Martinez-Vilalta J, Mencuccini M, Alvarez X, Camacho J, Loepfe J, Pinol J (2012) Spatial distribution and packing of xylem conduits. 7th Plant Biomechanics International Conference, Clermont-Ferrand, France, 20-24 August 2012. (Comunicació oral).
Loepfe L., Martinez-Vilalta J., Piñol J. (2012) Management alternatives to offset climate change effects on Mediterranean fire regimes in NE Spain. Climatic Change. 115: 693-707.EnllaçDoi: 10.1007/s10584-012-0488-3
Fire regime is affected by climate and human settlements. In the Mediterranean, the predicted climate change is likely to exacerbate fire prone weather conditions, but the mid- to long-term impact of climate change on fire regime is not easily predictable. A negative feedback via fuel reduction, for instance, might cause a non-linear response of burned area to fire weather. Also, the number of fires escaping initial control could grow dramatically if the fire meteorology is just slightly more severe than what fire brigades are prepared for. Humans can directly influence fire regimes through ignition frequency, fire suppression and land use management. Here we use the fire regime model FIRE LADY to assess the impacts of climate change and local management options on number of fires, burned area, fraction of area burned in large fires and forest area during the twenty-first century in three regions of NE Spain. Our results show that currently fuel-humidity limited regions could suffer a drastic shift of fire regime with an up to 8 fold increase of annual burned area, due to a combination of fuel accumulation and severe fire weather, which would result in a period of unusually large fires. The impact of climate change on fire regime is predicted to be less pronounced in drier areas, with a gradual increase of burned area. Local fire prevention strategies could reduce but not totally offset climate induced changes in fire regimes. According to our model, a combination of restoring the traditional rural mosaic and classical fire prevention would be the most effective strategy, as a lower ignition frequency reduces the number of fires and the creation of agricultural fields in marginal areas reduces their extent. © 2012 Springer Science+Business Media B.V.
Martínez-Vilalta J., Mencuccini M., Álvarez X., Camacho J., Loepfe L., Piñol J. (2012) Spatial distribution and packing of xylem conduits. American Journal of Botany. 99: 1189-1196.EnllaçDoi: 10.3732/ajb.1100384
Premise of the study: The hydraulic properties of the xylem determine the ability of plants to transport water from the soil to the leaves and to cope with important stress factors such as frost and drought. Hydraulic properties have usually been studied as a function of the anatomy of xylem conduits and their pits, but recent studies have proposed that system-level properties, related to the topology of the xylem network, may also play a role. Here we study how the spatial arrangement of conduits in xylem cross sections affects the relationship between mean conduit lumen area and conduit density (packing function) across species.Methods: Point pattern analysis was used to describe the spatial distribution of xylem conduits in 97 woody species. The effect of conduit aggregation on the packing function was tested using phylogenetic generalized least squares. A hydraulic model with an explicit description of the topology of the xylem network was used to interpret the functional significance of our findings.Key results: The spatial arrangement of conduits affected the packing function across species, so that species with aggregated distributions tended to have lower conduit densities for a given conduit size and lower conduit lumen fractions. According to our modeling results, the higher conduit-to-conduit connectivity of species with aggregated distributions allows them to achieve higher hydraulic conductivity. Species with aggregated conduits, however, pay a cost in terms of increased vulnerability to embolism.Conclusions: The spatial arrangement of conduits affects the fundamental structural and functional attributes of the xylem. © 2012 Botanical Society of America.
Loepfe L., Martinez-Vilalta J., Piñol J. (2011) An integrative model of human-influenced fire regimes and landscape dynamics. Environmental Modelling and Software. 26: 1028-1040.EnllaçDoi: 10.1016/j.envsoft.2011.02.015
Fire regimes depend on climate, vegetation structure and human influences. Climate determines the water content in fuel and, in the longer term, the amount of biomass. Humans alter fire regimes through increased ignition frequency and by hindering the spread of fire through fire suppression and fuel fragmentation. Here, we present FIRE LADY (FIre REgime and LAndscape DYnamics), a spatially explicit fire regime model that takes into account daily weather data, topography, vegetation growth, fire behaviour, fire suppression and land use changes. In this model, vegetation growth depends on water availability, and stem diameter and stand density are the fundamental parameters. Fire behaviour is modelled using the Rothermel equations and taking into account both crown fire and spotting. Human influences on fire regime, such as ignition frequency, fire suppression and land use changes, are explicitly modelled. The model was calibrated for three regions in NE Spain and reproduces fire regimes, changes in land cover distribution and tree biomass with promising accuracy. The explicit modelling of human influences makes the model a useful and unique tool for assessing the impacts of climate change and informing local fire regime management strategies. © 2011 Elsevier Ltd.
