[show abstract][hide abstract] ABSTRACT: Owing to the peculiarities of forest net primary production humans would appropriate ca. 60% of the global
increment of woody biomass if forest biomass were to produce 20% of current global primary energy supply. We argue that such an increase in biomass harvest would result in younger forests, lower biomass pools, depleted soil nutrient stocks and a loss of other ecosystem functions. The proposed strategy is likely to miss its main objective, i.e. to reduce greenhouse gas (GHG) emissions, because it would result in a reduction of biomass pools that may take decades to centuries to be paid back by fossil fuel substitution, if paid back at all. Eventually, depleted soil fertility will make the production unsustainable and require fertilization, which in turn increases GHG emissions due to N2O emissions. Hence, large-scale production of bioenergy from forest biomass is neither sustainable nor GHG neutral.
[show abstract][hide abstract] ABSTRACT: Conceptual models suggest that stability and age of organic carbon (OC)
in soil depends on the source of plant litter, occlusion within
aggregates, incorporation in organo-mineral complexes, and location
within the soil profile. Various tools like density fractionation,
mineralization experiments, and radiocarbon analyses have been used to
study the importance of these mechanisms. We systematically apply them
to a range of European soils to test whether general controls emerge
even for soils that vary in vegetation, soil types, parent material, and
land use. At each of the 12 study sites, 10 soil cores were sampled in
10 cm depth intervals to 60 cm depth and subjected to density
separation. Bulk soil samples and density fractions (free light
fractions - fLF, occluded light fractions - oLF, heavy fractions - HF)
were analysed for OC, total nitrogen (TN), δ13C, and
Δ14C. Bulk samples were also incubated to determine
mineralizable OC. Declining OC-normalized CO2
release and increasing age with soil depth confirm greater stability of
OC in subsoils across sites. Depth profiles of LF-OC matched those of
roots, which in turn reflect plant functional types in soil profiles not
subject to ploughing. Modern Δ14C signatures and
positive correlation between mineralizable C and fLF-OC indicate the fLF
is an easily available energy and nutrient source for subsurface
microbes. Fossil C derived from the geogenic parent material affected
the age of OC especially in the LF at three study sites. The overall
importance of OC stabilization by binding to minerals was demonstrated
by declining OC-normalized CO2 release rates with increasing
contributions of HF-OC to bulk soil OC and the low Δ14C
values of HF-OC. The stability of HF-OC was greater in subsoils than in
topsoils; nevertheless, a portion of HF-OC was active throughout the
profile. The decrease in Δ14C (increase in age) of
HF-OC with soil depth was related to soil pH as well as to dissolved OC
fluxes. This indicates that dissolved OC translocation contributes to
the formation of subsoil HF-OC and shapes the Δ14C
profiles. While quantitatively less important than OC in the HF,
consistent older ages of oLF-OC than fLF-OC indicate that occlusion of
LF-OC in aggregates also contributes to OC stability in subsoils.
Overall, our results showed that association with minerals is the most
important factor in stabilization of OC in soils.
[show abstract][hide abstract] ABSTRACT: We determine the net land to atmosphere flux of carbon in Russia, including Ukraine, Belarus and Kaza-khstan, using inventory-based, eddy covariance, and inver-sion methods. Our high boundary estimate is −342 Tg C yr −1 from the eddy covariance method, and this is close to the upper bounds of the inventory-based Land Ecosys-tem Assessment and inverse models estimates. A lower boundary estimate is provided at −1350 Tg C yr −1 from the inversion models. The average of the three methods is −613.5 Tg C yr −1 . The methane emission is estimated sepa-rately at 41.4 Tg C yr −1 . These three methods agree well within their respective er-ror bounds. There is thus good consistency between bottom-up and top-down methods. The forests of Russia primar-ily cause the net atmosphere to land flux (−692 Tg C yr −1 from the LEA. It remains however remarkable that the three methods provide such close estimates (−615, −662, −554 Tg C yr −1) for net biome production (NBP), given the inherent uncertainties in all of the approaches. The lack of recent forest inventories, the few eddy covariance sites and associated uncertainty with upscaling and undersampling of concentrations for the inversions are among the prime causes of the uncertainty. The dynamic global vegetation models (DGVMs) suggest a much lower uptake at −91 Tg C yr −1 , and we argue that this is caused by a high estimate of het-erotrophic respiration compared to other methods.
