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Nitrogen and water availability are two of staple environmental elements in agroecosystems that can substantially alter canopy structure and physiology then crop growth, yielding large impacts on ecosystem regulating/production provisions. However, to date, explicitly quantifying such impacts remains challenging partially due to lack of adequate methodology to capture spatial dimensions of ecosystem changes associated with nitrogen and water effects. A data assimilation, where close-range remote sensing measurements of vegetation indices derived from a hand-held instrument and an unmanned aerial vehicle (UAV) system are linked to leaf and canopy photosynthetic traits quantified at plot level by portable chamber systems, was applied to capture and interpret inter- and intra-field variations in gross primary productivity (GPP) in lowland rice grown under flooded condition (paddy rice, PD) subject to three available nutrient availability and under rainfed condition (RF) in East-Asian monsoon region, South Korea. Spatial variations (SVs) in both GPP and light use efficiency (LUEcabs) early in growing season were amplified by nitrogen addition, and such nutritional effects narrowed over time. Shift planting culture from flooded to rainfed conditions strengthened SVs in GPP and LUEcabs. Intervention of prolonged drought event at late growing season dramatically intensified their SVs that are supposed to seasonally decrease. Nevertheless, nitrogen addition effects on SV of LUEcabs at early growth stage made PD field exert greater SVs than RF field. SV of GPP across PD and RF rice were likely related to LAI development less to LUEcabs while, numerical analysis suggested that consider spatial variation and strength in LUEcabs for the same crop type tends to be vital for better evaluation in landscape/regional patterns of ecosystem photosynthetic productivity at critical phenology stages.

Overgrazing by livestock and the changing patterns of rainfall, characterized by severe drought and floods during dry and wet seasons, respectively, threaten the sustainable productivity of the savannas. To understand the implications of such changes in Lambwe Valley—Kenya, we simulated 50% decrease (50%) and increase (150%) in ambient rainfall (100%), respectively, in grazed (G) and ungrazed (U) sites during dry and wet months. CO2 exchange and biomass production were quantified using chamber method and direct biomass sampling technique, respectively. Plots were named by combining the first letters of the sites followed by rainfall amount, i.e., U150%. Soil moisture (VWC) increased along a rainfall gradient of 50–150%. Grazing reduced the VWC, net ecosystem exchange (NEE), and total biomass by 19.07%, 57.14%, and 37.03%, respectively, with severe effects during the dry months. 50% rainfall strongly influenced the VWC, NEE (negative and positive signs indicate CO2 uptake and net carbon loss, respectively), and biomass compared to 150% rainfall. The U150% plot reported the highest mean NEE (– 8.80 ± 2.26 µmol m−2 s−1), AGB (1208.41 g m−2), and total biomass (1589.06 g m−2) during the wet months. Lower VWC in the G50% plot triggered a net carbon loss of 3.68 ± 0.81 µmol m−2 s−1 (NEE). Our results show that livestock grazing during the dry months hinders herbaceous CO2 uptake and standing biomass. Proper understanding of the interaction between livestock grazing and rainfall variability in humid savannas is essential for sustainable management strategies to regulate the herbaceous productivity.

Purpose: The effect of uncontrolled grazing and unpredictable rainfall pattern on future changes in soil properties and processes of savanna ecosystems is poorly understood. This study investigated how rainfall amount at a gradient of 50%, 100%, and 150% would influence soil bulk density (ρ), volumetric water content (θv), carbon (C), and nitrogen (N) contents in grazed (G) and ungrazed (U) areas. Materials and methods: Rainfall was manipulated by 50% reduction (simulating drought—50%) and 50% increase (simulating abundance—150%) from the ambient (100%) in both G and U areas. Plots were named by combining the first letter of the area followed by rainfall amount, i.e., G150%. Samples for soil ρ, C, and N analysis were extracted using soil corer (8 cm diameter and 10 cm height). Real-time θv was measured using 5TE soil probes (20 cm depth). The EA2400CHNS/O and EA2410 analyzers were used to estimate soil C and N contents respectively. Results and discussion: The interaction between grazing and rainfall manipulation increased θv and C but decreased N with no effect on ρ and C:N ratio. Rainfall reduction (50%) strongly affected most soil properties compared to an increase (150%). The highest (1.241 ± 0.10 g cm−3) and lowest (1.099 ± 0.05 g cm−3) ρ were in the G50% and U150% plots respectively. Soil θv decreased by 34.0% (grazed) and 25.8% (ungrazed) due to drought after rainfall cessation. Soil ρ increased with grazing due to trampling effect, therefore reducing infiltration of rainwater and soil moisture availability. Consequently, soil C content (11.45%) and C:N ratio (24.68%) decreased, whereas N increased (7.8%) in the grazed plots due to reduced C input and decomposition rate. Conclusions: The combined effect of grazing and rainfall variability will likely increase soil θv, thereby enhancing C and N input. Grazing during drought will induce water stress that will destabilize soil C and N contents therefore affecting other soil properties. Such changes are important in predicting the response of soil properties to extreme rainfall pattern and uncontrolled livestock grazing that currently characterize most savanna ecosystems.

