Gleason S.M., Westoby M., Jansen S., Choat B., Hacke U.G., Pratt R.B., Bhaskar R., Brodribb T.J., Bucci S.J., Cao K.-F., Cochard H., Delzon S., Domec J.-C., Fan Z.-X., Feild T.S., Jacobsen A.L., Johnson D.M., Lens F., Maherali H., Martinez-Vilalta J., Mayr S., Mcculloh K.A., Mencuccini M., Mitchell P.J., Morris H., Nardini A., Pittermann J., Plavcova L., Schreiber S.G., Sperry J.S., Wright I.J., Zanne A.E. (2015) Weak tradeoff between xylem safety and xylem-specific hydraulic efficiency across the world's woody plant species. New Phytologist. : 0-0.LinkDoi: 10.1111/nph.13646
The evolution of lignified xylem allowed for the efficient transport of water under tension, but also exposed the vascular network to the risk of gas emboli and the spread of gas between xylem conduits, thus impeding sap transport to the leaves. A well-known hypothesis proposes that the safety of xylem (its ability to resist embolism formation and spread) should trade off against xylem efficiency (its capacity to transport water). We tested this safety-efficiency hypothesis in branch xylem across 335 angiosperm and 89 gymnosperm species. Safety was considered at three levels: the xylem water potentials where 12%, 50% and 88% of maximal conductivity are lost. Although correlations between safety and efficiency were weak (r2
Guerrieri R., Vanguelova E.I., Michalski G., Heaton T.H.E., Mencuccini M. (2015) Isotopic evidence for the occurrence of biological nitrification and nitrogen deposition processing in forest canopies. Global Change Biology. 21: 4613-4626.LinkDoi: 10.1111/gcb.13018
This study examines the role of tree canopies in processing atmospheric nitrogen (Ndep) for four forests in the United Kingdom subjected to different Ndep: Scots pine and beech stands under high Ndep (HN, 13-19 kg N ha-1 yr-1), compared to Scots pine and beech stands under low Ndep (LN, 9 kg N ha-1 yr-1). Changes of NO3-N and NH4-N concentrations in rainfall (RF) and throughfall (TF) together with a quadruple isotope approach, which combines δ18O, Δ17O and δ15N in NO3 - and δ15N in NH4 +, were used to assess N transformations by the canopies. Generally, HN sites showed higher NH4-N and NO3-N concentrations in RF compared to the LN sites. Similar values of δ15N-NO3 - and δ18O in RF suggested similar source of atmospheric NO3 - (i.e. local traffic), while more positive values for δ15N-NH4 + at HN compared to LN likely reflected the contribution of dry NHx deposition from intensive local farming. The isotopic signatures of the N-forms changed after interacting with tree canopies. Indeed, 15N-enriched NH4 + in TF compared to RF at all sites suggested that canopies played an important role in buffering dry Ndep also at the low Ndep site. Using two independent methods, based on δ18O and Δ17O, we quantified for the first time the proportion of NO3 - in TF, which derived from nitrification occurring in tree canopies at the HN site. Specifically, for Scots pine, all the considered isotope approaches detected biological nitrification. By contrast for the beech, only using the mixing model with Δ17O, we were able to depict the occurrence of nitrification within canopies. Our study suggests that tree canopies play an active role in the N cycling within forest ecosystems. Processing of Ndep within canopies should not be neglected and needs further exploration, with the combination of multiple isotope tracers, with particular reference to Δ17O. © 2015 John Wiley & Sons Ltd.
