[Show abstract][Hide abstract] ABSTRACT: Forests strongly affect climate through the exchange of large amounts of atmospheric CO2. However, the main drivers of spatial variability in net ecosystem production (NEP) on a global scale are poorly known. We present our synthesis study of 92 forests in different climate zones revealing that nutrient availability indeed plays a crucial role in determining NEP and ecosystem carbon-use efficiency [CUEe, i.e. the ratio of NEP to gross primary production (GPP)]. Forests with high GPP exhibited high NEP only in nutrient-rich forests (CUEe = 33 ± 4%; mean ± SE). In nutrient-poor forests, a much larger proportion of GPP was released through ecosystem respiration, resulting in lower CUEe (6 ± 4%). We have also analyzed C-flux time series to check whether GPP, Re and NEP has increased during the last two decades. Our analysis of 23 temperate and boreal forests revealed a strong increase in GPP and NEP from 1992 to 2013, especially important during the first decade. Instead Re has remained rather constant. These increases in GPP and NEP were mostly attributable to the CO2 fertilization effect. However, increasing temperatures and nutrient limitation can constrain the effect of increasing CO2.
1st ICOS International Conference on Greenhouse Gases and Biogeochemical Cycles, Brussels; 09/2014
[Show abstract][Hide abstract] ABSTRACT: The traditional view of forest dynamics originated by Kira and Shidei [Kira T, Shidei T (1967) Jap J Ecol 17:70-87] and Odum [Odum EP (1969) Science 164(3877):262-270] suggests a decline in net primary productivity (NPP) in aging forests due to stabilized gross primary productivity (GPP) and continuously increased autotrophic respiration (Ra). The validity of these trends in GPP and Ra is, however, very difficult to test because of the lack of long-term ecosystem-scale field observations of both GPP and Ra. Ryan and colleagues [Ryan MG, Binkley D, Fownes JH (1997) Ad Ecol Res 27:213-262] have proposed an alternative hypothesis drawn from site-specific results that aboveground respiration and belowground allocation decreased in aging forests. Here, we analyzed data from a recently assembled global database of carbon fluxes and show that the classical view of the mechanisms underlying the age-driven decline in forest NPP is incorrect and thus support Ryan's alternative hypothesis. Our results substantiate the age-driven decline in NPP, but in contrast to the traditional view, both GPP and Ra decline in aging boreal and temperate forests. We find that the decline in NPP in aging forests is primarily driven by GPP, which decreases more rapidly with increasing age than Ra does, but the ratio of NPP/GPP remains approximately constant within a biome. Our analytical models describing forest succession suggest that dynamic forest ecosystem models that follow the traditional paradigm need to be revisited.
Proceedings of the National Academy of Sciences 06/2014; · 9.81 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: This study combined a process-based ecosystem model with a fire regime model to understand the effect of changes in fire regime and climate pattern on woody plants of miombo woodland in African savanna. Miombo woodland covers wide areas in Africa and is subject to frequent anthropogenic fires. The model was developed based on observations of tree topkill rates in individual tree size classes for fire intensity, and re-sprouting. Using current and near-future climate patterns, the model simulated the dynamics of miombo woodland for various fire return intervals and grass cover fractions, allowing fire intensity to be estimated. There was a significant relationship between aboveground woody biomass and long-term fire regimes. An abrupt increase in fire intensity and/or fire frequency applied as a model forcing led to reduced long-term average aboveground woody biomass and mean tree size. Fire intensity increased with increasing living-grass biomass (which provides increased flammable fuel), thereby affecting the relationship between fire regime and tree size, creating a demographic bottleneck on the route to tree maturity. For the current fire regime in miombo woodland, with a fire return interval of about 1.6–3 years, the model predicted fire intensity lower than 930–1700 kW m−1 is necessary to maintain today's aboveground woody biomass under current climate conditions. Future climate change was predicted to have a significant positive effect on woody plants in miombo woodland associated with elevated CO2 concentration and warming, allowing woody plants to survive more effectively against periodic fires.
