Bernard Longdoz

Influence of spring and autumn phenological transitions on forest ecosystem productivity

We use eddy covariance measurements of net ecosystem productivity (NEP) from 21 FLUXNET sites (153 site-years of data) to investigate relationships between phenology and productivity (in terms of both NEP and gross ecosystem photosynthesis, GEP) in temperate and boreal forests. Results are used to evaluate the plausibility of four different conceptual models. Phenological indicators were derived from the eddy covariance time series, and from remote sensing and models. We examine spatial patterns (across sites) and temporal patterns (across years); an important conclusion is that it is likely that neither of these accurately represents how productivity will respond to future phenological shifts resulting from ongoing climate change. In spring and autumn, increased GEP resulting from an ‘extra’ day tends to be offset by concurrent, but smaller, increases in ecosystem respiration, and thus the effect on NEP is still positive. Spring productivity anomalies appear to have carry-over effects that translate to productivity anomalies in the following autumn, but it is not clear that these result directly from phenological anomalies. Finally, the productivity of evergreen needleleaf forests is less sensitive to phenology than is productivity of deciduous broadleaf forests. This has implications for how climate change may drive shifts in competition within mixed-species stands.

Ten years of fluxes and stand growth in a young beech forest at Hesse, North-eastern France

Abstract• Water and carbon fluxes, as measured by eddy covariance, climate, soil water content, leaf area index, tree biomass, biomass increment (BI), litter fall and mortality were monitored for 10 successive years in a young beech stand in Hesse forest (north-eastern France) under contrasting climatic and management conditions.• Large year-to-year variability of net carbon fluxes (NEE) and to a lesser extent, of tree growth was observed. The variability in NEE (coefficient of variation, CV = 44%) was related to both gross primary production (GPP) and to variations in total ecosystem respiration (TER), each term showing similar and lower interannual variability (CV = 14%) than NEE. Variation in the annual GPP was related to: (i) the water deficit duration and intensity cumulated over the growing season, and (ii) the growing season length, i.e. the period of carbon uptake by the stand. Two thinnings occurring during the observation period did not provoke a reduction in either GPP, water fluxes, or in tree growth. Interannual variation of TER could not be explained by any annual climatic variables, or LAI, and only water deficit duration showed a poor correlation. Annual biomass increment was well correlated to water shortage duration and was significantly influenced by drought in the previous year.• The relationship between annual NEE and biomass increment (BI) was poor: in some years, the annual carbon uptake was much higher and in others much lower than tree growth. However this relationship was much stronger and linear (r2 = 0.93) on a weekly to monthly time-scale from budburst to the date of radial growth cessation, indicating a strong link between net carbon uptake and tree growth, while carbon losses by respiration occurring after this date upset this relationship.• Despite the lack of correlation between annual data, the NEE and BI cumulated over the 10 years of observations were very close.• On the annual time-scale, net primary productivity calculated from eddy fluxes and from biological measurements showed a good correlation.Résumé• Les flux d’eau et de dioxyde de carbone, mesurés par la méthode des corrélations turbulentes, le climat, le contenu en eau du sol, l’indice foliaire, la biomasse et l’accroissement en biomasse (BI) des arbres, les chutes de litière et la mortalité ont été suivis en continu pendant 10 années successives dans une jeune hêtraie de la forêt de Hesse (nord-est de la France) en conditions de climat et de gestion contrastées.• Une forte variabilité interannuelle des flux nets de carbone (NEE) et dans une moindre mesure de la croissance des arbres ont été observées. La variabilité de NEE (son coefficient de variation, CV = 44 %) a été mise en relation avec celles de la productivité primaire brute (GPP) et de la respiration totale de l’écosystème (TER), chacun de ces deux termes montrant une variabilité similaire et plus faible (CV = 14 %) que pour NEE. Les variations de la GPP annuelle étaient sous la dépendance : (i) de la durée et de l’intensité du déficit hydrique cumulé sur la saison de végétation, (ii) la longueur de la saison de végétation, définie comme la période où le peuplement absorbe du dioxyde de carbone. Deux éclaircies pratiquées pendant la période de mesures n’ont pas provoqué de réduction ni de GPP, ni du flux d’évapotranspiration, ni de l’accroissement en biomasse du peuplement. Les variations interannuelles de TER n’ont pu être expliquées par aucune des variables climatiques au pas de temps annuel, ni par le LAI, mais seulement par la durée du déficit hydrique du sol, mais avec une corrélation médiocre. L’accroissement annuel en biomasse était fortement corrélé à la durée de la contrainte hydrique de la même année mais aussi influencé de façon significative par la celle de l’année précédente.• La relation entre la NEE annuelle et l’accroissement en biomasse (BI) n’était pas significative : selon les années, l’absorption de carbone par le peuplement était beaucoup plus forte ou plus faible que l’accroissement du peuplement. Toutefois, nous avons observé une corrélation beaucoup plus élevée et linéaire (r2 = 0,93) sur une base de temps hebdomadaire à mensuelle pendant la période allant du débourrement à la date d’arrêt de croissance radiale, ce qui indique un couplage fort entre l’acquisition du carbone et la croissance des arbres, alors que la perte de carbone par respiration en dehors de cette période découple cette relation.• En dépit de l’absence de corrélation entre NEE et BI au pas de temps annuel, le cumul de NEE et celui de BI sur les 10 années ont été très proches.• Par contre, la productivité primaire nette annuelle, calculée à partir des mesures de flux et biométriques sur le peuplement a montré un bon accord.