[show abstract][hide abstract] ABSTRACT: We determine the carbon balance of Russia, including Ukraine, Belarus
and Kazakhstan using inventory based, eddy covariance, Dynamic Global
Vegetation Models (DGVM), and inversion methods. Our current best
estimate of the net biosphere to atmosphere flux is -0.66 Pg C
yr-1. This sink is primarily caused by forests that using two
independent methods are estimated to take up -0.69 Pg C yr-1.
Using inverse models yields an average net biopshere to atmosphere flux
of the same value with a interannual variability of 35 % (1σ). The
total estimated biosphere to atmosphere flux from eddy covariance
observations over a limited number of sites amounts to -1 Pg C
yr-1. Fires emit 137 to 121 Tg C yr-1 using two
different methods. The interannual variability of fire emissions is
large, up to a factor 0.5 to 3. Smaller fluxes to the ocean and inland
lakes, trade are also accounted for. Our best estimate for the Russian
net biosphere to atmosphere flux then amounts to -659 Tg C yr-1 as the
average of the inverse models of -653 Tg C yr-1, bottom up
-563 Tg C yr-1 and the independent landscape approach of -761
Tg C yr-1. These three methods agree well within their error
bounds, so there is good consistency between bottom up and top down
methods. The best estimate of the net land to atmosphere flux, including
the fossil fuel emissions is -145 to -73 Tg C yr-1. Estimated methane emissions vary considerably with one
inventory-based estimate providing a net land to atmosphere flux of 12.6
Tg C-CH4yr -1 and an independent model estimate
for the boreal and Arctic zones of Eurasia of 27.6 Tg C-CH4
[show abstract][hide abstract] ABSTRACT: We investigated the variability of tree-ring width, wood density and 13C/12C in beech tree rings (Fagus sylvatica L.), and analyzed the influence of climatic variables and carbohydrate storage on these parameters. Wood cores were taken from dominant beech trees in three stands in Germany and Italy. We used densitometry to obtain density profiles of tree rings and laser-ablation-combustion-GC-IRMS to estimate carbon isotope composition (δ
13C) of wood. The sensitivity of ring width, wood density and δ
13C to climatic variables differed; with tree-ring width responding to environmental conditions (temperature or precipitation) during the first half of a growing season and maximum density correlated with temperatures in the second part of a growing season (July–September). δ
13C variations indicate re-allocation and storage processes and effects of drought during the main growing season. About 20% of inter-annual variation of tree-ring width was explained by the tree-ring width of the previous year. This was confirmed by δ
13C of wood which showed a contribution of stored carbohydrates to growth in spring and a storage effect that competes with growth in autumn. Only mid-season δ
13C of wood was related to concurrent assimilation and climate. The comparison of seasonal changes in tree-ring maximum wood density and isotope composition revealed that an increasing seasonal water deficit changes the relationship between density and 13C composition from a negative relation in years with optimal moisture to a positive relationship in years with strong water deficit. The climate signal, however, is over-ridden by effects of stand density and crown structure (e.g., by forest management). There was an unexpected high variability in mid season δ
13C values of wood between individual trees (−31 to −24‰) which was attributed to competition between dominant trees as indicated by crown area, and microclimatological variations within the canopy. Maximum wood density showed less variation (930–990gcm−3). The relationship between seasonal changes in tree-ring structure and 13C composition can be used to study carbon storage and re-allocation, which is important for improving models of tree-ring growth and carbon isotope fractionation. About 20–30% of the tree-ring is affected by storage processes. The effects of storage on tree-ring width and the effects of forest structure put an additional uncertainty on using tree rings of broad leaved trees for climate reconstruction.
