The regulation of sapwood area, water transport and heartwood formation in Sitka spruce

Beauchamp K., Mencuccini M., Perks M., Gardiner B. (2013) The regulation of sapwood area, water transport and heartwood formation in Sitka spruce. Plant Ecology and Diversity. 6: 45-56.
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Doi: 10.1080/17550874.2012.702359

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Background: The mechanism by which water transport, tree growth and heartwood formation are balanced is poorly understood.Aims: To test the hypothesis that xylem formation drives heartwood production through changes in water transport to regulate sapwood area.Methods: We measured changes in sap flux at multiple depths across the sapwood to heartwood boundary in Sitka spruce (Picea sitchensis) from May 2009-October 2010 using the Heat Field Deformation method.Results: The radial sap flux profile was peaked with maximum flux occurring 1-2 cm below bark before reducing to the heartwood boundary (tail). Changes occurred in two stages. The depth of peak sap flux (D) extended outwards as new xylem formed during the growing season, giving an annual increment (AI). Water transport reduced in the tail sometime during dormancy, from November to March. The correlation between AI and D was good however, these variables and the increase in sapwood area correlated poorly with the extent of heartwood formed.Conclusions: Heartwood formed during the dormant period in Sitka spruce in Great Britain. Xylem formation did not directly drive heartwood production; however, changes in specific conductivity need consideration. Reduced transport in the inner sapwood could provide a temporal signal for heartwood formation in pre-conditioned cells. © 2013 Copyright Crown copyright 2013.

Llegeix més

Effects of Long-Term Nitrogen Addition and Atmospheric Nitrogen Deposition on Carbon Accumulation in Picea sitchensis Plantations

Gentilesca T., Vieno M., Perks M.P., Borghetti M., Mencuccini M. (2013) Effects of Long-Term Nitrogen Addition and Atmospheric Nitrogen Deposition on Carbon Accumulation in Picea sitchensis Plantations. Ecosystems. 16: 1310-1324.
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Doi: 10.1007/s10021-013-9685-9

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This study aimed to assess the combined effects of long-term nitrogen (N) supply and nitrogen deposition (N dep) on carbon (C) accumulation within Sitka spruce [Picea sitchensis (Bong.) Carr.] plantations in Scotland. Six study sites established from 1970 to 1982 were periodically N-fertilized, monitored over time and commonly surveyed in 2010. Soil, aboveground biomass, and ground vegetation C stock changes were analyzed; aboveground C stocks were correlated with total additional N experienced at each site, that is, the sum of experimental N supply (N add) and site-specific accumulated N dep from 1900 to 2010. Results showed a positive N effect on aboveground tree C stock and no decline in tree growth was observed either during fertilization or after the latest N addition. The amount of C in litter was significantly higher in experimentally N-treated plots, whereas the amount of C in understory vegetation was higher in control plots. Pooling all the compartments (that is, understory vegetation, litter, soil, and tree biomass) the total ecosystem C content was estimated for each site, and at most sites a higher C stock was estimated for N-treated plots. Differences in aboveground C accumulation rates between treated and control plots were lower at sites with high levels of accumulated N dep. Our results indicate that site-specific accumulated N dep should be considered to understand tree growth responses to N fertilization. © 2013 Springer Science+Business Media New York.

Llegeix més

Species mixing boosts root yield in mangrove trees

Lang'at J.K.S., Kirui B.K.Y., Skov M.W., Kairo J.G., Mencuccini M., Huxham M. (2013) Species mixing boosts root yield in mangrove trees. Oecologia. 172: 271-278.
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Doi: 10.1007/s00442-012-2490-x