Journal of Geophysical Research: Biogeosciences 05/2014; · 3.02 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Anthropogenic changes to land cover (LCC) remain common, but continuing land scarcity promotes the widespread intensification of land management changes (LMC) to better satisfy societal demand for food, fibre, fuel and shelter1. The biophysical effects of LCC on surface climate are largely understood2, 3, 4, 5, particularly for the boreal6 and tropical zones7, but fewer studies have investigated the biophysical consequences of LMC; that is, anthropogenic modification without a change in land cover type. Harmonized analysis of ground measurements and remote sensing observations of both LCC and LMC revealed that, in the temperate zone, potential surface cooling from increased albedo is typically offset by warming from decreased sensible heat fluxes, with the net effect being a warming of the surface. Temperature changes from LMC and LCC were of the same magnitude, and averaged 2 K at the vegetation surface and were estimated at 1.7 K in the planetary boundary layer. Given the spatial extent of land management (42–58% of the land surface) this calls for increasing the efforts to integrate land management in Earth System Science to better take into account the human impact on the climate8.
[Show abstract][Hide abstract] ABSTRACT: Forests strongly affect climate through the exchange of large amounts of atmospheric CO2. The main drivers of spatial variability in net ecosystem production (NEP) on a global scale are, however, poorly known. As increasing nutrient availability increases the production of biomass per unit of photosynthesis and reduces heterotrophic respiration in forests, we expected nutrients to determine carbon sequestration in forests. Our synthesis study of 92 forests in different climate zones revealed that nutrient availability indeed plays a crucial role in determining NEP and ecosystem carbon-use efficiency (CUEe; that is, the ratio of NEP to gross primary production (GPP)). Forests with high GPP exhibited high NEP only in nutrient-rich forests (CUEe = 33 ± 4%; mean ± s.e.m.). In nutrient-poor forests, a much larger proportion of GPP was released through ecosystem respiration, resulting in lower CUEe (6 ± 4%). Our finding that nutrient availability exerts a stronger control on NEP than on carbon input (GPP) conflicts with assumptions of nearly all global coupled carbon cycle–climate models, which assume that carbon inputs through photosynthesis drive biomass production and carbon sequestration. An improved global understanding of nutrient availability would therefore greatly improve carbon cycle modelling and should become a critical focus for future research.
[Show abstract][Hide abstract] ABSTRACT: Lateral transport of carbon plays an important role in linking the carbon cycles of terrestrial and aquatic ecosystems. There is, however, a lack of information on the factors controlling one of the main C sources of this lateral flux i.e. the concentration of dissolved organic carbon (DOC) in soil solution across large spatial scales and under different soil, vegetation and climate conditions. We compiled a database on DOC in soil solution down to 80 cm and analyzed it with the aim, firstly, to quantify the differences in DOC concentrations among terrestrial ecosystems, climate zones, soil and vegetation types at global scale and, secondly, to identify potential determinants of the site-to-site variability of DOC concentration in soil solution across European broadleaved and coniferous forests. We found that DOC concentrations were 75% lower in mineral than in organic soil and temperate sites showed higher DOC concentrations than boreal and tropical sites. The majority of the variation (R2 = 0.67-0.99) in DOC concentrations in mineral European forest soils correlates with NH4+, C/N, Al and Fe as the most important predictors. Overall, our results show that the magnitude (23% lower in broadleaved than in coniferous forests) and the controlling factors of DOC in soil solution differ between forest types, with site productivity being more important in broadleaved forests and water balance in coniferous stands.
