351 results found
Terrer C, Vicca S, Hungate BA, et al., 2017, Response to Comment on "Mycorrhizal association as a primary control of the CO2 fertilization effect", Science, Vol: 355, ISSN: 0036-8075
Norby et al. center their critique on the design of the data set and the response variable used. We address these criticisms and reinforce the conclusion that plants that associate with ectomycorrhizal fungi exhibit larger biomass and growth responses to elevated CO2 compared with plants that associate with arbuscular mycorrhizae.
Prentice IC, Cleator SF, Huang YH, et al., 2017, Reconstructing ice-age palaeoclimates: Quantifying low-CO2 effects on plants, Global and Planetary Change, Vol: 149, Pages: 166-176, ISSN: 0921-8181
We present a novel method to quantify the ecophysiological effects of changes in CO2 concentration during the reconstruction of climate changes from fossil pollen assemblages. The method does not depend on any particular vegetation model. Instead, it makes use of general equations from ecophysiology and hydrology that link moisture index (MI) to transpiration and the ratio of leaf-internal to ambient CO2 (χ). Statistically reconstructed MI values are corrected post facto for effects of CO2 concentration. The correction is based on the principle that e, the rate of water loss per unit carbon gain, should be inversely related to effective moisture availability as sensed by plants. The method involves solving a non-linear equation that relates e to MI, temperature and CO2 concentration via the Fu-Zhang relation between evapotranspiration and MI, Monteith's empirical relationship between vapour pressure deficit and evapotranspiration, and recently developed theory that predicts the response of χ to vapour pressure deficit and temperature. The solution to this equation provides a correction term for MI. The numerical value of the correction depends on the reconstructed MI. It is slightly sensitive to temperature, but primarily sensitive to CO2 concentration. Under low LGM CO2 concentration the correction is always positive, implying that LGM climate was wetter than it would seem from vegetation composition. A statistical reconstruction of last glacial maximum (LGM, 21±1 kyr BP) palaeoclimates, based on a new compilation of modern and LGM pollen assemblage data from Australia, is used to illustrate the method in practice. Applying the correction brings pollen-reconstructed LGM moisture availability in southeastern Australia better into line with palaeohydrological estimates of LGM climate.
Prentice IC, Rogers A, Medlyn BE, et al., 2016, A roadmap for improving the representation of photosynthesis in Earth system models, New Phytologist, Vol: 213, Pages: 22-42, ISSN: 1469-8137
Accurate representation of photosynthesis in terrestrial biosphere models (TBMs) is essential for robust projections of global change. However, current representations vary markedly between TBMs, contributing uncertainty to projections of global carbon fluxes. ● Here we compared the representation of photosynthesis in seven TBMs by examining leaf and canopy level responses of A to key environmental variables: light, temperature,carbon dioxide concentration, vapor pressure deficit and soil water content. ● We identified research areas where limited process knowledge prevents inclusion ofphysiological phenomena in current TBMs and research areas where data are urgently needed for model parameterization or evaluation.● We provide a roadmap for new science needed to improve the representation ofphotosynthesis in the next generation of terrestrial biosphere and Earth System Models.
Wang H, Prentice IC, Davis TW, et al., 2016, Photosynthetic responses to altitude: an explanation based on optimality principles, New Phytologist, Vol: 213, Pages: 976-982, ISSN: 1469-8137
Keenan TF, Prentice IC, Canadell JG, et al., 2016, Recent pause in the growth rate of atmospheric CO2 due to enhanced terrestrial carbon uptake., Nature Communications, Vol: 7, ISSN: 2041-1723
Terrestrial ecosystems play a significant role in the global carbon cycle and offset a large fraction of anthropogenic CO2 emissions. The terrestrial carbon sink is increasing, yet the mechanisms responsible for its enhancement, and implications for the growth rate of atmospheric CO2, remain unclear. Here using global carbon budget estimates, ground, atmospheric and satellite observations, and multiple global vegetation models, we report a recent pause in the growth rate of atmospheric CO2, and a decline in the fraction of anthropogenic emissions that remain in the atmosphere, despite increasing anthropogenic emissions. We attribute the observed decline to increases in the terrestrial sink during the past decade, associated with the effects of rising atmospheric CO2 on vegetation and the slowdown in the rate of warming on global respiration. The pause in the atmospheric CO2 growth rate provides further evidence of the roles of CO2 fertilization and warming-induced respiration, and highlights the need to protect both existing carbon stocks and regions, where the sink is growing rapidly.
