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.EnllaçDoi: 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.EnllaçDoi: 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.
Steppe K., Vandegehuchte M.W., Tognetti R., Mencuccini M. (2015) Sap flow as a key trait in the understanding of plant hydraulic functioning. Tree Physiology. 35: 341-345.EnllaçDoi: 10.1093/treephys/tpv033
[No abstract available]
Lonsdale J., Minunno F., Mencuccini M., Perks M. (2014) Bayesian calibration and Bayesian model comparison of a stand level dynamic growth model for Sitka spruce and Scots pine. Forestry. 88: 326-335.EnllaçDoi: 10.1093/forestry/cpv003
Growth models continue to be of importance in modern multi-functional forestry to provide forecasts. Bayesian calibrations allow both model structure and parameters to be assessed simultaneously in a probabilistic framework, providing a model with which forecasts and their uncertainty can be better understood and quantified using posterior probability distributions. A Bayesian calibration of a stand-level dynamic growth (SLeDG) model is carried out for both Sitka spruce and Scots pine in the UK for the first time. The calibration used the differential evolution Markov-Chain method to reduce the required number of iterations for inference. Two different model structures were considered for estimating local stand productivity: one using the measured height-age relationship, and one using estimated site yield class. The height-age relationship was shown to be more probable for both species in a Bayesian model comparison (total model probability$=$0.64 and 0.58 for Sitka spruce and Scots pine, respectively), although metrics of model performance were similar for both model structures ($R2 \geq 0.88$ in all variables). A complete calibration (using all data) of the more probable model structure was then completed, and excellent model fit was observed ($R2 >0.95$ for all variables in both species). Example forecasts using the output from the calibration were demonstrated, and are compatible with existing yield tables for both species. This method could be applied to other species or other model structures in the future. © Institute of Chartered Foresters, 2015.
Martinez-Vilalta J., Poyatos R., Aguade D., Retana J., Mencuccini M. (2014) A new look at water transport regulation in plants. New Phytologist. : 0-0.EnllaçDoi: 10.1111/nph.12912
Plant function requires effective mechanisms to regulate water transport at a variety of scales. Here, we develop a new theoretical framework describing plant responses to drying soil, based on the relationship between midday and predawn leaf water potentials. The intercept of the relationship (Λ) characterizes the maximum transpiration rate per unit of hydraulic transport capacity, whereas the slope (σ) measures the relative sensitivity of the transpiration rate and plant hydraulic conductance to declining water availability. This framework was applied to a newly compiled global database of leaf water potentials to estimate the values of Λ and σ for 102 plant species. Our results show that our characterization of drought responses is largely consistent within species, and that the parameters Λ and σ show meaningful associations with climate across species. Parameter σ was ≤1 in most species, indicating a tight coordination between the gas and liquid phases of water transport, in which canopy transpiration tended to decline faster than hydraulic conductance during drought, thus reducing the pressure drop through the plant. The quantitative framework presented here offers a new way of characterizing water transport regulation in plants that can be used to assess their vulnerability to drought under current and future climatic conditions. © 2014 New Phytologist Trust.
Mencuccini M. (2014) Temporal scales for the coordination of tree carbon and water economies during droughts. Tree Physiology. 34: 439-442.EnllaçDoi: 10.1093/treephys/tpu029
Mencuccini M., Onate M., Penuelas J., Rico L., Munne-Bosch S. (2014) No signs of meristem senescence in old scots pine. Journal of Ecology. 102: 555-565.EnllaçDoi: 10.1111/1365-2745.12219
Ageing and senescence in plants remain poorly understood. Although meristem totipotency may allow woody perennials to be immortal, relative growth and photosynthetic rates typically decline with age. Trees of ages between 129 and 534 years were selected in one of the oldest extant populations of Scots pine. Apical branches were propagated by grafting onto homogeneous juvenile rootstock to eliminate the effects of size and environmental variability and isolate those due to age. The hormonal profile of leaves and seeds along with markers of the physiological status of leaves and their pattern of DNA cytosine methylation were measured 15 years after grafting. The percentage of total methylated loci in nuclear DNA increased with increasing meristematic age. However, only very few significant relationships were found between levels of phyto-hormones, pigments or physiological markers either in leaves or seeds and age of the meristem. In addition, shoots grafted from old trees grew as fast as those from younger trees and produced the same number of germinable seeds. Synthesis. We conclude that changes in DNA methylation can occur in old trees. The lack of apparent physiological deterioration in the grafted plants suggests that meristem senescence is not the main factor triggering whole-plant ageing in Scots pine. We conclude that changes in DNA methylation can occur in old trees. The lack of apparent physiological deterioration in the grafted plants suggests that meristem senescence is not the main factor triggering whole-plant ageing in Scots pine. © 2014 British Ecological Society.