Global Biogeochemical Cycles 04/2014; 28(5):497-509. · 4.53 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Although forest management is one of the instruments proposed to mitigate climate change, the relationship between forest management and canopy albedo has been ignored so far by climate models. Here we develop an approach that could be implemented in Earth system models. A stand-level forest gap model is combined with a canopy radiation transfer model and satellite-derived model parameters to quantify the effects of forest thinning on summertime canopy albedo. This approach reveals which parameter has the largest affect on summer canopy albedo: we examined the effects of three forest species (pine, beech, oak) and four thinning strategies with a constant forest floor albedo (light to intense thinning regimes) and five different solar zenith angles at five different sites (40° N 9° E-60° N 9° E). During stand establishment, summertime canopy albedo is driven by tree species. In the later stages of stand development, the effect of tree species on summertime canopy albedo decreases in favour of an increasing influence of forest thinning. These trends continue until the end of the rotation, where thinning explains up to 50% of the variance in near-infrared albedo and up to 70% of the variance in visible canopy albedo. The absolute summertime canopy albedo of all species ranges from 0.03 to 0.06 (visible) and 0.20 to 0.28 (near-infrared); thus the albedo needs to be parameterised at species level. In addition, Earth system models need to account for forest management in such a way that structural changes in the canopy are described by changes in leaf area index and crown volume (maximum change of 0.02 visible and 0.05 near-infrared albedo) and that the expression of albedo depends on the solar zenith angle (maximum change of 0.02 visible and 0.05 near-infrared albedo). Earth system models taking into account these parameters would not only be able to examine the spatial effects of forest management but also the total effects of forest management on climate.
[Show abstract][Hide abstract] ABSTRACT: Key message Stand age, water availability, and the length of the warm period are the most influencing controls of forest structure, functioning, and efficiency. Abstract We aimed to discern the distribution and con-trols of plant biomass, carbon fluxes, and resource-use efficiencies of forest ecosystems ranging from boreal to tropical forests. We analysed a global forest database containing estimates of stand biomass and carbon fluxes (400 and 111 sites, respectively) from which we calculated resource-use efficiencies (biomass production, carbon sequestration, light, and water-use efficiencies). We used the WorldClim climatic database and remote-sensing data derived from the Moderate Resolution Imaging Spectro-radiometer to analyse climatic controls of ecosystem functioning. The influences of forest type, stand age, management, and nitrogen deposition were also explored. Tropical forests exhibited the largest gross carbon fluxes (photosynthesis and ecosystem respiration), but rather low net ecosystem production, which peaks in temperate for-ests. Stand age, water availability, and length of the warm period were the main factors controlling forest structure (biomass) and functionality (carbon fluxes and efficien-cies). The interaction between temperature and precipita-tion was the main climatic driver of gross primary production and ecosystem respiration. The mean resource-use efficiency varied little among biomes. The spatial variability of biomass stocks and their distribution among ecosystem compartments were strongly correlated with the variability in carbon fluxes, and both were strongly con-trolled by climate (water availability, temperature) and stand characteristics (age, type of leaf). Gross primary production and ecosystem respiration were strongly cor-related with mean annual temperature and precipitation only when precipitation and temperature were not limiting factors. Finally, our results suggest a global convergence in mean resource-use efficiencies.
[Show abstract][Hide abstract] ABSTRACT: Since 70% of global forests are managed and forests impact the global carbon cycle and the energy exchange with the overlying atmosphere, forest management has the potential to mitigate climate change. Yet, none of the land surface models used in Earth system models, and therefore none of today’s predictions of future climate, account for the interactions between climate and forest management. We addressed this gap in modelling capability by developing and parametrizing a version of the land
surface model ORCHIDEE to simulate the biogeochemical and biophysical effects of forest management. The most significant changes between the new branch called ORCHIDEE-CAN (SVN r2290) and the trunk version of ORCHIDEE (SVN r2243) are the allometric-based allocation of carbon to leaf, root, wood, fruit and reserve pools; the transmittance, absorbance and reflectance of radiation within the canopy; and the vertical
discretisation of the energy budget calculations. In addition, conceptual changes towards a better process representation occurred for the interaction of radiation with snow, the hydraulic architecture of plants, the representation of forest management and a numerical solution for the photosynthesis formalism of Farquhar, von Caemmerer and Berry. For consistency reasons, these changes were extensively linked throughout the code. Parametrization was revisited after introducing twelve new parameter sets that
represent specific tree species or genera rather than a group of unrelated species, as is the case in widely used plant functional types. Performance of the new model was compared against the trunk and validated against independent spatially explicit data for basal area, tree height, canopy strucure, GPP, albedo and evapotranspiration over Europe. For all tested variables ORCHIDEE-CAN outperformed the trunk regarding its ability to reproduce large-scale spatial patterns as well as their inter-annual variability over Europe. Depending on the data stream, ORCHIDEE-CAN had a 67 to 92% chance to reproduce the spatial and temporal variability of the validation data.
