Fraction Of Net Primary Production Research Articles

Changes in productivity partitioning induced by precipitation extremes increase inaccuracy of grassland carbon estimation.

The fraction of net primary productivity (NPP) allocated to belowground organs (fBNPP) in grasslands is a critical parameter in global carbon cycle models; moreover, understanding the effect of precipitation changes on this parameter is vital to accurately estimating carbon sequestration in grassland ecosystems. However, how fBNPP responds to temporal precipitation changes along a gradient from extreme drought to extreme wetness, remains unclear, mainly due to the lack of long-term data of belowground net primary productivity (BNPP) and the fact that most precipitation experiments did not have a gradient from extreme drought to extreme wetness. Here, by conducting both a precipitation gradient experiment (100-500 mm) and a long-term observational study (34 years) in the Inner Mongolia grassland, we showed that fBNPP decreased linearly along the precipitation gradient from extreme drought to extreme wetness due to stronger responses in aboveground NPP to drought and wet conditions than those of BNPP. Our further meta-analysis in grasslands worldwide also indicated that fBNPP increased when precipitation decreased, and the vice versa. Such a consistent pattern of fBNPP response suggests that plants increase the belowground allocation with decreasing precipitation, while increase the aboveground allocation with increasing precipitation. Thus, the linearly decreasing response pattern in fBNPP should be incorporated into models that forecast carbon sequestration in grassland ecosystems; failure to do so will lead to underestimation of the carbon stock in drought years and overestimation of the carbon stock in wet years in grasslands.

The Seasonal Flux and Fate of Dissolved Organic Carbon Through Bacterioplankton in the Western North Atlantic.

The oceans teem with heterotrophic bacterioplankton that play an appreciable role in the uptake of dissolved organic carbon (DOC) derived from phytoplankton net primary production (NPP). As such, bacterioplankton carbon demand (BCD), or gross heterotrophic production, represents a major carbon pathway that influences the seasonal accumulation of DOC in the surface ocean and, subsequently, the potential vertical or horizontal export of seasonally accumulated DOC. Here, we examine the contributions of bacterioplankton and DOM to ecological and biogeochemical carbon flow pathways, including those of the microbial loop and the biological carbon pump, in the Western North Atlantic Ocean (∼39–54°N along ∼40°W) over a composite annual phytoplankton bloom cycle. Combining field observations with data collected from corresponding DOC remineralization experiments, we estimate the efficiency at which bacterioplankton utilize DOC, demonstrate seasonality in the fraction of NPP that supports BCD, and provide evidence for shifts in the bioavailability and persistence of the seasonally accumulated DOC. Our results indicate that while the portion of DOC flux through bacterioplankton relative to NPP increased as seasons transitioned from high to low productivity, there was a fraction of the DOM production that accumulated and persisted. This persistent DOM is potentially an important pool of organic carbon available for export to the deep ocean via convective mixing, thus representing an important export term of the biological carbon pump.

Biogenic sinking particle fluxes and sediment trap collection efficiency at Ocean Station Papa

Comprehensive field observations characterizing the biological carbon pump (BCP) provide the foundation needed to constrain mechanistic models of downward particulate organic carbon (POC) flux in the ocean. Sediment traps were deployed three times during the EXport Processes in the Ocean from RemoTe Sensing campaign at Ocean Station Papa in August–September 2018. We propose a new method to correct sediment trap sample contamination by zooplankton “swimmers.” We consider the advantages of polyacrylamide gel collectors to constrain swimmer influence and estimate the magnitude of possible trap biases. Measured sediment trap fluxes of thorium-234 are compared to water column measurements to assess trap performance and estimate the possible magnitude of fluxes by vertically migrating zooplankton that bypassed traps. We found generally low fluxes of sinking POC (1.38 ± 0.77 mmol C m–2 d–1 at 100 m, n = 9) that included high and variable contributions by rare, large particles. Sinking particle sizes generally decreased between 100 and 335 m. Measured 234Th fluxes were smaller than water column 234Th fluxes by a factor of approximately 3. Much of this difference was consistent with trap undersampling of both small (<32 μm) and rare, large particles (>1 mm) and with zooplankton active migrant fluxes. The fraction of net primary production exported below the euphotic zone (0.1% light level; Ez-ratio = 0.10 ± 0.06; ratio uncertainties are propagated from measurements with n = 7–9) was consistent with prior, late summer studies at Station P, as was the fraction of material exported to 100 m below the base of the euphotic zone (T100, 0.55 ± 0.35). While both the Ez-ratio and T100 parameters varied weekly, their product, which we interpret as overall BCP efficiency, was remarkably stable (0.055 ± 0.010), suggesting a tight coupling between production and recycling at Station P.