Climatic Influences on Seasonal and Spatial Differences in Soil CO2 Efflux

The efflux of C02 from the soil is characterized by large seasonal fluctuations due to seasonal changes in root and microbial respiration. Although several biotic and abiotic factors influence root and microbial activity (see Chap. 3), the control exerted by temperature, and in some cases moisture, is usually dominant. In the absence of water stress, variation in soil temperature accounts for most of the seasonal and diurnal variation in soil C02 efflux. Where water stress frequently occurs, soil C02 efflux may not be correlated with soil temperature, but with its moisture content (Rout and Gupta 1989). Thus, C02 release from the soil appears to respond to temperature or moisture, whichever is most limiting at the time of measurement (Schlentner and van Cleve 1985).

Ground-Based Optical Measurements at European Flux Sites: A Review of Methods, Instruments and Current Controversies

This paper reviews the currently available optical sensors, their limitations and opportunities for deployment at Eddy Covariance (EC) sites in Europe. This review is based on the results obtained from an online survey designed and disseminated by the Co-cooperation in Science and Technology (COST) Action ESO903—“Spectral Sampling Tools for Vegetation Biophysical Parameters and Flux Measurements in Europe” that provided a complete view on spectral sampling activities carried out within the different research teams in European countries. The results have highlighted that a wide variety of optical sensors are in use at flux sites across Europe, and responses further demonstrated that users were not always fully aware of the key issues underpinning repeatability and the reproducibility of their spectral measurements. The key findings of this survey point towards the need for greater awareness of the need for standardisation and development of a common protocol of optical sampling at the European EC sites.

Seasonal and daily time course of the 13C composition in soil CO2 efflux recorded with a tunable diode laser spectrophotometer (TDLS).

