Báo cáo khoa học: "Variation in forest gas exchange at to continental scales landscape"

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Original article Variation in forest gas exchange at landscape to continental scales John D. Tenhunen Riccardo Valentini Barbara Köstner b a Reiner Zimmermann André Granier c a a of Plant Ecology II, Bayreuth Institute for Terrestrial Ecosystem Research, Department University of Bayreuth, 95440 Bayreuth, Germany b Department of Forest Science and Resources (DISAFRI), University of Tuscia, Via S. Camillo de Lellis, 01100 Viterbo, Italy c of Ecophysiology, Inra, 54280 Champenoux, France Department (Received18 August 1997; accepted 20 October 1997) Abstract - The European Community project EUROFLUX has established the first network for monitoring and comparing gas exchange of forest ecosystems via eddy covariance tech- niques at the continental scale, applying both standardized instrumentation and software. The EUROFLUX workshop entitled ’Water Flux Regulation in Forest Stands’ reviewed at the start of the project our current understanding of water relations and water balances in European forests. Recent studies of transpiration via sapflow monitoring methods were highlighted and the view of water flux regulation that they provide was examined. Studies of sapflow are being car- ried out at EUROFLUX sites together with above canopy flux measurements in order to char- acterize function of the tree canopy compartment. Sapflow studies at additional European sites extend the environmental gradients along which water fluxes are being observed, e.g. by includ- ing forests of riparian zones and of high elevation. Achieving an understanding of forest gas exchange response and forest acclimation potential along climate gradients, and especially in response to environmental stresses at the extreme of the gradients, is essential for integrating information on fluxes and biogeochemistry at landscape, regional and continental scales. (© Inra/Elsevier, Paris.) forest gas exchange / landscape models / global models / heterogeneity / scaling Résumé — Variations des échanges gazeux des forêts de l’échelle locale à l’échelle conti- nentale. Le projet européen Euroflux a mis en place le premier réseau de mesure et de comparaison des échanges gazeux au-dessus des écosystèmes forestiers à l’échelle continentale, au moyen de la méthode des corrélations turbulentes, en utilisant une instrumentation et des procédures * Correspondence and reprints E-mail: john.tenhunen@bitoek.uni-bayreuth.de
de traitement standardisées. L’atelier de travail Euroflux intitulé « Régulation des flux hydriques dans les peuplements forestiers » a évalué au départ du projet les connaissances actuelles sur les relations hydriques et les bilans hydriques dans les forêts européennes. Les études récentes de la transpiration des arbres via les techniques de mesure du flux de sève brute ont été mises en avant, et les résultats concernant la régulation des flux hydriques ont été examinés. Dans les différents sites Euroflux, des mesures de flux de sève sont mises en &oelig;uvre parallèlement à la mesure des flux au-dessus des couverts, dans le but de caractériser le fonctionnement du compartiment foliaire des arbres. Des mesures de flux de sève réalisées dans des sites européens additionnels accroissent l’étendue du gradient d’observations des flux hydriques, en incluant par exemple des forêts allu- viales et d’altitude. Parvenir à une meilleurs compréhension des échanges gazeux par les forêts, et de leur acclimatation potentielle le long des gradients climatiques, et notamment de leur réponse aux contraintes en situations extrêmes, est essentiel pour pouvoir faire la synthèse des infor- mations sur les flux et sur la biogéochimie aux échelles locale, régionale et continentale. (© Inra/Elsevier, Paris.) forêts / modèles régionaux / modèles globaux / hétérogénéités / échanges gazeux des changement d’échelle 1. CO-ORDINATED FOREST GAS vegetation/atmosphere exchanges and their EXCHANGE STUDIES AND feedbacks are parameterized in both global CURRENT RESEARCH TRENDS circulation models (GCMs) and models for regional and landscape assessments. The exchange of water vapor, CO and 2 other gaseous materials between the atmo- Surface exchange varies due to the sphere and forest ecosystems is affected in which specific vegetation devel- manner by the successional stage of the vegeta- opment modifies 1) the interception of tion [1, 32], the stage of canopy closure, precipitation and storage of water in the and by growth activity as related to site canopy, 2) surface roughness and micro- quality or influenced by atmospheric nitro- climate profiles, 3) overstory and under- gen deposition [20, 29, 38]. Additionally, story stomatal conductance, and 4) soil both drought and cold temperature- water extraction and coupling to soil water induced limitations on structure, physiol- stores [4, 9, 19, 40]. GCMs have purported ogy, phenology and nutrition limit forest to reasonably represent these processes at exchange capacities [18, 37, 39]. Given the grid square scale (approximately 50 x that climate model simulations are sensi- 50 km). To date, however, model param- tive to vegetation effects on evapotran- eterization has been based on stand level spiration (ET - [12, 27]), that vegetation studies or relatively local aircraft mea- function is strongly influenced by surements, which are assumed to apply increases in atmospheric CO concentra- 2 homogeneously at larger scales. Due to tion at sites with limiting water and nutri- the ubiquitous influence of man on land- ent availability [7, 24, 33], and that the use in all parts of the globe [45], the need structure of regional vegetation mosaics for dynamic vegetation models that eval- is being modified by changing frequen- uate the vegetation mosaic and, thus, cies in natural and anthropogenic distur- achieve a reasonable representation of the bance regimes [49], heterogeneity as well heterogeneity in vegetation/atmosphere as shifts in forest ecosystem function along exchange and a basis for translating fluxes landscape, regional and continental scale and balances into currencies relevant to gradients must be better understood. Infor- human concerns is recognized [26, 45, mation on shifts in process regulation must be used to improve the manner in which 49].
