CARbon Assimilation In The Biosphere Research Articles

Refining Species Traits in a Dynamic Vegetation Model to Project the Impacts of Climate Change on Tropical Trees in Central Africa

African tropical ecosystems and the services they provide to human society suffer from an increasing combined pressure of land use and climate change. How individual tropical tree species respond to climate change remains relatively unknown. In this study, we refined the species characterization in the CARAIB (CARbon Assimilation In the Biosphere) dynamic vegetation model by replacing plant functional type morpho-physiological traits by species-specific traits. We focus on 12 tropical tree species selected for their importance in both the plant community and human society. We used CARAIB to simulate the current species net primary productivity (NPP), biomass and potential distribution and their changes in the future. Our results indicate that the use of species-specific traits does not necessarily result in an increase of predicted current NPPs. The model projections for the end of the century highlight the large uncertainties in the future of African tropical species. Projected changes in species distribution vary greatly with the general circulation model (GCM) and, to a lesser extent, with the concentration pathway. The question about long-term plant response to increasing CO2 concentrations also leads to contrasting results. In absence of fertilization effect, species are exposed to climate change and might lose 25% of their current distribution under RCP8.5 (12.5% under RCP4.5), considering all the species and climatic scenarios. The vegetation model projects a mean biomass loss of −21.2% under RCP4.5 and −34.5% under RCP8.5. Potential range expansions, unpredictable due to migration limitations, are too limited for offsetting range contraction. By contrast, if the long-term species response to increasing [CO2] is positive, the range reduction is limited to 5%. However, despite a mean biomass increase of 12.2%, a positive CO2 feedback might not prevent tree dieback. Our analysis confirms that species will respond differently to new climatic and atmospheric conditions, which may induce new competition dynamics in the ecosystem and affect ecosystem services.

Contrasting climate risks predicted by dynamic vegetation and ecological niche-based models applied to tree species in the Brazilian Atlantic Forest

Climate change is a threat to natural ecosystems. To evaluate this threat and, where possible, respond, it is useful to understand the potential impacts climate change could have on species’ distributions, phenology, and productivity. Here, we compare future-scenario outcomes between a dynamic vegetation model (DVM; CARbon Assimilation In the Biosphere (CARAIB)) and an ecological niche-based model (ENM; maximum entropy model) to outline the risks to tree species in the Brazilian Atlantic Forest, comprising the habitats of several endemic species, including the endangered primate Leontopithecus chrysomelas (golden-headed lion tamarin; GHLT), our species of interest. Compared to MaxENT, the DVM predicts larger present-day species ranges. Conversely, MaxENT ranges are closer to sampled distributions of the realised niches. MaxENT results for two future scenarios in four general circulation models suggest that up to 75% of the species risk losing more than half of their original distribution. CARAIB simulations are more optimistic in scenarios with and without accounting for potential plant-physiological effects of increased CO2, with less than 10% of the species losing more than 50% of their range. Potential gains in distribution outside the original area do not necessarily diminish risks to species, as the potential new zones may not be easy to colonise. It will also depend on the tree species’ dispersal ability. So far, within the current range of L. chrysomelas, CARAIB continues to predict persistence of most resource trees, while MaxENT predicts the loss of up to 19 species out of the 59 simulated. This research highlights the importance of choosing the appropriate modelling approach and interpretation of results to understand key processes.

Bayesian inversions of a dynamic vegetation model at four European grassland sites