Thomas RT, Prentice IC, Graven H, et al., 2016, Increased light-use efficiency in northern terrestrial ecosystems indicated by CO2 and greening observations, Geophysical Research Letters, Vol: 43, Pages: 11339-11349, ISSN: 1944-8007
Observations show an increasing amplitude in the seasonal cycle of CO2 (ASC) north of 45°N of 56 ± 9.8% over the last 50 years and an increase in vegetation greenness of 7.5–15% in high northern latitudes since the 1980s. However, the causes of these changes remain uncertain. Historical simulations from terrestrial biosphere models in the Multiscale Synthesis and Terrestrial Model Intercomparison Project are compared to the ASC and greenness observations, using the TM3 atmospheric transport model to translate surface fluxes into CO2 concentrations. We find that the modeled change in ASC is too small but the mean greening trend is generally captured. Modeled increases in greenness are primarily driven by warming, whereas ASC changes are primarily driven by increasing CO2. We suggest that increases in ecosystem-scale light use efficiency (LUE) have contributed to the observed ASC increase but are underestimated by current models. We highlight potential mechanisms that could increase modeled LUE.
De Kauwe MG, Lin Y-S, Wright IJ, et al., 2016, A test of the 'one-point method' for estimating maximum carboxylation capacity from field-measured, light-saturated photosynthesis (vol 210, pg 1130, 2016), NEW PHYTOLOGIST, Vol: 212, Pages: 792-792, ISSN: 0028-646X
Li G, Harrison SP, Prentice IC, 2016, A model analysis of climate and CO<inf>2</inf> controls on tree growth and carbon allocation in a semi-arid woodland, Ecological Modelling, Vol: 342, Pages: 175-185, ISSN: 1872-7026
Many studies have failed to show an increase in the radial growth of trees in response to increasing atmospheric CO2 concentration [CO2] despite the expected enhancement of photosynthetic rates and water-use efficiency at high [CO2]. A global light use efficiency model of photosynthesis, coupled with a generic carbon allocation and tree-growth model based on mass balance and tree geometry principles, was used to simulate annual ring-width variations for the gymnosperm Callitris columellaris in the semi-arid Great Western Woodlands, Western Australia, over the past 100 years. Parameter values for the tree-growth model were derived from independent observations except for sapwood specific respiration rate, fine-root turnover time, fine-root specific respiration rate and the ratio of fine-root mass to foliage area (ζ), which were calibrated to the ring-width measurements by approximate Bayesian optimization. This procedure imposed a strong constraint on ζ. Modelled and observed ring-widths showed quantitatively similar, positive responses to total annual photosynthetically active radiation and soil moisture, and similar negative responses to vapour pressure deficit. The model also produced enhanced radial growth in response to increasing [CO2] during recent decades, but the data do not show this. Recalibration in moving 30-year time windows produced temporal shifts in the estimated values of ζ, including an increase by ca 12% since the 1960s, and eliminated the [CO2]-induced increase in radial growth. The potential effect of CO2 on ring-width was thus shown to be small compared to effects of climate variability even in this semi-arid climate. It could be counteracted in the model by a modest allocation shift, as has been observed in field experiments with raised [CO2].
