus availability within Florida Bay (USA). We measured organic carbon stocks in soils and above- and belowground seagrass biomass after 17 months of experimental nutrient addition. At the nutrient-limited sites, phosphorus addition increased the carbon stock in aboveground seagrass biomass by more than 300 %; belowground seagrass carbon stock increased by 50–100 %. Soil carbon content slightly decreased ( ∼ 10 %) in response to phosphorus addition. There was a strong but non-linear relationship between soil carbon and Thalassia testudinum leaf nitrogen : phosphorus (N : P) or belowground seagrass carbon stock. When seagrass leaf N : P exceeded an approximate threshold of 75 : 1, or when belowground seagrass carbon stock was less than 100 g m−2, there was less than 3 % organic carbon in the sediment. Despite the marked difference in soil carbon between phosphorus-limited and phosphorus-replete areas of Florida Bay, all areas of the bay had relatively high soil carbon stocks near or above the global median of 1.8 % organic carbon. The relatively high carbon content in the soils indicates that seagrass beds have extremely high carbon storage potential, even in nutrient-limited areas with low biomass or productivity.

Balanced exploitation and coexistence of interacting, size-structured, fish species

Abstract: This paper examines some effects of exploitation on a simple ecosystem containing two interacting fish species, with life histories similar to mackerel (Scomber scombrus) and cod (Gadus morhua), using a dynamic, size-spectrum model. Such models internalize body growth and mortality from predation, allowing bookkeeping of biomass at a detailed level of individual predation and growth and enabling scaling up to the mass balance of the ecosystem. Exploitation set independently for each species with knife-edge, size-at-entry fishing can lead to collapse of cod. Exploitation to achieve a fixed ratio of yield to productivity across species can also lead to collapse of cod. However, harvesting balanced to the overall productivity of species in the exploited ecosystem exerts a strong force countering such collapse. If balancing across species is applied to a fishery with knife-edge selection, size distributions are truncated, changing the structure of the system and reducing its resilience to perturbations. If balancing is applied on the basis of productivity at each body size as well as across species, there is less disruption to size-structure, resilience is increased, and substantially greater biomass yields are possible. We note an identity between the body size at which productivity is maximized and the age at which cohort biomass is maximized. In our numerical results based on detailed bookkeeping of biomass, cohort biomass reaches its maximum at body masses

Diversity and productivity of photosynthetic picoeukaryotes in biogeochemically distinct regions of the South East Pacific Ocean

Abstract: Picophytoplankton, including photosynthetic picoeukaryotes (PPE) and unicellular cyanobacteria, are important contributors to plankton biomass and primary productivity. In this study, phytoplankton composition and rates of carbon fixation were examined across a large trophic gradient in the South East Pacific Ocean (SEP) using a suite of approaches: photosynthetic pigments, rates of 14C-primary productivity, and phylogenetic analyses of partial 18S rRNA genes PCR amplified and sequenced from flow cytometrically sorted cells. While phytoplankton >10 μm (diatoms and dinoflagellates) were prevalent in the upwelling region off the Chilean coast, picophytoplankton consistently accounted for 55–92% of the total chlorophyll a inventories and >60% of 14C-primary productivity throughout the sampling region. Estimates of rates of 14C-primary productivity derived from flow cytometric sorting of radiolabeled cells revealed that the contributions of PPE and Prochlorococcus to euphotic zone depth-integrated picoplankton productivity were nearly equivalent (ranging 36–57%) along the transect, with PPE comprising a larger share of picoplankton productivity than cyanobacteria in the well-lit (>15% surface irradiance) region compared with in the lower regions (1–7% surface irradiance) of the euphotic zone. 18S rRNA gene sequence analyses revealed the taxonomic identities of PPE; e.g., Mamiellophyceae (Ostreococcus) were the dominant PPE in the upwelling-influenced waters, while members of the Chrysophyceae, Prymnesiophyceae, Pelagophyceae, and Prasinophyceae Clades VII and IX flourished in the oligotrophic South Pacific Subtropical Gyre. Our results suggest that, despite low numerical abundance in comparison to cyanobacteria, diverse members of PPE are significant contributors to carbon cycling across biogeochemically distinct regions of the SEP.