Lonsdale J., Xenakis G., Mencuccini M., Perks M. (2015) A comparison of models for quantifying growth and standing carbon in UK Scots pine forests. IForest. 8: 596-605.LinkDoi: 10.3832/ifor1403-008
Scots pine is the most abundant native conifer in the UK. A stand level dynamic growth (SLeDG) model is parametrised for British Scots pine stands for the first time. This model predicts stands annually based on their current state, and allows for changes in forest management. Stand growth and carbon storage predictions using this model were compared with those of the yield look-up package Forest Yield, and a process-based model (3PGN). Predictions were compared graphically over an 100 year rotation, and strengths and weaknesses of each were considered. The SLeDG parametrisation provided forecasts of Scots pine growth with percentage mean absolute difference
Meir P., Mencuccini M., Dewar R.C. (2015) Drought-related tree mortality: Addressing the gaps in understanding and prediction. New Phytologist. 207: 28-33.LinkDoi: 10.1111/nph.13382
Increased tree mortality during and after drought has become a research focus in recent years. This focus has been driven by: the realisation that drought-related tree mortality is more widespread than previously thought; the predicted increase in the frequency of climate extremes this century; and the recognition that current vegetation models do not predict drought-related tree mortality and forest dieback well despite the large potential effects of these processes on species composition and biogeochemical cycling. To date, the emphasis has been on understanding the causal mechanisms of drought-related tree mortality, and on mechanistic models of plant function and vegetation dynamics, but a consensus on those mechanisms has yet to emerge. In order to generate new hypotheses and to help advance the modelling of vegetation dynamics in the face of incomplete mechanistic understanding, we suggest that general patterns should be distilled from the diverse and as-yet inconclusive results of existing studies, and more use should be made of optimisation and probabilistic modelling approaches that have been successfully applied elsewhere in plant ecology. The outcome should inform new empirical studies of tree mortality, help improve its prediction and reduce model complexity. © 2015 The Authors. New Phytologist © 2015 New Phytologist Trust.
Mencuccini M., Minunno F., Salmon Y., Martinez-Vilalta J., Holtta T. (2015) Coordination of physiological traits involved in drought-induced mortality of woody plants. New Phytologist. : 0-0.LinkDoi: 10.1111/nph.13461
Accurate modelling of drought-induced mortality is challenging. A steady-state model is presented integrating xylem and phloem transport, leaf-level gas exchange and plant carbohydrate consumption during drought development. A Bayesian analysis of parameter uncertainty based on expert knowledge and a literature review is carried out. The model is tested by combining six data compilations covering 170 species using information on sensitivities of xylem conductivity, stomatal conductance and leaf turgor to water potential. The possible modes of plant failure at steady state are identified (i.e. carbon (C) starvation, hydraulic failure and phloem transport failure). Carbon starvation occurs primarily in the parameter space of isohydric stomatal control, whereas hydraulic failure is prevalent in the space of xylem susceptibility to embolism. Relative to C starvation, phloem transport failure occurs under conditions of low sensitivity of photosynthesis and high sensitivity of growth to plant water status. These three failure modes are possible extremes along two axes of physiological vulnerabilities, one characterized by the balance of water supply and demand and the other by the balance between carbohydrate sources and sinks. Because the expression of physiological vulnerabilities is coordinated, we argue that different failure modes should occur with roughly equal likelihood, consistent with predictions using optimality theory. © 2015 New Phytologist Trust.
