ALBERT

Description: Thermal adaptation of gross primary production and ecosystem respiration has been well documented over broad thermal gradients. However, no study has examined their interaction as a function of temperature, i.e. the thermal responses of net ecosystem exchange of carbon (NEE). In this study, we constructed temperature response curves of NEE against temperature using 380 site-years of eddy covariance data at 72 forest, grassland and shrubland ecosystems located at latitudes ranging from ~29° N to 64° N. The response curves were used to define two critical temperatures: transition temperature (Tb) at which ecosystem transferring from carbon source to sink and optimal temperature (To) at which carbon uptake is maximized. Tb was strongly correlated with annual mean air temperature. To was strongly correlated with mean temperature during the net carbon uptake period across the study ecosystems. Our results suggested that ecosystem CO2 flux switched from source to sink when air temperature reached annual mean temperature in spring and reached maximum uptake at mean temperature of the net carbon uptake period. Our results imply that the net ecosystem exchange of carbon adapt to the temperature across the geographical range due to intrinsic connections between vegetation primary production and ecosystem respiration.

Description: Dissolved organic carbon (DOC) in soil solution is connected to DOC in surface waters through hydrological flows. Therefore, it is expected that long-term dynamics of DOC in surface waters reflect DOC trends in soil solution. However, a multitude of site-studies has failed so far to establish consistent trends in soil solution DOC, whereas increasing concentrations in European surface waters over the past decades appear to be the norm, possibly as a result from acidification recovery. The objectives of this study were therefore to understand the long-term trends of soil solution DOC from a large number of European forests (ICP Forests Level II plots) and determine their main physico-chemical and biological controls. We applied trend analys is at two levels: 1) to the entire European dataset and 2) to the individual time series and related trends with plot characteristics, i.e., soil and vegetation properties, soil solution chemistry and atmospheric deposition loads. Analyses of the entire dataset showed an overall increasing trend in DOC concentrations in the organic layers, but, at individual plots and depths, there was no clear overall trend in soil solution DOC across Europe with temporal slopes of soil solution DOC ranging between −16.8 % yr−1 and +23 % yr−1 (median= +0.4 % yr−1). The non-significant trends (40 %) outnumbered the increasing (35 %) and decreasing trends (25 %) across the 97 ICP Forests Level II sites. By means of multivariate statistics, we found increasing DOC concentrations with increasing mean nitrate (NO3−) deposition and decreasing DOC concentrations with decreasing me an sulphate (SO42−) deposition, with the magnitude of these relationships depending on plot deposition history. While the attribution of increasing trends in DOC to the reduct ion of SO42− deposition could be confirmed in N-poorer forests, in agreement with observations in surface waters, this was not the case in N-richer forests. In conclusion, long-term trends of soil solution DOC reflected the interactions between controls acting at local (soil and vegetation properties) and regional (atmospheric deposition of SO42− and inorganic N) scales.

Description: Carbon taken up by the forest canopy is allocated to tree organs for biomass production and respiration. Because tree organs have different life span and decomposition rate, the tree C allocation determines the residence time of C in the ecosystem and its C cycling rate. The study of the carbon-use efficiency, or ratio between net primary production (NPP) and gross primary production (GPP), represents a convenient way to analyse the C allocation at the stand level. Previous studies mostly focused on comparison of the annual NPP-GPP ratio among forests of different functional types, biomes and age. In this study, we extend the current knowledge by assessing (i) the annual NPP-GPP ratio and its interannual variability (for five years) for five tree organs (leaves, fruits, branches, stem and coarse roots), and (ii) the seasonal dynamic of NPP-GPP ratio of leaves and stems, for two stands dominated by European beech and Scots pine. The average NPP-GPP ratio for the beech stand (38%) was similar to previous estimates for temperate deciduous forests, whereas the NPP-GPP ratio for the pine stand (17%) is the lowest recorded till now in the literature. The proportion of GPP allocated to leaf NPP was similar for both species, whereas beech allocated a remarkable larger proportion of GPP to wood NPP than pine (29% vs. 6%, respectively). The interannual variability of the NPP-GPP ratio for wood was substantially larger than the interannual variability of the NPP-GPP ratio for leaves, fruits and overall stand and it is likely to be controlled by previous year air temperature (both species), previous year drought intensity (beech) and thinning (pine). Seasonal pattern of NPP-GPP ratio greatly differed between beech and pine, with beech presenting the largest ratio in early season, and pine a more uniform ratio along the season. For beech, NPP-GPP ratio of leaves and stems peaked during the same period in the early season, whereas they peaked in opposite periods of the growing season for pine. Seasonal differences in C allocation are likely due to functional differences between deciduous and evergreen species and temporal variability of the sink strength. The similar GPP and autotrophic respiration between stands and the remarkable larger C allocation to wood at the beech stand indicate that at the beech ecosystem C has a longer residence time than at the pine ecosystem. Further research on belowground production and particularly on fine roots and ectomycorrhizal fungi likely represents the most important step to progress our knowledge on C allocation dynamics.