Mizunuma T., Mencuccini M., Wingate L., Ogée J., Nichol C., Grace J. (2014) Sensitivity of colour indices for discriminating leaf colours from digital photographs. Methods in Ecology and Evolution. 5: 1078-1085.EnllaçDoi: 10.1111/2041-210X.12260
Digital images of tree canopies have been analysed to understand how forest phenology responds to climate change. Researchers have used different colour indices to carry out quantitative analyses, but uncertainties over the performance of the various indices are hampering progress in their use. To compare the various indices under controlled conditions, we carried out experiments using a low-cost off-the-shelf digital camera with a set of standard colour charts as model leaves for different stages: emerging leaves, yellowish green; newly expanded leaves, green; fully mature leaves, dark green; senescent leaves, yellow. Two models of cameras, a compact digital camera and a surveillance 'live image' camera were used, and photographs were taken by two cameras for each model under clear or overcast sky conditions with two colour balance settings. The indices were also compared with those derived from spectral reflectance. Colour indices based on hue distinguished leaf colour samples with only a small influence of camera models, balance setting and sky conditions, while indices based on green were strongly influenced by camera models and were relatively insensitive to leaf colours. The strength of the green channel relative to the total of digital numbers took similar values for the mature and senescent replica leaves, highlighting its poor ability to identify the change of colour in autumn. Spectral-based hue was also sensitive to the gradation of leaf colours and showed a good correlation with the digital representation of hue regardless of camera models and balance setting. Remarkably, the primitive digital number of red, Nred, also discriminated leaf colours well, with a small influence of the factors investigated here, showing a good correlation with the reflectance of the red band, except from images taken by the surveillance cameras with auto balance. Hue was a robust index across the image set, while the green-based indices often used to quantify canopy phenology in previous studies performed poorly. Hue was well correlated with spectral reflectance indices and worked better than all other indices to discriminate leaf colours. We recommend using hue as a colour index for tracking different stages of leaf development. © 2014 The Authors.
Nair R., Weatherall A., Perks M., Mencuccini M. (2014) Stem injection of 15N-NH4NO3 into mature Sitka spruce (Picea sitchensis). Tree Physiology. 34: 1130-1140.EnllaçDoi: 10.1093/treephys/tpu084
Stem injection techniques can be used to introduce 15N into trees to overcome a low variation in natural abundance and label biomass with a distinct 15N signature, but have tended to target small and young trees, of a variety of species, with little replication. We injected 98 atom% 15N ammonium nitrate (NH4NO3) solution into 13 mature, 9- to 13-m tall edge-profile Sitka spruce trees in order to produce a large quantity of labelled litter, examining the distribution of the isotope throughout the canopy after felling in terms of both total abundance of 15N and relative distribution of the isotope throughout individual trees. Using a simple mass balance of the canopy alone, based on observed total needle biomass and modelled branch biomass, all of the isotope injected was accounted for, evenly split between needles and branches, but with a high degree of variability both within individual trees, and among trees. Both 15N abundance and relative within-canopy distribution were biased towards the upper and middle crown in foliage. Recovery of the label in branches was much more variable than in needles, possibly due to differences in nitrogen allocation for both growth and storage, which differ seasonally between foliage and woody biomass. © The Author 2014. Published by Oxford University Press.
Sala A., Mencuccini M. (2014) Ecosystem science: Plump trees win under drought. Nature Climate Change. 4: 666-667.EnllaçDoi: 10.1038/nclimate2329
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