Geoscientific Model Development Discussions 01/2014; 7:8565-8647.
[Show abstract][Hide abstract] ABSTRACT: Climate mitigation activities in forests need to be quantified in terms of the long-term effects on forest carbon stocks, accumulation, and emissions. The impacts of future environmental change and bioenergy harvests on regional forest carbon storage have not been quantified. We conducted a comprehensive modeling study and life-cycle assessment of the impacts of projected changes in climate, CO2 concentration, and N deposition, and region-wide forest management policies on regional forest carbon fluxes. By 2100, if current management strategies continue, the warming and CO2 fertilization effect in the given projections result in a 32-68% increase in net carbon uptake, overshadowing increased carbon emissions from projected increases in fire activity and other forest disturbance factors. To test the response to new harvesting strategies, repeated thinnings were applied in areas susceptible to fire to reduce mortality, and two clearcut rotations were applied in productive forests to provide biomass for wood products and bioenergy. The management strategies examined here lead to long-term increased carbon emissions over current harvesting practices, although semi-arid regions contribute little to the increase. The harvest rates were unsustainable. This comprehensive approach could serve as a foundation for regional place-based assessments of management effects on future carbon sequestration by forests in other locations.
[Show abstract][Hide abstract] ABSTRACT: A substantial fraction of the terrestrial carbon sink, past and present,
may be incorrectly attributed to environmental change rather than
changes in forest management.
[Show abstract][Hide abstract] ABSTRACT: Despite an emerging body of literature linking canopy albedo to forest
management, understanding of the process is still fragmented. We
combined a stand-level forest gap model with a canopy radiation transfer
model and satellite-derived model parameters to quantify the effects of
forest thinning, that is removing trees at a certain time during the
forest rotation, on summertime canopy albedo. The effects of different
forest species (pine, beech, oak) and four thinning strategies (light to
intense thinning regimes) were examined. During stand
establishment, summertime canopy albedo is driven by tree species. In
the later stages of stand development, the effect of tree species on
summertime canopy albedo decreases in favour of an increasing influence
of forest thinning on summertime canopy albedo. These trends continue
until the end of the rotation where thinning explains up to 50% of the
variance in near-infrared canopy albedo and up to 70% of the variance in
visible canopy albedo. More intense thinning lowers the
summertime shortwave albedo in the canopy by as much as 0.02 compared to
unthinned forest. The structural changes associated with forest thinning
can be described by the change in LAI in combination with crown volume.
However, forests with identical canopy structure can have different
summertime albedo values due to their location: the further north a
forest is situated, the more the solar zenith angle increases and thus
the higher is the summertime canopy albedo, independent of the
wavelength. Despite the increase of absolute summertime canopy albedo
values with latitude, the difference in canopy albedo between managed
and unmanaged forest decreases with increasing latitude. Forest
management thus strongly altered summertime forest albedo.
[Show abstract][Hide abstract] ABSTRACT: Background/Question/Methods
Pacific Northwest forest diversity, productivity and sustainability are shaped by climate and anthropogenic influences, including legacy land use. These forests are among the highest in productivity and biomass globally. Climate change is expected to include warming, changes in precipitation regimes, and lengthen forest growing seasons which can exacerbate drought stress and contribute to disturbance from insects, pathogens and wildfire. These disturbance agents often occur in sequence, further complicating understanding of potential trajectories of change in carbon cycling. In addition, natural disturbances can be amplified by anthropogenic activities, increasing vulnerability of forests. There is much uncertainty about how and where to mitigate these effects at the local to regional scales. To simulate possible effects of changing climate, N deposition, and forest mitigation options, ecoregion-specific parameterization and fire combustion factors in the NCAR CLM4 model, regional atmospheric forcing datasets, IPCC RCP4.5 and RCP 8.5 scenarios, and a life cycle assessment (LCA) were applied across the region. In addition, a physiology-based model and decision rule analysis produced estimates of species richness and presence/absence by the 21st century.
