André Granier

Does Drought Influence the Relationship Between Biodiversity and Ecosystem Functioning in Boreal Forests

In mixed forests, interactions among species influence ecosystem functioning but environmental conditions also play an important role in shaping relationships between biodiversity and ecosystem functioning. In the context of climate change, the carbon and water balance in pure versus mixed forest stands may be differentially influenced by changing soil water availability. To test this hypothesis, we compared the influence of biodiversity on stand water use efficiency (WUES) in boreal forests between wet and dry years. We assessed the carbon isotope composition (δ13C) of tree rings in Betula pendula, Pinus sylvestris, and Picea abies growing in pure versus mixed stands. In addition, we tested whether differences in WUES affected patterns of stand basal area increment (BAIS). No biodiversity effect was found for stand δ13C (δ13CS) during the wet year. However, there was a significant increase in δ13CS between the wet and the dry year and a significant effect of biodiversity on δ13CS in the dry year. The increase in δ13CS in mixed stands was associated with both selection and complementarity effects. Although BAIS decreased significantly in the dry year, changes in δ13CS did not translate into variations in BAIS along the biodiversity gradient. Our results confirmed that the physiological response of boreal forest ecosystems to changing soil water conditions is influenced by species interactions and that during dry growing seasons, species interactions in mixed stands can lead to lower soil moisture availability. This illustrates that biodiversity effects can also be negative in mixed stands in the sense that soil resources can be more intensively exhausted. Overall, our results confirm that in boreal forests, the biodiversity–ecosystem functioning relationship depends on local environmental conditions.

Drought responses by individual tree species are not often correlated with tree-species diversity in European forests

Drought frequency and intensity are predicted to increase in many parts of the Northern Hemisphere and the effects of such changes on forest growth and tree mortality are already evident in many regions around the world. Mixed-species forests and increasing tree species diversity have been put forward as important risk reduction and adaptation strategies in the face of climate change. However, little is known about whether the species interactions that occur in diverse forests will reduce drought susceptibility or water stress. In this study, we focused on the effect of drought on individual tree species (n=16) within six regions of Europe and assessed whether this response was related to tree species diversity and stand density, and whether community-level responses resulted from many similar or contrasting species-level responses. For each species in each plot, we calculated the increase in carbon isotope composition of latewood from a wet to a dry year (C-13) as an estimate of its drought stress level. When significant community-level relationships occurred (three of six regions), there was only one species within the given community that showed a significant relationship (three of 25 species-region combinations), showing that information about a single species can be a poor indicator of the response of other species or the whole community. There were many two-species mixtures in which both species were less water-stressed compared with their monocultures, but also many mixtures where both species were more stressed compared with their monocultures. Furthermore, a given species combination responded differently in different regions.Synthesis and applications. Our study shows that drought stress may sometimes be reduced in mixed-species forests, but this is not a general pattern, and even varies between sites for a given combination. The management or prediction of drought stress requires consideration of the physiological characteristics of the mixed species, and how this complements the water-related climatic and edaphic features of the site, rather than species richness.nnOur study shows that drought stress may sometimes be reduced in mixed-species forests, but this is not a general pattern, and even varies between sites for a given combination. The management or prediction of drought stress requires consideration of the physiological characteristics of the mixed species, and how this complements the water-related climatic and edaphic features of the site, rather than species richness.

Differential response to soil drought among co-occurring broad-leaved tree species growing in a 15- to 25-year-old mixed stand

Abstract• IntroductionIn the context of global environmental changes, better understanding of tree response to soil drought in young mixed species stands is needed to anticipate forest adaptation and management practices for the future.• Materials and methodsWe investigated the functional response of five co-occurring broad-leaved tree species growing in a 15- to 25-year-old mixed stand in northeastern France during the 2006 summer drought. We measured functional traits related to water acquisition (phenology, rooting pattern and vulnerability of xylem to cavitation) and the ecophysiological response (sap flux density, leaf water potential) of these species to soil water shortage.• ResultsOur study highlights contrasted drought response strategies among these species and a trade-off between leaf phenology, resistance of xylem to cavitation and root system depth.• ConclusionAt this site, a deep root system seemed to be a key functional trait for the species to cope with drought. Quercus robur and Salix capreae can be characterised as drought-avoidance species as they possess a deep root system and therefore did not strongly experience soil drought. Despite deep rooting capacity, Betula pendula did not really avoid soil drought and strongly regulated transpiration during dry periods. Nevertheless, the earliness of budburst of this species contributes to high annual growth rate. In contrast, Carpinus betulus and Fagus sylvatica both displayed typical characteristics of drought-sensitive species.

