The relevance of xylem network structure for plant hydraulic efficiency and safety

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.
Enllaç
Doi: 10.1016/j.jtbi.2007.03.036

Resum:

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.

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Conditioning uncertainty in ecological models: Assessing the impact of fire management strategies

Piñol J., Castellnou M., Beven K.J. (2007) Conditioning uncertainty in ecological models: Assessing the impact of fire management strategies. Ecological Modelling. 207: 34-44.
Enllaç
Doi: 10.1016/j.ecolmodel.2007.03.020

Resum:

A simple simulation model has been used to investigate whether large fires in Mediterranean regions are a result of extreme weather conditions or the cumulative effect of a policy of fire suppression over decades. The model reproduced the fire regime characteristics for a wide variety of regions of Mediterranean climate in California, France and Spain. The Generalised Likelihood Uncertainty Estimation (GLUE) methodology was used to assess the possibility of multiple model parameter sets being consistent with the available calibration data. The resulting set of behavioural models was used to assess uncertainty in the predictions. The results suggested that (1) for a given region, the total area burned is much the same whether suppression or prescribed fire policies are used or not; however fire suppression enhances fire intensity and prescribed burning reduces it; (2) the proportion of large fires can be reduced, but not eliminated, using prescribed fires, especially in areas which have the highest proportion of large fires. © 2007 Elsevier B.V. All rights reserved.

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Modelling evapotranspiration in a Scots pine stand under Mediterranean mountain climate using the GLUE methodology

Poyatos R., Villagarcía L., Domingo F., Piñol J., Llorens P. (2007) Modelling evapotranspiration in a Scots pine stand under Mediterranean mountain climate using the GLUE methodology. Agricultural and Forest Meteorology. 146: 13-28.
Enllaç
Doi: 10.1016/j.agrformet.2007.05.003

Resum:

Canopy transpiration (Ec) and soil evaporation (Es) in a Mediterranean Scots pine stand were simulated using a two-layer model, with a Jarvis-type submodel of canopy stomatal conductance (Gs) and a soil resistance to evaporation expressed as a function of superficial soil moisture. Sap flow measurements and soil evaporation data, together with meteorological and soil moisture variables were used to calibrate the model. Gs was calibrated using the generalized likelihood uncertainty estimation (GLUE) methodology, first with data from the year 2004, a year characterised by mild meteorological conditions. Then, data from the year 2003, which included an intense summer drought, was used to update the results from the previous calibration. The discrepancy between the diurnal courses of modelled and measured Ec using best-fit parameters was not related to any particular situation of meteorology or soil moisture. Model performance improved at the daily scale, but the model failed to simulate Ec adequately during the year 2005. Maximum modelled Es rates were 0.7 mm day-1 with the ratio Es/Ec being typically under 0.3 during the growing season. The GLUE analysis revealed that parameters representing reference stomatal aperture at a vapour pressure deficit (D) value equal to 1 kPa (Gs,ref), and sensitivity to D (m) were the most relevant, and were consistent with the hydraulic theory of stomatal regulation. Parameters controlling the response to superficial soil moisture deficit only appeared sensitive in the calibration with data from the year 2003, suggesting that response to deeper soil layers should also be considered in the model. Updating the original calibration reduced predictive uncertainty and constrained the value of some parameters. Nevertheless, it seems that representations of variable plant and soil hydraulic resistances, are required to simulate long-term Ec in seasonally-dry Mediterranean forest stands. © 2007 Elsevier B.V. All rights reserved.

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