Trees 04/2012; 20(5):571-586. · 1.93 Impact Factor
[show abstract][hide abstract] ABSTRACT: Trees with sufficient nutrition are known to allocate carbon preferentially to aboveground plant parts. Our global study of 49 forests revealed an even more fundamental carbon allocation response to nutrient availability: forests with high-nutrient availability use 58 ± 3% (mean ± SE; 17 forests) of their photosynthates for plant biomass production (BP), while forests with low-nutrient availability only convert 42 ± 2% (mean ± SE; 19 forests) of annual photosynthates to biomass. This nutrient effect largely overshadows previously observed differences in carbon allocation patterns among climate zones, forest types and age classes. If forests with low-nutrient availability use 16 ± 4% less of their photosynthates for plant growth, what are these used for? Current knowledge suggests that lower BP per unit photosynthesis in forests with low- versus forests with high-nutrient availability reflects not merely an increase in plant respiration, but likely results from reduced carbon allocation to unaccounted components of net primary production, particularly root symbionts.
[show abstract][hide abstract] ABSTRACT: The relative role of fire and of climate in determining canopy species
composition and aboveground carbon stocks were investigated.
Measurements were made along a transect extending from the dark taiga
zone of central Siberia, where Picea and Abies dominate the canopy, into
the Larix zone of eastern Siberia. We test the hypotheses that the
change in canopy species composition is based (1) on climate-driven
performance only, (2) on fire only, or (3) on fire-performance
interactions. We show that the evergreen conifers Picea obovata and
Abies sibirica are the natural late-successional species both in central
and eastern Siberia, provided there has been no fire for an extended
period of time. There are no changes in performance of the observed
species along the transect. Fire appears to be the main factor
explaining the dominance of Larix and of soil carbon. Of lesser
influence were longitude as a proxy for climate, local hydrology and
active-layer thickness. We can only partially explain fire return
frequency, which is not only related to climate and land cover, but also
to human behavior. Stand-replacing fires decreased from 300
to 50 yrs between the Yenisei Ridge and the upper Tunguska. Repeated
non-stand-replacing surface fires eliminated the regeneration of Abies
and Picea. With every 100 yrs since the last fire, the percentage of
Larix decreased by 20%. Biomass of stems of single trees
did not show signs of age-related decline. Relative diameter increment
was 0.41 ± 0.20% at breast height and stem volume increased
linearly over time with a rate of about 0.36 t C ha-1
yr-1 independent of age class and species. Stand biomass
reached about 130 t C ha-1(equivalent to about 520
m3 ha-1). Individual trees of Larix were older
than 600 yrs. The maximum age and biomass seemed to be limited by fungal
rot of heart wood. 60% of old Larix and Picea and 30% of Pinus sibirica
trees were affected by stem rot. Implications for the future role of
fire and of plant diseases are discussed.
[show abstract][hide abstract] ABSTRACT: The relative roles of fire and climate in determining canopy species
composition and aboveground carbon stocks were investigated.
Measurements were made along a transect extending from the dark taiga
zone of Central Siberia, where Picea and Abies dominate the canopy, into
the Larix zone of Eastern Siberia. We test the hypotheses that the
change in canopy species composition is based (1) on climate-driven
performance only, (2) on fire only, or (3) on fire-performance
interactions. We show that the evergreen conifers Picea obovata and
Abies sibirica are the natural late-successional species both in Central
and Eastern Siberia, provided there has been no fire for an extended
period of time. There are no changes in the climate-driven performance
of the observed species. Fire appears to be the main factor explaining
the dominance of Larix. Of lesser influence were longitude, hydrology
and active-layer thickness. Stand-replacing fires decreased
from 300 to 50 yr between the Yenisei Ridge and the upper Tunguska.
Repeated non-stand-replacing surface fires eliminated the regeneration
of Abies and Picea. With every 100 yr since the last fire, the
percentage of Larix decreased by 20 %. Biomass of stems of
single trees did not show signs of age-related decline. Relative
diameter increment was 0.41 ± 0.20 % at breast height and stem
volume increased linearly over time with a rate of about 0.36 t C
ha-1 yr-1 independent of age class and species.