Watershed-scale modeling can be a valuable tool to aid in quantification of water quality and yield; however, several challenges remain. In many watersheds, it is difficult to adequately quantify hydrologic partitioning. Data scarcity is prevalent, accuracy of spatially distributed meteorology is difficult to quantify, forest encroachment and land use issues are common, and surface water and groundwater abstractions substantially modify watershed-based processes. Our objective is to assess the capability of the Soil and Water Assessment Tool (SWAT) model to capture event-based and long-term monsoonal rainfall–runoff processes in complex mountainous terrain. To accomplish this, we developed a unique quality-control, gap-filling algorithm for interpolation of high-frequency meteorological data. We used a novel multi-location, multi-optimization calibration technique to improve estimations of catchment-wide hydrologic partitioning. The interdisciplinary model was calibrated to a unique combination of statistical, hydrologic, and plant growth metrics. Our results indicate scale-dependent sensitivity of hydrologic partitioning and substantial influence of engineered features. The addition of hydrologic and plant growth objective functions identified the importance of culverts in catchment-wide flow distribution. While this study shows the challenges of applying the SWAT model to complex terrain and extreme environments; by incorporating anthropogenic features into modeling scenarios, we can enhance our understanding of the hydroecological impact.

Forest ecosystem service (FES) provisioning and management in Vietnam is a priority in the Vietnamese environmental agenda. The main rationale of private forest management is to maximise profits from timber and non-timber forest product (NTFP) production. From a social point of view an under-supply of positive forest externalities (or non-marketed ecosystem services) exists. This paper therefore contributes to the ecosystem service (ES) literature by assessing the production cost structure, in other words, the cost of marketed production and provision of carbon and biodiversity, based on a survey of private forest owners in Hoa Binh Province in Vietnam. The econometric analysis was carried out using a dual cost function approach to analyse the trade-off between forestry costs and ecological performance. This is, to our knowledge, the first time such an approach has been used to estimate the production relationship between marketed outputs and non-marketed ES in the forest sector. This approach appears to be appropriate for handling the multiple joint outputs of forest production and allows us to estimate marginal costs and other cost measures such as cost complementarities in the production of multiple ES. Our results indicate that there is complementarity in the provision of timber and carbon sequestration and, consequently, policies that enhance carbon sequestration in private forests in Vietnam can be implemented without additional costs for the forest owner. We also found that keeping deadwood (to favour biodiversity) had no significant cost and was complementary with NTFP (also an indicator of biodiversity in our study), but could increase the marginal cost of producing timber. This means that biodiversity can be enhanced at no additional cost, provided that the quantity of deadwood does not significantly increase.

In this study, we use the Contingent Valuation (CV) method to estimate households’ willingness to pay (WTP) for the aquatic ecosystem health (biodiversity) improvement. This paper extends CV studies by dealing with the endogenous effect of a proxy variable, namely the subjective experience of negative environmental quality changes. The results show that the correction for the endogeneity bias facilitates the efficiency of parameter estimation in the empirical model. The mean WTP per household accounts for around 46.8% (KRW 79.6) of the current water use charge (KRW 170 per cubic meter). The total benefit from conserving the biodiversity is around KRW 198.62 billion. We found several factors that affect households’ WTP for fish biodiversity conservation, suggesting the importance of these factors in the formulation of water policies associated with aquatic biodiversity. In addition, the inefficient water management costs should be redistributed to other projects or new programs such as for the fish biodiversity conservation.

This study examines the willingness to pay (WTP) for the highland agriculture restriction policy which aims to stabilize the water quality in the Han River basin, South Korea. To estimate the WTP, we use a double-bounded contingent valuation method and a random-effects interval-data regression. We extend contingent valuation studies by dealing with the potential preference anomalies (shift, anchoring, and inconsistent response effects). The result indicates that after the preference anomalies are corrected, the statistical precision of parameter estimates is improved. After correcting the potential preference anomalies, estimated welfare gains are on average South Korean currency (KRW) 2,861 per month per household. Based on the WTP estimate, the total benefits from the land use restriction policy are around KRW 297.73 billion and the total costs are around KRW 129.44 billion. The net benefit is, thus, around KRW 168.29 billion. This study suggests several practical solutions that would be useful for the water management. First, a priority should be given to the valid compensation for the highland farmers’ expected income loss. Second, it is necessary to increase in the unit cost of the highland purchase. Third, wasted or inefficiently used costs (e.g., overinvestment in waste treatment facilities, and temporary upstream community support) should be transferred to the program associated with high mountainous agriculture field purchase. Results of our analysis support South Korean legislators and land use policy makers with useful information for the approval and operationalization of the policy.

Background: Positive plant-plant interactions similar to specialised plant growth forms are potential strategies to overcome the environmental harshness of Afro-alpine ecosystems. However, knowledge about plant-plant interactions is limited for African alpine regions. Aims: We investigated the ameliorative effect of the densely leaved dwarf shrub Helichrysum citrispinum on two frequently co-occurring herbaceous plant species in the alpine zone of Mt. Kilimanjaro. Methods: We recorded microclimatic conditions, plant water potentials and gross primary production (GPP) for plants of the low-growing perennial Alchemilla johnstonii and the tussock grass Festuca abyssinica and compared these parameters between open sites and under H. citrispinum shrubs between July and August 2012. Results: Shrubs significantly buffered daily variation and extreme values of irradiation, air-, plant surface- and soil-temperatures as well as vapour pressure deficit. We found enhanced plant water potentials and gross primary production for shaded plants of both species investigated; ameliorative effects were higher for A. johnstonii than for F. abyssinica. Conclusions: Habitat amelioration of H. citrispinum significantly improves the productivity of plant species that grow under the shrub, although the net outcome may be affected by interspecific growth form differences. Future studies on positive plant-plant interactions should more strongly focus on the ecophysiological consequences of habitat amelioration.