Temporal variations of carbon isotope composition of soil CO2 efflux (FS and δ13CFS) at different time scales should reflect both temporal variations of the climate conditions that affect canopy functioning and temporal changes in the relative contribution of autotrophic respiration to total FS. A tunable diode laser spectrophotometer (TDLS) was installed in the Hesse forest (northeast of France) early during the 2007 growing season to determine the seasonal and daily variability in δ13CFS. This method, based on the measurement of the absorption of an infrared laser emission at specific wave lengths of the 13CO2 and 12CO2, allows the continuous monitoring of the two isotopologues. The concentrations of the two isotopologues in FS were continuously monitored from June to November 2007 using chamber method and Keeling plots drawn from nocturnal accumulation of CO2 below the canopy. These TDLS measurements and isotope ratio mass spectrometer based Keeling plots gave very similar values of δ13CFS, showing the reliability of the TDLS system in this context. Results were analysed with regard to seasonal and daily changes in climatic and edaphic variables and compared with the δ13C of CO2 respired by roots, litter and soil incubated under controlled conditions. Pronounced daily as well as seasonal variations in δ13CFS were recorded (up to 1.5‰). The range of variation of δ13CFS was of the same order of magnitude at both diurnal and seasonal scales. δ13CFS observed in the field fluctuated between values of litter and of root respiration recorded during incubation, suggesting that temporal (and probably spatial) variations were associated with changes in the relative contribution of the two compartments during the day and during the season.

TRAP: a modelling approach to below-ground carbon allocation in temperate forests

Tree root systems, which play a major role in below-ground carbon (C) dynamics, are one of the key research areas for estimating long-term C cycling in forest ecosystems. In addition to regulating major C fluxes in the present conditions, tree root systems potentially hold numerous controls over forest responses to a changing environment. The predominant contribution of tree root systems to below-ground C dynamics has been given little emphasis in forest models. We developed the TRAP model, i.e. Tree Root Allocation of Photosynthates, to predict the partitioning of photosynthates between the fine and coarse root systems of trees among series of soil layers. TRAP simulates root system responses to soil stress factors affecting root growth. Validation data were obtained from two Belgian experimental forests, one mostly composed of beech (Fagus sylvatica L.) and the other of Scots pine (Pinus sylvestris L.). TRAP accurately predicted (R = 0.88) night-time CO2 fluxes from the beech forest for a 3-year period. Total fine root biomass of beech was predicted within 6% of measured values, and simulation of fine root distribution among soil layers was accurate. Our simulations suggest that increased soil resistance to root penetration due to reduced soil water content during summer droughts is the major mechanism affecting the distribution of root growth among soil layers of temperate Belgian forests. The simulated annual rate of C input to soil litter due to the fine root turnover of the Scots pine was 207 g C m−2 yr−1. The TRAP model predicts that fine root turnover is the single most important source of C to the temperate forest soils of Belgium.

Remotely-sensed detection of effects of extreme droughts on gross primary production.

Severe droughts strongly impact photosynthesis (GPP), and satellite imagery has yet to demonstrate its ability to detect drought effects. Especially changes in vegetation functioning when vegetation state remains unaltered (no browning or defoliation) pose a challenge to satellite-derived indicators. We evaluated the performance of different satellite indicators to detect strong drought effects on GPP in a beech forest in France (Hesse), where vegetation state remained largely unaffected while GPP decreased substantially. We compared the results with three additional sites: a Mediterranean holm oak forest (Puéchabon), a temperate beech forest (Hainich), and a semi-arid grassland (Bugacpuszta). In Hesse, a three-year reduction in GPP following drought was detected only by the Enhanced Vegetation Index (EVI). The Photochemical Reflectance Index (PRI) also detected this drought effect, but only after normalization for absorbed light. In Puéchabon normalized PRI outperformed the other indicators, while the short-term drought effect in Hainich was not detected by any tested indicator. In contrast, most indicators, but not PRI, captured the drought effects in Bugacpuszta. Hence, PRI improved detection of drought effects on GPP in forests and we propose that PRI normalized for absorbed light is considered in future algorithms to estimate GPP from space.