Phytoplankton and Primary Production

Abstract: The northern part of the East Sea (Japan Sea) is dominated by the cold waters of the Liman Current, whereas in the southern part, the dominant current is the Tsushima Warm Current. Together with these major current systems, subpolar fronts and mesoscale eddies that form in the region exert a strong influence on the phytoplankton ecosystem in the East Sea. These conspicuous physical forcings are manifested as often clearly discernible features in temporal and spatial distributions, composition, abundance, biomass and production of phytoplankton. In short, diatoms and dinoflagellates are found to be the most diverse phytoplankton groups and are occasionally responsible for blooms in coastal waters. Diatoms are also abundant in frontal areas and in the rings of warm core eddies. Picophytoplankton groups are also found to be important phytoplankton in the East Sea, especially in warm seasons, and photosynthetic picoeukaryotes and Synechococcus show distinct seasonal and vertical distribution patterns. Recent field measurements indicated that the spatial distribution of primary productivity in the Ulleung Basin (UB) of the East Sea ranged from 172 to 358 g C m−2 year−1. This range of primary productivity is relatively higher than in other regions in the East Sea. The East Sea is a body of dynamic “non-oceanic” water with its own particular oceanic characteristics. Coastal upwelling and mesoscale eddies over a wide range of horizontal scales contribute to the high primary productivity in the UB. This vibrant primary production provides the foundation for a biological “hot” spot and strong support for an energetic biological pump cycle in the East Sea. Despite much progress in expanding knowledge of phytoplankton ecology in the East Sea, more studies on diversity, productivity, niche, and physiological adaptation to dynamic environments should be conducted to better understand ecological roles of phytoplankton in changing oceans.

Variation in biochemical composition of Saccharina latissima and Laminaria digitata along an estuarine salinity gradient in inner Danish waters

Abstract: In European kelp cultivation, knowledge on the spatial variation in biomass productivity and quality needs to be established. The present study provides a detailed overview of the biochemical composition and biomass production potential of Saccharina latissima and Laminaria digitata along a salinity gradient (16–31 PSU) in inner Danish waters. We discuss the results in a cultivation perspective, and evaluate the potential use of Laminariales as an energy feedstock, a feed additive and a bioremediation tool for mitigating eutrophication. We found the highest biomass production potential, the highest protein content (7.5% of dry matter), and the highest capacity for bio-remediation of nitrogen (1.88% N of dry matter) at high salinities, as opposed to the highest concentrations of fermentable sugars (90% of dry matter) and pigments at low salinities. Thus, areas suitable for high biomass production are not necessarily optimal for producing a specific biomass quality such as high carbohydrate concentration for bioenergy conversion, and this challenges the cultivation practice. Furthermore, concentrations of arsenic in the biomass were generally higher (up to 88 ppm) than allowed for animal diet (40 ppm) and could therefore impose challenges for utilizing S. latissima and L. digitata as animal feed additives.

Adding value to the treatment of municipal wastewater through the intensive production of freshwater macroalgae

Abstract: Municipal wastewater treatment plants discharge large quantities of treated water that, in many regions, is not productively used and is instead released directly into the environment. In this study we examine the use of freshwater macroalgae as an in-line tertiary treatment process for existing municipal treatment plants. We examine the suitability of using the treated discharge water from a 29,000 m3.day− 1 municipal wastewater treatment plant as the sole source of water and nutrients for the intensive cultivation of the freshwater macroalga Oedogonium intermedium. A monoculture of algae was initially cultivated for a 3 month period in which water quality and biomass productivity were quantified and the composition of the biomass characterized. These cultures were then maintained for a further 9 months to determine the average monthly biomass productivity, and seasonal variation, over a 12 month period. The cultivation of Oedogonium significantly improved the quality of the discharged water with a 36% reduction in total nitrogen and a 65% reduction in total phosphorous. The average monthly biomass productivity of Oedogonium ranged between a minimum of 8.9 g DW·m− 2·day− 1 in June (austral winter – dry season) and a maximum of 15.8 g DW·m− 2·day− 1 in January (austral summer – wet season) with an average annual rate of 12.5 g DW·m− 2·day− 1. The biomass produced was of a high quality with a total protein content of 23 g·100 g− 1 and a total lipid content of 10 g·100 g− 1. Both the protein (10 g·100 g− 1 of essential amino acids) and lipid (4.5 g·100 g− 1 of polyunsaturated fatty acids) provide product opportunities for animal nutrition. This study demonstrates that the production of algae integrated with the operation of conventional wastewater treatment can complement and add value to existing processes by recovering residual nutrients and metals and, at the same time, create a high-quality biomass resource for product development.