Loepfe L., Martinez-Vilalta J., Oliveres J., Piñol J., Lloret F. (2010) Feedbacks between fuel reduction and landscape homogenisation determine fire regimes in three Mediterranean areas. Forest Ecology and Management. 259: 2366-2374.EnllaçDoi: 10.1016/j.foreco.2010.03.009
In densely populated areas like the Mediterranean, wildfire extent is mostly limited by fire suppression and fuel fragmentation. Fire is known to spread more easily through high fuel loads and homogenous terrain and it is supposed to reduce fuel amount and continuity, creating a negative feedback. Here we combine information from administration fire records, satellite imagery fire scars and land use/cover maps to asses the effects of fire on landscape structure and vice versa for three areas in Catalonia (NE Spain). We worked with three spatial focuses: the actual fire scar, 1 km2 squares and 10 km2 squares. In these regions agriculture land abandonment has lead to increased fuel continuity, paralleled by an increment of fire size. We confirm that fire spread is facilitated by land use/cover types with high fuel load and by homogeneous terrain and that fire reduces fuel load by transforming forests into shrublands. But we also found that fire increased landscape homogeneity, creating a positive feedback on fire propagation. We argue that this is possible in landscapes with finer grain than fire alone would create. The lack of discontinuities in the fuel bed diminishes the extinction capacity of fire brigades and increases the risk of large fires. We recommend that fire management should focus more on conservation of the traditional rural mosaic in order to prevent further increases in fuel continuity and fire risk. © 2010 Elsevier B.V. All rights reserved.
Mencuccini M., Martinez-Vilalta J., Piñol J., Loepfe L., Mireia B., Alvarez X., Camacho J., Gil D. (2010) A quantitative and statistically robust method for the determination of xylem conduit spatial distribution. American Journal of Botany. 97: 1247-1259.EnllaçDoi: 10.3732/ajb.0900289
Premise of the study: Because of their limited length, xylem conduits need to connect to each other to maintain water transport from roots to leaves. Conduit spatial distribution in a cross section plays an important role in aiding this connectivity. While indices of conduit spatial distribution already exist, they are not well defi ned statistically. Methods: We used point pattern analysis to derive new spatial indices. One hundred and fi ve cross-sectional images from different species were transformed into binary images. The resulting point patterns, based on the locations of the conduit centersof-area, were analyzed to determine whether they departed from randomness. Conduit distribution was then modeled using a spatially explicit stochastic model. Key results: The presence of conduit randomness, uniformity, or aggregation depended on the spatial scale of the analysis. The large majority of the images showed patterns signifi cantly different from randomness at least at one spatial scale. A strong phylogenetic signal was detected in the spatial variables. Conclusions: Conduit spatial arrangement has been largely conserved during evolution, especially at small spatial scales. Species in which conduits were aggregated in clusters had a lower conduit density compared to those with uniform distribution. Statistically sound spatial indices must be employed as an aid in the characterization of distributional patterns across species and in models of xylem water transport. Point pattern analysis is a very useful tool in identifying spatial patterns. © 2010 Botanical Society of America.
Piñol J., Espadaler X., Cañellas N., MartíNez-Vilalta J., Barrientos J.A., Sol D. (2010) Ant versus bird exclusion effects on the arthropod assemblage of an organic citrus grove. Ecological Entomology. 35: 367-376.EnllaçDoi: 10.1111/j.1365-2311.2010.01190.x
Predation-exclusion experiments have highlighted that top-down control is pervasive in terrestrial communities, but most of these experiments are simplistic in that they only excluded a single group of predators and the effect of removal was evaluated on a few species from the community. The main goal of our study was to experimentally establish the relative effects of ants and birds on the same arthropod assemblage of canopy trees. We conducted 1-year long manipulative experiments in an organic citrus grove intended to quantify the independent effects of bird and ant predators on the abundance of arthropods. Birds were excluded with plastic nets whereas ants were excluded with sticky barriers on the trunks. The sticky barrier also excluded other ground dwelling insects, like the European earwig Forficula auricularia L. Both the exclusion of ants and birds affected the arthropod community of the citrus canopies, but the exclusion of ants was far more important than the exclusion of birds. Indeed, almost all groups of arthropods had higher abundance in ant-excluded than in control trees, whereas only dermapterans were more abundant in bird-excluded than in control trees. A more detailed analysis conducted on spiders also showed that the effect of ant exclusion was limited to a few families rather than being widespread over the entire diverse spectrum of spiders. Our results suggest that the relative importance of vertebrate and invertebrate predators in regulating arthropod populations largely depends on the nature of the predator-prey system. © 2010 The Authors. Journal compilation © 2010 The Royal Entomological Society.
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, Piñol J (2004) A plumber's-eye view of xylem water transport in woody plants. Journal of Biological Education 38:137-141.
Martínez-Vilalta J, Sala A, Piñol J (2004) The hydraulic arquitecture of Pinaceae - a review. Plant Ecology 171:3-13.
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