Jürgen Kreuzwieser

Potential risks for European beech (Fagus sylvatica L.) in a changing climate

Over large areas of Europe, coniferous monocultures are being transformed into mixed forests by the re-introduction of broadleaf tree species belonging to the potential natural vegetation. One important species of interest in this changing forest policy is European beech (Fagus sylvatica). However, at present, this forest management directive has ignored potential adverse effects of global climate change on wide-spread re-introduction of beech to these areas. Average global surface temperatures have risen by approx. 0.8°C in the period between 1861 and 2005 and are expected to continue to increase until the end of this century by 1.5–5.8°C above the 1990 value. To estimate the climate change in the southern part of central Europe in future, we reviewed calculations from regional climate models. Temperature increase for the southern part of central Europe is projected to be up to 2°C within the next 40 years. In contrast, the annual precipitation will most likely remain constant over the same time period, but will experience significant changes in seasonal patterns. Rising intensities of individual precipitation events may result in increasing number and intensities of flooding events and reduced precipitation during the growing season in a higher frequency of summer droughts. Growth and competitive ability of European beech will not, necessarily, respond to increasing CO2 concentrations but may be strongly impacted by intensive drought that occurs during the growing season. Seedlings as well as adult trees may suffer from xylem embolism, restricted nutrient uptake capacity and reduced growth under limited water availability. However, it remains uncertain to what extent other environmental factors (e.g. soil properties, competitive interactions) may modify the drought response of beech, thus either enhancing susceptibility or increasing drought tolerance and resilience potential. Water-logged soils, predicted during the spring for several regions due to higher than average precipitation, could negatively impact nutrient uptake and growth of beech. Whereas other dominant species as, e.g. oak are well adapted to that environmental stress, beech is known to be sensitive to water-logging and flooding. Thus, the competitive capacity of beech might—depending on the other environmental conditions—be reduced under the expected future climate conditions. Silvicultural practices must be aware today of the potential risks which a changing climate may impose on sustainable forest development.

Differential Response of Gray Poplar Leaves and Roots Underpins Stress Adaptation during Hypoxia

The molecular and physiological responses of gray poplar (Populus × canescens) following root hypoxia were studied in roots and leaves using transcript and metabolite profiling. The results indicate that there were changes in metabolite levels in both organs, but changes in transcript abundance were restricted to the roots. In roots, starch and sucrose degradation were altered under hypoxia, and concurrently, the availability of carbohydrates was enhanced, concomitant with depletion of sucrose from leaves and elevation of sucrose in the phloem. Consistent with the above, glycolytic flux and ethanolic fermentation were stimulated in roots but not in leaves. Various messenger RNAs encoding components of biosynthetic pathways such as secondary cell wall formation (i.e. cellulose and lignin biosynthesis) and other energy-demanding processes such as transport of nutrients were significantly down-regulated in roots but not in leaves. The reduction of biosynthesis was unexpected, as shoot growth was not affected by root hypoxia, suggesting that the up-regulation of glycolysis yields sufficient energy to maintain growth. Besides carbon metabolism, nitrogen metabolism was severely affected in roots, as seen from numerous changes in the transcriptome and the metabolome related to nitrogen uptake, nitrogen assimilation, and amino acid metabolism. The coordinated physiological and molecular responses in leaves and roots, coupled with the transport of metabolites, reveal important stress adaptations to ensure survival during long periods of root hypoxia.

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

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.

Global climate change and tree nutrition: influence of water availability

The effects of global climate change will regionally be very different, mainly causing considerable changes in temperature and water availability. For Central Europe, for example, increased temperatures are predicted, which will cause increased frequencies and durations of summer drought events. On the other hand, the predicted changes in precipitation patterns will lead to enhanced rainfall during winter and spring, thereby increasing the risk of flooding in Central and Northern Europe. Depending on the sensitivity to reduced water availability on the one hand and oxygen depletion due to waterlogging on the other, physiological performance, growth and competitive ability of trees may be adversely affected. Both drought and excess water availability impair the mineral nutrition of trees by influencing on the one hand the nutrient availability in the soil and on the other hand the physiology of the uptake systems mainly of the mycorrhizal tree roots. Extreme water regimes also change interaction patterns among plants and between plants and microorganisms, and alter the carbon balance of trees and ecosystems. Here we summarize and discuss the present knowledge on tree nutrition under altered water availability as expected to be more common in the future. The focus is on tree mineral nutrient uptake and metabolism as well as on the interaction between carbon allocation and the mineral nutrient balance as affected by reduced and excess water availability.