The dry season intensity as a key driver of NPP trends

Abstract: We analyze the impacts of changing dry season length and intensity on vegetation productivity and biomass. Our results show a wetness asymmetry in dry ecosystems, with dry seasons becoming drier and wet seasons becoming wetter, likely caused by climate change. The increasingly intense dry seasons were consistently correlated with a decreasing trend in net primary productivity (NPP) and biomass from different products and could potentially mean a reduction of 10–13% in NPP by 2100. We found that annual NPP in dry ecosystems is particularly sensitive to the intensity of the dry season, whereas an increase in precipitation during the wet season has a smaller effect. We conclude that changes in water availability over the dry season affect vegetation throughout the whole year, driving changes in regional NPP. Moreover, these results suggest that usage of seasonal water fluxes is necessary to improve our understanding of the link between water availability and the land carbon cycle.

Arctic foxes as ecosystem engineers: increased soil nutrients lead to increased plant productivity on fox dens.

Abstract: Top predators can provide fundamental ecosystem services such as nutrient cycling, and their impact can be even greater in environments with low nutrients and productivity, such as Arctic tundra. We estimated the effects of Arctic fox (Vulpes lagopus) denning on soil nutrient dynamics and vegetation production near Churchill, Manitoba in June and August 2014. Soils from fox dens contained higher nutrient levels in June (71% more inorganic nitrogen, 1195% more extractable phosphorous) and in August (242% more inorganic nitrogen, 191% more extractable phosphorous) than adjacent control sites. Inorganic nitrogen levels decreased from June to August on both dens and controls, whereas extractable phosphorous increased. Pup production the previous year, which should enhance nutrient deposition (from urine, feces, and decomposing prey), did not affect soil nutrient concentrations, suggesting the impact of Arctic foxes persists >1 year. Dens supported 2.8 times greater vegetation biomass in August, but δ15N values in sea lyme grass (Leymus mollis) were unaffected by denning. By concentrating nutrients on dens Arctic foxes enhance nutrient cycling as an ecosystem service and thus engineer Arctic ecosystems on local scales. The enhanced productivity in patches on the landscape could subsequently affect plant diversity and the dispersion of herbivores on the tundra.

A carbon budget for the Amundsen Sea Polynya, Antarctica: Estimating net community production and export in a highly productive polar ecosystem

Abstract: Polynyas, or recurring areas of seasonally open water surrounded by sea ice, are foci for energy and material transfer between the atmosphere and the polar ocean. They are also climate sensitive, with both sea ice extent and glacial melt influencing their productivity. The Amundsen Sea Polynya (ASP) is the greenest polynya in the Southern Ocean, with summertime chlorophyll a concentrations exceeding 20 μg L−1. During the Amundsen Sea Polynya International Research Expedition (ASPIRE) in austral summer 2010–11, we aimed to determine the fate of this high algal productivity. We collected water column profiles for total dissolved inorganic carbon (DIC) and nutrients, particulate and dissolved organic matter, chlorophyll a, mesozooplankton, and microbial biomass to make a carbon budget for this ecosystem. We also measured primary and secondary production, community respiration rates, vertical particle flux and fecal pellet production and grazing. With observations arranged along a gradient of increasing integrated dissolved inorganic nitrogen drawdown (ΔDIN; 0.027–0.74 mol N m−2), changes in DIC in the upper water column (ranging from 0.2 to 4.7 mol C m−2) and gas exchange (0–1.7 mol C m−2) were combined to estimate early season net community production (sNCP; 0.2–5.9 mol C m−2) and then compared to organic matter inventories to estimate export. From a phytoplankton bloom dominated by Phaeocystis antarctica, a high fraction (up to ∼60%) of sNCP was exported to sub-euphotic depths. Microbial respiration remineralized much of this export in the mid waters. Comparisons to short-term (2–3 days) drifting traps and a year-long moored sediment trap capturing the downward flux confirmed that a relatively high fraction (3–6%) of the export from ∼100 m made it through the mid waters to depth. We discuss the climate-sensitive nature of these carbon fluxes, in light of the changing sea ice cover and melting ice sheets in the region.