Potential yield simulated by global gridded crop models: using a process-based emulator to explain their differences

Abstract. How global gridded crop models (GGCMs) differ in their simulation of potential yield and reasons for those differences have never been assessed. The GGCM Intercomparison (GGCMI) offers a good framework for this assessment. Here, we built an emulator (called SMM for simple mechanistic model) of GGCMs based on generic and simplified formalism. The SMM equations describe crop phenology by a sum of growing degree days, canopy radiation absorption by the Beer–Lambert law, and its conversion into aboveground biomass by a radiation use efficiency (RUE). We fitted the parameters of this emulator against gridded aboveground maize biomass at the end of the growing season simulated by eight different GGCMs in a given year (2000). Our assumption is that the simple set of equations of SMM, after calibration, could reproduce the response of most GGCMs so that differences between GGCMs can be attributed to the parameters related to processes captured by the emulator. Despite huge differences between GGCMs, we show that if we fit both a parameter describing the thermal requirement for leaf emergence by adjusting its value to each grid-point in space, as done by GGCM modellers following the GGCMI protocol, and a GGCM-dependent globally uniform RUE, then the simple set of equations of the SMM emulator is sufficient to reproduce the spatial distribution of the original aboveground biomass simulated by most GGCMs. The grain filling is simulated in SMM by considering a fixed-in-time fraction of net primary productivity allocated to the grains (frac) once a threshold in leaves number (nthresh) is reached. Once calibrated, these two parameters allow for the capture of the relationship between potential yield and final aboveground biomass of each GGCM. It is particularly important as the divergence among GGCMs is larger for yield than for aboveground biomass. Thus, we showed that the divergence between GGCMs can be summarized by the differences in a few parameters. Our simple but mechanistic model could also be an interesting tool to test new developments in order to improve the simulation of potential yield at the global scale.

Impacts of human appropriation of net primary production on ecosystem regulating services in Tibet

Humans are transforming the ecology of Earth and have reshaped the form and processes of ecosystems across the terrestrial biosphere. Net primary production (NPP) is a key ecosystem process, and humans interfere with this process by appropriating a fraction of NPP. This fraction is defined as human appropriation of net primary production (HANPP). HANPP could potentially be an important indicator of ecological sustainability. We provide a framework that elucidates the relationship between HANPP and ecosystem regulating services by revealing hypothetical trends of regulating services impacted by HANPP. We performed an empirical study in the Tibet Autonomous Region of China (hereafter, Tibet) by assessing the impacts of HANPP on an important regulating service, the wind erosion prevention (WEP) service. We estimated HANPP in percent of potential NPP (hereafter HANPP%) and the WEP service in Tibet from 1989 to 2015, and interpolated wind speed, annual precipitation, and mean annual temperature as climatic factors. The results showed that, at the county level, HANPP% became the dominant factor in south-central Tibet after the year 1998. Temporal change and space-for-time substitution results suggested a HANPP turning point of approximately 27% of the potential NPP with regard to the WEP service. On the basis of these results, we suggested a hypothesis that HANPP had two thresholds with regard to its impact on regulating services, the sustainability threshold and the upper limit threshold. Before HANPP reaches the sustainability threshold, ecosystems are considered to be sustainable; while after HANPP reaches the upper limit threshold, ecosystems will be at risk of collapse. The findings of this study provide an insight into how human activities affect ecosystems, and also provide an operational framework that enables the use of HANPP as a sustainability indicator.

Copepods Boost the Production but Reduce the Carbon Export Efficiency by Diatoms

The fraction of net primary production that is exported from the euphotic zone as sinking particulate organic carbon (POC) varies notably through time and from region to region. Phytoplankton containing biominerals, such as silicified diatoms have long been associated with high export fluxes. However, recent reviews point out that the magnitude of export is not controlled by diatoms alone, but determined by the whole plankton community structure. The combined effect of phytoplankton community composition and zooplankton abundance on export flux dynamics, were explored using a set of 12 large outdoor mesocosms. All mesocosms received a daily addition of minor amounts of nitrate and phosphate, while only 6 mesocosms received silicic acid (dSi). This resulted in a dominance of diatoms and dinoflagellate in the +Si mesocosms and a dominance of dinoflagellate in the –Si mesocosms. Simultaneously, half of the mesocosms had decreased mesozooplankton populations whereas the other half were supplemented with additional zooplankton. In all mesocosms, POC fluxes were positively correlated to Si/C ratios measured in the surface community and additions of dSi globally increased the export fluxes in all treatments highlighting the role of diatoms in C export. The presence of additional copepods resulted in higher standing stocks of POC, most probably through trophic cascades. However it only resulted in higher export fluxes for the –Si mesocosms. In the +Si with copepod addition (+Si +Cops) export was dominated by large diatoms with higher Si/C ratios in sinking material than in standing stocks. During non-bloom situations, the grazing activity of copepods decrease the export efficiency in diatom dominated systems by changing the structure of the phytoplankton community and/or preventing their aggregation. However, in flagellate-dominated system, the copepods increased phytoplankton growth, aggregation and fecal pellet production, with overall higher net export not always visible in term of export efficiency.