Simulations of future distribution of major forest species predict potential decline in the presence of ponderosa pine and increase in Douglas-fir in parts of the PNW by 2050, and a shift in species richness to the north. By the end of the 21st century, CLM4 simulations suggest the regional net biome production (NBP) will increase by 58% despite fire increases, indicating that enhanced productivity from a warmer climate and CO2 fertilization compensates for disturbance losses if current management intensity is continued. After simulating typical harvest and thinning treatments intended to sustain forest carbon, and applying a complete Life Cycle Assessment, none of the thinning scenarios reduced net emissions to the atmosphere by the end of the 21st century. The ecoregion-specific model-data integration and LCA provides a framework for regional analysis of future forest biogeochemistry, distribution, and sustainability, and potential effects of forest management mitigation options.
[Show abstract][Hide abstract] ABSTRACT: A substantial amount of the atmospheric carbon taken up on land through photosynthesis and chemical weathering is transported laterally along the aquatic continuum from upland terrestrial ecosystems to the ocean. So far, global carbon budget estimates have implicitly assumed that the transformation and lateral transport of carbon along this aquatic continuum has remained unchanged since pre-industrial times. A synthesis of published work reveals the magnitude of present-day lateral carbon fluxes from land to ocean, and the extent to which human activities have altered these fluxes. We show that anthropogenic perturbation may have increased the flux of carbon to inland waters by as much as 1.0 Pg C yr−1 since pre-industrial times, mainly owing to enhanced carbon export from soils. Most of this additional carbon input to upstream rivers is either emitted back to the atmosphere as carbon dioxide (~0.4 Pg C yr−1) or sequestered in sediments (~0.5 Pg C yr−1) along the continuum of freshwater bodies, estuaries and coastal waters, leaving only a perturbation carbon input of ~0.1 Pg C yr−1 to the open ocean. According to our analysis, terrestrial ecosystems store ~0.9 Pg C yr−1 at present, which is in agreement with results from forest inventories but significantly differs from the figure of 1.5 Pg C yr−1 previously estimated when ignoring changes in lateral carbon fluxes. We suggest that carbon fluxes along the land–ocean aquatic continuum need to be included in global carbon dioxide budgets.
[Show abstract][Hide abstract] ABSTRACT: Forest management is considered to be one of the more easy to implement
instruments available to mitigate climate change as it can lead to
increased sequestration of atmospheric carbon dioxide. However, the
changes in canopy albedo, and hence surface energy balance, may
neutralise or offset the climate benefits of carbon sequestration.
Although there is an emerging body of literature linking canopy albedo
to management, understanding is still fragmented. We here make use of
a generally applicable approach: we combine a stand-level forest gap
model with a canopy radiation transfer model and satellite-derived model
parameters in order to quantify the effects of forest management on
canopy albedo for different forest species and management strategies.
The structural changes associated with forest management can be
described by the change in LAI in combination with crown volume.
However, not only the removal of trees but also the type of understorey
affects the canopy albedo. The relationship between the canopy cover and
the canopy albedo is explained by the influence of the albedo of the
forest floor. A low canopy cover allows more light to penetrate to the
forest floor and is more likely to support an abundant understorey
vegetation. When the albedo of the understorey vegetation exceeds the
albedo of the overstorey canopy, a low canopy cover is likely to result
in a high albedo. We find that the most intensive management measures
lead to the largest albedo change, which can reach up to 9% compared to
unmanaged forest. The choice of species, in combination with thinning,
dominates the variation in canopy albedo. During stand establishment,
albedo is driven by tree species. Following canopy closure, the effect
of tree species on albedo decreases in favour of an increasing
importance of forest management on albedo. These trends continue until
the end of the rotation where management finally explains up to 80% of
the variance in canopy albedo. In summary, forest albedo, and hence
surface energy balance, is strongly altered by humans.