De Kauwe MG, Keenan TF, Medlyn BE, et al., 2016, Satellite based estimates underestimate the effect of CO2 fertilization on net primary productivity, Nature Climate Change, Vol: 6, Pages: 892-893, ISSN: 1758-678X
Ukkola AM, Keenan TF, Kelley DI, et al., 2016, Vegetation plays an important role in mediating future water resources, Environmental Research Letters, Vol: 11, ISSN: 1748-9326
Future environmental change is expected to modify the global hydrological cycle, with consequences for the regional distribution of freshwater supplies. Regional precipitation projections, however, differ largely between models, making future water resource projections highly uncertain. Using two representative concentration pathways and nine climate models, we estimate 21st century water resources across Australia, employing both a process-based dynamic vegetation model and a simple hydrological framework commonly used in water resource studies to separate the effects of climate and vegetation on water resources. We show surprisingly robust, pathway-independent regional patterns of change in water resources despite large uncertainties in precipitation projections. Increasing plant water use efficiency (due to the changing atmospheric CO2) and reduced green vegetation cover (due to the changing climate) relieve pressure on water resources for the highly populated, humid coastal regions of eastern Australia. By contrast, in semi-arid regions across Australia, runoff declines are amplified by CO2-induced greening, which leads to increased vegetation water use. These findings highlight the importance of including vegetation dynamics in future water resource projections.
Thomas R, Graven H, Hoskins B, et al., 2016, What is meant by ‘balancing sources and sinks of greenhouse gases’ to limit global temperature rise?, Grantham Institute Briefing Note, Imperial College London, 3
In an effort to limit global temperature rise to well below 2˚C, the COP21 Paris Agreement stipulates that a ‘balance’ between anthropogenic (man-made) sources and sinks of greenhouse gases must be reached by 2050-2100. An overall greenhouse gas ‘balance’ must consider individual gases in terms of how strongly they absorb solar infrared radiation, their concentration in the atmosphere, and their lifetime in the atmosphere.• Long-lived greenhouse gases, including carbon dioxide (CO2), accumulate in the atmosphere and continue to affect the climate for many centuries. To stabilise the concentrations of these long-lived gases, and thereby their effect on the climate, their sources must be progressively reduced towards zero. • For short-lived greenhouse gases that remain in the atmosphere for less than 100 years, including methane, stable or decreasing concentrations could be achieved within decades if emissions were stabilised or decreased. However, these gases currently only contribute about 20% of the total warming from greenhouse gases, so their reduction alone cannot successfully stabilise global temperature.• An overall ‘balance’ of sources and sinks of greenhouse gases could be facilitated by deliberate removal of CO2 from the atmosphere, for example, by combining biomass energy production with carbon capture and storage. Most current greenhouse gas emission scenarios that keep global temperature rise below 2˚C include some deliberate removal of CO2 to compensate for continued emissions of CO2 and other greenhouse gases
Le Quéré C, Buitenhuis ET, Moriarty R, et al., 2016, Role of zooplankton dynamics for Southern Ocean phytoplankton biomass and global biogeochemical cycles, Biogeosciences, Vol: 13, Pages: 4111-4133, ISSN: 1726-4170
Global ocean biogeochemistry models currently employed in climate change projections use highly simplified representations of pelagic food webs. These food webs do not necessarily include critical pathways by which ecosystems interact with ocean biogeochemistry and climate. Here we present a global biogeochemical model which incorporates ecosystem dynamics based on the representation of ten plankton functional types (PFTs): six types of phytoplankton, three types of zooplankton, and heterotrophic procaryotes. We improved the representation of zooplankton dynamics in our model through (a) the explicit inclusion of large, slow-growing macrozooplankton (e.g. krill), and (b) the introduction of trophic cascades among the three zooplankton types. We use the model to quantitatively assess the relative roles of iron vs. grazing in determining phytoplankton biomass in the Southern Ocean high-nutrient low-chlorophyll (HNLC) region during summer. When model simulations do not include macrozooplankton grazing explicitly, they systematically overestimate Southern Ocean chlorophyll biomass during the summer, even when there is no iron deposition from dust. When model simulations include a slow-growing macrozooplankton and trophic cascades among three zooplankton types, the high-chlorophyll summer bias in the Southern Ocean HNLC region largely disappears. Our model results suggest that the observed low phytoplankton biomass in the Southern Ocean during summer is primarily explained by the dynamics of the Southern Ocean zooplankton community, despite iron limitation of phytoplankton community growth rates. This result has implications for the representation of global biogeochemical cycles in models as zooplankton faecal pellets sink rapidly and partly control the carbon export to the intermediate and deep ocean.