La sécheresse de 2003 dans le contexte climatique des 54 dernières années : analyse écophysiologique et influence sur les arbres forestiers

Apres avoir rappele comment caracteriser une secheresse pour en evaluer ses consequences sur le fonctionnement des arbres, lanalyse du bilan hydrique de lannee 2003 en Lorraine est presentee, pour des peuplements feuillus et resineux, et sur des sols a reserve utile moyenne ou forte. La secheresse 2003 est replacee dans la chronologie des 54 dernieres annees, avec une comparaison detaillee avec lannee 1976. Le deficit hydrique de lannee 2003 apparait comme le plus fort des 54 dernieres annees pour les feuillus. Pour les resineux, 2003 se classe au second rang pour les sols a reserve utile moyenne, et seulement en 5 e rang pour les sols profonds. Les consequences sur la physiologie des arbres sont presentees, et les potentiels hydriques atteints par les arbres en 2003 sont discutes. Cet article compare en particulier un large eventail despeces autochtones ou introduites en termes de strategies de reponses a la secheresse edaphique (vulnerabilite et risque de cavitation, regulation stomatique, developpement du systeme racinaire).

Influence of species interactions on transpiration of Mediterranean tree species during a summer drought

Recent research has shown that interactions between species with different functional traits can promote forest ecosystem processes. In the context of climate change, understanding whether species interactions in mixed-species ecosystems can improve the adaptation of these ecosystems to extreme climatic events is crucial to developing new management strategies. In this study, we investigated the impact of species interactions on the sap flux density of three Mediterranean tree species (Quercus faginea, Pinus nigra and Pinus sylvestris) during a summer drought. Measurements of foliar carbon isotopic composition (δ13C) were also conducted on the same trees. The decline in transpiration during drought was the greatest for P. sylvestris and the least pronounced for Q. faginea. For P. nigra and Q. faginea, the decrease in transpiration as the drought progressed was lower when these species where interacting with another tree species, particularly with P. sylvestris. In contrast, the decrease for P. sylvestris was higher when this species was interacting with another species. Differing drought effects were consistent with the δ13C values. We showed that the identity of the species present in the direct neighbourhood of a given tree can differentially influence water availability and water-use of these three co-existing Mediterranean tree species during a summer drought. Our findings suggest that species interactions play an important role in modulating the response of tree species to drought. Favouring tree species diversity in this region does not seem to be systematically beneficial in terms of soil water availability and water-use for all the interacting species.

Testing the applicability of BIOME-BGC to simulate beech gross primary production in Europe using a new continental weather dataset

Key messageA daily 1-km Pan-European weather dataset can drive the BIOME-BGC model for the estimation of current and future beech gross primary production (GPP). Annual beech GPP is affected primarily by spring temperature and more irregularly by summer water stress.ContextThe spread of beech forests in Europe enhances the importance of modelling and monitoring their growth in view of ongoing climate changes.AimsThe current paper assesses the capability of a biogeochemical model to simulate beech gross primary production (GPP) using a Pan-European 1-km weather dataset.MethodsThe model BIOME-BGC is applied in four European forest ecosystems having different climatic conditions where the eddy covariance technique is used to measure water and carbon fluxes. The experiment is in three main steps. First, the accuracy of BIOME-BGC GPP simulations is assessed through comparison with flux observations. Second, the influence of two major meteorological drivers (spring minimum temperature and growing season dryness) on observed and simulated inter-annual GPP variations is analysed. Lastly, the impacts of two climate change scenarios on beech GPP are evaluated through statistical analyses of the ground data and model simulations.ResultsThe weather dataset can drive BIOME-BGC to simulate most of the beech GPP evolution in all four test areas. Both observed and simulated inter-annual GPP variations are mainly dependent on minimum temperature around the beginning of the growing season, while spring/summer dryness exerts a secondary role. BIOME-BGC can also reasonably predict the impacts of the examined climate change scenarios.ConclusionThe proposed modelling approach is capable of approximately reproducing spatial and temporal beech GPP variations and impacts of expected climate changes in the examined European sites.

