Transpiration of montane Pinus sylvestris L. and Quercus pubescens Willd. forest stands measured with sap flow sensors in NE Spain

Abstract

Abstract. Stand transpiration was measured during the 2003 and 2004 growing seasons using heat dissipation sap flow sensors in a Scots pine (Pinus sylvestris L.) and a pubescent oak (Quercus pubescens Willd.) forests located in a montane area of the Eastern Pyrenees (NE Spain). The first aim of the study was to assess the differences in quantitative estimates of transpiration (E c ) and the response to evaporative demand of the two stands. Over the studied period of 2003, characterised by a severe drought episode during the summer, the oak stand E c was only 110 mm compared to the 239 mm transpired by the Scots pine stand, although the ratio of transpiration to reference evapotranspiration (E c /ET 0 ) in the oak stand compares well with the expected values predicted for low leaf area index (LAI) oak forests in southern Europe. Scots pine showed a strong reduction in E c /ET 0 as the drought developed, whereas pubescent oak was less affected by soil moisture deficits in the upper soil. As a second objective, and given the contrasting meteorological con-

Combining remote sensing and GIS climate modelling to estimate daily forest evapotranspiration in a Mediterranean mountain area

“…As we mentioned in Sect. 2.1, the study area has high spatial heterogeneity, mosaic of afforestation patches overgrowing old agricultural terraces (Poyatos et al, 2003), at smaller scales than the coarse TERRA/AQUA MODIS pixel, which makes it difficult to validate the ET d model results. However, it has to be noted that validating a TERRA/AQUA MODIS in a heterogeneous mountain landscape is not easy due to the extensive instrumentation needed to measure the energy flux in each of the landscape covers.…”

“…The vegetation in the area is sub-Mediterranean oak forest (Buxo-sempervirentis-Quercetum pubescentis association), but most of the land was terraced and deforested for cultivation in the past, and then progressively abandoned during the second half of the twentieth century. The present landscape is mainly a mosaic of mesophylous grasslands and patches of Scots pine, which colonized old agricultural terraces after they were abandoned (Poyatos et al, 2003). Figure 1 shows the location of the Vallcebre research catchments.…”

“…Instantaneous values (15-min averages) were then summed to compute daily stand transpiration. Further details on the methodology and results of sap flow measurements used in this study can be found in Poyatos et al (2005Poyatos et al ( , 2008.…”

“…A planimetric accuracy (obtained with an independent set of GCP) of less than 15 m (half pixel) was obtained. Radiometric correction was carried out following the methodology proposed by Pons and Solé-Sugrañes (1994), which allows us to reduce the number of undesired artifacts due to the atmospheric effects or differential illumination that are results of the time of day, the location on the Earth and the relief (zones being more illuminated than others, shadows, etc). The digital numbers were converted to radiances by means of image header parameters, taking into account the considerations presented by Cristóbal et al (2004) and Chander et al (2009).…”

“…In the case of the Scots pine stand, air temperature (HMP35C, Vaisala, Vantaa, Finland), wind speed (A100R, Vector InstrumentsRhyl, UK) and net radiation (NR-Lite ,Kipp & Zonen, Delft, The Netherlands) were measured at a height of 2 m above the canopy. Data were recorded every 10 s and stored as 15-min averages in a datalogger, DT500, DataTaker, Australia (Poyatos et al, 2005).…”