Terrer C, Vicca S, Hungate BA, et al., 2016, Mycorrhizal association as a primary control of the CO2 fertilization effect, Science, Vol: 353, Pages: 72-74, ISSN: 1095-9203
Plants buffer increasing atmospheric CO2 concentrations through enhanced growth, but the question whether nitrogen availability constrains the magnitude of this ecosystem service remains unresolved. Synthesizing experiments from around the world, we show that CO2 fertilization is best explained by a simple interaction between nitrogen availability and mycorrhizal association. Plant species that associate with ectomycorrhizal fungi show a strong biomass increase (30 ± 3%, P<0.001) in response to elevated CO2 regardless of nitrogen availability, whereas low nitrogen availability limits CO2 fertilization (0 ± 5%, P=0.946) in plants that associate with arbuscular mycorrhizal fungi. The incorporation of mycorrhizae in global carbon cycle models is feasible, and crucial if we are to accurately project ecosystem responses and feedbacks to climate change.
Harrison SP, Bartlein PJ, Prentice IC, 2016, What have we learnt from palaeoclimate simulations?, Journal of Quaternary Science, Vol: 31, Pages: 363-385, ISSN: 1099-1417
There has been a gradual evolution in the way that palaeoclimate modelling and palaeoenvironmental data are used together to understand how the Earth System works, from an initial and largely descriptive phase through explicit hypothesis testing to diagnosis of underlying mechanisms. Analyses of past climate states are now regarded as integral to the evaluation of climate models, and have become part of the toolkit used to assess the likely realism of future projections. Palaeoclimate assessment has demonstrated that changes in large-scale features of climate that are governed by the energy and water balance show consistent responses to changes in forcing in different climate states, and these consistent responses are reproduced by climate models. However, state-of-the-art models are still largely unable to reproduce observed changes in climate at a regional scale reliably. While palaeoclimate analyses of state-of-the-art climate models suggest an urgent need for model improvement, much work is also needed on extending and improving palaeoclimate reconstructions and quantifying and reducing both numerical and interpretative uncertainties.
Biomass burning impacts vegetation dynamics, biogeochemical cycling, atmospheric chemistry, and climate, with sometimes deleterious socio-economic impacts. Under future climate projections it is often expected that the risk of wildfires will increase. Our ability to predict the magnitude and geographic pattern of future fire impacts rests on our ability to model fire regimes, using either well-founded empirical relationships or process-based models with good predictive skill. While a large variety of models exist today, it is still unclear which type of model or degree of complexity is required to model fire adequately at regional to global scales. This is the central question underpinning the creation of the Fire Model Intercomparison Project (FireMIP), an international initiative to compare and evaluate existing global fire models against benchmark data sets for present-day and historical conditions. In this paper we review how fires have been represented in fire-enabled dynamic global vegetation models (DGVMs) and give an overview of the current state of the art in fire-regime modelling. We indicate which challenges still remain in global fire modelling and stress the need for a comprehensive model evaluation and outline what lessons may be learned from FireMIP.