A comprehensive evaluation of energy balance closure is performed across 22 sites and 50 site-years in FLUXNET, a network of eddy covariance sites measuring long-term carbon and energy fluxes in contrasting ecosystems and climates. Energy balance closure was evaluated by statistical regression of turbulent energy fluxes (sensible and latent heat (LE)) against available energy (net radiation, less the energy stored) and by solving for the energy balance ratio, the ratio of turbulent energy fluxes to available energy. These methods indicate a general lack of closure at most sites, with a mean imbalance in the order of 20%. The imbalance was prevalent in all measured vegetation types and in climates ranging from Mediterranean to temperate and arctic. There were no clear differences between sites using open and closed path infrared gas analyzers. At a majority of sites closure improved with turbulent intensity (friction velocity), but lack of total closure was still prevalent under most conditions. The imbalance was greatest during nocturnal periods. The results suggest that estimates of the scalar turbulent fluxes of sensible and LE are underestimated and/or that available energy is overestimated. The implications on interpreting long-term CO2 fluxes at FLUXNET sites depends on whether the imbalance results primarily from general errors associated

Heightened awareness of global change issues within both science and political communities has increased interest in using the global network of eddy covariance flux towers to more fully understand the impacts of natural and anthropogenic phenomena on the global carbon balance. Comparisons of net ecosystem exchange ( F NEE ) responses are being made among biome types, phenology patterns, and stress conditions. The comparisons are usually performed on annual sums of F NEE ; however, the average data coverage during a year is only 65%. Therefore, robust and consistent gap filling methods are required. We review several methods of gap filling and apply them to data sets available from the EUROFLUX and AmeriFlux databases. The methods are based on mean diurnal variation (MDV), look-up tables (LookUp), and nonlinear regressions (Regr.), and the impact of different gap filling methods on the annual sum of F NEE is investigated. The difference between annual F NEE filled by MDV compared to F NEE filled by Regr. ranged from −45 to +200 g C m −2 per year (MDV−Regr.). Comparing LookUp and Regr. methods resulted in a difference (LookUp−Regr.) ranging from −30 to +150 g C m −2 per year. We also investigated the impact of replacing measurements at night, when turbulent mixing is insufficient. The nighttime correction for low friction velocities ( u ∗ ) shifted annual F NEE on average by +77 g C m −2 per year, but in certain cases as much as +185 g C m −2 per year. Our results emphasize the need to standardize gap filling-methods for improving the comparability of flux data products from regional and global flux networks.

By using topographic indices as derived from a Digital Terrain Models (DTM), it is possible to represent the heterogeneity within a landscape. This heterogeneity can reflect both long term evolutionary patterns seen in a landscape and the short term forcing of flow dynamics during storm events. By spatial analysis, the linkage between the geomorphological- hydrological-plant physiological phenomena can be examined. In this study, a direct link will be established between the topographically-driven hydrological phenomena and the eco-physiological response. The topographic distribution function of TOPMODEL is used to control the spatial and temporal flux of the channel flow and water table. The plant physiological model GAS-FLUX is used to give a spatially and temporally dissaggregated species-sensitive estimate of evapotranspiration flux. Evapotranspiration is sensitive to the vegetation phonology, to tundra community physiology and to the temperature regime. A simple linking of TOPMODEL and the GAS-FLUX model is applied to a summer snow-free period to the Imnavait catchment, Alaska (2.2 km2). A species-sensitive evapotranspiration model proved to give the highest quality results when validated against flow observations. Predicted dynamics of variable source area and the component hydrological processes are illustrated.

Detailed data on land use and land cover constitute important information for Earth system models, environmental monitoring and ecosystem services research. Global land cover products are evolving rapidly; however, there is still a lack of information particularly for heterogeneous agricultural landscapes. We censused land use and land cover field by field in the agricultural mosaic catchment Haean in South Korea. We recorded the land cover types with additional information on agricultural practice. In this paper we introduce the data, their collection and the post-processing protocol. Furthermore, because it is important to quantitatively evaluate available land use and land cover products, we compared our data with the MODIS Land Cover Type product (MCD12Q1). During the studied period, a large portion of dry fields was converted to perennial crops. Compared to our data, the forested area was underrepresented and the agricultural area overrepresented in MCD12Q1. In addition, linear landscape elements such as waterbodies were missing in the MODIS product due to its coarse spatial resolution. The data presented here can be useful for earth science and ecosystem services research. The data are available at the public repository Pangaea (doi:110.1594/PANGAEA.823677).