On the metabolic origin of the carbon isotope composition of CO2 evolved from darkened light‐acclimated leaves in Ricinus communis

The (13)C isotopic signature (delta(13)C) of CO(2) respired from plants is widely used to assess carbon fluxes and ecosystem functioning. There is, however, a lack of knowledge of the metabolic basis of the delta(13)C value of respired CO(2). To elucidate the physiological mechanisms driving (12)C/(13)C fractionation during respiration, the delta(13)C of respired CO(2) from dark-acclimated leaves during the night, from darkened leaves during the light period, and from stems and roots of Ricinus communis was analysed. The delta(13)C of potential respiratory substrates, the respiratory quotient and the activities of phosphoenolpyruvatecarboxylase (PEPc) and key respiratory enzymes were also measured. It is shown here that the CO(2) evolved from darkened light-acclimated leaves during the light period is (13)C-enriched, and that this correlates with malate accumulation in the light and rapid malate decarboxylation just after the onset of darkness. Whilst CO(2) evolved from leaves was generally (13)C-enriched (but to a lesser extent during the night), CO(2) evolved from stems and roots was depleted compared with the putative respiratory substrates; the difference was mainly caused by intensive PEPc-catalysed CO(2) refixation in stems and roots. These results provide a physiological explanation for short-term variations of delta(13)C in CO(2), illustrating the effects of variations of metabolic fluxes through different biochemical pathways.

Contribution of Different Carbon Sources to Isoprene Biosynthesis in Poplar Leaves

This study was performed to test if alternative carbon sources besides recently photosynthetically fixed CO2 are used for isoprene formation in the leaves of young poplar (Populus × canescens) trees. In a 13CO2 atmosphere under steady state conditions, only about 75% of isoprene became 13C labeled within minutes. A considerable part of the unlabeled carbon may be derived from xylem transported carbohydrates, as may be shown by feeding leaves with [U-13C]Glc. As a consequence of this treatment approximately 8% to 10% of the carbon emitted as isoprene was 13C labeled. In order to identify further carbon sources, poplar leaves were depleted of leaf internal carbon pools and the carbon pools were refilled with 13C labeled carbon by exposure to 13CO2. Results from this treatment showed that about 30% of isoprene carbon became 13C labeled, clearly suggesting that, in addition to xylem transported carbon and CO2, leaf internal carbon pools, e.g. starch, are used for isoprene formation. This use was even increased when net assimilation was reduced, for example by abscisic acid application. The data provide clear evidence of a dynamic exchange of carbon between different cellular precursors for isoprene biosynthesis, and an increasing importance of these alternative carbon pools under conditions of limited photosynthesis. Feeding [1,2-13C]Glc and [3-13C]Glc to leaves via the xylem suggested that alternative carbon sources are probably derived from cytosolic pyruvate/phosphoenolpyruvate equivalents and incorporated into isoprene according to the predicted cleavage of the 3-C position of pyruvate during the initial step of the plastidic deoxyxylulose-5-phosphate pathway.

Transient Release of Oxygenated Volatile Organic Compounds during Light-Dark Transitions in Grey Poplar Leaves

In this study, we investigated the prompt release of acetaldehyde and other oxygenated volatile organic compounds (VOCs) from leaves of Grey poplar [Populus x canescens (Aiton) Smith] following light-dark transitions. Mass scans utilizing the extremely fast and sensitive proton transfer reaction-mass spectrometry technique revealed the following temporal pattern after light-dark transitions: hexenal was emitted first, followed by acetaldehyde and other C6-VOCs. Under anoxic conditions, acetaldehyde was the only compound released after switching off the light. This clearly indicated that hexenal and other C6-VOCs were released from the lipoxygenase reaction taking place during light-dark transitions under aerobic conditions. Experiments with enzyme inhibitors that artificially increased cytosolic pyruvate demonstrated that the acetaldehyde burst after light-dark transition could not be explained by the recently suggested pyruvate overflow mechanism. The simulation of light fleck situations in the canopy by exposing leaves to alternating light-dark and dark-light transitions or fast changes from high to low photosynthetic photon flux density showed that this process is of minor importance for acetaldehyde emission into the Earths atmosphere.

Molecular and physiological responses of trees to waterlogging stress.

