The heat wave of summer 2003 was the largest and the most persistent ever experienced in Central Europe and has fuelled concern about the effects of climate change on European ecosystems. Since forests constitute the most important European ecosystems, in this review article we assess current knowledge on the effects of heat and drought on key metabolic processes for growth and productivity of forest trees. In particular, the general consequences of heat and drought on (1) photosynthesis and respiration at the cellular and community level, and (2) on nutrient uptake, partitioning and competition for nutrients are summarized. The latter are a major sink for photosynthetic energy and, therefore, are indirectly but strongly connected to the performance of photosynthesis. In addition, the interaction of heat and drought with stress compensation mechanisms and emission of biogenic volatile organic compounds (BVOC) are discussed, since these processes are directly connected to carbon metabolism. Effects on the emission of BVOC are also included because they constitute an important feedback mechanism on ozone formation and, thus, on atmospheric pollution. As far as available, data collected during the 2003 heat wave are included and discussed.

Physiological responses of forest trees to heat and drought.

Drought and heat stress are among the two most important environmental factors influencing crop growth, development, and yield processes. A comprehensive understanding of the impact of drought and heat stress will be critical in evaluating the impact of climate change and climate variabil ity on crop production. Both drought and heat stress influence an array of processes including physiological, growth, developmental, yield, and quality of crop. The objective of this review is to provide an overview of the influences of these two stresses on the above processes independently and in combination. Our review suggests a clear need of information on interactive effects of stresses particularly of drought and heat stress which mostly occur in combination. Both short- and long-term stresses can signifi cantly influence growth and yield processes when stress occurs at sensitive stages. Crops are generally more sensitive to drought and/or heat stress during reproductive stages of development, which mainly influences seed numbers. Some of the important traits associated with drought- and/or heat-stress tolerance are indicated and discussed. The impacts of drought and heat stress are often different, and tolerance mechanisms may also be different. There is a wide range of crop modeling approaches (simple empirical models and more mechanistic models) that try to quantify the impact of stresses on growth, development, and yield and yield quality traits. These crop models should have the capability to quantify the impact of both short- and long-term stress events on growth, development, and yield processes. Modeling growth, development, sink-source relation, grain yield, and grain quality of crops can improve understanding of physiological and genetic nature of tolerance which can lead to increased grain yield and quality of crops. Improved models can enhance our capacity to predict crop performance in future climates and also to identify traits that can potentially be improved or exploited to obtain higher and more stable crop yields under stressed environments.

Impacts of Drought and/or Heat Stress on Physiological, Developmental, Growth, and Yield Processes of Crop Plants

Climate teleconnections drive highly variable and synchronous seed production (masting) over large scales. Disentangling the effect of high-frequency (inter-annual variation) from low-frequency (decadal trends) components of climate oscillations will improve our understanding of masting as an ecosystem process. Using century-long observations on masting (the MASTREE database) and data on the Northern Atlantic Oscillation (NAO), we show that in the last 60 years both high-frequency summer and spring NAO, and low-frequency winter NAO components are highly correlated to continent-wide masting in European beech and Norway spruce. Relationships are weaker (non-stationary) in the early twentieth century. This finding improves our understanding on how climate variation affects large-scale synchronization of tree masting. Moreover, it supports the connection between proximate and ultimate causes of masting: indeed, large-scale features of atmospheric circulation coherently drive cues and resources for masting, as well as its evolutionary drivers, such as pollination efficiency, abundance of seed dispersers, and natural disturbance regimes.

/pdf/inter-annual-and-decadal-changes-in-teleconnections-drive-54chim11h0.pdf

Inter-annual and decadal changes in teleconnections drive continental-scale synchronization of tree reproduction

Forest ecosystems with low soil nitrogen (N) availability are characterized by direct competition for this growth-limiting resource between several players, i.e. various components of vegetation, such as old-growth trees, natural regeneration and understorey species, mycorrhizal fungi, free-living fungi and bacteria. With the increase in frequency and intensity of extreme climate events predicted in current climate change scenarios, also competition for N between plants and/or soil microorganisms will be affected. In this review, we summarize the present understanding of ecosystem N cycling in N-limited forests and its interaction with extreme climate events, such as heat, drought and flooding. More specifically, the impacts of environmental stresses on microbial release and consumption of bioavailable N, N uptake and competition between plants, as well as plant and microbial uptake are presented. Furthermore, the consequences of drying-wetting cycles on N cycling are discussed. Additionally, we highlight the current methodological difficulties that limit present understanding of N cycling in forest ecosystems and the need for interdisciplinary studies.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1438-8677.2009.00241.x