Variations in microclimate and soil characteristics on mountain slopes influence forest structure and function. Precipitation, incoming solar radiation and relative humidity change along a mountain slope. Equally, soil depth and the amount of stored soil moisture vary. The objective of this study was to examine the impacts of these factors on forest water use in mountainous terrains. Transpiration of four temperate deciduous forest stands located at different elevations in South Korea was monitored with a sap flow technique throughout the growing season in 2010. The study sites were located on the north slope at 450 m (450 N), 650 m (650 N), and 950 m (950 N). To examine the effect of aspect, an additional site with a southern aspect was studied at 650 m (650S). All the sites were dominated by Quercus species, with leaf area index (L) ranging between 5 − 6 m2 m−2. Rainfall increased, while air temperature (T A ) and daytime vapor pressure deficit (D) decreased with increasing elevation. We did not observe any gradients in solar radiation (R S ), soil moisture and sap flux density of the individual trees (J st ) with an elevational gradient. Sapwood area (A S ), i.e., hydro-active xylem area, and daily maximum tree water use (max TWU) increased non-linearly with increasing diameter at breast height (DBH). Neither A S nor max TWU varied among tree species or along the elevation. The total annual canopy transpiration (E C ) was 175, 115, 110, and 90 mm for 450 N, 650 N, 650S, and 950 N, respectively. E C declined with increasing elevation as a result of decreasing length of the growing season, D, and T A along the elevation. Significantly (P < 0.001) higher stomatal sensitivity to changes in D was found at the 950 N, leading to lower annual E C and lower water use efficiency (WUE) at this elevation. We conclude that differences in E C exist along the mountain slope studied, corresponding to changing T A , D, length of the growing season, and stomatal sensitivity to D, which should be considered when establishing forest catchment water balances.

International audience; Functional convergence in water use of trees across species from diverse geographic locations was examined using data on tree water use parameters, with the intention of gaining an understanding on the capacity for water transport for trees with varying structural and functional traits. Wood density (rho(w)), which is reported to have a negative exponential relation with sap flow density (SFD), showed a bell-shaped curve when the daily SFD data from 101 tree species belonging to 35 angiosperm and gymnosperm families were plotted. The species came from 23 different geographical locations representing all continents. Trees were most efficient in water transport when the rho(w) was between 0.51 and 0.65 g cm(-3). When the rho(w) increased or decreased from this range, there was a gradual fall in their water transport rate as indicated by lower daily SFD. The unexpected reduction in SFD with decreasing rho(w) is explained in terms of reduced conductance in the transport pathway, which is a precaution taken by the tree for avoiding cavitation or implosion in larger conducting tubes, which is characteristic of low density wood. The development of severe leaf water potential variations, which is frequently reported in such trees, supports this notion. The SFD versus rho(w) relation has a potentially wide applicability in predicting water use by forest stands with varying rho(w). In addition, the occurrence of a high number of tree species with rho(w) values in the range of 0.51-0.65 g cm(-3) across all continents examined points towards the importance of rho(w) in the evolutionary process as related to efficient functioning of the water transport mechanism.

Understanding the relationships between climate and carbon exchange by terrestrial ecosystems is critical to predict future levels of atmospheric carbon dioxide because of the potential accelerating effects of positive climate-carbon cycle feedbacks. However, directly observed relationships between climate and terrestrial CO2 exchange with the atmosphere across biomes and continents are lacking. Here we present data describing the relationships between net ecosystem exchange of carbon (NEE) and climate factors as measured using the eddy covariance method at 125 unique sites in various ecosystems over six continents with a total of 559 site-years. We find that NEE observed at eddy covariance sites is (1) a strong function of mean annual temperature at mid-and high-latitudes, (2) a strong function of dryness at mid-and low-latitudes, and (3) a function of both temperature and dryness around the mid-latitudinal belt (45 degrees N). The sensitivity of NEE to mean annual temperature breaks down at similar to 16 degrees C (a threshold value of mean annual temperature), above which no further increase of CO2 uptake with temperature was observed and dryness influence overrules temperature influence.

Soil respiration constitutes the second largest flux of carbon (C) between terrestrial ecosystems and the atmosphere. This study provides a synthesis of soil respiration (R s) in 20 European grasslands across a climatic transect, including ten meadows, eight pastures and two unmanaged grasslands. Maximum rates of R s ( $$ R_{{{\text{s}}_{{{\text{max}}}} }} $$ ), R s at a reference soil temperature (10°C; $$ R_{{{\text{s}}_{{{\text{10}}}} }} $$ ) and annual R s (estimated for 13 sites) ranged from 1.9 to 15.9 μmol CO2 m−2 s−1, 0.3 to 5.5 μmol CO2 m−2 s−1 and 58 to 1988 g C m−2 y−1, respectively. Values obtained for Central European mountain meadows are amongst the highest so far reported for any type of ecosystem. Across all sites $$ R_{{{\text{s}}_{{{\text{max}}}} }} $$ was closely related to $$ R_{{{\text{s}}_{{{\text{10}}}} }} $$ . Assimilate supply affected R s at timescales from daily (but not necessarily diurnal) to annual. Reductions of assimilate supply by removal of aboveground biomass through grazing and cutting resulted in a rapid and a significant decrease of R s. Temperature-independent seasonal fluctuations of R s of an intensively managed pasture were closely related to changes in leaf area index (LAI). Across sites $$ R_{{{\text{s}}_{{{\text{10}}}} }} $$ increased with mean annual soil temperature (MAT), LAI and gross primary productivity (GPP), indicating that assimilate supply overrides potential acclimation to prevailing temperatures. Also annual R s was closely related to LAI and GPP. Because the latter two parameters were coupled to MAT, temperature was a suitable surrogate for deriving estimates of annual R s across the grasslands studied. These findings contribute to our understanding of regional patterns of soil C fluxes and highlight the importance of assimilate supply for soil CO2 emissions at various timescales.