Root exclusion through trenching does not affect the isotopic composition of soil CO2 efflux

Disentangling the autotrophic and heterotrophic components of soil CO2 efflux is critical to understanding the role of soil system in terrestrial carbon (C) cycling. In this study, we combined a stable C-isotope natural abundance approach with the trenched plot method to determine if root exclusion significantly affected the isotopic composition (δ13C) of soil CO2 efflux (RS). This study was performed in different forest ecosystems: a tropical rainforest and two temperate broadleaved forests, where trenched plots had previously been installed. At each site, RS and its δ13C (δ13CRs) tended to be lower in trenched plots than in control plots. Contrary to RS, δ13CRs differences were not significant. This observation is consistent with the small differences in δ13C measured on organic matter from root, litter and soil. The lack of an effect on δ13CRs by root exclusion could be from the small difference in δ13C between autotrophic and heterotrophic soil respirations, but further investigations are needed because of potential artefacts associated with the root exclusion technique.

The faint young sun climatic paradox: Influence of the continental configuration and of the seasonal cycle on the climatic stability

Abstract A quasi-three-dimensional climate model is used to study the early state of the Earth when the solar luminosity was 70% of the present value. Usually, climatic simulations going back to this period lead to a completely frozen planet contrasting with the geologic evidences of sedimentary rock formation and thus of the presence of liquid water at the surface of the continents during the Archean (4.6−2.5 billion years before present). Here, several model simulations are performed for solar luminosities varying between 0.7 and 1 times the present value. Using the present-day continental configuration and taking the seasonal cycle into account, a steady state is found in which glaciation is complete but snow covers only some oceanic coasts, leaving the continents essentially snow-free. As a result, the albedo of the continental area is strongly reduced compared to that of the frozen ocean. Some continental temperatures can almost reach the freezing point of water in summer −1°C in the center of Eurasia). This result can be explained by the behavior of the detailed hydrologic cycle included in the model. During the decrease of the solar luminosity, the jump to a completely frozen Earth occurs when the solar luminosity reaches 0.86 times its present value. The behavior of the climatic system is substantially different with a global ocean configuration. In the absence of land surfaces, the meridional heat transport, explicitly calculated, is less effective and the glaciation of a model latitude zone does not lead to the glaciation of its equatorward neighbor. The climate instability is relatively local and the jump to the completely frozen state is much more progressive than in the case of the modem continental configuration. The role of the seasonal cycle in the paleoclimatic simulation is also studied. Due to the non-linearity of the model, removing the seasonal cycle drives the system to an increase of the annual mean planetary albedo and to a decrease of the relative value (0.82) of the critical solar luminosity at which the jump to the completely frozen solution occurs.

Field measurements of soil respiration: principles and constraints, potentials and limitations of different methods

Soil respiration is a major component in the carbon balance of terrestrial ecosystems and has been measured in the field for more than eight decades. In this chapter, we will describe the measurement of soil CO2 efflux at the soil surface that can be considered as equivalent to soil CO2 production when integrated over long time periods (week, month or season). At shorter time scales the transport of CO2 may uncouple the soil CO2 efflux from its production inside the soil. Different methods have been developed to measure this efflux. These methods can affect the object being measured by disturbing the biochemical processes involved in CO2 production, the physical properties influencing CO2 movement towards the soil surface, or by changing the environmental conditions in the soil. Therefore, soil respiration measurements in the field are one of the most difficult among the ecosystem flux measurements. So far, no single method has been established as the standard but comparisons, which give important indications on their accuracy, have been performed. The choice of the measurement methodology is not limited to that of a measurement system. The experimenter has to elaborate a protocol depending on the temporal and spatial scales studied. In this chapter, we will describe the most commonly used methodologies for measuring soil CO2 efflux and present their history, principles and constraints (Section 2.2). In addition, we will present a number of major error sources associated with the different methods and the ways to avoid them (Section 2.3), describe a comparison between different systems (Section 2.4) and give recommendations for the measurement protocol (Section 2.5). 2.2 MEASUREMENT PRINCIPLES AND HISTORY OF TECHNICAL DEVELOPMENTS