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Enhanced species richness can stimulate the productivity of plant communities; however, its effect on the belowground production of forests has scarcely been tested, despite the role of tree roots in carbon storage and ecosystem processes. Therefore, we tested for the effects of tree species richness on mangrove root biomass: thirty-two 6 m by 6 m plots were planted with zero (control), one, two or three species treatments of six-month-old Avicennia marina (A), Bruguiera gymnorrhiza (B) and Ceriops tagal (C). A monoculture of each species and the four possible combinations of the three species were used, with four replicate plots per treatment. Above- and belowground biomass was measured after three and four years' growth. In both years, the all-species mix (ABC) had significant overyielding of roots, suggesting complementarity mediated by differences in rhizosphere use amongst species. In year four, there was higher belowground than aboveground biomass in all but one treatment. Belowground biomass was strongly influenced by the presence of the most vigorously growing species, A. marina. These results demonstrate the potential for complementarity between fast- and slow-growing species to enhance belowground growth in mangrove forests, with implications for forest productivity and the potential for belowground carbon sequestration. © 2012 Springer-Verlag Berlin Heidelberg.

Llegeix més

Evaluating theories of drought-induced vegetation mortality using a multimodel-experiment framework

Mcdowell N.G., Fisher R.A., Xu C., Domec J.C., Holtta T., Mackay D.S., Sperry J.S., Boutz A., Dickman L., Gehres N., Limousin J.M., Macalady A., Martinez-Vilalta J., Mencuccini M., Plaut J.A., Ogee J., Pangle R.E., Rasse D.P., Ryan M.G., Sevanto S., Waring R.H., Williams A.P., Yepez E.A., Pockman W.T. (2013) Evaluating theories of drought-induced vegetation mortality using a multimodel-experiment framework. New Phytologist. 200: 304-321.
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Doi: 10.1111/nph.12465

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Summary: Model-data comparisons of plant physiological processes provide an understanding of mechanisms underlying vegetation responses to climate. We simulated the physiology of a piñon pine-juniper woodland (Pinus edulis-Juniperus monosperma) that experienced mortality during a 5 yr precipitation-reduction experiment, allowing a framework with which to examine our knowledge of drought-induced tree mortality. We used six models designed for scales ranging from individual plants to a global level, all containing state-of-the-art representations of the internal hydraulic and carbohydrate dynamics of woody plants. Despite the large range of model structures, tuning, and parameterization employed, all simulations predicted hydraulic failure and carbon starvation processes co-occurring in dying trees of both species, with the time spent with severe hydraulic failure and carbon starvation, rather than absolute thresholds per se, being a better predictor of impending mortality. Model and empirical data suggest that limited carbon and water exchanges at stomatal, phloem, and below-ground interfaces were associated with mortality of both species. The model-data comparison suggests that the introduction of a mechanistic process into physiology-based models provides equal or improved predictive power over traditional process-model or empirical thresholds. Both biophysical and empirical modeling approaches are useful in understanding processes, particularly when the models fail, because they reveal mechanisms that are likely to underlie mortality. We suggest that for some ecosystems, integration of mechanistic pathogen models into current vegetation models, and evaluation against observations, could result in a breakthrough capability to simulate vegetation dynamics. © 2013 New Phytologist Trust.

Llegeix més

Concurrent measurements of change in the bark and xylem diameters of trees reveal a phloem-generated turgor signal

Mencuccini M., Holtta T., Sevanto S., Nikinmaa E. (2013) Concurrent measurements of change in the bark and xylem diameters of trees reveal a phloem-generated turgor signal. New Phytologist. 198: 1143-1154.
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Doi: 10.1111/nph.12224

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Currently, phloem transport in plants under field conditions is not well understood. This is largely the result of the lack of techniques suitable for the measurement of the physiological properties of phloem. We present a model that interprets the changes in xylem diameter and live bark thickness and separates the components responsible for such changes. We test the predictions from this model on data from three mature Scots pine trees in Finland. The model separates the live bark thickness variations caused by bark water capacitance from a residual signal interpreted to indicate the turgor changes in the bark. The predictions from the model are consistent with processes related to phloem transport. At the diurnal scale, this signal is related to patterns of photosynthetic activity and phloem loading. At the seasonal scale, bark turgor showed rapid changes during two droughts and after two rainfall events, consistent with physiological predictions. Daily cumulative totals of this turgor term were related to daily cumulative totals of canopy photosynthesis. Finally, the model parameter representing radial hydraulic conductance between phloem and xylem showed a temperature dependence consistent with the temperature-driven changes in water viscosity. We propose that this model has potential for the continuous field monitoring of tree phloem function. © 2013 The Authors. New Phytologist © 2013 New Phytologist Trust.

