Helfter C, Shephard JD, Martinez-Vilalta J, Mencuccini M, Hand DP (2007) A non-invasive optical system for the measurement of xylem and phloem sap flow in woody plants. Tree physiology 27: 169-179.
Mencuccini M, Martínez-Vilalta J, Hamid HA, Korakaki E, Vanderklein D (2007) Theoretical and methodological considerations on the importance of size-related versus age-related changes in tree growth Tree physiology 27: 463-473.
Loepfe L., Martinez-Vilalta J., Piñol J., Mencuccini M. (2007) The relevance of xylem network structure for plant hydraulic efficiency and safety. Journal of Theoretical Biology. 247: 788-803.EnllaçDoi: 10.1016/j.jtbi.2007.03.036
The xylem is one of the two long distance transport tissues in plants, providing a low resistance pathway for water movement from roots to leaves. Its properties determine how much water can be transported and transpired and, at the same time, the plant's vulnerability to transport dysfunctions (the formation and propagation of emboli) associated to important stress factors, such as droughts and frost. Both maximum transport efficiency and safety against embolism have classically been attributed to the properties of individual conduits or of the pit membrane connecting them. But this approach overlooks the fact that the conduits of the xylem constitute a network. The topology of this network is likely to affect its overall transport properties, as well as the propagation of embolism through the xylem, since, according to the air-seeding hypothesis, drought-induced embolism propagates as a contact process (i.e., between neighbouring conduits). Here we present a model of the xylem that takes into account its system-level properties, including the connectivity of the xylem network. With the tools of graph theory and assuming steady state and Darcy's flow we calculated the hydraulic conductivity of idealized wood segments at different water potentials. A Monte Carlo approach was adopted, varying the anatomical and topological properties of the segments within biologically reasonable ranges, based on data available from the literature. Our results showed that maximum hydraulic conductivity and vulnerability to embolism increase with the connectivity of the xylem network. This can be explained by the fact that connectivity determines the fraction of all the potential paths or conduits actually available for water transport and spread of embolism. It is concluded that the xylem can no longer be interpreted as the mere sum of its conduits, because the spatial arrangement of those conduits in the xylem network influences the main functional properties of this tissue. This brings new arguments into the long-standing discussion on the efficiency vs. safety trade-off in the plants' xylem. © 2007 Elsevier Ltd. All rights reserved.
Martínez-Vilalta J., Korakaki E., Vanderklein D., Mencuccini M. (2007) Below-ground hydraulic conductance is a function of environmental conditions and tree size in Scots pine. Functional Ecology. 21: 1072-1083.EnllaçDoi: 10.1111/j.1365-2435.2007.01332.x
1. Variations in water tension in a transpiring tree cause elastic changes in stem diameter. To better understand the dynamics of these variations, stem diameter changes and sap flow rates were monitored simultaneously in trees from two Scots pine chronosequences in Scotland. 2. Tree below-ground hydraulic conductance (kbg) was estimated from the relationship between leaf-specific sap flow rates and the difference between stem and soil water potentials estimated from diameter variations in the stem. 3. In a given tree, kbg varied both within and among days, with conductance increasing as a function of sap flow and evaporative demand. These patterns could be explained in terms of a composite model of root water transport and possible changes in the gating of aquaporins. 4. We interpreted these trends of increasing kbg with evaporative demand as a mechanism to enhance the ability of trees to control leaf water potential and keep it within physiologically acceptable limits, with potential implications for our general understanding of plant water relations, and for the estimation and modelling of ecosystem water fluxes. 5. Across trees, kbg declined with increasing tree age/size, but the proportional contribution of below-ground to whole-tree hydraulic resistance also declined. This is consistent with an increase in below-ground carbon allocation in old/tall trees and a partial acclimation of tall trees to hydraulic limitations. It is argued that these trends have to be considered when discussing the importance of tree height for water transport and growth. © 2007 The Authors.
Martínez-Vilalta J., Vanderklein D., Mencuccini M. (2007) Tree height and age-related decline in growth in Scots pine (Pinus sylvestris L.). Oecologia. 150: 529-544.EnllaçDoi: 10.1007/s00442-006-0552-7
Growth and seasonal water use was measured amongst trees growing in an old growth Scots pine forest in the Scottish Highlands. Three sites which differed in their recent management history and contained old and naturally regenerated young trees growing together were monitored in the field. Our results showed a clear decrease in growth efficiency with age, from values of around 0.25 kg m-2 leaves year-1 in approximately 25-year-old trees to less than 0.1 kg m-2 leaves year-1 in trees over 200 years old. When the old trees in one of the field sites were released from competition by thinning, their growth efficiency reverted to that of coexisting young trees, indicating that the decline in growth was reversible. This is consistent with the results of a parallel study showing that cambial age had no effect on the physiology or growth of grafted seedlings originating from the same population studied here (Mencuccini et al. 2005). Our detailed study of tree water use in the field showed an overall decrease in whole-tree hydraulic conductance and stomatal canopy conductance with tree height in the unthinned stands, in agreement with the hydraulic limitation hypothesis. However, the effect of this reduction in hydraulic efficiency on growth was comparatively small, and old trees also showed consistently lower nitrogen concentrations in needles, suggesting that hydraulic and nutritional factors combined to produce the decline in growth efficiency with age observed in the studied populations. © 2006 Springer-Verlag.
Vanderklein D, Martínez-Vilalta J, Lee S, Mencuccini M (2007) Plant size, not age, regulates growth and gas exchange in grafted Scots pine trees. Tree physiology 27: 71-79.
Poyatos R., Martínez-Vilalta J., Čermák J., Ceulemans R., Granier A., Irvine J., Köstner B., Lagergren F., Meiresonne L., Nadezhdina N., Zimmermann R., Llorens P., Mencuccini M. (2007) Plasticity in hydraulic architecture of Scots pine across Eurasia. Oecologia. 153: 245-259.EnllaçDoi: 10.1007/s00442-007-0740-0
Widespread tree species must show physiological and structural plasticity to deal with contrasting water balance conditions. To investigate these plasticity mechanisms, a meta-analysis of Pinus sylvestris L. sap flow and its response to environmental variables was conducted using datasets from across its whole geographical range. For each site, a Jarvis-type, multiplicative model was used to fit the relationship between sap flow and photosynthetically active radiation, vapour pressure deficit (D) and soil moisture deficit (SMD); and a logarithmic function was used to characterize the response of stomatal conductance (G s) to D. The fitted parameters of those models were regressed against climatic variables to study the acclimation of Scots pine to dry/warm conditions. The absolute value of sap flow and its sensitivity to D and SMD increased with the average summer evaporative demand. However, relative sensitivity of G s to D (m/G s,ref, where m is the slope and G s,ref is reference G s at D = 1 kPa) did not increase with evaporative demand across populations, and transpiration per unit leaf area at a given D increased accordingly in drier/warmer climates. This physiological plasticity was linked to the previously reported climate- and size-related structural acclimation of leaf to sapwood area ratios. G s,ref, and its absolute sensitivity to D (m), tended to decrease with age/height of the trees as previously reported for other pine species. It is unclear why Scots pines have higher transpiration rates at drier/warmer sites, at the expense of lower water-use efficiency. In any case, our results suggest that these structural adjustments may not be enough to prevent lower xylem tensions at the driest sites. © 2007 Springer-Verlag.
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