Sustaining the sequestration efficiency of the European forest sector

The optimal forest management strategies for mitigating climate change are hotly debated during political negotiations , because afforestation and forest management can increase atmospheric CO 2 removal, and the wood produced can provide a substitute for fossil fuel. Studies quantifying the carbon balance of the forest sector apply a wide variety of management and wood-use scenarios. Some model studies include future climate change effects on forest growth, but others ignore them. Here, a conceptual empirical model of sequestration efficiency, the fraction of net primary production stored in the biosphere and anthroposhere, simulates European forest carbon pools and fluxes. The sensitivity of the sequestration efficiency of European forests was quantified by varying model parameters along the forest growth and wood transformation chain: environment and climate change, harvest intensity, rotation length, fraction of harvest residues left on site and substitution efficiency. Irrespective of the evolution of the sink, the forest sector as a whole remains a net carbon absorber in 99% of the simulations at a time horizon of 100 years, even if in 25% of the simulations the forests themselves become sources. However, if the goal is to enhance the current sequestration efficiency to mitigate emissions, only in 25% of the simulations the sink efficiency was found to be enhanced. If the current sink were to reverse to a source, no management action or change in wood use would result in an increase in the current forest sequestration efficiency. In all other cases, increasing harvest levels would lead to an increase in forest sector carbon emissions, highlighting the pivotal role of the baseline used to set the emission reduction targets. Our results show that the uncertainty on the response of European forest to climate change undermines the quest for a carbon-optimal management strategy. The uncertainty in whether climate change will maintain the current forest sink or turn it into a carbon source is largely overlooked in the debate over the best forest management strategy to reduce the growth of atmospheric CO 2 concentration, yet it is large enough to change the merit order of different alternatives .

Seasonality constraints to livestock grazing intensity.

Increasing food production is essential to meet the future food demand of a growing world population. In light of pressing sustainability challenges such as climate change and the importance of the global livestock system for food security as well as GHG emissions, finding ways to increasing food production sustainably and without increasing competition for food crops is essential. Yet, many unknowns relate to livestock grazing, in particular grazing intensity, an essential variable to assess the sustainability of livestock systems. Here, we explore ecological limits to grazing intensity (GI; i.e. the fraction of net primary production consumed by grazing animals) by analysing the role of seasonality in natural grasslands. We estimate seasonal limitations to GI by combining monthly net primary production data and a map of global livestock distribution with assumptions on the length of nonfavourable periods that can be bridged by livestock (e.g. by browsing dead standing biomass, storage systems or biomass conservation). This allows us to derive a seasonality-limited potential GI, which we compare with the GI prevailing in 2000. We find that GI in 2000 lies below its potential on 39% of the total global natural grasslands, which has a potential for increasing biomass extraction of up to 181 MtC/yr. In contrast, on 61% of the area GI exceeds the potential, made possible by management. Mobilizing this potential could increase milk production by 5%, meat production by 4% or contribute to free up to 2.8 Mio km² of grassland area at the global scale if the numerous socio-ecological constraints can be overcome. We discuss socio-ecological trade-offs, which may reduce the estimated potential considerably and require the establishment of sound monitoring systems and an improved understanding of livestock system's role in the Earth system.

Incorporation of plant traits in a land surface model helps explain the global biogeographical distribution of major forest functional types