Abstract. Eddy covariance data from four European grassland sites are used to probabilistically invert the CARAIB (CARbon Assimilation In the Biosphere) dynamic vegetation model (DVM) with 10 unknown parameters, using the DREAM(ZS) (DiffeRential Evolution Adaptive Metropolis) Markov chain Monte Carlo (MCMC) sampler. We focus on comparing model inversions, considering both homoscedastic and heteroscedastic eddy covariance residual errors, with variances either fixed a priori or jointly inferred together with the model parameters. Agreements between measured and simulated data during calibration are comparable with previous studies, with root mean square errors (RMSEs) of simulated daily gross primary productivity (GPP), ecosystem respiration (RECO) and evapotranspiration (ET) ranging from 1.73 to 2.19, 1.04 to 1.56 g C m−2 day−1 and 0.50 to 1.28 mm day−1, respectively. For the calibration period, using a homoscedastic eddy covariance residual error model resulted in a better agreement between measured and modelled data than using a heteroscedastic residual error model. However, a model validation experiment showed that CARAIB models calibrated considering heteroscedastic residual errors perform better. Posterior parameter distributions derived from using a heteroscedastic model of the residuals thus appear to be more robust. This is the case even though the classical linear heteroscedastic error model assumed herein did not fully remove heteroscedasticity of the GPP residuals. Despite the fact that the calibrated model is generally capable of fitting the data within measurement errors, systematic bias in the model simulations are observed. These are likely due to model inadequacies such as shortcomings in the photosynthesis modelling. Besides the residual error treatment, differences between model parameter posterior distributions among the four grassland sites are also investigated. It is shown that the marginal distributions of the specific leaf area and characteristic mortality time parameters can be explained by site-specific ecophysiological characteristics.

Modelling Late Miocene vegetation in Europe: Results of the CARAIB model and comparison with palaeovegetation data

The CARAIB (CARbon Assimilation In the Biosphere) model is used to study the vegetation distribution during the Late Miocene (Tortonian). In this version, the plant classification is specifically adapted to best represent Miocene European vegetation. Compared to other plant classifications used in global models, this adapted classification is more refined, since it is specifically developed for European vegetation and it includes various thermophylous tree types, which were present in Europe during the Miocene. The corresponding climatic tolerance parameters are based on the study of Laurent et al. (Journal of Vegetation Science, 15, 739-746, 2004) for the tree types currently present in Europe and on the distribution of analogue species in southeastern Asia and North/Central America for the thermophylous (sub-tropical) trees. The same classification is used to characterize the palaeoflora at the available Late Miocene localities, allowing a model–data comparison at the plant functional type level, rather than at the biome level. The climatic inputs to CARAIB are obtained from the COSMOS atmosphere–ocean general circulation model. The climatic anomalies (Tortonian minus Present) derived from COSMOS are interpolated to a higher spatial resolution before being used in the vegetation model. These anomalies are combined with a modern climatology to produce climatic fields with high spatial resolution (10′ × 10′). This procedure has the advantage of making apparent relief features smaller than the grid cells of the climate model and, hence, makes easier the comparison with local vegetation data, although it does not really improve the quality of the Tortonian climate reconstruction. The new version of CARAIB was run over Europe at this higher spatial resolution. It calculates the potential distribution of 13 different classes of trees (including cold/cool/warm-temperate, sub-tropical and tropical types), together with their cover fractions, net primary productivities and biomasses. The resulting model vegetation distribution reconstructed for the Tortonian is compared to available palaeovegetation and pollen data. Before performing this comparison, the tree taxa present at the various data sites are assigned to one or several model classes, depending on the identification level of the taxa. If several classes are possible for a taxon, only those that can co-exist with the other tree classes identified at the site are retained. This methodology is similar to the co-existence approach used in palaeoclimatic reconstructions based on vegetation data. It narrows the range of tree types present at the various sites, by suppressing in the data the extreme types, such as the cold boreal/temperate and tropical trees. The method allows a comparison with the model simulation on a presence/absence basis. This comparison provides an overall agreement of 53% between the model and the data, when all sites and tree types are considered. The agreement is high (> 85%) for needle-leaved summergreen boreal/temperate cold trees ( Larix sp.) and for tropical trees, intermediate (> 40%) for other boreal/temperate cold trees and for needle-leaved evergreen temperate cool trees, broadleaved summergreen temperate cool trees and broadleaved evergreen warm-temperate trees, and poor (< 40%) for most temperate perhumid warm trees. In many cases, the model is shown to be better at predicting the absence than the presence, as observed for tropical trees. The modelled distributions of cold boreal/temperate trees tend to extend too much towards the south compared to the data. By contrast, model sub-tropical trees (temperate perhumid warm and needle-leaf summergreen temperate warm trees) appear to be restricted to some limited areas in southern Europe, while they are present in the data from central Europe up to at least 50°N. Consequently, modelled Late Miocene climate appears to remain too cold to produce assemblages of trees consistent with the data. The predicted modelled trends from the past to the present are in the right direction, but the amplitude remains too small. For the simulations to be in a better agreement with the data, higher CO 2 levels may be necessary in the climate simulations, or possibly other oceanic boundary conditions may be required, such as different bathymetry in the Panama seaway.