Stocker BD, Prentice IC, Cornell SE, et al., 2016, Terrestrial nitrogen cycling in Earth system models revisited., New Phytologist, Vol: 210, Pages: 1165-1168, ISSN: 1469-8137
Gallego-Sala AV, Charman DJ, Harrison SP, et al., 2016, Climate-driven expansion of blanket bogs in Britain during the Holocene, Climate of the Past, Vol: 12, Pages: 129-136, ISSN: 1814-9332
Blanket bog occupies approximately 6 % of the area of the UK today. The Holocene expansion of this hyperoceanic biome has previously been explained as a consequence of Neolithic forest clearance. However, the present distribution of blanket bog in Great Britain can be predicted accurately with a simple model (PeatStash) based on summer temperature and moisture index thresholds, and the same model correctly predicts the highly disjunct distribution of blanket bog worldwide. This finding suggests that climate, rather than land-use history, controls blanket-bog distribution in the UK and everywhere else. We set out to test this hypothesis for blanket bogs in the UK using bioclimate envelope modelling compared with a database of peat initiation age estimates. We used both pollen-based reconstructions and climate model simulations of climate changes between the mid-Holocene (6000 yr BP, 6 ka) and modern climate to drive PeatStash and predict areas of blanket bog. We compiled data on the timing of blanket-bog initiation, based on 228 age determinations at sites where peat directly overlies mineral soil. The model predicts that large areas of northern Britain would have had blanket bog by 6000 yr BP, and the area suitable for peat growth extended to the south after this time. A similar pattern is shown by the basal peat ages and new blanket bog appeared over a larger area during the late Holocene, the greatest expansion being in Ireland, Wales, and southwest England, as the model predicts. The expansion was driven by a summer cooling of about 2 °C, shown by both pollen-based reconstructions and climate models. The data show early Holocene (pre-Neolithic) blanket-bog initiation at over half of the sites in the core areas of Scotland and northern England. The temporal patterns and concurrence of the bioclimate model predictions and initiation data suggest that climate change provides a parsimonious explanation for the early Holocene distribution and later expansion of bl
Hoogakker BAA, Smith RS, Singarayer JS, et al., 2016, Terrestrial biosphere changes over the last 120 kyr, Climate of the Past, Vol: 12, Pages: 51-73, ISSN: 1814-9332
A new global synthesis and biomization of long (> 40 kyr) pollen-data records is presented and used with simulations from the HadCM3 and FAMOUS climate models and the BIOME4 vegetation model to analyse the dynamics of the global terrestrial biosphere and carbon storage over the last glacial–interglacial cycle. Simulated biome distributions using BIOME4 driven by HadCM3 and FAMOUS at the global scale over time generally agree well with those inferred from pollen data. Global average areas of grassland and dry shrubland, desert, and tundra biomes show large-scale increases during the Last Glacial Maximum, between ca. 64 and 74 ka BP and cool substages of Marine Isotope Stage 5, at the expense of the tropical forest, warm-temperate forest, and temperate forest biomes. These changes are reflected in BIOME4 simulations of global net primary productivity, showing good agreement between the two models. Such changes are likely to affect terrestrial carbon storage, which in turn influences the stable carbon isotopic composition of seawater as terrestrial carbon is depleted in 13C.
© British Ecological Society 2016. Introduction The fundamental reason for the presence of peatlands is a positive balance between plant production and decomposition of organic matter. Organic matter accumulates in these systems because prolonged waterlogged conditions result in soil anoxia (i.e. exclusion of oxygen), and under these conditions decomposition rates can be lower than those of primary production, as seen in Figure 8.1. Climate therefore plays an important role in peat accumulation, both directly by affecting plant productivity and decomposition of organic matter, and indirectly through its effects on hydrology, water balance and vegetation composition (for a summary, refer to Yu, Beilman and Jones (2009)). Climate provides broad-scale controls on peatland extent, types and vegetation, and ultimately, ecosystem services such as carbon sequestration and storage, as well as water and hazard regulation (Chapters 4 and 5). Peatlands can therefore play a vital role in ecosystem-based adaptation in helping society mitigate and adapt to climate change. Future climate change is likely to alter the hydrology and soil temperature of peatlands, with far-reaching consequences for their biodiversity, ecology and biogeochemistry, and interactions with the Earth system. For example, the possibility of drier conditions allowing peat erosion and increases in CO2 emissions that would result in a positive feedback to climate change (Turetsky 2010). Peatlands that have been damaged by human activity are more vulnerable to climate-induced changes in hydrology and temperature, but suitable management strategies may make them more resilient to changes and help to stabilise the delivery of ecosystem services (Chapter 1). This chapter describes the interactions between climate and peatlands in three sections. The first section explains how present climate influences peatlands, by documenting how climate limits peatland geographical extent globally, and how bioclimatic enve
De Kauwe MG, Lin Y-S, Wright IJ, et al., 2015, A test of the 'one-point method' for estimating maximum carboxylation capacity from field-measured, light-saturated photosynthesis, New Phytologist, Vol: 210, Pages: 1130-1144, ISSN: 1469-8137
Earth is home to a remarkable diversity of plant forms and life histories, yet comparatively few essential trait combinations have proved evolutionarily viable in today’s terrestrial biosphere. By analysing worldwide variation in six major traits critical to growth, survival and reproduction within the largest sample of vascular plant species ever compiled, we found that occupancy of six-dimensional trait space is strongly concentrated, indicating coordination and trade-offs. Three-quarters of trait variation is captured in a two-dimensional global spectrum of plant form and function. One major dimension within this plane reflects the size of whole plants and their parts; the other represents the leaf economics spectrum, which balances leaf construction costs against growth potential. The global plant trait spectrum provides a backdrop for elucidating constraints on evolution, for functionally qualifying species and ecosystems, and for improving models that predict future vegetation based on continuous variation in plant form and function.
