Living on borrowed time – Amazonian trees use decade‐old storage carbon to survive for months after complete stem girdling

Nonstructural carbon (NSC) reserves act as buffers to sustain tree activity during periods when carbon (C) assimilation does not meet C demand, but little is known about their age and accessibility; we designed a controlled girdling experiment in the Amazon to study tree survival on NSC reserves. We used bomb-radiocarbon (14C) to monitor the time elapsed between C fixation and release (‘age’ of substrates). We simultaneously monitored how the mobilization of reserve C affected δ13CO2. Six ungirdled control trees relied almost exclusively on recent assimilates throughout the 17 months of measurement. The Δ14C of CO2 emitted from the six girdled stems increased significantly over time after girdling, indicating substantial remobilization of storage NSC fixed up to 13–14 yr previously. This remobilization was not accompanied by a consistent change in observed δ13CO2. These trees have access to storage pools integrating C accumulated over more than a decade. Remobilization follows a very clear reverse chronological mobilization with younger reserve pools being mobilized first. The lack of a shift in the δ13CO2 might indicate a constant contribution of starch hydrolysis to the soluble sugar pool even outside pronounced stress periods (regular mixing). © 2018 The Authors. New Phytologist © 2018 New Phytologist Trus

Oxygen in tree stems and possible relations to pathogens and red heart formation

Neu entwickelte Sauerstoffoptoden, die auf der Technik des Fluoreszenzquenching basieren, ermöglichten es Messungen direkt im Holzkörper von Bäumen durchzuführen. Es wurde untersucht, wie lebende Zellen im Splint mit Sauerstoff versorgt werden. O2 kann entweder durch radiale Diffusion oder in gelöster Form mit dem Saftstrom transportiert werden. In einem Laborversuch wurde die Diffusionsgeschwindigkeit von O2 in axialer und radialer Richtung im Holz gemessen. Die gemessenen Diffusionskoeffizienten waren stark vom anatomischen Bau und dem Wassergehalt abhängig, Modellrechnungen zeigten, dass Splintholz unter bestimmten Bedingungen allein durch radiale Diffusion mit O2 versorgt werden kann. Langzeitmessungen in Buchen und Fichten zeigten deutliche Tagesschwankungen und Unterschiede zwischen verschiedenen Positionen im Stamm (Splint/Kernholz, Höhe im Baum). Es wurden Hinweise auf die Bedeutung beider Transportwege für die O2-Versorgung des Splints gefunden. Bei Versuchen mit eingetopften Fichten wurde getestet, ob Trockenstress oder Überflutung O2 im Stamm reduziert. Nach einer Hypothese sollte unter Sauerstoffmangel im Stamm gebildetes Ethanol als Signalstoff für Borkenkäfer dienen. Gestresste Bäume zeigten jedoch weder reduzierte Sauerstoffkonzentrationen, noch erhöhte Emissionen. Es wurde getested, ob erhöhte Sauerstoffzufuhr der auslösende Faktor für die fakultative Ausbildung von Rotkernen in Buchen ist. Dazu wurden Messungen an verkernten und nicht verkernten Buchen durchgeführt und Holz mikrobiologisch untersucht. Bei Lagerungsversuchen zeigte sich, dass das Ausmaß der Rotfärbung von der Sauerstoffkonzentration abhängig ist. Es verfärbten sich allerdings auch Holzstücke bei 0% O2 und auch bereits gebildetes Kernholz. O2 in stehenden Bäumen mit und ohne Farbkern unterschied sich nur selten, aber sowohl in verkernten als auch in nicht verkernten Buchen wurde O2 im Stamm in einer genügend hohen Konzentration festgestellt um potentiell Farbkernbildung auszulösen. O2 ist ein essentieller, aber nicht der einzige Faktor bei der fakultativen Rotkernbildung ist, und eine Beteiligung von Mikroorganismen wird vermutet.Aerobic processes in plant metabolism require a constant supply with oxygen. Whereas gas exchange of leaves and roots is a central topic in plant eco-physiology and has been studied in much detail, only few studies have dealt with gas exchange and especially the oxygen content of stems. This was probably due to the lack of a proper methodology. Recently developed micro-sensors, based on fluorescence quenching, allowed the direct measurements of oxygen in the stem of standing trees. The research conducted in this study measured the oxygen concentration in standing spruce and beech trees, investigating how the living parts of the sapwood are supplied with enough oxygen for respiration. It was hypothesized that oxygen is either transported to the parenchyma cells by means of radial diffusion through bark, phloem and cambium or in dissolved form with the transpiration stream upwards from the roots to the crown. It was further investigated what factors can influence the oxygen content of a stem, and if and how a reduction of oxygen can affects a tree. First, the velocity of axial and radial oxygen diffusion was measured in wood of various native trees to determine if diffusion allone can supply the living sapwood with oxygen. Results showed that oxygen diffusion is strongly dependent on wood anatomy and water content, and model calculations implied that active sapwood can be supplied with oxygen under certain conditions. However, if water content and respiration are too high to assure a sufficient supply by radial diffusion, respiration in the sapwood is reduced by oxygen deficiency. Continuous long-term measurements in stems of spruce and beech trees showed distinct diurnal fluctuation of oxygen, and differences in oxygen between various depths and positions in the stem (sapwood versus heartwood, stem height). These results suggest that both transport pathways play a role. On the one hand, the decrease of oxygen with stem height found in spruce and the dependency of oxygen in beech on soil moisture indicated the importance of the transpiration stream as medium of oxygen transport. On the other hand the fact that oxygen replenished even in times of zero sapflow, and it did not decrease when sapflow was reduced suggests that at least some oxygen enters by radial diffusion. Experiments with potted spruce saplings were conducted to investigate if drought stress or flooding induces oxygen deficiency in the stem. A hypothesis was tested, suggesting that the emissions of ethanol and volatile terpenes are enhanced under conditions of hypoxia or anoxia, which successively could lead to an increased attractiveness for bark-beetles that locate stressed individuals by their emission spectrum. In this study oxygen was not reduced in stressed individuals and emissions of ethanol and volatile terpenes were not increased. Further the hypothesis was tested that oxygen, penetrating into the heartwood through injuries in beech trees, induces the formation of red heart, while bacteria and fungi are irrelevant in the process. Oxygen was measured in beech trees with and without red heart and wood samples were secreened for the presence of micro-organisms. In exposure experiments beech wood was stored at various oxygen concentrations in the laboratory. Although wood discolorations were stronger in the presence of elevated oxygen concentrations in the laboratory, discolorations were also found in beech wood exposed to 0% oxygen and in already formed red heart. In the standing tree a difference in oxygen concentration between beeche trees with and without red heart was found only on few occasions, and oxygen was present in both in a sufficient concentration to induce red heart. The results of this study imply that oxygen is not the only factor necessary for the formation of red heart in beech, and the involvement of micro-organisms needs to be studied in more detail.submitted by Johannes SorzZsfassung in dt. SpracheWien, Univ. für Bodenkultur, Diss., 2006OeBB(VLID)191837