[Show abstract][Hide abstract] ABSTRACT: Stand-replacing fires are the dominant fire type in North American
boreal forest and leave a historical legacy of a mosaic landscape of
different aged forest cohorts. To accurately quantify the role of fire
in historical and current regional forest carbon balance using models,
one needs to explicitly simulate the new forest cohort that is
established after fire. The present study adapted the global
process-based vegetation model ORCHIDEE to simulate boreal forest fire
CO2 emissions and follow-up recovery after a stand-replacing
fire, with representation of postfire new cohort establishment, forest
stand structure and the following self-thinning process. Simulation
results are evaluated against three clusters of postfire forest
chronosequence observations in Canada and Alaska. Evaluation variables
for simulated postfire carbon dynamics include: fire carbon emissions,
CO2 fluxes (gross primary production, total ecosystem
respiration and net ecosystem exchange), leaf area index (LAI), and
biometric measurements (aboveground biomass carbon, forest floor carbon,
woody debris carbon, stand individual density, stand basal area, and
mean diameter at breast height). The model simulation results, when
forced by local climate and the atmospheric CO2 history on
each chronosequence site, generally match the observed CO2
fluxes and carbon stock data well, with model-measurement mean square
root of deviation comparable with measurement accuracy (for
CO2 flux ~100 g C m-2 yr-1, for biomass
carbon ~1000 g C m-2 and for soil carbon ~2000 g C
m-2). We find that current postfire forest carbon sink on
evaluation sites observed by chronosequence methods is mainly driven by
historical atmospheric CO2 increase when forests recover from
fire disturbance. Historical climate generally exerts a negative effect,
probably due to increasing water stress caused by significant
temperature increase without sufficient increase in precipitation. Our
simulation results demonstrate that a global vegetation model such as
ORCHIDEE is able to capture the essential ecosystem processes in
fire-disturbed boreal forests and produces satisfactory results in terms
of both carbon fluxes and carbon stocks evolution after fire, making it
suitable for regional simulations in boreal regions where fire regimes
play a key role on ecosystem carbon balance.
[Show abstract][Hide abstract] ABSTRACT: The lateral transport of dissolved organic carbon (DOC) is an important
and not well-understood process linking terrestrial and aquatic
ecosystems. Up to day very few Earth System Models (ESMs) represent
explicitly this process despite its crucial role in the global carbon
cycle. However, to be able to integrate DOC leaching in ESMs, more
accurate information is needed in order to better understand and predict
DOC dynamics. DOC concentrations mainly vary by geographical location,
soil and vegetation types, topography, season and climate. Within this
framework, a database was designed to compile data on DOC in soil
solution at different depths in different ecosystems around the world,
with special focus on European sites. The database contains information
on 349 sites, with 304 being forest, gathered from published literature
and datasets accessible on the internet. A substantial dataset was
provided by International Co-operative Programme on Assessment and
Monitoring of Air Pollution Effects on Forests (ICP Forests). The
database also includes other meta-data related to the sites, such as
land cover, soil properties, climate, annual water balance and other
soil solution parameters. The analysis of the database has been
focused on: 1) the study of the environmental and physical factors that
are acting as drivers of DOC concentrations changes in soil solution
across sites at European level , and 2) the DOC distribution through the
soil profile and how this varies with different vegetation types and
soil properties. The preliminary results show that variables related to
biological processes (Dry weight of the organic layer, for example) are
the most important in explaining the spatial distribution of the DOC
concentration in soil solution at the European scale. However, the
interactions between variables are complex and we will need further
analysis in order to draw more robust conclusions. With regards to the
vertical profile of DOC, we found that there is a pronounced decrease of
DOC concentrations with depth in forests with specific patterns for
broadleaved and coniferous forests, while in organic soils the opposite
pattern is observed. This new database offers an unique opportunity to
improve our understanding of the process controlling the DOC dynamics.
Based on such results we developed statistic models linking DOC content
and biophysics parameters that can be easily used in ESMs. The
relationships achieved from this analysis will shed light on the most
important drivers of DOC concentrations variability, and can therefore
help in the design, parameterization and validation of current and
future DOC models. This model development will allow to account for
lateral DOC fluxes in ESMs and thus will improve the predictions of
responses to climate change.