De Kauwe MG, Zhou S-X, Medlyn BE, et al., 2015, Do land surface models need to include differential plant species responses to drought? Examining model predictions across a mesic-xeric gradient in Europe, Biogeosciences, Vol: 12, Pages: 7503-7518, ISSN: 1726-4189
Future climate change has the potential to increase drought in many regions of the globe, making it essential that land surface models (LSMs) used in coupled climate models realistically capture the drought responses of vegetation. Recent data syntheses show that drought sensitivity varies considerably among plants from different climate zones, but state-of-the-art LSMs currently assume the same drought sensitivity for all vegetation. We tested whether variable drought sensitivities are needed to explain the observed large-scale patterns of drought impact on the carbon, water and energy fluxes. We implemented data-driven drought sensitivities in the Community Atmosphere Biosphere Land Exchange (CABLE) LSM and evaluated alternative sensitivities across a latitudinal gradient in Europe during the 2003 heatwave. The model predicted an overly abrupt onset of drought unless average soil water potential was calculated with dynamic weighting across soil layers. We found that high drought sensitivity at the most mesic sites, and low drought sensitivity at the most xeric sites, was necessary to accurately model responses during drought. Our results indicate that LSMs will over-estimate drought impacts in drier climates unless different sensitivity of vegetation to drought is taken into account.
Zhou S-X, Medlyn BE, Prentice IC, 2015, Long-term water stress leads to acclimation of drought sensitivity of photosynthetic capacity in xeric but not riparian Eucalyptus species, Annals of Botany, Vol: 117, Pages: 133-144, ISSN: 1095-8290
Background and Aims Experimental drought is well documented to induce a decline in photosynthetic capacity. However, if given time to acclimate to low water availability, the photosynthetic responses of plants to low soil moisture content may differ from those found in short-term experiments. This study aims to test whether plants acclimate to long-term water stress by modifying the functional relationships between photosynthetic traits and water stress, and whether species of contrasting habitat differ in their degree of acclimation.Methods Three Eucalyptus taxa from xeric and riparian habitats were compared with regard to their gas exchange responses under short- and long-term drought. Photosynthetic parameters were measured after 2 and 4 months of watering treatments, namely field capacity or partial drought. At 4 months, all plants were watered to field capacity, then watering was stopped. Further measurements were made during the subsequent ‘drying-down’, continuing until stomata were closed.Key Results Two months of partial drought consistently reduced assimilation rate, stomatal sensitivity parameters (g1), apparent maximum Rubisco activity (V′cmaxVcmax′) and maximum electron transport rate (J′maxJmax′). Eucalyptus occidentalis from the xeric habitat showed the smallest decline in V′cmaxVcmax′ and J′maxJmax′; however, after 4 months, V′cmaxVcmax′ and J′maxJmax′ had recovered. Species differed in their degree of V′cmaxVcmax′ acclimation. Eucalyptus occidentalis showed significant acclimation of the pre-dawn leaf water potential at which the V′cmaxVcmax′ and ‘true’ Vcmax (accounting for mesophyll conductance) declined most steeply during drying-down.Conclusions The findings indicate carbon loss under prolonged drought could be over-estimated without accounting for acclimation. In particular, (1) species from contrasting habitats differed in th
Ukkola AM, Prentice IC, Keenan TF, et al., 2015, Reduced streamflow in water-stressed climates consistent with CO2 effects on vegetation, Nature Climate Change, Vol: 6, Pages: 75-78, ISSN: 1758-6798