Description: Gross primary productivity (GPP) is the largest and most variable component of the global terrestrial carbon cycle. Repeatable and accurate monitoring of terrestrial GPP is therefore critical for quantifying dynamics in regional-to-global carbon budgets. Remote sensing provides high frequency observations of terrestrial ecosystems and is widely used to monitor and model spatiotemporal variability in ecosystem properties and processes that affect terrestrial GPP. We used data from the Moderate Resolution Imaging Spectroradiometer (MODIS) and FLUXNET to assess how well four metrics derived from remotely sensed vegetation indices (hereafter referred to as proxies) and six remote sensing-based models capture spatial and temporal variations in annual GPP. Specifically, we used the FLUXNET "La Thuile" data set, which includes several times more sites (144) and site years (422) than previous efforts have used. Our results show that remotely sensed proxies and modeled GPP are able to capture statistically significant amounts of spatial variation in mean annual GPP in every biome except croplands, but that the total variance explained differed substantially across biomes (R2 ≈ 0.1−0.8). The ability of remotely sensed proxies and models to explain interannual variability GPP was even more limited. Remotely sensed proxies explained 40–60% of interannual variance in annual GPP in moisture-limited biomes including grasslands and shrublands. However, none of the models or remotely sensed proxies explained statistically significant amounts of interannual variation in GPP in croplands, evergreen needleleaf forests, and deciduous broadleaf forests. Because important factors that affect year-to-year variation in GPP are not explicitly captured or included in the remote sensing proxies and models we examined (e.g., interactions between biotic and abiotic conditions, and lagged ecosystems responses to environmental process), our results are not surprising. Nevertheless, robust and repeatable characterization of interannual variability in carbon budgets is critically important and the carbon cycle science community is increasingly relying on remotely sensing data. As larger and more comprehensive data sets derived from the FLUXNET community become available, additional systematic assessment and refinement of remote sensing-based methods for monitoring annual GPP is warranted.

Description: Plant phenological development is orchestrated through subtle changes in photoperiod, temperature, soil moisture and nutrient availability. Presently, the exact timing of plant development stages and their response to climate and management practices are crudely represented in land surface models. As visual observations of phenology are laborious, there is a need to supplement long-term observations with automated techniques such as those provided by digital repeat photography at high temporal and spatial resolution. We present the first synthesis from a growing observational network of digital cameras installed on towers across Europe above deciduous and evergreen forests, grasslands and croplands, where vegetation and atmosphere CO2 fluxes are measured continuously. Using colour indices from digital images and using piecewise regression analysis of time-series, we explored whether key changes in canopy phenology could be detected automatically across different land use types in the network. The piecewise regression approach could capture the start and end of the growing season, in addition to identifying striking changes in colour signals caused by flowering and management practices such as mowing. Exploring the dates of green up and senescence of deciduous forests extracted by the piecewise regression approach against dates estimated from visual observations we found that these phenological events could be detected adequately (RMSE 〈 8 and 11 days for leaf out and leaf fall respectively). We also investigated whether the seasonal patterns of red, green and blue colour fractions derived from digital images could be modelled mechanistically using the PROSAIL model parameterised with information of seasonal changes in canopy leaf area and leaf chlorophyll and carotenoid concentrations. From a model sensitivity analysis we found that variations in colour fractions, and in particular the late spring "green hump" observed repeatedly in deciduous broadleaf canopies across the network, are essentially dominated by changes in the respective pigment concentrations. Using the model we were able to explain why this spring maximum in green signal is often observed out of phase with the maximum period of canopy photosynthesis in ecosystems across Europe. Coupling such quasi-continuous digital records of canopy colours with co-located CO2 flux measurements will improve our understanding of how changes in growing season length are likely to shape the capacity of European ecosystems to sequester CO2 in the future.