Intertidal resource use over millennia enhances forest productivity

Abstract: Human occupation is usually associated with degraded landscapes but 13,000 years of repeated occupation by British Columbia’s coastal First Nations has had the opposite effect, enhancing temperate rainforest productivity. This is particularly the case over the last 6,000 years when intensified intertidal shellfish usage resulted in the accumulation of substantial shell middens. We show that soils at habitation sites are higher in calcium and phosphorous. Both of these are limiting factors in coastal temperate rainforests. Western redcedar (Thuja plicata) trees growing on the middens were found to be taller, have higher wood calcium, greater radial growth and exhibit less top die-back. Coastal British Columbia is the first known example of long-term intertidal resource use enhancing forest productivity and we expect this pattern to occur at archaeological sites along coastlines globally. Human settlements are often associated with degraded landscapes. Trant and colleagues now show that near-shore settlements in British Columbia have locally enhanced forest productivity over millennia by enriching soils with calcium and phosphorous derived from shellfish remnants.

Transient effects of an invasive kelp on the community structure and primary productivity of an intertidal assemblage

Abstract: Invasive species can have significant impacts on the diversity and productivity of recipient ecological communities. The kelp Undaria pinnatifida (Harvey) Suringar is one of the world's most successful invasive species but, although itspurported impactsarestrong, thereis littleempirical evidence that itdisplacesnative species.Furthermore, as thisspeciesnaturalisesinlocalcommunities,itspotentialeffectsoncommunitydynamicshavenotbeenwelltested.Here, we test the ecological impacts of Undaria in intertidal communities in southern New Zealand using a combination of surveys, a 2.5-year press-removal experiment and in situ measures of net primary production to gauge its impact on community structure and productivity. Undaria had transient effects on the composition of communities, affecting two seasonally abundant species in 1 year, but these impacts did not persist into the following year. Overall, there were only small effects of Undaria removal on diversity and abundance of native algae and invertebrates at two sites. However, the presence of Undaria more than doubled net primary production of recipient communities during its annual peak abundance when it increased biomass by 606 g DW m � 2 . We conclude that the invasion of Undaria represents an additionalandsubstantialcarbonsubsidytocoastalecosystemswithpotentiallypositiveeffectsonnearshoreproductivity. Additional keywords: intertidal biodiversity, net primary production, NPP, Undaria pinnatifida.

Precipitation Pattern Determines the Inter-annual Variation of Herbaceous Layer and Carbon Fluxes in a Phreatophyte-Dominated Desert Ecosystem

Abstract: Arid and semi-arid ecosystems dominated by shrubby species are an important component in the global carbon cycle but are largely under-represented in studies of the effect of climate change on carbon flux. This study synthesizes data from long-term eddy covariance measurements and experiments to assess how changes in ecosystem composition, driven by precipitation patterns, affect inter-annual variability of carbon flux and their components in a halophyte desert community dominated by deep-rooted shrubs (phreatophytes, which depend on groundwater as their primary water source). Our results demonstrated that the carbon balance of this community responded strongly to precipitation variations. Both pre-growing season precipitation and growing season precipitation frequency significantly affected inter-annual variations in ecosystem carbon flux. Heavy pre-growing season precipitation (November–April, mostly as snow) increased annual net ecosystem carbon exchange, by facilitating the growth and carbon assimilation of shallow-rooted annual plants, which used spring and summer precipitation to increase community productivity. Sufficient pre-growing season precipitation led to more germination and growth of shallow-rooted annual plants. When followed by high-frequency growing season precipitation, community productivity of this desert ecosystem was lifted to the level of grassland or forest ecosystems. The long-term observations and experimental results confirmed that precipitation patterns and the herbaceous component were dominant drivers of the carbon dynamics in this phreatophyte-dominated desert ecosystem. This study illustrates the importance of inter-annual variations in climate and ecosystem composition for the carbon flux in arid and semi-arid ecosystems. It also highlights the important effect of changing frequency and seasonal pattern of precipitation on the regional and global carbon cycle in the coming decades.

