Measuring and modelling transpiration of pine and oak forest stands in a Mediterranean mountain area (Vallcebre, NE Spain)

Abstract

This doctoral dissertation contains five applied reseaches about these subjectes:<br/><br/>1. "Transpiration of montane Pinus sylvestris L. and Quercus pubescens Willd. forest stands measured with sap flow sensors in NE Spain."<br/>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.) forest located in a montane area of the Eastern Pyrenees (NE Spain). Over the studied period of 2003, characterised by a severe drought episode during the summer, the oak stand Ec was only 110 mm compared to the 239 mm transpired by the Scots pine stand. Scots pine showed a strong reduction in Ec/ET0 as the drought developed, whereas pubescent oak was less affected by soil moisture deficits in the upper soil. Rainfall during the summer months (June-September) in 2003 was almost 40% less than in the same interval in 2004. Accordingly, transpiration was also reduced about 25% in 2003. <br/><br/>2."Variation in the radial patterns of sap flux density in pubescent oak (Quercus pubescens Willd.) and its implications for tree and stand transpiration measurements".<br/><br/>Radial variation in sap flux density along the sapwood was assessed in several Quercus pubescens Wild. trees with the Heat Field Deformation method. Sapwood depth could be delimited by identifying the point of zero flow in radial patterns of sap flow, yielding tree sapwood areas which were 1.5-2 times larger than previous visual identifications on wood cores. Radial patterns varied among trees and diurnally, with a significant contribution from the inner sapwood during the early morning and late in the day. Stand transpiration calculated using the new sapwood areas and radial correction coefficients was similar (year 2003) or about 25% higher (year 2004) than previous uncorrected values.<br/><br/>3. "Parameterisation and validation of a two-source evapotranspiration model for Pinus sylvestris L. stands."<br/><br/>We used a two-layer evapotranspiration model to predict canopy transpiration and soil evaporation in a Scots pine stand. A Generalized Likelihood Uncertainty Estimation (GLUE) calibration was carried out using 2003 and 2004 data. Parameters representing reference stomatal aperture and sensitivity to vapour pressure deficit were the most relevant, whereas those controlling the response to soil moisture deficit only appeared sensitive when calibration against 2004 data was done. Combining calibration results from 2003 and 2004 showed how predictive uncertainty was reduced and the value of some parameters was constrained. <br/><br/>4. "Response of co-occurring Scots pine (Pinus sylvestris L.) and pubescent oak (Quercus pubescens Willd.) to soil and atmospheric water deficits."<br/><br/>The physiological responses to water deficits of Scots pine (Pinus sylvestris L.) and pubescent oak (Quercus pubescens Willd.) under similar climatic conditions have been studied in two stands located in the Eastern Pyrenees. Leaf water potentials at midday were generally lower for pubescent oak. Sap flow relative to its species-specific maximum value was shown to decline with increasing vapour pressure deficit above a threshold of ca. 1.2 kPa for both species. Extreme drought conditions during the summer of 2003, promoted an enhanced stomatal control in Scots pine.<br/><br/>5. "Plasticity in structural and functional hydraulic characters in Scots pine: Analysis of variability across Eurasia."<br/><br/>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. Transpiration per unit leaf area at a given vapour pressure deficit increased in drier/warmer climates. This physiological acclimation was linked to the previously reported climate- and size-related structural acclimation of leaf to sapwood area ratios. Our results suggest that these structural adjustments may not be enough to prevent lower xylem tensions at the driest sites.

<i>La tesi tracta diversos aspectes ecohidrològics d'especial interès en àrees de muntanya mitjana Mediterrània i s'engloba en les investigacions de tipus hidrològic i geomorfològic dutes a terme a les conques experimentals de Vallcebre. Aquestes conques són representatives dels processos d'aforestació que han tingut lloc a muntanya des del declivi de l'activitat agrícola. Els estudis presentats es centren concretament en la transpiració de boscos, el principal component de l'evapotranspiració en els ambients estudiats. La transpiració en aquest estudi ha estat avaluada mitjançant mesures de flux de saba, principalment el mètode de dissipació tèrmica. La regulació fisiològica de la transpiració s'ha avaluat des del punt de vista de la regulació estomàtica, el seguiment dels potencials hídrics foliars i de la capacitat conductora del xilema. En els últims anys s'ha demostrat que donades les interaccions entre el control estomàtic de la transpiració i les característiques estructurals que caracteritzen el camí de l'aigua des de les arrels fins a les fulles, cal estudiar les respostes de la planta als dèficits hídrics a nivell d'organisme. És important comprendre com funciona aquesta relació entre regulació fisiològica i estructural, tant en espècies diferents que conviuen en un mateix hàbitat, com per a una mateixa espècie sotmesa a diferents condicions ambientals. </i>