Nitrogen balance in forest soils: nutritional limitation of plants under climate change stresses

*Recent studies have highlighted a direct, fast transfer of recently assimilated C from the tree canopy to the soil. However, the effect of environmental changes on this flux remains largely unknown. *We investigated the effects of drought on the translocation of recently assimilated C, by pulse-labelling 1.5-yr-old beech tree mesocosms with (13)CO(2). (13)C signatures were then measured daily for 1 wk in leaves, twigs, coarse and fine root water-soluble and total organic matter, phloem organic matter, soil microbial biomass and soil CO(2) efflux. *Drought reduced C assimilation and doubled the residence time of recently assimilated C in leaf biomass. In phloem organic matter, the (13)C label peaked immediately after labelling then decayed exponentially in the control treatment, while under drought it peaked 4 d after labelling. In soil microbial biomass, the label peaked 1 d after labelling in the control treatment, whereas under drought no peak was measured. Two days after labelling, drought decreased the contribution of recently assimilated C to soil CO(2) efflux by 33%. *Our study showed that drought reduced the coupling between canopy photosynthesis and belowground processes. This will probably affect soil biogeochemical cycling, with potential consequences including slower soil nitrogen cycling and changes in C-sequestration potential under future climate conditions.

Drought effects on allocation of recent carbon: From beech leaves to soil CO2 efflux

The effects of local climate and silvicultural treatment on the inorganic N availability, net N uptake capacity of mycorrhizal beech roots and microbial N conversion were assessed in order to characterise changes in the partitioning of inorganic N between adult beech and soil microorganisms. Fine root dynamics, inorganic N in the soil solution and in soil extracts, nitrate and ammonium uptake kinetics of beech as well as gross ammonification, nitrification and denitrification rates were determined in a beech stand consisting of paired sites that mainly differed in aspect (SW vs. NE) and stand density (controls and thinning treatments). Nitrate was the only inorganic N form detectable in the soil water. Its concentration was high in control plots of the NE aspect, but only in canopy gaps and not influenced by thinning. Neither thinning nor aspect affected the abundance of root tips in the soil. Maximum nitrate net uptake by mycorrhizal fine roots of beech, however, differed with aspect, showing significantly lower values at the SW aspect with warm–dry local climate. There were no clear-cut significant effects of local climate or thinning on microbial N conversion, but a tendency towards higher ammonification and nitrification and lower denitrification rates on the untreated controls of the SW as compared to the NE aspect. Apparently, the observed sensitivity of beech towards reduced soil water availability is at least partially due to impaired N acquisition. This seems to be mainly a consequence of reduced N uptake capacity rather than of limited microbial re-supply of inorganic N or of changed patterns of inorganic N partitioning between soil bacteria and roots.

Climate and forest management influence nitrogen balance of European beech forests: microbial N transformations and inorganic N net uptake capacity of mycorrhizal roots

Foresters from many countries are seeking for tree species or provenances able to cope with expected climate change. While it becomes clear that some temperate tree species will increasingly suffer from climate warming, the fate of the ecologically and economically important silver fir (Abies alba Mill.) remains uncertain and debated because the ecological requirements of this species, as well as its resilience to drought, are still unclear. On the one hand, paleoecological studies reveal that this species was widely distributed under much warmer climate, suggesting a high potential to cope with ongoing and future climate warming. On the other hand, species distribution models generally predict a strong decline of its climatic suitability in the future. This paper aims to clarify the potential of this species to thrive in central and western Europe under predicted climate warming by reviewing the knowledge gained from different fields. Based on insight from different fields, we argue that silver fir has a great potential to thrive under warmer conditions in western and central Europe provided sufficient rainfall, as forecasted by climate models for most regions by 2100. For instance, dendroecological studies demonstrate that silver fir is more resistant and resilient to drought compared to co-occurring species such as Norway spruce, European beech and larch. The most prominent obstacle for increasing the proportion of fir in mixed forests nowadays is ungulate browsing that often prevents its upgrowth.