Greater soil carbon stocks and faster turnover rates with increasing agricultural productivity

Abstract: . Devising agricultural management schemes that enhance food security and soil carbon levels is a high priority for many nations. However, the coupling between agricultural productivity, soil carbon stocks and organic matter turnover rates is still unclear. Archived soil samples from four decades of a long-term crop rotation trial were analyzed for soil organic matter (SOM) cycling-relevant properties: C and N content, bulk composition by nuclear magnetic resonance (NMR) spectroscopy, amino sugar content, short-term C bioavailability assays, and long-term C turnover rates by modeling the incorporation of the bomb spike in atmospheric 14C into the soil. After > 40 years under consistent management, topsoil carbon stocks ranged from 14 to 33 Mg C ha−1 and were linearly related to the mean productivity of each treatment. Measurements of SOM composition demonstrated increasing amounts of plant- and microbially derived SOM along the productivity gradient. Under two modeling scenarios, radiocarbon data indicated overall SOM turnover time decreased from 40 to 13 years with increasing productivity – twice the rate of decline predicted from simple steady-state models or static three-pool decay rates of measured C pool distributions. Similarly, the half-life of synthetic root exudates decreased from 30.4 to 21.5 h with increasing productivity, indicating accelerated microbial activity. These findings suggest that there is a direct feedback between accelerated biological activity, carbon cycling rates and rates of carbon stabilization with important implications for how SOM dynamics are represented in models.

Trophic ecology of groundwater species reveals specialization in a low‐productivity environment

Abstract: 1. Identifying feeding strategies at lower bounds of habitat productivity is fundamental to understand the relationship between energy availability and trophic specialization. Low pro- ductivity is expected to severely constrain trophic specialization because organisms may no longer be able to fulfil their energy requirements by feeding on a reduced set of resources. However, species living in low-productivity habitats often exhibit particular biological traits such as low metabolic rates and high food-finding abilities, which may release constraints on trophic specialization. 2. In the present study, we used carbon (C) and nitrogen (N) stable isotopes to measure the degree of trophic specialization in two species of isopods (Proasellus valdensis and Proasellus cavaticus) living in groundwater, one of the most energy-limited environments on earth. Fun- damental specialization was obtained from a 13C/15N-labelling experiment in the laboratory: we measured separately the carbon and nitrogen assimilation rates of the two species across the three food sources encountered in their natural cave habitats (fine and coarse particulate organic matter and sedimentary biofilm). Then, for each species, we tested for variation in diet composition among individuals and populations by quantifying the relative contribution of the three food sources to the diet of multiple individuals within 5 cave populations. 3. The labelling experiment showed that both species assimilated about 10 times more carbon and at least 4 times more nitrogen from sedimentary biofilm than from both forms of particu- late organic matter. Field samplings showed that sedimentary biofilm made up, on average, 83% of the diet of isopods. Moreover, we found almost no variation in diet among individuals of a cave population as well as among cave populations within species. 4. This study provides the first evidence of a high degree of trophic specialization in a low- productivity cave environment. Both species exhibited a strong fundamental specialization on sedimentary biofilm and most probably fed selectively on this food source in their natural environment. Our findings challenge the prediction that species would adopt generalist feeding strategies at lower bounds of habitat productivity.

The contribution of trees and grasses to productivity of an Australian tropical savanna

Abstract: . Savanna ecosystems cover 20 % of the global land surface and account for 25 % of global terrestrial carbon uptake. They support one fifth of the world's human population and are one of the most important ecosystems on our planet. Savanna productivity is a product of the interplay between trees and grass that co-dominate savanna landscapes and are maintained through interactions with climate and disturbance (fire, land use change, herbivory). In this study, we evaluate the temporally dynamic partitioning of overstory and understory carbon dioxide fluxes in Australian tropical savanna using overstory and understory eddy covariance measurements. Over a 2-year period (September 2012 to October 2014) the overall net ecosystem productivity (NEP) of the savanna was 506.2 (±22 SE) g C m−2 yr−1. The total gross primary productivity (GPP) was 2267.1 (±80 SE) g C m−2 yr−1, of which the understory contributed 32 %. The understory contribution was strongly seasonal, with most GPP occurring in the wet season (40 % of total ecosystem in the wet season and 18 % in the dry). This study is the first to elucidate the temporal dynamics of savanna understory and overstory carbon flux components explicitly using observational information. Understanding grass productivity is crucial for evaluating fuel loads, as is tree productivity for quantifying the tree carbon sink. This information will contribute to a significant refinement of the representation of savannas in models, as well as improved understanding of relative tree-grass productivity and competition for resources.