Stand volumes reached about 130 t C ha-1 (equivalent to about
520 m3 ha-1). Individual trees of Larix were older
than 600 yr. The maximum age and biomass seemed to be limited by fungal
rot of heart wood. 60 % of old Larix and Picea and 30 % of Pinus
sibirica trees were affected by stem rot. Implications for the future
role of fire and of plant diseases are discussed.
[show abstract][hide abstract] ABSTRACT: Globally, terrestrial ecosystems have absorbed about 30% of anthropogenic greenhouse gas emissions over the period 2000–2007 and inter-hemispheric gradients indicate that a significant fraction of terrestrial carbon sequestration must be north of the Equator. We present a compilation of the CO2, CO, CH4 and N2O balances of Europe following a dual constraint approach in which (1) a land-based balance derived mainly from ecosystem carbon inventories and (2) a land-based balance derived from flux measurements are compared to (3) the atmospheric data-based balance derived from inversions constrained by measurements of atmospheric GHG (greenhouse gas) concentrations. Good agreement between the GHG balances based on fluxes (1294 ± 545 Tg C in CO2-eq yr−1), inventories (1299 ± 200 Tg C in CO2-eq yr−1) and inversions (1210 ± 405 Tg C in CO2-eq yr−1) increases our confidence that the processes underlying the European GHG budget are well understood and reasonably sampled. However, the uncertainty remains large and largely lacks formal estimates. Given that European net land to atmosphere exchanges are determined by a few dominant fluxes, the uncertainty of these key components needs to be formally estimated before efforts could be made to reduce the overall uncertainty. The net land-to-atmosphere flux is a net source for CO2, CO, CH4 and N2O, because the anthropogenic emissions by far exceed the biogenic sink strength. The dual-constraint approach confirmed that the European biogenic sink removes as much as 205 ± 72 Tg C yr−1 from fossil fuel burning from the atmosphere. However, This C is being sequestered in both terrestrial and inland aquatic ecosystems. If the C-cost for ecosystem management is taken into account, the net uptake of ecosystems is estimated to decrease by 45% but still indicates substantial C-sequestration. However, when the balance is extended from CO2 towards the main GHGs, C-uptake by terrestrial and aquatic ecosystems is offset by emissions of non-CO2 GHGs. As such, the European ecosystems are unlikely to contribute to mitigating the effects of climate change.
[show abstract][hide abstract] ABSTRACT: Precise determination of changes in organic carbon (OC) stocks is prerequisite to understand the role of soils in the global cycling of carbon and to verify changes in stocks due to management. A large dataset was collected to form base to repeated soil inventories at 12 CarboEurope sites under different climate and land-use, and with different soil types. Concentration of OC, bulk density (BD), and fine earth fraction were determined to 60 cm depth at 100 sampling points per site. We investigated (1) time needed to detect changes in soil OC, assuming future re-sampling of 100 cores; (2) the contribution of different sources of uncertainties to OC stocks; (3) the effect of OC stock calculation on mass rather than volume base for change detection; and (4) the potential use of pedotransfer functions (PTF) for estimating BD in repeated inventories. The period of time needed for soil OC stocks to change strongly enough to be detectable depends on the spatial variability of soil properties, the depth increment considered, and the rate of change. Cropland sites, having small spatial variability, had lower minimum detectable differences (MDD) with 100 sampling points (105 ± 28 kg C m−2 for the upper 10 cm of the soil) than the grassland (206 ± 64 kg C m−2) and forest (246 ± 64 kg C m−2) sites. Expected general trends in soil OC indicate that changes could be detectable after 2–15 years with 100 samples if changes occurred in the upper 10 cm of stone-poor soils. Error propagation analyses showed that in undisturbed soils with low stone contents, OC concentrations contributed most to OC stock variability while BD and fine earth fraction were more important in upper soil layers of croplands and in stone rich soils. Though the calculation of OC stocks based on equivalent soil masses slightly decreases the chance to detect changes with time at most sites except for the croplands, it is still recommended to account for changing bulk densities with time. Application of PTF for the estimation of bulk densities caused considerable underestimation of total variances of OC stocks if the error associated with the PTF was not accounted for, which rarely is done in soil inventories. Direct measurement of all relevant parameters approximately every 10 years is recommended for repeated soil OC inventories.