In this new generation of global, scape and regional perspectives, compar- ative analysis and modelling of the and landscape models, parame- regional repeated observations within stands of terization of ecosystem function must be derived either from remote sensing [21, Picea abies, Pinus sylvestris, Fagus syl- vatica, and Quercus ilex (table I) will help 28, 36] or for global models by upscaling and simplifying landscape vegetation formulate hypotheses about the acclima- tion potential of major woody vegetation dynamics to represent corresponding pro- elements along regional and continental cesses at grid square scales [50]. Both environmental gradients. Studies at addi- research efforts focus attention on the tional European sites (some of which are understanding of aggregation or process described in the contributions to this issue) integration within real landscapes. The can be referenced to the EUROFLUX net- analysis of ecosystem energy exchange work, enriching the spectrum and value processes along landscape and regional of both sets of investigations. The work- scale gradients is extremely important, shop ’Water Flux Regulation in Forest since such studies are carried out at the largest scale utilized to date for ’ground Stands’ established new contacts between truth’ verification of ecosystem-related EUROFLUX research groups and others involved in forest water balance studies. concepts [14, 30, 31, 42]. Thus, landscape and regional studies provide a solid basis for formulating ecosystem models for The dual potentials for use of application at large scales. Sound ecosys- EUROFLUX data (figure 1) suggests that tem models at landscape and regional vegetation/atmosphere exchange models scales provide a link between land-use (SVATs as described by Lee et al. [19] change and socio-economic problems [45], and Dolman [10]) should satisfy one of will aid resource management [6, 41],and two separate sets of criteria, i.e. should allow us to test the assumptions of global function according to technical restric- models. tions and should be designed to accom- plish the needs of either GCM or land- Recent advances in measurement tech- scape models. With respect to future development of SVAT models at both now permit long-term observa- nologies scales, there is now a concensus opinion tions of water and carbon dioxide exchange of forest ecosystems [2, 16, 17]. that exchange processes should be related The European Community funded to canopy physiological and ecosystem research project EUROFLUX has estab- respiration potentials, thus, preparing an appropriate link to ecosystem dynamics lished the first measurement network for monitoring and comparing gas exchange and to biogeochemistry [40]. Similarly, of forest ecosystems at the continental SVAT-model sensitivities with respect to scale, using standardized instrumentation water stress, phenological stages and site- specific nutrient availability is being and software. The data base now being improved. At both global and landscape assembled and to be complemented from a world-wide flux measurement network scales, the importance of remote sensing for parameterization and ultimately for promoted by the IGBP core project BAHC provides for two imperative needs of validation is unquestionable [23, 28, 36, 40]. Differences in global versus regional ecosystem modellers and resource man- and landscape scale SVATs may be agers (figure I). Viewed from a global perspective, a well-distributed network of expected in the structural representation of ecosystems. While it may suffice for flux sites will allow comparisons with cur- rent ET calculated within GCMs along GCM applications to differentially define continental climate gradients. From land- the parallel flux contributions of two or
izontal dimension of 10 m to 1 km. Cur- maximally three functional elements per grid square (each with minimum layer- rent restrictions on the assumption of ing), the assignment in development of homogeneity are usually imposed by the SVATs at the landscape level is to realis- resolution of remotely sensed data, e.g. tically assess differences in flux regula- 30 m size of Landsat TM pixels, or by tion by recognizable landscape elements. potentials for coupling stand level analy- The simplifications of ecosystem struc- ses with other models, e.g. 1 x 1 km grid ture and function at both scales should be size of some mesoscale climate models carried out explicitly. versus small grid sizes in hydrological models. Whereas global-oriented SVATs landscape scales, the actual perfor- At must consider large scale disturbance of individual species should be mance effects on surface exchange, landscape described. Such models must attempt to SVATs and landscape ecosystem models reasonably describe average function in will be required to distinguish and alter- ’homogeneous’ landscape units with a hor-