European bioclimatic affinity groups: Data-model comparisons

Global vegetation models are remarkably effective when considering large areas such as Europe. However, their accuracy at finer scales remains to be tested. In this paper, we validate the simulation of modern potential vegetation by the CARbon Assimilation In the Biosphere (CARAIB) model in Europe. Then, in order to evaluate the simulation of tree group distributions at a finer scale, in France, we present a comparison between observed distributions, distributions reconstructed from palynological data, and model simulated ranges. The results will help to validate past vegetation simulations. For this analysis, we use Bioclimatic Affinity Groups (BAGs), based on vegetation groups' climatic tolerances and requirements. The CARAIB model was adapted to simulate the net primary productivity (NPP), biomass and range of the arboreal BAGs. In Europe, at a 30′ latitude/longitude grid scale, simulated NPP of BAGs are used to define classes of vegetation as being present or absent, with a classification rule, based on Kappa statistics. In France, at a 10′ lat./long. scale, a second discriminant analysis, based on Classification And Regression Tree (CART), allows for a similar classification with BAG pollen percentages. At each palynological sampling site, we then compared the simulation to the reconstruction from pollen data. With 30′ lat./long. resolution, most thresholds that discriminate NPP into absence or presence classes are low, ranging from 1 to 77 g/m 2. Agreement indices between observed and simulated distributions range from 0.4 to 0.83, with broad scale BAG potential patterns and boundaries being accurately simulated by CARAIB. In France, on the 10′ lat./long. scale, pollen percentages correctly account for BAG presence/absence despite non-linear pollen-vegetation relationships. Agreement ratios between observed and reconstructed patterns range from 0.53 to 0.95. At the 10′ lat./long. scale, the validation of simulated ranges with pollen data is reliable for 9 of 13 arboreal BAGs and acceptable for three more BAGs. The discrepancies highlight the gap between potential and actual distribution areas. The filling of simulated potential ranges, such as the Atlantic coast and near Mediterranean border, are uncompleted as actual ranges are limited by a number of climate and dispersal constraints related to competition as well as historical, geographical and anthropogenic factors. Our results suggest that the simulation of these constraints would be a major improvement for the CARAIB model.

Biospheric carbon stocks reconstructed at the Last Glacial Maximum: comparison between general circulation models using prescribed and computed sea surface temperatures