AbstractAimsContrasting leaf types with different leaf life spans represent different adaptive strategies in plants. Previous studies explained the adaptive advantages of different strategies on the basis of environmental climatic limits, but could not account for the observed co‐dominance of multiple plant functional types (PFTs) in many regions. Here we used a process‐based model to explore whether observed inter‐ and intra‐PFT variation in plant traits can explain global biogeographical variation in the dominance and co‐dominance of major forest types.LocationWorld‐wide.MethodsWe identified four important plant traits: leaf N concentration, leaf life span, fraction of net primary production (NPP) allocated to leaves and plant basal respiration rate. We incorporated means and variances of these plant traits from trait databases into the Community Atmosphere–Biosphere–Land Exchange model. We then predicted the dominant PFT or PFT mixture for global forested grid cells, using NPP as a proxy for growth rate, and considering three PFTs: evergreen needleleaf forests (ENF), evergreen broadleaf forests (EBF) and deciduous broadleaf forests (DBF). We compared results with: (1) those from simulations that did not account for trait variance; (2) results from an empirical model based purely on mean annual temperature; and (3) data from remotely sensed observations.ResultsOur estimates of the fractions of land area covered by major forest types were consistent with observation; i.e. ENFs dominate in boreal regions, EBFs dominate in tropical regions and DBFs are distributed widely across a broad range of environmental conditions. We also showed that co‐dominance of different forest PFTs cannot be reproduced without considering variation in plant traits.Main conclusionsGlobal trait data are useful for representing underlying plant strategies and functional diversity. Variation in key plant traits explains significant fractions of global biogeographical variation of three major forest types. Future developments in dynamic global vegetation modelling will benefit from the inclusion of plant trait variation.

Drought and fire stress influence seedling competition in oak forests: fine-root dynamics as indicator of adaptation strategies to climate change

Increased summer drought and wildfires as a consequence of continuing climate change are expected to lead to disturbance of Mediterranean ecosystems. Seedlings recruitment is sensitive to both stresses and, therefore, any adaptation and restoration strategy devised to protect these forests should take into account a careful study on their effects on seedling development. As a substantial fraction of net primary productivity of forested ecosystems is channelled in the belowground compartments, the knowledge of how roots behave under stressful conditions becomes of primary importance to select the right management strategy to be implemented. This work tries to enlighten the events occurring in the fine root portion of the root system in young seedlings of three co-existing oak species (Quercus ilex, Quercus trojana and Quercus virgiliana) under controlled conditions. We have made a comparative analysis of the effect of these two stresses, alone or in combination, with the aim to evaluate the tolerance level of these seedlings and, therefore, to obtain an indication of their recruitment potential in the field. The parameters investigated were biomass and a number of morphological traits. Data obtained suggest that a decrease in diameter could be part of a tolerance strategy in all three oaks tested together with a reduction of root length. In addition, tolerance to water shortage could require a reduction of carbon allocated belowground, in particular in the very fine roots, which leads to an overall reduction of the root system dimension. Q. trojana seedlings seem to be the fastest in resuming growth after stress interruption but a good recovery was also found for the remaining two oak species. Although our study provides interesting information regarding a possible tolerance strategy taking place in the fine root compartment when seedlings of these three oak species undergo water stress and fire treatment, more information is needed before any suggestion can be made as to which species would be best suited to make these forests more resistant to global changes.

Response of carbon fluxes to drought in a coastal plain loblolly pine forest

AbstractFull accounting of ecosystem carbon (C) pools and fluxes in coastal plain ecosystems remains less studied compared with upland systems, even though the C stocks in these systems may be up to an order of magnitude higher, making them a potentially important component in regional C cycle. Here, we report C pools and CO2 exchange rates during three hydrologically contrasting years (i.e. 2005–2007) in a coastal plain loblolly pine plantation in North Carolina, USA. The daily temperatures were similar among the study years and to the long‐term (1971–2000) average, whereas the amount and timing of precipitation differed significantly. Precipitation was the largest in 2005 (147 mm above normal), intermediate in 2006 (48 mm below) and lowest in 2007 (486 mm below normal). The forest was a strong C sink during all years, sequestering 361 ± 67 (2005), 835 ± 55 (2006) and 724 ± 55 (2007) g C m−2 yr−1 according to eddy covariance measurements of net ecosystem CO2 exchange (NEE). The interannual differences in NEE were traced to drought‐induced declines in canopy and whole tree hydraulic conductances, which declined with growing precipitation deficit and decreasing soil volumetric water content (VWC). In contrast, the interannual differences were small in gross ecosystem productivity (GEP) and ecosystem respiration (ER), both seemingly insensitive to drought. However, the drought sensitivity of GEP was masked by higher leaf area index and higher photosynthetically active radiation during the dry year. Normalizing GEP by these factors enhanced interannual differences, but there were no signs of suppressed GEP at low VWC during any given year. Although ER was very consistent across the 3 years, and not suppressed by low VWC, the total respiratory cost as a fraction of net primary production increased with annual precipitation and the contribution of heterotrophic respiration (Rh) was significantly higher during the wettest year, exceeding new litter inputs by 58%. Although the difference was smaller during the other 2 years (Rh : litterfall ratio was 1.05 in 2006 and 1.10 in 2007), the soils lost about 109 g C m−2 yr−1, outlining their potential vulnerability to decomposition, and pointing to potential management considerations to protect existing soil C stocks.