Description: In this study, we quantified the predictive accuracy loss involved with omitting photosynthetic capacity variation for a Scots pine (Pinus sylvestris L.) stand in Flanders, Belgium. Over the course of one phenological year, we measured the maximum carboxylation capacity at 25 °C (Vm25), the maximum electron transport capacity at 25 °C (Jm25), and the leaf area index (LAI) of different-aged needle cohorts in the upper and lower canopy. We used these measurements as input for a process-based multi-layer canopy model with the objective to quantify the difference in yearly gross ecosystem productivity (GEP) and canopy transpiration (Ecan) simulated under scenarios in which the observed needle age-related and/or seasonal variation of Vm25 and Jm25 was omitted. We compared simulated GEP with estimations obtained from eddy covariance measurements. Additionally, we measured summer needle N content to investigate the relationship between photosynthetic capacity parameters and needle N content along different needle ages. Results show that Vm25 and Jm25 were, respectively, 27% and 13% higher in current-year than in one-year old needles. A significant seasonality effect was found on Vm25, but not on Jm25. Summer needle N content was considerably lower in current-year than in one-year-old needles. As a result, the correlations between Vm25 and needle N content and Jm25 and needle N content were negative and non-significant, respectively. Some explanations for these unexpected correlations were brought forward. Yearly GEP was overestimated by the canopy model by ±15% under all scenarios. The inclusion and omission of the observed needle age-related Vm25 and Jm25 variation in the model simulations led to statistically significant but ecologically irrelevant differences in simulated yearly GEP and Ecan. Omitting seasonal variation did not yield significant simulation differences. Our results indicate that intensive photosynthetic capacity measurements over the full growing season and separate simulation of needle age classes were no prerequisites for accurate simulations of yearly canopy gas exchange. This is true, at least, for the studied stand, which has a very sparse canopy and is exposed to high N deposition and, hence, is not fully representative for temperate Scots pine stands. Nevertheless, we believe well-parameterized process-based canopy models – as applied in this study – are a useful tool to quantify losses of predictive accuracy involved with canopy simplification in modelling.

Description: The allocation of carbon (C) taken up by the tree canopy for respiration and production of tree organs with different construction and maintenance costs, life span and decomposition rate, crucially affects the residence time of C in forests and their C cycling rate. The carbon-use efficiency, or ratio between net primary production (NPP) and gross primary production (GPP), represents a convenient way to analyse the C allocation at the stand level. In this study, we extend the current knowledge on the NPP-GPP ratio in forests by assessing the temporal variability of the NPP-GPP ratio at interannual (for 8 years) and seasonal (for 1 year) scales for a young temperate beech stand, reporting dynamics for both leaves and woody organs, in particular stems. NPP was determined with biometric methods/litter traps, whereas the GPP was estimated via the eddy covariance micrometeorological technique. The interannual variability of the proportion of C allocated to leaf NPP, wood NPP and leaf plus wood NPP (on average 11% yr−1, 29% yr−1 and 39% yr−1, respectively) was significant among years with up to 12% yr−1 variation in NPP-GPP ratio. Studies focusing on the comparison of NPP-GPP ratio among forests and models using fixed allocation schemes should take into account the possibility of such relevant interannual variability. Multiple linear regressions indicated that the NPP-GPP ratio of leaves and wood significantly correlated with environmental conditions. Previous year drought and air temperature explained about half of the NPP-GPP variability of leaves and wood, respectively, whereas the NPP-GPP ratio was not decreased by severe drought, with large NPP-GPP ratio on 2003 due mainly to low GPP. During the period between early May and mid June, the majority of GPP was allocated to leaf and stem NPP, whereas these sinks were of little importance later on. Improved estimation of seasonal GPP and of the contribution of previous-year reserves to stem growth, as well as reduction of data uncertainty, will be of relevance to increase the accuracy of the seasonal assessment of the NPP-GPP ratio in forests.