The terrestrial biosphere model Carbon Assimilation in the Biosphere (CARAIB) was improved by introducing two vegetation storeys and implementing a new module which simulates the equilibrium distribution of the vegetation inferred from physiological processes and climatic constraints. In this fourth version of CARAIB, we differentiate ground-level grasses from tree canopies, which allows us to determine the light available to grasses as a direct function of the leaf area index (LAI) of the forest canopy. Both of these storeys are potentially composed of several plant functional types (PFT). The cover fraction of each PFT within each storey is estimated according to its respective net primary productivity (NPP). A biome is assigned to each grid cell on the basis of three physiological criteria: (1) the cover fraction, (2) the NPP, and (3) the LAI; and two climatic constraints: (1) the growing degree-days (GDD) and (2) the lowest temperature reached during the cold season ( T min), which are well-known indices of vegetation expansion boundaries. Total biospheric carbon stocks (vegetation+soil) are reconstructed by forcing the model with eight climatic scenarios of the Last Glacial Maximum (LGM, 21 ka BP), which were obtained from the Paleo-Modelling Intercomparison Project (PMIP) from four general circulation models (MRI2, UGAMP, LMD4, and GEN2) using prescribed and computed sea surface temperatures (SSTs). The model was also forced with a current climate together with a preindustrial atmospheric CO 2 level of 280 ppm as reference simulation. To validate the model, current biome distribution is reconstructed and compared, for the modern climate, with two distributions of potential vegetation and, for the LGM, with pollen data. The model simulations are in good agreement with broad-scale patterns of vegetation distribution. The results indicate an increase in the total biospheric carbon stock of 827.8–1106.1 Gt C since the LGM. Sensitivity analyses were performed to discriminate the relative effects of the atmospheric CO 2 level (“fertilization effect”), the climate (present or LGM), and the sea level. Our results suggest that the CO 2 fertilization effect is mostly responsible for the total increase in vegetation and soil carbon stocks. The four GCMs diverged in their predicted responses of continental climate to calculated SSTs. Only one of them, i.e., MRI2, predicted a marked decline of the continental temperatures in response to lower calculated SSTs. For this GCM, the effect of reduced SSTs on continental biospheric carbon stocks was a decrease of 544.1 Gt for the soil carbon stock and of 283.7 for the vegetation carbon stock, which means a decrease in the total biopsheric carbon stock of 827.8 Gt.

Carbon stocks and isotopic budgets of the terrestrial biosphere at mid-Holocene and last glacial maximum times

The carbon fluxes, stocks and isotopic budgets of the land biosphere at mid-Holocene (6 ka BP) and last glacial maximum (21 ka BP) times are reconstructed with the CARbon Assimilation In the Biosphere (CARAIB) model forced with two different sets of climates simulated by the European Centre-HAMburg (ECHAM) and LMD general circulation models. It is found that the trends predicted on the basis of both sets of GCM climatic fields are generally consistent with each other, although substantial discrepancies in the magnitude of the changes may be observed. Actually, these discrepancies in the biospheric results associated with the use of different GCM climatic fields are usually smaller than the differences between biospheric runs performed while considering or neglecting the CO2 fertilization effect (which might, however, be overestimated by the model due to uncertainties concerning changes in nutrient availability). The calculated changes with respect to the present of the biosphere carbon stock range from −132 to +92 Gt C for the mid-Holocene and from −710 to +70 Gt C for the last glacial maximum. It is also shown that the relative contribution of the material synthesized by C4 plants to the total biomass of vegetation, litter and soils was substantially larger at mid-Holocene and last glacial maximum times than today. This change in the relative importance of the C3 and C4 photosynthetic pathways induced changes in the 13C fractionation of the land biosphere. These changes in the average biospheric fractionation resulting from the redistribution of C3 and C4 plants were partly compensated for by changes of opposite sign in the fractionation of C3 plants due to the modification of the intercellular CO2 pressure within their leaves. With respect to present times, the combination of both processes reduced the 13C discrimination (i.e., less negative fractionation) of the land biosphere by 0.03 to 0.32‰ during the mid-Holocene and by 0.30 to 1.86‰ at the last glacial maximum.

The interannual change of atmospheric CO2: Contribution of subtropical ecosystems?

The global terrestrial carbon cycle model CARAIB (CARbon Assimilation In the Biosphere) is used to study the response of the terrestrial ecosystems to the large scale climate variations over the period 1980–1993. The global net carbon exchange flux with the atmosphere is calculated and compared with the terrestrial contribution derived from the deconvolution of the atmospheric CO2 and δ13C measurements. A fairly large CO2 biospheric source is predicted during the strong El Niño events of 1982–83 and 1986–87 as a consequence of the induced global warming. The direct and indirect temperature controls of the primary production and respiration dominate the CO2 anomaly. An analysis of the relative contribution by latitudinal bands and ecosystems shows that low‐latitude vegetation dominates the variability at the El Niño time scale. In savannas, the model indicates that the interannual changes result, to a large extent, from the control of soil water content on gross primary production (GPP). In the tropical rain forests, both respiration and GPP contribute to the response of the net biospheric flux.