Nitrogen limitation in a sweetgum plantation: implications for carbon allocation and storage

The N status of temperate forests is closely linked to their C fluxes, and altered C or N availability may affect ecosystem C storage through changes in forest production and C allocation. We proposed that increased fine-root production previously observed in a sweetgum ( Liquidambar styraciflua L.) forest in response to elevated [CO2] was a physiological response to N limitation. To examine this premise, we fertilized plots in the sweetgum plantation adjacent to the Oak Ridge National Laboratory free-air CO2-enrichment (FACE) experiment. We hypothesized that N fertilization would increase sweetgum net primary production, leaf [N], and the relative flux of C to wood production. Annual additions of 200 kg·ha–1 of N as urea increased soil N availability, which increased stand net primary production, stand N uptake, and N requirement by about one-third. Increased leaf [N] and leaf area production in the fertilized plots increased stem production and shifted relative flux of C to wood production. We conclude that sweetgum production on this site is limited by soil N availability and a decreased fraction of net primary production in fine-root production with N addition is consistent with the premise that increased fine-root production in the adjacent FACE experiment is in response to N limitation.

Seasonal patterns of fine-root productivity and turnover in a tropical savanna of northern Australia

Fine roots and their turnover represent a dynamic aspect of below-ground biomass (BGB) and nutrient capital in forest ecosystems, and account for a significant fraction of net primary productivity (NPP) (Cuevas 1995, Vogt et al. 1990). On a weight basis, coarse roots contribute more to total ecosystem biomass than fine roots, but they account for only a small portion of annual root production (Eamus et al. 2002). Despite the fact that fine roots may compose less than 2% of total ecosystem biomass, they may contribute up to 40% of total ecosystem production (Vogt et al. 1990). Therefore, estimates of root production, like estimates of root biomass, should differentiate between coarse- and fine-root production.

Seasonal Water and Energy Metabolism of the Desert-Dwelling Kangaroo Rat (Dipodomys merriami)

Free-living Merriam's kangaroo rats in the Mojave Desert did not drink, even when abundant rainwater was available for short periods in winter and summer, but they still obtained much more water than expected from a diet of dry seeds alone. Yearround measurements of water influx rates and field metabolic rates (via doubly labeled water), diet composition (stomach content analysis), and body mass changes were used to evaluate water and energy balance during each month of the year. From late spring (May) through midwinter (December), kangaroo rats ate primarily the seeds of creosote bush (Larrea tridentata) and were able to maintain water and energy balance on this diet even during the summer drought. Dry seeds collected at the surface apparently were cached in burrows where hygroscopic uptake of water from humid air increased the succulence of seeds before ingestion. In late winter and early spring kangaroo rats obtained abundant water and energy from their diet of green vegetation (90% of diet dry mass) and arthropods (10%). Only in late April and early May did these animals lose body mass, but that was due primarily to reproductive effort (lactation) by females. Thus, at no time of year were kangaroo rats distressed by their comparatively dry, hot, and barren desert habitat. Field metabolic rates were highest in winter, which suggests that these animals did not use torpor or hibernation. On an annual basis, an individual free-living kangaroo rat (mean body mass, 33.9 g) consumed nearly I kg (dry matter) of food, and the population (density, 1 Z5 animals per hectare) consumed over 17 kg ha⁻¹. This is a small fraction of net primary productivity but can be a large proportion of seed production.

Comparative Analyses of Carbon Dynamics in Native and Cultivated Ecosystems.

Plant productivity, litter decomposition rates, and soil organic matter accumulation for winter wheat and for unbroken tallgrass prairie in Central Missouri were analyzed and compared. As measured in this 3-yr study, annual production of dry matter above- and belowground for the cultivated system averaged slightly higher than that for the natural vegetation burned every 3-4 yr. Prairie was characterized by greater year-to-year fluctuation of productivity, which in some years exceeded productivity of the cultivated system. More litter and structural biomass older than 1 yr occurred in the prairie, reflecting the perennial nature of the root system. Principal pathways of carbon transfer in both ecosystems were modeled, and mean residence time in major storage compartments estimated. Combined CO2 losses from above- and belowground litter decay, expressed as a percentage of total soil respiration, were twice as great under wheat as for native prairie. A relatively greater fraction of net primary production was available for synthesis of soil organic matter in native prairie. Under equilibrium conditions, decomposition constants for this compartment, however, were similar. Contrasts in community structure and phenology, seasonal dynamics of litter decay, and levels of soil disturbance are suggested influences affecting transfer and storage characteristics.