Global environmental change has implications for the spatial and temporal distribution of water resources, but quantifying its effects remains a challenge. The impact of vegetation responses to increasing atmospheric CO2 concentrations on the hydrologic cycle is particularly poorly constrained1, 2, 3. Here we combine remotely sensed normalized difference vegetation index (NDVI) data and long-term water-balance evapotranspiration (ET) measurements from 190 unimpaired river basins across Australia during 1982–2010 to show that the precipitation threshold for water limitation of vegetation cover has significantly declined during the past three decades, whereas sub-humid and semi-arid basins are not only ‘greening’ but also consuming more water, leading to significant (24–28%) reductions in streamflow. In contrast, wet and arid basins show nonsignificant changes in NDVI and reductions in ET. These observations are consistent with expected effects of elevated CO2 on vegetation. They suggest that projected future decreases in precipitation4 are likely to be compounded by increased vegetation water use, further reducing streamflow in water-stressed regions.
Prentice I, Williams S, Friedlingstein P, 2015, Biosphere feedbacks and climate change, Grantham Institute Briefing Paper, Imperial College London, Publisher: Imperial College London, 12
Prentice IC, Liang X, Medlyn BE, et al., 2015, Reliable, robust and realistic: the three R's of next-generation land-surface modelling, Atmospheric Chemistry and Physics, Vol: 15, Pages: 5987-6005, ISSN: 1680-7316
Land-surface models (LSMs) are increasinglycalled upon to represent not only the exchanges of energy,water and momentum across the land–atmosphere interface(their original purpose in climate models), but also howecosystems and water resources respond to climate, atmospheric environment, land-use and land-use change, and howthese responses in turn influence land–atmosphere fluxes ofcarbon dioxide (CO2), trace gases and other species that affect the composition and chemistry of the atmosphere. However, the LSMs embedded in state-of-the-art climate modelsdiffer in how they represent fundamental aspects of the hydrological and carbon cycles, resulting in large inter-modeldifferences and sometimes faulty predictions. These “thirdgeneration” LSMs respect the close coupling of the carbonand water cycles through plants, but otherwise tend to beunder-constrained, and have not taken full advantage of robust hydrological parameterizations that were independentlydeveloped in offline models. Benchmarking, combining multiple sources of atmospheric, biospheric and hydrologicaldata, should be a required component of LSM development,but this field has been relatively poorly supported and intermittently pursued. Moreover, benchmarking alone is not sufficient to ensure that models improve. Increasing complexity may increase realism but decrease reliability and robustness, by increasing the number of poorly known model parameters. In contrast, simplifying the representation of complex processes by stochastic parameterization (the representation of unresolved processes by statistical distributions ofvalues) has been shown to improve model reliability and realism in both atmospheric and land-surface modelling contexts. We provide examples for important processes in hydrology (the generation of runoff and flow routing in heterogeneous catchments) and biology (carbon uptake by speciesdiverse ecosystems). We propose that the way forward fornext-generation complex LSM
Maire V, Wright IJ, Prentice IC, et al., 2015, Global effects of soil and climate on leaf photosynthetic traits and rates, Global Ecology and Biogeography, Vol: 24, Pages: 706-717, ISSN: 1466-822X
AimThe influence of soil properties on photosynthetic traits in higher plants is poorly quantified in comparison with that of climate. We address this situation by quantifying the unique and joint contributions to global leaf‐trait variation from soils and climate.LocationTerrestrial ecosystems world‐wide.MethodsUsing a trait dataset comprising 1509 species from 288 sites, with climate and soil data derived from global datasets, we quantified the effects of 20 soil and 26 climate variables on light‐saturated photosynthetic rate (Aarea), stomatal conductance (gs), leaf nitrogen and phosphorus (Narea and Parea) and specific leaf area (SLA) using mixed regression models and multivariate analyses.ResultsSoil variables were stronger predictors of leaf traits than climatic variables, except for SLA. On average, Narea, Parea and Aarea increased and SLA decreased with increasing soil pH and with increasing site aridity. gs declined and Parea increased with soil available P (Pavail). Narea was unrelated to total soil N. Joint effects of soil and