Stochastic generation of meteorological variables and effects on global models of water and carbon cycles in vegetation and soils

Global models of water and carbon cycles in continental vegetation and soils are usually forced with monthly mean climatic data-sets and thus neglect day to day variations of the weather. This treatment may be justified for empirical models based on parametrizations validated at a monthly timescale. Mechanistic models handling hydrological and biological processes at much shorter timescales might, however, be largely affected by such an approximation, since the various processes described are highly nonlinear. A random generator of daily precipitations and temperatures applicable at the global scale has thus been developed from worldwide meteorological data covering 6 years of observations. The probability of a wet day is correlated to the weather encountered the previous day. The amount of precipitation, the daily mean temperature and the diurnal range of temperature are described from the statistical point of view by the cumulative distribution functions (CDF) of three random variables. The CDFs relative to temperatures are different for rainy and dry days. This stochastically generated weather field is used as input to IBM (Improved Bucket Model) and CARAIB (CARbon Assimilation In the Biosphere), two global models of respectively soil hydrology and vegetation productivity. Large differences in both the geographical distribution and the global value of soil water, vegetation productivity and carbon stocks are obtained between the model runs using monthly uniform weather on one side and randomly generated weather on the other. The main contribution to this difference at the global scale arises from the precipitation generation occurring as a result of high degree of nonlinearity of the interception scheme used in IBM.

Modelling the Interactions Between Biospheric and Weathering Processes: Towards a Mechanistic Description of the Land Environment

Recently, some tentative attempts to calculate the distribution of the rock chemical weathering rate over the continents and its variation from the last glacial maximum to the present have been published in the literature (e.g. Gibbs and Kump, 1994). These investigators used empirical relationships between the weathering rate of most important rock types and water runoff. The reconstruction of past weathering can then be performed from the water budget predicted by general circulation models. Although runoff is one of the most important environmental factors contolling chemical weathering at large spatial scales, many complex processes may influence weathering and its response to climatic forcings. For instance, temperature, orogeny or mechanical weathering (and the associated exposure of fresh mineral surface), and vegetation may exert non negligible controls on weathering, as widely discussed in recent years. In particular the effect of vegetation and soil microbial activity is recognized as important in promoting mineral dissolution (e.g. Schwartzman and Volk, 1989), through enhancement of the soil CO2 pressure and through organic secretion from plant roots and fungi. It is unlikely that simple statistical relationships calibrated on the present-day system can be successful in predicting chemical weathering rates under past climatic conditions when the atmospheric CO2 level and the vegetation distribution were very different from today. For this reason, it is necessary to build more mechanistic models of the coupled vegetation-soilrock system, which should describe in some detail the various processes involved in rock weathering, plant growth and soil microbial activity. However, the drawback of such a process-oriented approach is that it must involve an upscaling methodology from small (site, catchment) to large (river basin, continent) spatial units and from short (day, season) to long (103-106 years) time periods, including a validation at each level. Such an initiative has been pursued over the last few years in the biospheric community and global process-oriented models of the land biosphere are currently available. A similar effort may be conducted within the scientific community studying soil formation and weathering. Here we present a preliminary attempt towards a mechanistic description of rock weathering and its interactions with vegetation and soil biogeochemistry. Our approach is based on the chemical weathering model developed by Gwiazda and Broecker (1994) coupled to the CARAIB (CARbon Assimilation In the Biosphere; Warnant et al., 1994) model of the land biosphere. The CARAIB model couples various modules associated with the budget of soil water, photosynthesis, plant growth and respiration, and soil microbial oxidation of organic matter. The most important outputs are the soil water anaount, evapotranspiration, surface runoff, drainage at the bottom of the root zone, net primary productivity (NPP) of vegetation, autotrophic and heterotrophic respirations, and the contents of vegetation, litter and soil carbon reservoirs. The model is designed to be used at the global scale and, in such applications, a resolution of 1~ ~ in longitude-latitude is usually adopted. Nevertheless, in the case of local applications, the spatial resolution may easily be modified or eventually the calculation may only be performed for one single grid cell (test site). The temporal resolution is 1 day for the calculation of every water and carbon reservoirs. If input climatic data are provided monthly, a weather generator can be used to transform these data into daily inputs. This allows to take into account the high degree of non-linearity of