Pfautsch S., Holtta T., Mencuccini M. (2015) Hydraulic functioning of tree stems - Fusing ray anatomy, radial transfer and capacitance. Tree Physiology. 35: 706-722.LinkDoi: 10.1093/treephys/tpv058
Not long ago, textbooks on plant physiology divulged the view that phloem and xylem are separate transport systems with exclusive functions. Phloem was flowing downwards providing roots with carbohydrates. Xylem transported water upwards from roots to leaves. This simplified view has changed forever. Today we have a much-refined understanding of the complex transport mechanisms, regulatory functions and surprisingly ingenuous solutions trees have evolved to distribute carbohydrates and water internally to fuel growth and help mediate biotic and abiotic stresses. This review focuses on functional links between tissues of the inner bark region (i.e., more than just phloem) and the xylem, facilitated by radially aligned and interconnected parenchyma cells, called rays. Rays are usually found along the entire vertical axis of tree stems, mediating a number of transport processes. We use a top-down approach to unveil the role of rays in these processes. Due to the central role of rays in facilitating the coupling of inner bark and xylem we dedicate the first section to ray anatomy, pathways and control mechanisms involved in radial transport. In the second section, basic concepts and models for radial movement through rays are introduced and their impacts on water and carbon fluxes at the whole-tree level are discussed. This section is followed by a closer look at the capacitive function of composite tissues in stems where gradual changes in water potential generate a diurnal 'pulse'. We explain how this pulse can be measured and interpreted, and where the limitations of such analyses are. Towards the end of this review, we include a brief description of the role of radial transport during limited availability of water. By elucidating the strong hydraulic link between inner bark and xylem, the traditional view of two separate transport systems dissolves and the idea of one interconnected, yet highly segregated transport network for carbohydrates and water arises. © 2015 The Author 2015. Published by Oxford University Press. All rights reserved.
Rowland L., Da Costa A.C.L., Galbraith D.R., Oliveira R.S., Binks O.J., Oliveira A.A.R., Pullen A.M., Doughty C.E., Metcalfe D.B., Vasconcelos S.S., Ferreira L.V., Malhi Y., Grace J., Mencuccini M., Meir P. (2015) Death from drought in tropical forests is triggered by hydraulics not carbon starvation. Nature. 528: 119-122.LinkDoi: 10.1038/nature15539
Drought threatens tropical rainforests over seasonal to decadal timescales, but the drivers of tree mortality following drought remain poorly understood. It has been suggested that reduced availability of non-structural carbohydrates (NSC) critically increases mortality risk through insufficient carbon supply to metabolism ('carbon starvation'). However, little is known about how NSC stores are affected by drought, especially over the long term, and whether they are more important than hydraulic processes in determining drought-induced mortality. Using data from the world's longest-running experimental drought study in tropical rainforest (in the Brazilian Amazon), we test whether carbon starvation or deterioration of the water-conducting pathways from soil to leaf trigger tree mortality. Biomass loss from mortality in the experimentally droughted forest increased substantially after >10 years of reduced soil moisture availability. The mortality signal was dominated by the death of large trees, which were at a much greater risk of hydraulic deterioration than smaller trees. However, we find no evidence that the droughted trees suffered carbon starvation, as their NSC concentrations were similar to those of non-droughted trees, and growth rates did not decline in either living or dying trees. Our results indicate that hydraulics, rather than carbon starvation, triggers tree death from drought in tropical rainforest. © 2015 Macmillan Publishers Limited. All rights reserved.
Rowland L., Lobo-do-Vale R.L., Christoffersen B.O., Melém E.A., Kruijt B., Vasconcelos S.S., Domingues T., Binks O.J., Oliveira A.A.R., Metcalfe D., da Costa A.C.L., Mencuccini M., Meir P. (2015) After more than a decade of soil moisture deficit, tropical rainforest trees maintain photosynthetic capacity, despite increased leaf respiration. Global Change Biology. 21: 4662-4672.LinkDoi: 10.1111/gcb.13035
Determining climate change feedbacks from tropical rainforests requires an understanding of how carbon gain through photosynthesis and loss through respiration will be altered. One of the key changes that tropical rainforests may experience under future climate change scenarios is reduced soil moisture availability. In this study we examine if and how both leaf photosynthesis and leaf dark respiration acclimate following more than 12 years of experimental soil moisture deficit, via a through-fall exclusion experiment (TFE) in an eastern Amazonian rainforest. We find that experimentally drought-stressed trees and taxa maintain the same maximum leaf photosynthetic capacity as trees in corresponding control forest, independent of their susceptibility to drought-induced mortality. We hypothesize that photosynthetic capacity is maintained across all treatments and taxa to take advantage of short-lived periods of high moisture availability, when stomatal conductance (gs) and photosynthesis can increase rapidly, potentially compensating for reduced assimilate supply at other times. Average leaf dark respiration (Rd) was elevated in the TFE-treated forest trees relative to the control by 28.2 ± 2.8% (mean ± one standard error). This mean Rd value was dominated by a 48.5 ± 3.6% increase in the Rd of drought-sensitive taxa, and likely reflects the need for additional metabolic support required for stress-related repair, and hydraulic or osmotic maintenance processes. Following soil moisture deficit that is maintained for several years, our data suggest that changes in respiration drive greater shifts in the canopy carbon balance, than changes in photosynthetic capacity. © 2015 John Wiley & Sons Ltd.