natively evaluate the effects of differing over large land areas within European coun- anthropogenic impacts on integrated land- tries means that response under sub-opti- scape function [26]. Thus, mechanistically mal conditions often contributes to occur- based model hierarchies must be devel- Wide-scale plantings ring heterogeneity. oped that permit an understanding of func- have contributed to the world-wide dis- tion within important ecosystem com- semination of knowledge of the physiol- partments as well as overall flux rates. ogy and production of such species as Pinus sylvestris and Picea abies (e.g. Gholz et al. While the EUROFLUX project sup- [15]). While certain principles influencing ports research efforts at several scales, the variation in forest ecosystem function have research papers subsequent in this issue become apparent in examining these data, derive from an activity primarily related to dependence of phenological events or e.g. landscape and regional perspectives. The changes in rates of biomass accumulation workshop entitled ’Water Flux Regula- on climate gradients (cf. Bugmann [5]), tion in Forest Stands’ was held in Thur- nutrient availability effects on leaf area nau, Germany during September 1996 to index, and the strong correlation of canopy assess our current understanding of water carbon gain with changes in light intercep- relations and water balances in European tion [15], continental scale patterns in the forests. More specifically, recent studies of actual exchange of materials between for- transpiration via the application of sapflow est vegetation and the atmosphere are much monitoring methods were highlighted and less clear due to interactive effects of nutri- the new view of water flux regulation that ent deposition, uncertainty in describing they provide was examined. We hope that water balance, as yet undefined responses the picture presented here will be broad- to temperature stress, and incomplete ened during the course of EUROFLUX knowledge of the structural changes that and that a new understanding of the range occur in trees along with these conditions. of behavior possible for European forest stands will result. As might be expected, the extensive of only a few major species has use resulted in numerous European studies of 2. SIMILARITY forest water balance in stands of pine, AND HETEROGENEITY spruce, beech and oak. A recent review IN EUROPEAN FOREST of European forest literature by Peck and ECOSYSTEM FUNCTION Mayer [25] revealed a reported range in annual transpiration (maximum annual estimate minus minimum estimate) of Our understanding of the current forest approximately 600, 400 and 300 mm for vegetation of Europe can be related first to Pinus, Picea and Fagus, respectively, and the reinvasion of the continent by forest of 720, 690 and 540 mm in mean ET for species after the last glaciation [13], but subsequently and more importantly to land the same species. Attempts to generalize these results demonstrate that our under- clearing and later to broad-scale, intensive standing of shifts in water flux regulation forest management practices. While at landscape to continental scales is vague. species-specific traits, ecological prefer- and competitive potentials provide Large differences in transpirational water ences use that are reported among stands are not ecological restrictions on variation in pro- cess rates, e.g. potential growth in relation systematically well-explained in terms of to soil characteristics or atmospheric fac- 1) experimental difficulties resulting from tors [5, 11 ], the ’experimental planting’ of different methodologies, 2) differences in only a few commercially useful species weather conditions, 3) differences in struc-
affected by age and management ing of the multiple influences affecting ture as practices, and 4) differences in stand nutri- function in the EUROFLUX stands will tion, understory flux contributions and be difficult to achieve owing to process interactions, non-linear responses, long- interception. term ecosystem adjustments and difficul- Intensive study but lack of generaliz- ties in evaluating the importance of able results provides a contradiction that Nevertheless, compara- extreme events. occurs because of differing methods, exper- tive analyses along environmental gradi- imental design and scales of observation. ents provide the best clues for explana- Sapflow methods that are now becoming tions (cf. Magill et al. [20]), even though increasingly a ’standard tool’ in studies of several gradients may overlap in complex water balance will aid our understanding fashion and sharp transitions in function for forest function by clarifying flux regu- should not be expected. A number of the lation at the individual tree level. Never- papers included in this issue extend the theless, ’standardization’ of sapflow mea- environmental gradients associated with surements must be discussed and attention observations of water fluxes in forest must be focused on errors and short-com- stands, e.g. by including forests of riparian ings of the method. We hope that this goal zones and at high elevation mountain sites. will be promoted by the papers of the pro- The importance of combining informa- ceedings which follow, by new commu- tion from these sites with information from nication networks established at the Thur- EUROFLUX locations should not be nau workshop, and through the interaction underestimated. Fundamental information among research groups of EUROFLUX. on ecological potentials of plants and reg- Additional contributions from the ulatory mechanisms has often been gained EUROFLUX project to clarification of in habitats that are extreme with respect continental scale heterogeneity in forest to particular environmental factors. vegetation/atmosphere exchanges and in comparative analysis of flux regulation is Achieving an understanding of forest