Climate seasonality limits carbon assimilation and storage in tropical forests

Abstract: The seasonal climate drivers of the carbon cycle in tropical forests remain poorly known, although these forests account for more carbon assimilation and storage than any other terrestrial ecosystem. Based on a unique combination of seasonal pan-tropical data sets from 89 experimental sites (68 include aboveground wood productivity measurements and 35 litter productivity measurements), their associate canopy photosynthetic capacity (enhanced vegetation index, EVI) and climate, we ask how carbon assimilation and aboveground allocation are related to climate seasonality in tropical forests and how they interact in the seasonal carbon cycle. We found that canopy photosynthetic capacity seasonality responds positively to precipitation when rainfall is < 2000 mm.yr−1 (water-limited forests) and to radiation otherwise (light-limited forests); on the other hand, independent of climate limitations, wood productivity and litterfall are driven by seasonal variation in precipitation and evapotranspiration respectively. Consequently, light-limited forests present an asynchronism between canopy photosynthetic capacity and wood productivity. Precipitation first-order control indicates an overall decrease in tropical forest productivity in a drier climate. (Resume d'auteur)

Data from: Conservative species drive biomass productivity in tropical dry forests

Abstract: Data of above-ground biomass dynamics over a five-year period in eight seasonal tropical dry forests in Minas Gerais state, Southeastern Brazil. In each forest, 25 permanent sample plots (20 × 20 m) were established totaling one ha per site. Biomass dynamics, structural, floristic, functional and soil variables used in the biomass models are given. AGBi = stand biomass, AGBnet = net biomass change, AGBsurv = biomass growth of surviving trees, AGBmort = biomass mortality, AGBrecr = biomass recruitment, NI = tree-density (ha-1), CWM = community weighed mean, WD = wood density, Dmax = maximum stem diameter, SLA = specific leaf area, Dec = deciduousness, S = rarefied species richness, H’ = Shannon-Wiener index, J’ = Pielou’s index, Fric = functional richness, Feve = functional evenness, Fdiv = functional divergence, P = phosphorous, K = potassium, Ca = calcium, Mg = magnesium, Al = aluminum, CEC = cation exchange capacity.

Belowground plant biomass allocation in tundra ecosystems and its relationship with temperature

Abstract: Climatewarming is known to increase the aboveground productivity of tundra ecosystems. Recently, belowground biomass is receiving more attention, but the effects of climate warming on belowground productivity remain unclear. Enhanced understanding of the belowground component of the tundra is important in the context of climate warming, sincemost carbon is sequestered belowground in these ecosystems. In this study we synthesized published tundra belowground biomass data from36 field studies spanning amean annual temperature (MAT) gradient from −20 °C to 0 °C across the tundra biome, and determined the relationships between different plant biomass pools andMAT. Our results show that the plant community biomass–temperature relationships are significantly different between above and belowground. Aboveground biomass clearly increased withMAT, whereas total belowground biomass and fine root biomass did not show a significant increase over the broadMATgradient. Our results suggest that biomass allocation of tundra vegetation shifts towards aboveground in warmer conditions,which could impact on the carbon cycling in tundra ecosystems through altered litter input and distribution in the soil, aswell as possible changes in root turnover.Climatewarming is known to increase the aboveground productivity of tundra ecosystems. Recently, belowground biomass is receiving more attention, but the effects of climate warming on belowground productivity remain unclear. Enhanced understanding of the belowground component of the tundra is important in the context of climate warming, sincemost carbon is sequestered belowground in these ecosystems. In this study we synthesized published tundra belowground biomass data from36 field studies spanning amean annual temperature (MAT) gradient from −20 °C to 0 °C across the tundra biome, and determined the relationships between different plant biomass pools andMAT. Our results show that the plant community biomass–temperature relationships are significantly different between above and belowground. Aboveground biomass clearly increased withMAT, whereas total belowground biomass and fine root biomass did not show a significant increase over the broadMATgradient. Our results suggest that biomass allocation of tundra vegetation shifts towards aboveground in warmer conditions,which could impact on the carbon cycling in tundra ecosystems through altered litter input and distribution in the soil, aswell as possible changes in root turnover.