Extreme droughts affecting Mediterranean tree species’ growth and water-use efficiency: the importance of timing

It has been known for a long time that drought intensity is a critical variable in determining water stress of Mediterranean tree species. However, not as much attention has been paid to other drought characteristics, for example the timing of the dry periods. We investigated the impact of the timing and intensity of extreme droughts on growing season length, growth and water-use efficiency of three tree species, Pinus nigra ssp. Salzmannii J.F. Arnold, Quercus ilex ssp. ballota (Desf.) Samp. and Quercus faginea Lam. coexisting in a continental Mediterranean ecosystem. Over the study period (2009-13), intense droughts were observed at annual and seasonal scales, particularly during 2011 and 2012. In 2012, an atypically dry winter and spring was followed by an intense summer drought. Quercus faginea growth was affected more by drought timing than by drought intensity, probably because of its winter-deciduous leaf habit. Pinus nigra showed a lower decrease in secondary growth than observed in the two Quercus species in extremely dry years. Resilience to extreme droughts was different among species, with Q. faginea showing poorer recovery of growth after very dry years. The highest intra- and inter-annual plasticity in water-use efficiency was observed in P. nigra, which maintained a more water-saving strategy. Our results revealed that the timing of extreme drought events can affect tree function to a larger extent than drought intensity, especially in deciduous species. Legacy effects of drought over months and years significantly strengthened the impact of drought timing and intensity on tree function.

Évolutions possibles des contraintes climatiques et conséquences pour la croissance des arbres

Les consequences probables de levolution des principales variables climatiques (pluie, temperature, evapotranspiration potentielle) sur la croissance et le deperissement des arbres forestiers sont analysees, dans la gamme de variation predite par les modeles climatiques globaux. La sensibilite de la phenologie (effets thermiques) et des niveaux de contrainte hydrique pendant la saison de vegetation (modification de pluviometrie et devapotranspiration potentielle) sont developpees. A partir des relations experimentales entre la croissance radiale et le climat, etablies par dendroclimatologie (approche retrospective), les consequences des variations des conditions climatiques sur la croissance radiale du Hetre et la mortalite des Chenes sont discutees.

Les forêts tempérées face aux conséquences du changement climatique: Est-il primordial de favoriser une plus forte diversité d'arbres dans les peuplements forestiers?

Une plus forte biodiversite dans les peuplements forestiers est suggeree comme etant un mode de gestion permettant d’adapter les forets a l’accentuation des secher esses dans le futur. Nous presentons ici une synthese sur la relation entre diversite en essences d’arbre et resistance a la secher esse des peuplements forestiers temperes. Des tendances contrastees pour cette relation ont ete observees en fonction des types forestiers etudies, des essences presentes dans le peuplement, ou du contexte pedoclimatique local. La generalisation de modes de gestion forestiere integrant des melanges de plusieurs essences ne semble donc pas necessaire ment assurer une meilleur e resistance a la secher esse des peuplements forestiers temperes.

L’eau forestière de qualité : comment agir sur les quantités produites ?

Forests have a key influence on the water cycle through properties related to their structure, in particular the ndevelopment of the canopy and of root systems. Any examination of forests as suppliers of water must nconsider the following two elements: n— overall, the more productive the forest, the greater its water consumption, which accordingly reduces ndrainage flow. In other words, there is a trade-off between biomass production and the amount of water nreturned to the environment; n— water drained under forest canopies to streams, springs or the water table is generally of good quality. nA self-evidence worth remembering is that quality and quantity of water cannot be dissociated: the service nof “quality water” implies availability of sufficient quantities over time. nKnowledge and management of forest-sourced water are based on knowledge of the water balance and its nvariations in space and time. This article discusses the various causes of variations in water balance connected nwith climate, soil conditions, forest species, silvicultural practices, all of which have an impact on the amount nof water drained. It also provides some comparisons with other major vegetation types. To answer the nquestion of how to quantitatively assess the amount of water supplied by forests and how to modulate that nquantity by management, we implemented a modelling approach that uses a water balance model with a ndaily time step. Using site-specific parameters and meteorological data, the model simulates the flow from nactual evapotranspiration, rainwater at ground level, the water content of the soil and drainage. Two forest nsites were selected for the study described in this article. These are forests in eastern France with contrasting nrainfall patterns for which we simulated, under current climate conditions, the effect on the volume of nwater drained of five different silvicultural scenarios.