International audience; The drought of 2003 was exceptionally severe in many regions of Europe, both in duration and in intensity. In some areas, especially in Germany and France, it was the strongest drought for the last 50 years, lasting for more than 6 months. We used continuous carbon and water flux measurements at 12 European monitoring sites covering various forest ecosystem types and a large climatic range in order to characterise the consequences of this drought on ecosystems functioning. As soil water content in the root zone was only monitored in a few sites, a daily water balance model was implemented at each stand to estimate the water balance terms: trees and understorey transpiration, rainfall interception, throughfall, drainage in the different soil layers and soil water content. This model calculated the onset date, duration and intensity of the soil water shortage (called water stress) using measured climate and site properties: leaf area index and phenology that both determine tree transpiration and rainfall interception, soil characteristics and root distribution, both influencing water absorption and drainage. At sites where soil water content was measured, we observed a good agreement between measured and modelled soil water content. Our analysis showed a wide spatial distribution of drought stress over Europe, with a maximum intensity within a large band extending from Portugal to NE Germany. Vapour fluxes in all the investigated sites were reduced by drought, due to stomatal closure, when the relative extractable water in soil (REW) dropped below ca. 0.4. Rainfall events during the drought, however, typically induced rapid restoration of vapour fluxes. Similar to the water vapour fluxes, the net ecosystem production decreased with increasing water stress at all the sites. Both gross primary production (GPP) and total ecosystem respiration (TER) also decreased when REW dropped below 0.4 and 0.2, for GPP and TER, respectively. A higher sensitivity to drought was found in the beech, and surprisingly, in the broadleaved Mediterranean forests; the coniferous stands (spruce and pine) appeared to be less drought-sensitive. The effect of drought on tree growth was also large at the three sites where the annual tree growth was measured. Especially in beech, this growth reduction was more pronounced in the year following the drought (2004). Such lag effects on tree growth should be considered an important feature in forest ecosystems, which may enhance vulnerability to more frequent climate extremes. (c) 2007 Elsevier B.V. All rights reserved.

International audience; This paper discusses the advantages and disadvantages of the different methods that separate net ecosystem exchange (NEE) into its major components, gross ecosystem carbon uptake (GEP) and ecosystem respiration (Reco). In particular, we analyse the effect of the extrapolation of night-time values of ecosystem respiration into the daytime; this is usually done with a temperature response function that is derived from long-term data sets. For this analysis, we used 16 one-year-long data sets of carbon dioxide exchange measurements from European and US-American eddy covariance networks. These sites span from the boreal to Mediterranean climates, and include deciduous and evergreen forest, scrubland and crop ecosystems. We show that the temperature sensitivity of Reco, derived from long-term (annual) data sets, does not reflect the short-term temperature sensitivity that is effective when extrapolating from night- to daytime. Specifically, in summer active ecosystems the long-term temperature sensitivity exceeds the short-term sensitivity. Thus, in those ecosystems, the application of a long-term temperature sensitivity to the extrapolation of respiration from night to day leads to a systematic overestimation of ecosystem respiration from halfhourly to annual time-scales, which can reach 425% for an annual budget and which consequently affects estimates of GEP. Conversely, in summer passive (Mediterranean) ecosystems, the long-term temperature sensitivity is lower than the short-term temperature sensitivity resulting in underestimation of annual sums of respiration. We introduce a new generic algorithm that derives a short-term temperature sensitivity of Reco from eddy covariance data that applies this to the extrapolation from night- to daytime, and that further performs a filling of data gaps that exploits both, the covariance between fluxes and meteorological drivers and the temporal structure of the fluxes. While this algorithm should give less biased estimates of GEP and Reco, we discuss the remaining biases and recommend that eddy covariance measurements are still backed by ancillary flux measurements that can reduce the uncertainties inherent in the eddy covariance data.

Sap flux density (SFD) was measured continuously during 1999 with the heat dissipation method in natural Fagus crenata Blume (Japanese beech) forests growing at 900 m on the northern slope of the Kagura Peak of the Naeba Mountains near the Sea of Japan. Radial variations in xylem daily SFD (SFDday) on three trees were investigated during the growing season. The radial pattern of SFDday that reached a maximum just behind of the cambium layer and then decreased exponentially was described by applying the Weibull function based on sensor measurements at 20 mm intervals. SFDday ratio of 20-40 mm depth (the value of 0-20 mm depth was 100%) increased by 10-32% because of soil drying. The peak value of the Weibull function shifted to 2-10% interior by those changes in the relative xylem depth. The variation of the radial pattern of SFDday under different environmental conditions was expressible as the shift of the peak position of the Weibull function., Influences des conditions environnementales sur les patrons radiaux de densités de flux de sève de Fagus crenata âgés de 70 ans dans les montagnes de Naeba au Japon. La densité de flux de sève (SFD) a été mesurée en continu pendant l'année 1999 avec la méthode de dissipation de chaleur dans une forêt naturelle de Fagus crenata Blume (hêtre du Japon) située à 900 m d'altitude sur un versant nord prés de la mer du Japon. Les variations radiales journalières de SFD (SFDjour) de trois arbres ont été étudiées pendant la saison de croissance. Le patron radial de SFDjour atteint un maximum juste derrière la couche du cambium et puis décroît de façon exponentielle et est décrit par la fonction Weibull sur la base des mesures des capteurs à des intervalles de temps de 20 mm. Le rapport de 20 à 40 mm (la valeur de 0 à 20 mm était égale à 100 %) s'est accru de 10 à 32 % à cause du dessèchement du sol. Le pic de la valeur de la fonction Weibull passe de 2 à 10 % par ces changements de valeur relative de l'épaisseur du xylème. La variation du patron radial de SFDjour sous différentes conditions environnementales était exprimable par le déplacement de la position du pic de la fonction Weibull.