Inter-annual variability in water table depth controls net ecosystem carbon dioxide exchange in a boreal bog

Abstract: The net ecosystem carbon dioxide (CO2) exchange (NEE) between boreal bogs and the atmosphere and its environmental drivers remains understudied despite the large carbon store of these northern ecosystems. We present NEE measurements using the eddy covariance technique in a boreal ombrotrophic bog over five growing seasons and four winters. Inter-annual variability in CO2 uptake was most pronounced in June–September (−4 to −122 g CO2–C m−2), less in March–May (−1 to −21 g CO2–C m−2) and very small in October–November (−2 to −4 g CO2–C m−2). Variability in NEE between years was linked primarily to changes in water table depth (WTD). Strong and significant relationships (r2 > 0.89, p ≤ 0.05) were found between summer (June–September) maximum photosynthetic rate (A max), net ecosystem productivity (NEP), gross ecosystem productivity and WTD. Adding air temperature through multiple regression analysis further increased correlation between summer A max, NEP, and WTD (r2 = 0.96, p = 0.05). In contrast to previous studies examining controls on peatland CO2 exchange, no relationships were found between productivity or cumulative exchange and early season temperature, timing of the snowmelt or growing season length.

The effect of fire intensity, nutrients, soil microbes, and spatial distance on grassland productivity

Abstract: Variation in fire intensity within an ecosystem is likely to moderate fire effects on plant and soil properties. We tested the effect of fire intensity on grassland biomass, soil microbial biomass, and soil nutrients. Additional tests determined plant-microbe, plant-nutrient, and microbe-nutrient associations. A replicated field experiment produced a fire intensity gradient. We measured plant and soil microbial biomasses at peak plant productivity the first growing season after fire. We concurrently measured flux in 11 soil nutrients and soil moisture. Fire intensity positively affected soil nitrogen, phosphorus (P), and zinc but did not appreciably affect plant biomass, microbial biomass, and other soil nutrients. Plant biomass was seemingly (co-)limited by boron, manganese, and P. Microbial biomass was (co-)limited mainly by P and also iron. In the Northern Great Plains, plant and soil microbial biomasses were limited mainly by P and some micronutrients. Fire intensity affected soil nutrients, however, pulsed P (due to fire) did not result in appreciable fire intensity effects on plant and microbial biomasses. Variable responses in plant productivity to fire are common and indicate the complexity of factors that regulate plant production after fire.

Optimal Environmental Conditions and Anomalous Ecosystem Responses: Constraining Bottom-up Controls of Phytoplankton Biomass in the California Current System

Abstract: In Eastern Boundary Current systems, wind-driven upwelling drives nutrient-rich water to the ocean surface, making these regions among the most productive on Earth. Regulation of productivity by changing wind and/or nutrient conditions can dramatically impact ecosystem functioning, though the mechanisms are not well understood beyond broad-scale relationships. Here, we explore bottom-up controls during the California Current System (CCS) upwelling season by quantifying the dependence of phytoplankton biomass (as indicated by satellite chlorophyll estimates) on two key environmental parameters: subsurface nitrate concentration and surface wind stress. In general, moderate winds and high nitrate concentrations yield maximal biomass near shore, while offshore biomass is positively correlated with subsurface nitrate concentration. However, due to nonlinear interactions between the influences of wind and nitrate, bottom-up control of phytoplankton cannot be described by either one alone, nor by a combined metric such as nitrate flux. We quantify optimal environmental conditions for phytoplankton, defined as the wind/nitrate space that maximizes chlorophyll concentration and present a framework for evaluating ecosystem change relative to environmental drivers. The utility of this framework is demonstrated by (i) elucidating anomalous CCS responses in 1998–1999, 2002 and 2005 and (ii) providing a basis for assessing potential biological impacts of projected climate change.