climate dominated over their unique effects on Narea and Parea, while unique effects of soils dominated for Aarea and gs. Path analysis indicated that variation in Aarea reflected the combined independent influences of Narea and gs, the former promoted by high pH and aridity and the latter by low Pavail.Main conclusionsThree environmental variables were key for explaining variation in leaf traits: soil pH and Pavail, and the climatic moisture index (the ratio of precipitation to potential evapotranspiration). Although the reliability of global soil datasets lags behind that of climate datasets, our results nonetheless provide compelling evidence that both can be jointly used in broad‐scale analyses, and that effects uniquely attributable to soil properties are important determinants of leaf photosynthetic traits and rates. A significant future challenge is to better disentangle the covarying physiological, ecological and evolutionary
Prentice I, Dickenson M, Mace G, 2015, Climate change and challenges for conservation, Grantham Institute Briefing Paper, Imperial College London, Publisher: Imperial College London, 13
The headlines• Biodiversity has been significantly depleted by non-climatic factors, such as land-use change. Climate change will exacerbate thisloss and compromise ecosystem integrity.• Integrated approaches will be necessary to evaluate species’ responses to climate change, which will be more complex and moreuncertain than range shifts alone.• This paper recommends applying new perspectives to traditional conservation practices. A global and flexible approach tobiodiversity protection and resource management may be needed for successful conservation policy and planning.
Medlyn BE, Zaehle S, De Kauwe MG, et al., 2015, Using ecosystem experiments to improve vegetation models, Nature Climate Change, Vol: 5, Pages: 528-534, ISSN: 1758-678X
Ecosystem responses to rising CO2 concentrations are a major source of uncertainty in climate change projections. Data from ecosystem-scale Free-Air CO2 Enrichment (FACE) experiments provide a unique opportunity to reduce this uncertainty. The recent FACE Model–Data Synthesis project aimed to use the information gathered in two forest FACE experiments to assess and improve land ecosystem models. A new 'assumption-centred' model intercomparison approach was used, in which participating models were evaluated against experimental data based on the ways in which they represent key ecological processes. By identifying and evaluating the main assumptions causing differences among models, the assumption-centred approach produced a clear roadmap for reducing model uncertainty. Here, we explain this approach and summarize the resulting research agenda. We encourage the application of this approach in other model intercomparison projects to fundamentally improve predictive understanding of the Earth system.
Calvo MM, Prentice IC, 2015, Effects of fire and CO2 on biogeography and primary production in glacial and modern climates, New Phytologist, Vol: 208, Pages: 987-994, ISSN: 0028-646X
Dynamic global vegetation models (DGVMs) can disentangle causes and effects in the control of vegetation and fire. We used a DGVM to analyse climate, CO2 and fire influences on biome distribution and net primary production (NPP) in last glacial maximum (LGM) and pre-industrial (PI) times.The Land surface Processes and eXchanges (LPX) DGVM was run in a factorial design with fire ‘off’ or ‘on’, CO2 at LGM (185 ppm) or PI (280 ppm) concentrations, and LGM (modelled) or recent climates. Results were analysed by Stein–Alpert decomposition to separate primary effects from synergies.Fire removal causes forests to expand and global NPP to increase slightly. Low CO2 greatly reduces forest area (dramatically in a PI climate; realistically under an LGM climate) and global NPP. NPP under an LGM climate was reduced by a quarter as a result of low CO2. The reduction in global NPP was smaller at low temperatures, but greater in the presence of fire.Global NPP is controlled by climate and CO2 directly through photosynthesis, but also through biome distribution, which is strongly influenced by fire. Future vegetation simulations will need to consider the coupled responses of vegetation and fire to CO2 and climate.
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