What is the potential of silver fir to thrive under warmer and drier climate

Le programme de recherche « Forêts et changements climatiques » a eu pour principal objet d’estimer les conséquences des changements climatiques pour la forêt et ses prestations. Dans cet ouvrage, les résultats des 42 projets sont intégrés aux connaissances de la recherche internationale et résumés en cinq messages principaux : 1) des connaissances solides sont nécessaires pour évaluer les changements climatiques et en déduire des mesures d’adaptation ; 2) les changements climatiques agissent différemment sur la forêt selon la station et le peuplement ; 3) les forêts ne peuvent pas amortir les changements climatiques que dans une certaine mesure ; 4) une gestion adaptée contribue à maintenir les prestations forestières menacées ; et 5) une gestion adaptative contribue à développer en permanence la gestion forestière. Le programme de recherche a permis d’améliorer sensiblement les connaissances sur la forêt dans le contexte des changements climatiques. Les résultats ont renforcé les certitudes quant aux domaines dans lesquels il est nécessaire d’agir, et quant aux stratégies d’adaptation et aux mesures les plus prometteuses. Il appartient désormais aux gestionnaires forestiers d’intégrer ces connaissances à leur propre expérience du terrain, et si nécessaire d’adapter la gestion. Les scientifiques devront continuer de fournir des bases et des connaissances précises sans perdre de vue le contexte général. Il faut en outre garantir le transfert des connaissances vers la pratique. Quant aux acteurs de la politique et de l’administration, il leur revient à présent de concevoir des conditions cadres qui permettent et favorisent la gestion adaptative. Pour que la forêt fournisse des prestations productrices, protectrices et récréatives de qualité malgré les changements climatiques, la gestion doit miser encore davantage sur la répartition des risques et – si ce n’est pas déjà le cas – viser à maintenir et renforcer la capacité de résistance et d’adaptation de la forêt.

Messages et recommandations concernant la forêt dans le contexte des changements climatiques

Although the composition of the soil solution has important ecological information, there is no general consensus for obtaining and analyzing of the soil solution.  This study presents an alternative procedure to obtain the soil solution and determine all relevant anions and cations. The soil samples are taken with an auger. 10—20 g of field moist soil are desorbed in a pressure chamber at 170 kPa (pF 3.2), with a cellulose acetate membrane filter (∅ < 0.45 μm) as capillar bridge between the interior and exterior of the chamber. The desorption procedure is performed at 4°C for 24 hours and yields up to 1.0 ml soil solution, depending on the actual water potential. If more soil solution is needed, the soil may be replaced by another aliquot of the same sample. 0.15 ml of soil solution is sufficient for analysing all cations and anions, which account quantitatively for the ion balance with a capillary electrophoresis. Compared with suction cups, ion concentrations in desorption solutions are, although generally lower, in the same order of magnitude. The advantage of this method is that no field equipment is needed, apart from the auger. Even in heterogeneous forest soils, water soluble ions can be monitored with a high spatial resolution and without any dilution effects, which are common in the most laboratory methods. The problem of lacking spatial representativity in stationary lysimeter stations is also overcome. Additionally it is possible to obtain and analyze soil solutions in a suction range where suction cups fail.



Die Desorptionslosung — ein alternativer Ansatz zur Bestimmung wasserloslicher Ionen in Boden