[show abstract][hide abstract] ABSTRACT: Overviewing the European carbon (C), greenhouse gas (GHG), and non-GHG fluxes, gross primary productivity (GPP) is about 9.3 Pg yr−1, and fossil fuel imports are 1.6 Pg yr−1. GPP is about 1.25% of solar radiation, containing about 360 × 1018 J energy – five times the energy content of annual fossil fuel use. Net primary production (NPP) is 50%, terrestrial net biome productivity, NBP, 3%, and the net GHG balance, NGB, 0.3% of GPP. Human harvest uses 20% of NPP or 10% of GPP, or alternatively 1‰ of solar radiation after accounting for the inherent cost of agriculture and forestry, for production of pesticides and fertilizer, the return of organic fertilizer, and for the C equivalent cost of GHG emissions. C equivalents are defined on a global warming potential with a 100-year time horizon. The equivalent of about 2.4% of the mineral fertilizer input is emitted as N2O. Agricultural emissions to the atmosphere are about 40% of total methane, 60% of total NO-N, 70% of total N2O-N, and 95% of total NH3-N emissions of Europe. European soils are a net C sink (114 Tg yr−1), but considering the emissions of GHGs, soils are a source of about 26 Tg CO2 C-equivalent yr−1. Forest, grassland and sediment C sinks are offset by GHG emissions from croplands, peatlands and inland waters. Non-GHGs (NH3, NOx) interact significantly with the GHG and the C cycle through ammonium nitrate aerosols and dry deposition. Wet deposition of nitrogen (N) supports about 50% of forest timber growth. Land use change is regionally important. The absolute flux values total about 50 Tg C yr−1. Nevertheless, for the European trace-gas balance, land-use intensity is more important than land-use change. This study shows that emissions of GHGs and non-GHGs significantly distort the C cycle and eliminate apparent C sinks.
[show abstract][hide abstract] ABSTRACT: This paper responds to the Forum contribution by Piovesan & Adams (2000) who criticized the results obtained by the EUROFLUX network on carbon fluxes of several European forests. The major point of criticism was that the data provided by EUROFLUX are inconsistent with current scientific understanding. It is argued that understanding the terrestrial global carbon cycle requires more than simply restating what was known previously, and that Piovesan & Adams have not been able to show any major conflicts between our findings and ecosystem or atmospheric-transport theories.
[show abstract][hide abstract] ABSTRACT: The long-term carbon balance (NBP) of grasslands is estimated by combining scarce multi-year eddy-covariance observations at ecosystem observation sites where information on carbon inputs and harvesting removals is available. Following accounting for carbon leached to rivers, we estimated grasslands to be net carbon sinks of 74±10 g C m−2 yr−1. Uncertainties arise from the small number of sites and the short measurement period. Only 11 sites, out of a total of 20 grassland sites in Europe where eddy covariance systems are installed, were set-up for estimating NBP. These 11 selected sites are representative of intensive management practice and we lack information on disturbance history, such as plowing. This suggests that the grassland NBP estimate is likely biased towards overestimating the sink, compared to the European average. Direct measurements of Net Primary Productivity (NPP) are not possible in grasslands given permanent biomass removal by grazing and mowing, uncertainties in rhizodeposition and production of volatile organic carbon compounds lost to the atmosphere. Therefore, the grassland process-based ecosystem model PASIM was used to estimate the spatial-temporal distribution of NPP, providing a European average value of 750±150 g C across extensively grazed, intensively grazed pastures, and forage production systems. In Europe the NPP of grasslands seems higher than that of croplands and forests. The carbon sequestration efficiency of grasslands, defined as the ratio of NBP to NPP, amounts to 0.09±0.10. Therefore, per unit of carbon input, grasslands sequester 3–4 times more carbon in the soil than forests do, making them a good candidate for managing onsite carbon sinks. When using the 100 yr greenhouse warming potential for CH4 and N2O, their emissions due to management of grasslands together offset roughly 70–80% of the carbon sink. Uncertainties on the European grassland greenhouse gas balance, including CO2, CH4 and N2O fluxes are likely to be reduced in the near future, with data being collected from more sites, and improved up-scaling methods.
