Cristóbal J., Poyatos R., Ninyerola M., Llorens P., Pons X. (2011) Combining remote sensing and GIS climate modelling to estimate daily forest evapotranspiration in a Mediterranean mountain area. Hydrology and Earth System Sciences. 15: 1563-1575.EnlaceDoi: 10.5194/hess-15-1563-2011
Evapotranspiration monitoring allows us to assess the environmental stress on forest and agricultural ecosystems. Nowadays, Remote Sensing and Geographical Information Systems (GIS) are the main techniques used for calculating evapotranspiration at catchment and regional scales. In this study we present a methodology, based on the energy balance equation (B-method), that combines remote sensing imagery with GIS-based climate modelling to estimate daily evapotranspiration (ETd) for several dates between 2003 and 2005. The three main variables needed to compute ETd were obtained as follows: (i) Land surface temperature by means of the Landsat-5 TM and Landsat-7 ETM+ thermal band, (ii) air temperature by means of multiple regression analysis and spatial interpolation from meteorological ground stations data at satellite pass, and (iii) net radiation by means of the radiative balance. We calculated ETd using remote sensing data at different spatial and temporal scales (Landsat-7 ETM+, Landsat-5 TM and TERRA/AQUA MODIS, with a spatial resolution of 60, 120 and 1000 m, respectively) and combining three different approaches to calculate the parameter, which represents an average bulk conductance for the daily-integrated sensible heat flux. We then compared these estimates with sap flow measurements from a Scots pine (Pinus sylvestris L.) stand in a Mediterranean mountain area. This procedure allowed us to better understand the limitations of ETd modelling and how it needs to be improved, especially in heterogeneous forest areas. The method using Landsat data resulted in a good agreement, R2 test of 0.89, with a mean RMSE value of about 0.6 mm day-1 and an estimation error of ±30 %. The poor agreement obtained using TERRA/AQUA MODIS, with a mean RMSE value of 1.8 and 2.4 mm day-1 and an estimation error of about ±57 and 50 %, respectively. This reveals that ETd retrieval from coarse resolution remote sensing data is troublesome in these heterogeneous areas, and therefore further research is necessary on this issue. Finally, implementing regional GIS-based climate models as inputs in ETd retrieval have has provided good results, making possible to compute ETd at regional scales. © 2011 Author(s).
Fletcher BJ, Gornall JL, Poyatos R, Press MC, Stoy PC, Huntley B, Baxter R, Phoenix GK (2011) Photosynthesis and productivity in heterogeneous arctic tundra: consequences for ecosystem function of mixing vegetation types at stand edges. Journal of Ecology doi: 10.1111/j.1365-2745.2011.01913.x.
Arnan X, López BC, Martínez-Vilalta J, Estorach M, Poyatos R (2011) The age of monumental olive trees (Olea europaea) in northeastern Spain. Dendrochronología doi: 10.1016/j.dendro.2011.02.002.
Llorens P., Poyatos R., Latron J., Delgado J., Oliveras I., Gallart F. (2010) A multi-year study of rainfall and soil water controls on Scots pine transpiration under Mediterranean mountain conditions. Hydrological Processes. 24: 3053-3064.EnlaceDoi: 10.1002/hyp.7720
This study is focused on the analysis of the relationship between sap-flow-derived transpiration measured in a Scots pine stand in the Vallcebre research catchments (NE Iberian Peninsula) and meteorological and rainfall data. The first part of the study is focused on the analysis of temperature and rainfall anomalies. Then, the Scots pine transpiration response to inter-annual rainfall variability, soil water stress and water table depth variations during the period 1997-2000 is analysed. This period includes the extremely dry year of 1998, which allows us to infer the response of Scots pine transpiration to severe droughts. Scots pine transpiration during the summer presented a high inter-annual variability, largely related to rainfall amounts. Daily transpiration during dry summers was 40% of the transpiration of a summer day with average rainfall. Moreover, during dry summers, transpiration rates were not fully recovered even after significant rainfall events. The analysis of the dependence of Scots pine transpiration on available water indicated the strong limitation on transpiration induced by water content in the whole soil profile as well as by water table position. Under these drought conditions, a reduction of runoff and deep water stores was observed at the catchment scale, suggesting that the predicted increase in the frequency of severe summer droughts may threaten the current role of Mediterranean mountain catchments as suppliers of water resources for lowland areas. © 2010 John Wiley & Sons, Ltd.
Poyatos R, Martínez-Vilalta J, Poyatos R, Martínez-Vilalta J, Curiel J, Barba J, ,Aguadé D, Mencuccini M,Lloret F (2010) Canvis recents en els fluxos d’aigua i carboni a les pinedes de pi roig del bosc de Poblet: implicacions en un escenari de canvi climàtic. III Jornades sobre el bosc de Poblet i les muntanyes de Prades (en prensa).
Martínez-Vilalta J, Claramunt B, Arnan X, Estorach M, Poyatos R (2009) L’edat de les oliveres monumentals i singulars del Montsià. Raïls 25: 208-221.
Martínez-Vilalta J., Cochard H., Mencuccini M., Sterck F., Herrero A., Korhonen J.F.J., Llorens P., Nikinmaa E., Nolè A., Poyatos R., Ripullone F., Sass-Klaassen U., Zweifel R. (2009) Hydraulic adjustment of Scots pine across Europe. New Phytologist. 184: 353-364.EnlaceDoi: 10.1111/j.1469-8137.2009.02954.x
Summary The variability of branch-level hydraulic properties was assessed across 12 Scots pine populations covering a wide range of environmental conditions, including some of the southernmost populations of the species. The aims were to relate this variability to differences in climate, and to study the potential tradeoffs between traits. Traits measured included wood density, radial growth, xylem anatomy, sapwood- and leaf-specific hydraulic conductivity (KS and KL), vulnerability to embolism, leaf-to-sapwood area ratio (AL : AS), needle carbon isotope discrimination (Δ13C) and nitrogen content, and specific leaf area. Between-population variability was high for most of the hydraulic traits studied, but it was directly associated with climate dryness (defined as a combination of atmospheric moisture demand and availability) only for A L : AS, KL and Δ13C. Shoot radial growth and AL : AS declined with stand development, which is consistent with a strategy to avoid exceedingly low water potentials as tree size increases. In addition, we did not find evidence at the intraspecific level of some associations between hydraulic traits that have been commonly reported across species. The adjustment of Scots pine's hydraulic system to local climatic conditions occurred primarily through modifications of AL : AS and direct stomatal control, whereas intraspecific variation in vulnerability to embolism and leaf physiology appears to be limited. © 2009 New Phytologist.
González-Rodríguez AM, Oncins JA, Peters J, Bocero JJ, Poyatos R (2009) Sap flow measurements in different young fig trees cultivars in Tenerife island. Acta Horticulturae. ISHS pp. 375-380.
Poyatos R, Llorens P, Piñol J, Rubio C (2008) Response of Scots pine (Pinus sylvestris L.) and pubescent oak (Quercus pubescens Willd.) to soil and atmospheric water deficits under Mediterranean moutain climate. Annals of Forest Science 65: 306
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.EnlaceDoi: 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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