Obwohl die Zusammensetzung der Bodenlosung hohen okologischen Informationsgehalt besitzt, gibt es keinen generellen Konsens uber ihre Gewinnung und Analyse. Es wird ein neues Verfahren zur Gewinnung und Analyse der Bodenlosung vorgestellt. Bodenproben werden mit einem Bohrstock entnommen, 10—20 g feldfrischen Bodens werden in einer Druckkammer bei 170 kPa (pF 3.2) desorbiert. Dabei dient ein Cellulose-Membranfilter (∅ < 0.45 μm) als kapillare Brucke zwischen dem Kammerinneren und dem Auslass. Die Desorption erfolgt bei 4°C, in 24 Stunden konnen — je nach Bodenfeuchte — bis zu 1.0 ml Bodenlosung gewonnen werden. 0.15 ml Bodenlosung sind fur die Analyse aller bilanzrelevanten Kationen und Anionen mittels Kapillarelektrophorese erforderlich. Die Konzentrationen in der Desorptionslosung sind im Vergleich zu Losungen aus Saugkerzenanlagen meistens geringer, liegen jedoch in der gleichen Grosenordnung. Der Vorteil der Methode ist, dass — abgesehen von einem Bohrstock — kein weiterer technischer Aufwand im Feld erforderlich ist. In heterogenen Waldboden kann so z.B. der Gehalt wasserloslicher Ionen mit hoher raumlicher Auflosung gemessen werden, ohne dass Verdunnungseffekte auftreten, wie sie bei den meisten Labormethoden ublich sind. Daruber hinaus gibt es — im Gegensatz zu Saugkerzenanlagen und Lysimetern — weniger Probleme mit unzureichender Flachenreprasentativitat. Es ist auch moglich, Bodenlosungen bei Wasserspannungen zu gewinnen, die auserhalb des Einsatzbereiches von Saugkerzen liegen.

The desorption solution - an alternative approach to measure water soluble ions in soils.

Eine Einschätzung der Auswirkungen des Klimawandels auf den Wald und die Waldleistungen stand im Mittelpunkt des Forschungsprogramms «Wald und Klimawandel». Die Ergebnisse aus den 42 Projekten wurden in diesem Buch mit dem Kenntnisstand der internationalen Forschung verknüpft und lassen sich in fünf Kernaussagen zusammenfassen: 1) Die Einschätzung des Klimawandels und die Ableitung von Anpassungsmassnahmen benötigen solide Wissensgrundlagen; 2) Je nach Standort und Bestockung wirkt sich der Klimawandel unterschiedlich auf den Wald aus; 3) Wälder können den Klimawandel nur in begrenztem Mass auffangen; 4) Ein angepasstes Management trägt zur Aufrechterhaltung von gefährdeten Waldleistungen bei; und 5) Adaptives Management hilft, die Waldbewirtschaftung fortlaufend weiterzuentwickeln. Im Forschungsprogramm konnte das Wissen zum Wald im Klimawandel erheblich verbessert werden. Die Forschungsergebnisse haben mehr Gewissheit geschaffen, in welchen Bereichen Handlungsbedarf besteht und welche Adaptationsstrategien und Einzelmassnahmen zielführend sein dürften. Die Waldbewirtschafter sind jetzt gefordert, diese Wissensgrundlagen unter Einbezug ihrer lokalen Erfahrungen zu nutzen und die Bewirtschaftung falls nötig anzupassen. Die Waldforschung ist gefordert, weiterhin Grundlagen und Detailwissen zu erarbeiten und dabei die übergeordneten Zusammenhänge im Auge zu behalten. Zudem ist der Wissenstransfer in die Praxis sicherzustellen. Die Politik und die Verwaltung sind gefordert, die Rahmenbedingungen so zu gestalten, dass adaptives Management durch die Waldbesitzenden und Forstbetriebe ermöglicht und gefördert wird. Um ein hohes Niveau an Nutz-, Schutzund Wohlfahrtsleistungen des Waldes auch im Klimawandel sicherzustellen, muss die Bewirtschaftung noch stärker auf Risikoverteilung setzen und – soweit dies nicht bereits der Fall ist – darauf abzielen, die Widerstandsund Anpassungsfähigkeit des Waldes zu erhalten und zu fördern.

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Sabine Augustin

Papers

Climate and forest management influence nitrogen balance of European beech forests: microbial N transformations and inorganic N net uptake capacity of mycorrhizal roots

What is the potential of silver fir to thrive under warmer and drier climate

Messages et recommandations concernant la forêt dans le contexte des changements climatiques

The desorption solution - an alternative approach to measure water soluble ions in soils.

Kernaussagen und Empfehlungen zum Wald im Klimawandel