[show abstract][hide abstract] ABSTRACT: Quantification of an ecosystem's carbon balance and its components is
pivotal for understanding both ecosystem functioning and global cycling.
Several methods are being applied in parallel to estimate the different
components of the CO2 balance. However, different methods are
subject to different sources of error. Therefore, it is necessary that
site level component estimates are cross-checked against each other
before being reported. Here we present a two-step approach for testing
the accuracy and consistency of eddy covariance-based gross primary
production (GPP) and ecosystem respiration (Re) estimates with biometric
measurements of net primary production (NPP), autotrophic (Ra) and
heterotrophic (Rh) respiration. The test starts with closing the
CO2 balance to account for reasonable errors in each of the
component fluxes. Failure to do so within the constraints will classify
the flux estimates on the site level as inconsistent. If the
CO2 balance can be closed, the test continues by comparing
the closed site level Ra/GPP with the Rh/GPP ratio. The consistency of
these ratios is then judged against expert knowledge. Flux estimates of
sites that pass both steps are considered consistent. An inconsistent
ratio is not necessarily incorrect but provides a signal for careful
data screening that may require further analysis to identify the
possible biological reasons of the unexpected ratios. We reviewed the
literature and found 16 sites, out of a total of 529 research forest
sites, that met the data requirements for the consistency test. Thirteen
of these sites passed both steps of the consistency cross-check.
Subsequently, flux ratios (NPP/GPP, Rh/NPP, Rh/Re, and Re/GPP) were
calculated for the consistent sites. Similar ratios were observed at
sites which lacked information to check consistency, indicating that the
flux data that are currently used for validating models and testing
ecological hypotheses are largely consistent across a wide range of site
productivities. Confidence in the output of flux networks could be
further enhanced if the required fluxes are independently estimated at
all sites for multiple years and harmonized methods are used.
Global Biogeochemical Cycles 09/2009; · 4.68 Impact Factor
[show abstract][hide abstract] ABSTRACT: We analyzed the magnitude, the trends and the uncertainties of fossil-fuel CO2 emissions in the European Union 25 member states (hereafter EU25), based on emission inventories from energy-use statistics. The stability of emissions during the past decade at EU25 scale masks decreasing trends in some regions, offset by increasing trends elsewhere. In the recent 4 years, the new Eastern EU25 member states have experienced an increase in emissions, reversing after a decade-long decreasing trend. Mediterranean and Nordic countries have also experienced a strong acceleration in emissions. In Germany, France and United Kingdom, the stability of emissions is due to the decrease in the industry sector, offset by an increase in the transportation sector. When four different inventories models are compared, we show that the between-models uncertainty is as large as 19% of the mean for EU25, and even bigger for individual countries. Accurate accounting for fossil CO2 emissions depends on a clear understanding of system boundaries, i.e. emitting activities included in the accounting. We found that the largest source of errors between inventories is the use of distinct systems boundaries (e.g. counting or not bunker fuels, cement manufacturing, nonenergy products). Once these inconsistencies are corrected, the between-models uncertainty can be reduced down to 7% at EU25 scale. The uncertainty of emissions at smaller spatial scales than the country scale was analyzed by comparing two emission maps based upon distinct economic and demographic activities. A number of spatial and temporal biases have been found among the two maps, indicating a significant increase in uncertainties when increasing the resolution at scales finer than ≈200 km. At 100 km resolution, for example, the uncertainty of regional emissions is estimated to be 60 g C/(m2 yr), up to 50% of the mean. The uncertainty on regional fossil-fuel CO2 fluxes to the atmosphere could be reduced by making accurate 14C measurements in atmospheric CO2, and by combining them with transport models.