anticipated, since a single methodology is response and forest acclimation potential used at the stand level for ET and CO 2 along climate gradients and in response Furthermore, exchange measurements. to environmental stresses is key to the above canopy flux observations are accom- development of realistic dynamic vegeta- panied by a suite of measurements which tion models. Available process informa- simultaneously characterize function within tion determines the structuring of such individual ecosystem compartments. models, the included parameterization, and, therefore, their overall behavior, e.g. whether transitions along continental level 3. CONTINENTAL SCALE transects are correctly described and GRADIENTS, FOREST whether important vegetation/atmosphere PLASTICITY AND feedbacks are quantified. Forest biologists RESEARCH NEEDS must examine and improve the assump- tions of such models via coordinated com- Climate, variation in species-specific parative process studies. With respect to potentials and nitrogen deposition [47] European forests, response ’strategies’ of produce a broad range of leaf area indices spruce, pine, beech and oak, as well as in the forest stands selected for study by those species occupying extreme situa- EUROFLUX, differences in light inter- tions or special habitats must be defined. ception and a broad range in annual wood The question of how phenology, structural increment (table I). A clear understand- change and physiological plasticity change
precipitation and temperature gradients in in along gradients availability resource the Alps has been summarized in the and, thus, control fluxes, biogeochemical be sys- model FORCLIM [5]. This summary serves cycles and competitiveness must as an interesting precursor model for tematically addressed. attempts to relate site conditions (monthly It is particularly important to obtain a temperatures, monthly precipitation, mean broader understanding of the effects of nitrogen availability, winter cold temper- water stress on forest gas exchange. atures and summer drought) to forest com- Decreased water availability significantly munity composition and biomass accu- influences ecosystem function of all major mulation at European continental scales. European forest types, from boreal forests The results of the simulation studies sug- of Scandinavia to Mediterranean forests gest that prediction of changing species and shrublands [8, 34, 35, 43, 44, 46]. dominance and of biomass accumulation From north to south in Europe, there are within the selected climate space is pos- obviously large differences in the dura- sible. Nevertheless, major problems occur tion and frequency of drought, its pre- in predicting forest response with limited dictability, and the depth to which soil water availability. Furthermore, only crude dries. While current summaries of infor- estimates of forest/atmosphere exchanges mation on forest gas exchange response (carbon gain, pollutant uptake, emission have generally defined the relationship of VOCs, etc.) and no quantification of between soil water availability and forest flux partitioning among species is cur- canopy conductance [18], there are few rently possible at regional to continental systematic studies of variability in this scale. response with respect to soil type or along climate gradients at landscape or conti- A much closer cooperation is needed, nental scales (as, for example, with respect proposed within the EUROFLUX pro- as to location on slopes for Quercus ilex; Sala ject, between research groups developing and Tenhunen [34]). Interpretation of dynamic vegetation models and those shifts in the response to water stress for quantifying forest ecosystem atmospheric selected forest stands along topographic exchanges and water balance. The short- gradients, e.g. changes in physiology ver- comings of dynamic vegetation models sus structure, will provide the basis for may be related in part to current inabil- our adjusting flux estimates applicable at large ity to adequately generalize water avail- scales. It should be noted that most ability effects due to rainfall patterning as descriptions of forest gas exchange well as exposition or landscape position response to water stress do not consider effects on forest ecosystem structure and the behavior of the understory and pro- function [3, 22, 34]. This collection of vide no information on potential changes papers resulting from the workshop ’Water in flux partitioning that may occur. Since Flux Regulation in Forest Stands’ repre- forest understory species appear differen- sents a step in the effort to assess current tially adapted to water stress and exhibit knowledge of forest water balances, to differing strategies of water use [48], addi- determine how to generalize this knowl- tional studies are required to clarify edge, to include it into simulation mod- changes in flux partitioning and changes in els, and to subsequently document our cur- total ecosystem gas exchange during the rent understanding with model tests. Thus, course of soil drying as well as after rehy- this issue represents work dedicated to dration. building new measurement and commu- Current knowledge of major processes nication networks, to developing ideas for affecting forest ecosystem function along upscaling, and for integrating information
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