The present study was undertaken to test for the hypothesis that the rate of development in the capacity for photosynthetic electron transport per unit area ( J max;A ), and maximum carboxylase activity of Rubisco ( V cmax;A ) is proportional to average integrated daily quantum flux density ( Q int ) in a mixed deciduous forest dominated by the shade-intolerant species Populus tremula L., and the shade-tolerant species Tilia cordata Mill. We distinguished between the age-dependent changes in net assimilation rates due to modifications in leaf dry mass per unit area ( M A ), foliar nitrogen content per unit dry mass ( N M ), and fractional partitioning of foliar nitrogen in the proteins of photosynthetic electron transport ( F B ), Rubisco ( F R ) and in light-harvesting chlorophyll-protein complexes ( V cmax;A µ M A N M F R ; J max;A µ M A N M F B ). In both species, increases in J max;A and V cmax;A during leaf development were primarily determined by nitrogen allocation to growing leaves, increases in leaf nitrogen partitioning in photosynthetic machinery, and increases in M A . Canopy differences in the rate of development of leaf photosynthetic capacity were mainly controlled by the rate of change in M A . There was only small within-canopy variation in the initial rate of biomass accumulation per unit Q int (slope of M A versus leaf age relationship per unit Q int ), suggesting that canopy differences in the rate of development of J max;A and V cmax;A are directly proportional to Q int . Nevertheless, M A , nitrogen, J max;A and V cmax;A of mature leaves were not proportional to Q int because of a finite M A in leaves immediately after budburst (light-independent component of M A ). M A , leaf chlorophyll contents and chlorophyll : N ratio of mature leaves were best correlated with the integrated average quantum flux density during leaf development, suggesting that foliar photosynthetic apparatus, once developed, is not affected by day-to-day fluctuations in Q int . However, for the upper canopy leaves of P. tremula and for the entire canopy of T. cordata , there was a continuous decline in N contents per unit dry mass in mature non-senescent leaves on the order of 15‐20% for a change of leaf age from 40 to 120 d, possibly manifesting nitrogen reallocation to bud formation. The decline in N contents led to similar decreases in leaf photosynthetic capacity and foliar chlorophyll contents. These data demonstrate that light-dependent variation in the rate of developmental changes in M A determines canopy differences in photosynthetic capacity, whereas foliar photosynthetic apparatus is essentially constant in fully developed leaves.

Drought is one of the most important limitations of photosynthesis in Mediterranean climates. However, Mediterranean sclerophyllous species with long-lived leaves also support extensive and dynamic canopies, with potentially large spatial and age-dependent gradients. We studied within-canopy and temporal patterns in foliage structure, chemistry and photosynthesis in the evergreen species Quercus coccifera L., Q. ilex L. subsp. ballota (Desf.) Samp. in Bol. and Q. suber L. and in the semi-deciduous marcescent species Q. faginea Lam. to determine the role of within-canopy shading and leaf age on foliage functioning. There was a 2.5-fold within-canopy gradient in leaf dry mass per unit area (MA) that was accompanied by a 3-fold range in area-based leaf nitrogen (N) content, the capacity for photosynthetic electron transport (Jmax) and maximum Rubisco carboxylase activity (Vcmax), while the fractional investments of leaf nitrogen in electron transport (FB) and in Rubisco (FR) were relatively constant within the canopy. Leaf aging led to increased MA, larger or constant mass-based N content, larger phosphorous (P) and structural carbon contents, but decreased movable cation contents. Age-dependent increases in MA and N per dry mass meant that Jmax and Vcmax per area were weakly related to leaf age, with a trend of decreasing values in older leaves. However, Jmax and Vcmax per unit dry mass decreased 4-fold across the range of leaf age, primarily owing to decreases in apparent N investments in photosynthetic machinery. This decrease in apparent N investments in photosynthetic machinery was possibly the result of a larger fraction of N bound to cell walls, or of an enhanced CO2 diffusion resistance from the outer surface of cell walls to the chloroplasts in older leaves with thicker and more lignified cell walls. The age-dependent variation in apparent fractional investments of N in photosynthetic machinery reduced the generality of leaf nitrogen v. photosynthesis relationships. Photosynthetic characteristics qualitatively fitted the same patterns with leaf age in all species, but at a common leaf age, area-based leaf photosynthetic potentials depended on species-specific values of MA. These data collectively demonstrate important canopy and age-dependent controls on leaf structure, chemistry and photosynthetic potentials that should be included in larger-scale photosynthesis simulations in Mediterranean climates.