Global Change Biology 05/2009; 16(2010-05-5):1395-1408. · 6.91 Impact Factor
[show abstract][hide abstract] ABSTRACT: Water-use efficiency (WUE) has been recognized as an important characteristic of vegetation productivity in various natural scientific disciplines for decades, but only recently at the ecosystem level, where different ways exist to characterize water-use efficiency. Hence, the objective of this research was (a) to systematically compare different ways of calculating ecosystem water-use efficiency (WUEe) from eddy-covariance measurements, (b) quantify the diurnal, seasonal and interannual variability of WUEe in relation to meteorological conditions, and (c) analyse between-site variability of WUEe as affected by vegetation type and climatic conditions, across sites in European forest ecosystems. Day-to-day variability of gross primary productivity (GPP) and evapotranspiration (ET) were more strongly coupled than net ecosystem production (NEP) and ET, obviously because NEP also depends on the respiration that is not heavily coupled to water fluxes. However, the slope of daytime NEP versus ET (mNEP) from half-hourly measurements of a single day may also be used as a WUEe-estimate giving very similar results to those of the GPP-ET slope (mGPP), since the diurnal variation is dominated by GPP. Since ET is the sum of transpiration (linked to GPP) and evaporation from wet vegetation and soil surfaces (not linked to GPP) we expected that WUEe is increasing when days after rain are excluded from the analysis. However only very minor changes were found, justifying an analysis of WUEe related to vegetation type. In most of the studied ecosystems the instantaneous WUEGPP was quite sensitive to diurnally varying meteorological conditions and tended to decline from the morning to the afternoon by more than 50% because of increasing vapour pressure deficits (VPD). Seasonally, WUEGPP increased with a rising monthly precipitation sum and rising average monthly temperatures up to a threshold of 11, 14 and 18°C in boreal, temperate and Mediterranean ecosystems, respectively. Across all sites, the highest monthly WUEGPP-values were detected at times of positive anomalies of summer-precipitation. During drought periods with high temperatures, high VPD, little precipitation and low soil water content, the water-use efficiency of gross carbon uptake (WUEGPP) tended to decrease in all forest types because of a stronger decline of GPP compared to ET. However the largest variation of growing season WUEGPP was found between-sites and significantly related to vegetation type: WUEGPP was highest in ecosystems dominated by deciduous trees ranging from 5.0 g CO2 kg H2O−1 for temperate broad-leaved deciduous forests (TD), to 4.5 for temperate mixed forests (TM), 3.5 for temperate evergreen conifers (TC), 3.4 for Mediterranean broad-leaved deciduous forests (MD), 3.3 for Mediterranean broad-leaved evergreen forests (Mbeg), 3.1 for Mediterranean evergreen conifers (MC), 2.9 for boreal evergreen conifers (BC) and only 1.2 g CO2 kg H2O−1 for a boreal wetland site (BT). Although vegetation type and meteorology co-vary, the WUEGPP variation was hardly related to meteorology, as we could show by comparing similar meteorological conditions only. Furthermore we compared across-site WUEGPP only under conditions when the 10% high GPP rates were exhibited. The between site differences remained, and at all sites ecosystem reached higher WUEGPP levels under this condition. This means when vegetation is most productive usually it also maximises the amount of carbon gained per water lost. Overall our results show that water-use efficiency exhibits a strong time-scale dependency in the sense that at longer time-scale meteorological conditions play a smaller role compared to shorter time scale. Moreover, we highlight the role of vegetation in determining carbon-water relation at ecosystem level. Consequently, all predictions of changing carbon-water cycle under changing climate should take into this role and the differences between vegetation types. These results show the strong time-scale dependency of water-use efficiency
[show abstract][hide abstract] ABSTRACT: European forests are intensively exploited for wood products, yet they also form a sink for carbon. European forest inventories, available for the past 50 years, can be combined with timber harvest statistics to assess changes in this carbon sink. Analysis of these data sets between 1950 and 2000 from the EU-15 countries excluding Luxembourg, plus Norway and Switzerland, reveals that there is a tight relationship between increases in forest biomass and forest ecosystem productivity but timber harvests grew more slowly. Encouragingly, the environmental conditions in combination with the type of silviculture that has been developed over the past 50 years can efficiently sequester carbon on timescales of decades, while maintaining forests that meet the demand for wood. However, a return to using wood as biofuel and hence shorter rotations in forestry could cancel out the benefits of carbon storage over the past five decades