Salmon Y., Torres-Ruiz J.M., Poyatos R., Martinez-Vilalta J., Meir P., Cochard H., Mencuccini M. (2015) Balancing the risks of hydraulic failure and carbon starvation: A twig scale analysis in declining Scots pine. Plant, Cell and Environment. : 0-0.LinkDoi: 10.1111/pce.12572
Understanding physiological processes involved in drought-induced mortality is important for predicting the future of forests and for modelling the carbon and water cycles. Recent research has highlighted the variable risks of carbon starvation and hydraulic failure in drought-exposed trees. However, little is known about the specific responses of leaves and supporting twigs, despite their critical role in balancing carbon acquisition and water loss. Comparing healthy (non-defoliated) and unhealthy (defoliated) Scots pine at the same site, we measured the physiological variables involved in regulating carbon and water resources. Defoliated trees showed different responses to summer drought compared with non-defoliated trees. Defoliated trees maintained gas exchange while non-defoliated trees reduced photosynthesis and transpiration during the drought period. At the branch scale, very few differences were observed in non-structural carbohydrate concentrations between health classes. However, defoliated trees tended to have lower water potentials and smaller hydraulic safety margins. While non-defoliated trees showed a typical response to drought for an isohydric species, the physiology appears to be driven in defoliated trees by the need to maintain carbon resources in twigs. These responses put defoliated trees at higher risk of branch hydraulic failure and help explain the interaction between carbon starvation and hydraulic failure in dying trees. Understanding the physiological responses of leaves to drought is crucial since they are the site of both photosynthesis and transpiration, and hence play key roles in balancing the risks of carbon starvation and hydraulic failure. Co-occurring healthy and unhealthy Scots pines showed different responses to summer drought: while healthy trees showed a typical response to drought for an isohydric species, atypical physiology in unhealthy trees appears to be driven by the need to maintain carbohydrate availability in needles and twigs. These responses put unhealthy trees at higher risk of branch hydraulic failure and help to explain the interaction between carbon-starvation and hydraulic failure in dying trees. © 2015 John Wiley & Sons Ltd.
Savage J.A., Clearwater M.J., Haines D.F., Klein T., Mencuccini M., Sevanto S., Turgeon R., Zhang C. (2015) Allocation, stress tolerance and carbon transport in plants: How does phloem physiology affect plant ecology?. Plant, Cell and Environment. : 0-0.LinkDoi: 10.1111/pce.12602
Despite the crucial role of carbon transport in whole plant physiology and its impact on plant-environment interactions and ecosystem function, relatively little research has tried to examine how phloem physiology impacts plant ecology. In this review, we highlight several areas of active research where inquiry into phloem physiology has increased our understanding of whole plant function and ecological processes. We consider how xylem-phloem interactions impact plant drought tolerance and reproduction, how phloem transport influences carbon allocation in trees and carbon cycling in ecosystems and how phloem function mediates plant relations with insects, pests, microbes and symbiotes. We argue that in spite of challenges that exist in studying phloem physiology, it is critical that we consider the role of this dynamic vascular system when examining the relationship between plants and their biotic and abiotic environment. © 2015 John Wiley & Sons Ltd.
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