This paper provides the first steps toward a regional-scale analysis of carbon (C) budgets. We explore the ability of the ecosystem model BIOME-BGC to estimate the daily and annual C dynamics of four European coniferous forests and shifts in these dynamics in response to changing environmental conditions. We estimate uncertainties in the model results that arise from incomplete knowledge of site management history (for example, successional stage of forest). These uncertainties are especially relevant in regional-scale simulations, because this type of information is difficult to obtain. Although the model predicted daily C and water fluxes reasonably well at all sites, it seemed to have a better predictive capacity for the photosynthesis-related processes than for respiration. Leaf area index (LAI) was modeled accurately at two sites but overestimated at two others (as a result of poor long-term climate drivers and uncertainties in model parameterization). The overestimation of LAI (and consequently gross photosynthetic production (GPP)), in combination with reasonable estimates of the daily net ecosystem productivity (NEP) of those forests, also illustrates the problem with modeled respiration. The model results suggest that all four European forests have been net sinks of C at the rate of 100-300 gC/m(2)/y and that this C sequestration capacity would be 30%-70% lower without increasing nitrogen (N) deposition and carbon dioxide (CO2) concentrations. The magnitude of the forest responses was dependent not only on the rate of changes in environmental factors, but also on sitespecific conditions such as climate and soil depth. We estimated that the modeled C exchange at the study sites was reduced by 50%-100% when model simulations were performed for climax forests rather than regrowing forests. The estimates of water fluxes were less sensitive to different initializations of state variables or environmental change scenarios than C fluxes.

Summary • The relationships between foliar photosynthetic variables and monoterpene emission rates (E) are reported here for two sclerophylls, Quercus coccifera L. and Q. ilex L., which lack specialized foliar monoterpene-storage structures. • Photosynthesis rates and E were measured within a forest canopy, and in leaves of different exposure in a garrigue ecosystem. Leaf dry mass per unit area (LMA) was used as a proxy of long-term leaf light availability. • Maximum photosynthetic electron transport rate (Jmax), Rubisco carboxylase activity (Vcmax) and light-saturated E (Emax) per unit area increased strongly with increasing LMA. The positive effect of LMA on area-based Emax resulted from an accumulation of enzymes responsible for monoterpenoid synthesis per unit area. Positive relationships were observed between both area- and mass-based Emax and Jmax, but not with photosynthesis rates, suggesting a possible control of terpene synthesis by foliar electron transport rates. Results suggested that monoterpene volatility limitations at low temperatures might also constrain E. Nonspecific storage may explain bursts of monoterpene emission on warm days following cold days, and monoterpene emission in low quantities during night. • Although the correlations between Emax and Jmax do not necessarily give mechanistic insight into the physiology of monoterpene emission, they provide a basis for scaling up leaf emissions to a canopy level. However, both the leaf-to-leaf differences in monoterpene synthase activities, and monoterpene storage may perplex the relations between the electron transport rates and E.

International audience; As length and timing of the growing season are major factors explaining differences in carbon exchange of ecosystems. we analyzed seasonal patterns of net ecosystem carbon exchange (F-NEE) using eddy covariance data of the FLUXNET data base (http://www-eosdis.ornl.gov/FLUXNET). The study included boreal and temperate. deciduous and coniferous forests, Mediterranean evergreen systems, rainforest, native and managed temperate grasslands, tundra, and C-3 and C-4 crops. Generalization of seasonal patterns are useful for identifying functional vegetation types for global dynamic vegetation models, as well as for global inversion studies, and can help improve phenological modules in SVAT or biogeochemical models. The results of this study have important validation potential for global carbon cycle modeling. The phasing of respiratory and assimilatory capacity differed within forest types: for temperate coniferous forests seasonal uptake and release capacities are in phase, for temperate deciduous and boreal coniferous forests, release was delayed compared to uptake. According to seasonal pattern of maximum nighttime release (evaluated over 15-day periods. F-max) the study sites can be grouped in four classes: (1) boreal and high altitude conifers and grasslands: (2) temperate deciduous and temperate conifers; (3) tundra and crops; (4) evergreen Mediterranean and tropical forest,,, Similar results are found for maximum daytime uptake (F-min) and the integral net carbon flux, but temperate deciduous forests fall into class 1. For forests, seasonal amplitudes of F-max and F-min increased in the order tropical < Mediterranean and temperate coniferous < temperate deciduous and boreal forests, and the pattern seems relatively stable for these groups. The seasonal amplitudes of F-max and F-min are largest for managed grasslands and crops. Largest observed values of F-min varied between -48 and -2 mumol m(-2) s(-1), decreasing in the order C-4-crops > C-3-crops > temperate deciduous forests > temperate conifers > boreal conifers > tundra ecosystems. Due to data restrictions, our analysis centered mainly on Northern Hemisphere temperate and boreal forest ecosystems. Grasslands, crops, Mediterranean ecosystems, and rainforests are under-represented. as are savanna systems, wooded grassland, shrubland, or year-round measurements in tundra systems. For regional or global estimates of carbon sequestration potentials, future investigations of eddy covariance should expand in these systems.