Comparison of methods to quantify loss of hydraulic conductivity in Norway spruce

$\bullet$ Percent loss of hydraulic conductivity (PLC) is an important measure in plant water relations, but the available methods differ and results have rarely been compared. $\bullet$ We compared PLC measured in Norway spruce (Picea abies) by quantifying conductivity before and after removing emboli, either by flushing with high water pressure or by infiltration under a partial vacuum, with relative water loss and staining conductive xylem to test advantages and possible problems of commonly applied methods. $\bullet$ Infiltration removed nearly all gas from sections of drought-stressed and unstressed twigs, and flushing and infiltration efficiently removed emboli. Infiltration appears less prone to producing clogged xylem elements than high pressure flushing. Relative water loss (RWL) and the proportion of xylem stained with phloxine B (PSX) was also highly correlated with PLC, the latter can be quantified by image analysis, and also shows the pattern of xylem dysfunction. Loss of conductivity was more common in the inner annual rings, in early wood within an annual ring, and in compression wood, though pattern differed substantially between branches. $\bullet$ Advantages and potential problems of these methods are discussed and it is suggested that RWL or PSX may be used as proxy measures for PLC in species when the correlations have been established.Comparaison de méthodes de quantification des pertes de conductivité hydraulique chez l'épicéa. $\bullet$ Le pourcentage de perte de conductivité hydraulique (PLC) est un paramètre important des relations hydriques des plantes, mais les méthodes de mesure disponibles diffèrent et les résultats ont rarement été comparés. $\bullet$ Nous avons comparé des valeurs de perte de conductivité chez Picea abies obtenues en quantifiant la conductivité hydraulique avant et après suppression de l'embolie, soit par un flush d'eau sous haute pression soit par infiltration sous un vide partiel, avec les pertes relatives en eau et avec des méthodes de coloration du xylème conducteur coloré ; cette comparaison a permis de tester les avantages et les inconvénients potentiels des méthodes couramment utilisées. $\bullet$ L'infiltration élimine presque tous les gaz des sections de petites branches, alors que de l'eau sous pression et l'infiltration suppriment l'embolie. L'infiltration paraît moins susceptible de provoquer une obstruction des éléments de xylème, qu'un flush à haute pression. Perte relative en eau (RWL) et proportion de xylème coloré avec de la phloxine B (PSX) étaient hautement corrélées avec la perte de conductivité. Une analyse d'image de xylème permet également de quantifier la perte de conductivité et de mettre en évidence la distribution spatiale des vaisseaux embolisés. La perte de conductivité a été très visible dans les cernes interne du tronc, dans le bois initial du cerne de l'année et dans le bois de compression, quoique les patrons diffèrent considérablement entre branches. $\bullet$ Les avantages et les inconvénients potentiels de ces méthodes sont discutés et il est proposé que RWL ou PSX puissent être utilisées comme des estimateurs indirects de perte de conductivité une fois les corrélations établies pour chaque espèce