One major effect of global climate change will be altered precipitation patterns in many regions of the world. This will cause a higher probability of long-term waterlogging in winter/spring and flash floods in summer because of extreme rainfall events. Particularly, trees not adapted at their natural site to such waterlogging stress can be impaired. Despite the enormous economic, ecological and social importance of forest ecosystems, the effect of waterlogging on trees is far less understood than the effect on many crops or the model plant Arabidopsis. There is only a handful of studies available investigating the transcriptome and metabolome of waterlogged trees. Main physiological responses of trees to waterlogging include the stimulation of fermentative pathways and an accelerated glycolytic flux. Many energy-consuming, anabolic processes are slowed down to overcome the energy crisis mediated by waterlogging. A crucial feature of waterlogging tolerance is the steady supply of glycolysis with carbohydrates, particularly in the roots; stress-sensitive trees fail to maintain sufficient carbohydrate availability resulting in the dieback of the stressed tissues. The present review summarizes physiological and molecular features of waterlogging tolerance of trees; the focus is on carbon metabolism in both, leaves and roots of trees.

Diurnal courses of ammonium net uptake by the roots of adult beech (Fagus sylvatica) and spruce (Picea abies) trees

Differences in ammonium net uptake by the roots of beech (Fagus sylvatica L.) and spruce (Picea abies (L.) Karst) trees between day and night were examined during the growing seasons in 1995 and 1996 using the depletion technique. In addition, diurnal courses of ammonium net uptake of both species were analysed in five sets of uptake experiments in May and September 1997 and were related (1) to the content of carbohydrates, organic acids and total soluble non protein N (TSNN) in the fine roots, and (2) to xylem flow densities and soil temperature. During the growing seasons 1995 and 1996, ammonium net uptake of beech was significantly lower during the night than during the day at 5 of 8 dates of measurement. On average, uptake rates during the night amounted to 50% of the uptake rates during the day. In spruce, the mean values of ammonium net uptake rates determined were similar between day and night during both growing seasons. In beech, the assessment of diurnal courses showed highest ammonium uptake rates during noon and in the afternoon and minima at midnight. In May 1997, comparable, but less pronounced diurnal patterns of ammonium uptake were observed in spruce, whereas in September 1997, ammonium uptake by spruce was constant during the day. Since no distinct differences in carbohydrate and organic acid contents in fine roots were observed during the diurnal courses and since the addition of sucrose into the artificial soil solutions root tips were exposed to did not alter ammonium uptake, depression of uptake by C- and/or energy limitation during night could be excluded. The TSNN contents in the fine roots of beech (May and September 1997) and spruce (May 1997) showed a diurnal pattern inverse to ammonium uptake. It is concluded that the enrichment of TSNN compounds during night that is apparently caused by a reduction of xylem transport is responsible for the down-regulation of ammonium net-uptake.

VOC emissions of Grey poplar leaves as affected by salt stress and different N sources

Nitrogen nutrition and salt stress experiments were performed in a greenhouse with hydroponic-cultured, salt-sensitive Grey poplar (Populus x canescens) plants to study the combined influence of different N sources (either 1 mm NO(3) (-) or NH(4)(+)) and salt (up to 75 mm NaCl) on leaf gas exchange, isoprene biosynthesis and VOC emissions. Net assimilation and transpiration proved to be highly sensitive to salt stress and were reduced by approximately 90% at leaf sodium concentrations higher than 1,800 microg Na g dry weight (dw)(-1). In contrast, emissions of isoprene and oxygenated VOC (i.e. acetaldehyde, formaldehyde and acetone) were unaffected. There was no significant effect of combinations of salt stress and N source, and neither NO(3)(-) or NH(4)(+) influenced the salt stress response in the Grey poplar leaves. Also, transcript levels of 1-deoxy-d-xylulose 5-phosphate reductoisomerase (PcDXR) and isoprene synthase (PcISPS) did not respond to the different N sources and only responded slightly to salt application, although isoprene synthase (PcISPS) activity was negatively affected at least in one of two experiments, despite high isoprene emission rates. A significant salt effect was the strong reduction of leaf dimethylallyl diphosphate (DMADP) content, probably due to restricted availability of photosynthates for DMADP biosynthesis. Further consequences of reduced photosynthetic gas exchange and maintaining VOC emissions are a very high C loss, up to 50%, from VOC emissions related to net CO(2) uptake and a strong increase in leaf internal isoprene concentrations, with maximum mean values up to 6.6 microl x l(-1). Why poplar leaves maintain VOC biosynthesis and emission under salt stress conditions, despite impaired photosynthetic CO(2) fixation, is discussed.