International audience; Differences in canopy exchange of water and carbon dioxide that occur due to changes in tree structure and density in montane Norway spruce stands of Central Germany were analyzed with a three dimensional microclimate and gas exchange model STANDFLUX. The model was used to calculate forest radiation absorption, the net photosynthesis and transpiration of single trees, and gas exchange of tree canopies. Model parameterizations were derived for six stands of Picea abies (L.) Karst. differing in age from 40 to 140 years and in density from 1680 to 320 trees per hectare. Parameterization included information on leaf area distribution from tree harvests, tree positions and tree sizes. Gas exchange was modelled using a single species-specific set of physiological parameters and assuming no influence of soil water availability. For our humid montane stands, these simplifying assumptions appeared to be acceptable. Comparisons of modelled daily tree transpiration with water use estimates from xylem sapflow measurements provided a test of the model. Estimates for canopy transpiration rate derived from the model and via xylem sapflow measurements agreed within $\pm$20% , especially at moderate to high air vapor pressure deficits. The results suggest that age and density dependent changes in canopy structure (changes in clumping of needles) and their effect on light exposure of the average needle lead to shifts in canopy conductance and determine tree canopy transpiration in these managed montane forests. Modelled canopy net photosynthesis rates are presented, but have not yet been verified at the canopy level.; Effet de l'âge et de la densité sur la modélisation des échanges gazeux dans la canopée de peuplements de Picea abies (L) Karst. dans le Fichtelgebirge (Allemagne). Les différences d'échange d'eau et de dioxyde de carbone au niveau de la canopée qui se produisent à la suite de changements dans la structure et la densité des arbres dans des peuplements d'Epicéa commun en zone de montagne de l'Allemagne centrale, ont été analysées selon le modèle tridimensionnel de microclimat et d'échange gazeux STANDFLUX. Le modèle a été utilisé pour calculer l'absorption du rayonnement par la forêt, le bilan photosynthétique net et la transpiration d'arbres individuels, et les échanges gazeux au niveau de la canopée. La paramétrisation du modèle a été effectuée à partir des mesures dans six peuplements de Picea abies (L.) Karst. dont l'âge variait de 40 à 140 ans et la densité des peuplements de 1680 à 320 arbres par hectare. La paramétrisation incluait l'information sur la distribution de la surface foliaire à partir de l'exploitation d'arbres, la position des arbres et leurs dimensions. Les échanges gazeux ont été modélisés en utilisant une série de paramètres physiologiques spécifiques de l'espèce, et en supposant qu'il n'y avait aucune influence de la disponibilité en eau. Pour les peuplements considérés en zone humide de montagne, ces hypothèses semblent acceptables. La comparaison de l'estimation par modélisation de la transpiration journalière avec l'estimation de l'eau absorbée par la mesure du flux de sève xylémique a permis de tester le modèle. L'estimation du taux de transpiration par la canopée dérivé du modèle par la mesure du flux xylémique concorde à $\pm20$% , spécialement lorsque les déficits de pression de vapeur sont modérés ou élevés. Les résultats suggèrent que les changements dans la structure de la canopée, dépendant de l'âge et la densité et leurs effets sur l'exposition à la lumière de l'aiguille moyenne conduisent à un décalage dans la conductance de la canopée et déterminent la transpiration dans la canopée de l'arbre de ces forêts aménagées de montagne. Les taux de photosynthèse nette modélisés de la canopée sont présentés, mais n'ont pas encore été validés au niveau des mesures.

Le projet europeen Euroflux a mis en place le premier reseau de mesure et de comparaison des echanges gazeux au-dessus des ecosystemes forestiers a l'echelle continentale, au moyen de la methode des correlations turbulentes, en utilisant une instrumentation et des procedures de traitement standardisees. L'atelier de travail Euroflux intitule « Regulation des flux hydriques dans les peuplements forestiers » a evalue au depart du projet les connaissances actuelles sur les relations hydriques et les bilans hydriques dans les forets europeennes. Les etudes recentes de la transpiration des arbres via les techniques de mesure du flux de seve brute ont ete mises en avant, et les resultats concernant la regulation des flux hydriques ont ete examines. Dans les differents sites Euroflux, des mesures de flux de seve sont mises en oeuvre parallelement a la mesure des flux au-dessus des couverts, dans le but de caracteriser le fonctionnement du compartiment foliaire des arbres. Des mesures de flux de seve realisees dans des sites europeens additionnels accroissent l'etendue du gradient d'observations des flux hydriques, en incluant par exemple des forets alluviales et d'altitude. Parvenir a une meilleurs comprehension des echanges gazeux par les forets, et de leur acclimatation potentielle le long des gradients climatiques, et notamment de leur reponse aux contraintes en situations extremes, est essentiel pour pouvoir faire la synthese des informations sur les flux et sur la biogeochimie aux echelles locale, regionale et continentale.

Tree transpiration was observed with sapflow methods in six Norway spruce (Picea abies) stands located in the Lehstenbach catchment, Fichtelgebirge, Germany, differing in age (40 years up to 140 years), structure, exposition and soil characteristics. The seasonal pattern in tree canopy transpiration, with the highest transpiration rates in July, was very similar among the stands. However, young dense stands had higher transpiration compared to older less dense stands. Because of forest management practices, stand density decreases with increasing stand age and provides the best predictor of canopy water use. Measured xylem sapflux density did not dif- fer significantly among stands, e.g. vary in correlation with stand density. Thus, differences in canopy transpiration were related to differences in cumulative sapwood area, which decreases with age and at lower tree density. While both total sapwood area and individual tree sapwood area decrease in older less dense stands, leaf area index of the stands remains high. Thus, transpiration or physiological activity of the average individual needle must decrease. Simulations with a three-dimensional stand model suggest that stand structural changes influence light climate and reduce the activity of the average needle in the stands. Nevertheless, age and nutrition must be con- sidered with respect to additional direct effects on canopy transpiration. (© Inra/Elsevier, Paris.) transpiration / canopy conductance / sapwood area / stand age / stand density / Picea abies