Seasonal and interannual influences of the terrestrial ecosystems on atmospheric CO 2: a model study

The prognostic CARAIB (Carbon Assimilation In the Biosphere) model has been used in conjunction with the Max-Planck Institut TM2 atmospheric transport model to calculate the atmospheric CO 2 fluctuations at the global scale. Two applications are briefly described. In the first one, the seasonal CO 2 variation is calculated and a Fourier analysis is performed to determine the relative contributions of the various vegetation types. It is found that the seasonal signal is dominated by the grasslands and needle leaf forests in the northern boreal and temperate zones. In the southern hemisphere, tropical deciduous forests and grasslands make the primary contribution. In the second application, the net primary productivity (NPP), soil heterotrophic respiration (SHR) and net ecosystem productivity (NEP) are calculated for years 1987 and 1988 with the model driven by observed climatic variables. Preliminary results indicate that the NEP variations between these two years are strongly dominated by tropical ecosystems. However, it is shown that the results are strongly dependent on the dataset used for the 1987-88 temperature record, raising the question of reliability of such modelling studies of the interannual variability of the biosphere.

The seasonality of the CO2 exchange between the atmosphere and the land biosphere: A study with a global mechanistic vegetation model

Two simulations of the seasonal variation of the global atmospheric CO2 distribution are obtained by combining an atmospheric transport model, two parameterizations of soil heterotrophic respiration (SHR), and a mechanistic model of carbon assimilation in the biosphere (CARAIB) that estimates the net primary production (NPP) of continental vegetation. The steady state hypothesis of the biosphere allows the spatial distribution and the global content of the soil carbon to be expressed as a function of the root fractions of soil respiration under forested and herbaceous vegetation covers. The sensitivity of the modeled CO2 signal to the wind field does not exceed the observed interannual variability. The influence of the various vegetation zones is quantified by the Fourier analysis of the modeled atmospheric signal. In the northern hemisphere, the temperate ecosystems dominate the seasonal atmospheric signal of the extratropical latitudes. The ecosystems of the tropical northern zone determine the local signal, while the southern tropical ecosystems influence largely the signal in the whole southern hemisphere. The results give credence to the mechanistic modeling of NPP since the simulated atmospheric signal is comparable with that obtained with normalized difference vegetation index (NDVI) based diagnostic models coupled with a parameterization of SHR fitted to optimize the atmospheric signal.

CARAIB: A global model of terrestrial biological productivity

CARAIB, a mechanistic model of carbon assimilation in the biosphere estimates the net primary productivity (NPP) of the continental vegetation on a grid of 1° × 1° in latitude and longitude. The model considers the annual and diurnal cycles. It is based on the coupling of the three following submodels; a leaf assimilation model including estimates of stomatal conductance and leaf respiration, a canopy model describing principally the radiative transfer through the foliage, and a wood respiration model. Present‐day climate and vegetation characteristics allow the discrimination between ecotypes. In particular, specific information on vegetation distribution and properties is successfully used at four levels; the leaf physiological level, the plant level, the ecosystem level, and the global level. The productivity determined by the CARAIB model is compared with local measurements and empirical estimates showing a good agreement with a global value of 65 Gt C yr−1. The sensitivity of the model to the diurnal cycle and to the abundance of C4 species is also tested. The productivity slightly decreases (10%) when the diurnal cycle of the temperature is neglected. By contrast, neglecting the diurnal cycle of solar irradiance produces unrealistically high values of NPP. Even if the importance of this increase would presumably be reduced by the coupling of CARAIB with a nutrient cycle model, this test emphasizes the key role of the diurnal cycle in a mechanistic model of the NPP. Uncertainties on the abundance and spatial distribution of C4 plants may cause errors in the NPP estimates, however, as demonstrated by two sensitivity tests, these errors are certainly lower than 10% at the global scale as shown by two tests.