Llegeix més

The relationship between carbon dioxide uptake and canopy colour from two camera systems in a deciduous forest in southern England

Mizunuma T., Wilkinson M., L. Eaton E., Mencuccini M., I. L. Morison J., Grace J. (2013) The relationship between carbon dioxide uptake and canopy colour from two camera systems in a deciduous forest in southern England. Functional Ecology. 27: 196-207.
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Doi: 10.1111/1365-2435.12026

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Carbon dioxide flux measurements using the eddy covariance (EC) methodology have helped researchers to develop models of ecosystem carbon balance. However, making reliable predictions of carbon fluxes is not straightforward due to phenological changes and possible abiotic/biotic stresses that profoundly influence tree functioning. To assess the influence of canopy phenological state on CO2 flux, we installed two different digital camera systems at different viewing angles (an outdoor webcam with a near-horizontal view and a commercial 'fisheye' digital camera with a downward view) on a flux measurement tower in southern England and tracked the visual change of the canopy in this oak-dominated (Quercus robur L.) forest over two growing seasons. Changes in the setting of the camera's white balance substantially affected the quality of the webcam images. However, the timing of the onset of greening and senescence was, nevertheless, detectable for the individual trees as well as the overall canopy for both years. The greening-up date assessed from the downward images from a hemispherical lens was ~5 days earlier than from the horizontal-view images, because of ground vegetation development (not visible in the horizontal view). The effects of a late air frost in 2010 were evident in the canopy greenness, and these led to reductions in daily gross primary productivity (GPP). The cameras recorded differences between individual tree crowns, showing their different responses to the late frost. A major new finding from this work is the strong relationship between GPP and Hue, which was stronger than the relationship between GPP and NDVI. © 2012 The Authors. Functional Ecology © 2012 British Ecological Society.

Llegeix més

Drought-induced defoliation and long periods of near-zero gas exchange play a key role in accentuating metabolic decline of Scots pine

Poyatos R., Aguade D., Galiano L., Mencuccini M., Martinez-Vilalta J. (2013) Drought-induced defoliation and long periods of near-zero gas exchange play a key role in accentuating metabolic decline of Scots pine. New Phytologist. 200: 388-401.
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Doi: 10.1111/nph.12278

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Summary: Drought-induced defoliation has recently been associated with the depletion of carbon reserves and increased mortality risk in Scots pine (Pinus sylvestris). We hypothesize that defoliated individuals are more sensitive to drought, implying that potentially higher gas exchange (per unit of leaf area) during wet periods may not compensate for their reduced photosynthetic area. We measured sap flow, needle water potentials and whole-tree hydraulic conductance to analyse the drought responses of co-occurring defoliated and nondefoliated Scots pines in northeast Spain during typical (2010) and extreme (2011) drought conditions. Defoliated Scots pines showed higher sap flow per unit leaf area during spring, but were more sensitive to summer drought, relative to nondefoliated pines. This pattern was associated with a steeper decline in soil-to-leaf hydraulic conductance with drought and an enhanced sensitivity of canopy conductance to soil water availability. Near-homeostasis in midday water potentials was observed across years and defoliation classes, with minimum values of -2.5 MPa. Enhanced sensitivity to drought and prolonged periods of near-zero gas exchange were consistent with low levels of carbohydrate reserves in defoliated trees. Our results support the critical links between defoliation, water and carbon availability, and their key roles in determining tree survival and recovery under drought. © 2013 New Phytologist Trust.

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