Pilar Llorens

Land Use and Land Cover Change After Agricultural Abandonment: The Case of a Mediterranean Mountain Area (Catalan Pre-Pyrenees)

Abstract Land cover mapping obtained from photointerpretation of aerial photographs and orthophotographs was used to quantify land cover changes between 1957 and 1996 in a Mediterranean middle mountain area. Expansion of forested area is clearly the main land cover change caused by the abandonment of traditional agricultural activities and by the use of other materials and energy sources instead of forest resources. As a result, about 64% of the area was covered by forest by 1996, whereas in 1957 forests accounted for only 40% of the land cover. Spontaneous afforestation of abandoned fields with Scots pine (Pinus sylvestris L.) in terraced areas and areas of sparse scrub vegetation, coupled with an increase in the density of forest canopies, has been responsible for this expansion of woodland. The influence of physiographic factors in land cover change processes in the terraced areas of the catchment was also considered. The results demonstrate that within the terraced areas, north-facing and more elevated steeper slopes are more intensely afforested. However, an accurate analysis of the role played by these factors in land cover change cannot be carried out because the pattern of land abandonment is not independent of these physiographic characteristics. Furthermore, field observations at the terrace scale are evidence of the relevant influence of local topography in afforestation dynamics.

Hydraulic adjustment of Scots pine across Europe.

* 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 (K(S) and K(L)), vulnerability to embolism, leaf-to-sapwood area ratio (A(L) : A(S)), needle carbon isotope discrimination (Delta13C) 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) : A(S), K(L) and Delta13C. Shoot radial growth and A(L) : A(S) 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 pines hydraulic system to local climatic conditions occurred primarily through modifications of A(L) : A(S) and direct stomatal control, whereas intraspecific variation in vulnerability to embolism and leaf physiology appears to be limited.

Rainfall interception by a Pinus sylvestris forest patch overgrown in a Mediterranean mountainous abandoned area I. Monitoring design and results down to the event scale

Monitoring (in 5 min steps) of precipitation, throughfall, stemflow and bulk canopy wetness, and also weather conditions and soil moisture, was carried out from July 1993 to December 1995, in a Pinus sylvestris forest patch located in a Mediterranean mountainous former agricultural basin subject to spontaneous change from pasture to forest. Throughfall collectors were designed to obtain hydrologically representative data and they consist of nine troughs with a total catchment area of 9 m2. The bulk interception rate measured after 30 months of monitoring was about 24%. Relative interception was irregular and decreased with the magnitude of the event; it was at least 15% for events of more than 20 mm. Multivariate analysis of the events demonstrates that their characteristics can be simplified in two main factors which respectively represent the duration of the event and its magnitude. The magnitude of the event biases the characterization because of the non-linearity of the rainfall-interception relationship. Long events do not produce higher interception rates than shorter ones because of the occurrence of low vapour pressure deficits during the former. In atmospheric dry conditions the rainfall intensity provides the main control on interception rates.

A simplified method for forest water storage capacity measurement

A simplification of direct methods to measure canopy storage capacity is presented. It is based on measurement of water retained by vegetal entities (needles, stems and branches) and the up-scaling of these measurements using the determination of the surface of canopy elements from common vertical photographs taken from the ground. The specific water retention capacities of Pinus sylvestris pine needles ranged between 0.104 and 0.043 mm, depending on the simulation of still air or windy conditions. These values are low when compared with the specific water retention capacity of branches and stems, 0.62 mm. The water retained in branches and stems, therefore, plays a key role in rainfall interception. The canopy storage results obtained are consistent with the spatial distribution of throughfall measured in five experimental plots located in a heterogeneous 40-year-old Pinus sylvestris stand in a Mediterranean mountain area of the South Eastern Pyrenees (Catalonia, Spain), and are 30% higher than the values estimated through indirect methods.

Studying the role of old agricultural terraces on runoff generation in a small Mediterranean mountainous basin

Abstract The small drainage basin of Cal Parisa was instrumented in early 1989 to study the hydrological and sediment routing behaviour of Mediterranean mountain areas formerly used for agriculture but now abandoned. Environmental changes produced by agricultural land use included the construction of terraces on the major part of the basin and subsequent artificial channelling of surface waters. Field observations and hydrological data suggest that storm runoff is generated by the contributing role of saturated areas, most of them originating from the terrace system. Modelling of the natural conditions before terracing, with the help of TOPMODEL, supports the hypothesis that terracing promotes the premature formation of saturated areas, increasing saturation overland flow at the expense of lower baseflow and actual evapotranspiration. Now, after land abandonment, the more relevant environmental hazard is linked to the spontaneous reorganization of the unmaintained artificial drainage network.

Catchment Management under Environmental Change: Impact of Land Cover Change on Water Resources

Abstract Since the second third of the 20th century, it has been clearly established in the hydrological science that an increase of the forest cover in a catchment determines a decrease in water resources from this catchment. Nevertheless, the application of this paradigm for catchment management has been delayed. In Spain, extensive mountain areas that were farmed or grazed in the past have been abandoned during the last 50 years and undergone spontaneous or induced afforestation, mainly by pine trees. The historical flow records of several Spanish rivers during the last 50 years show significant decreases that average an annual reduction of about 0.4 percent of mean annual flow, whereas in the Ebro River the annual decrease was as high as 0.63 percent of mean annual flow. About one-third of these decreases were not explained by increases in water consumption by irrigation or by climate variability, but should be attributed to an increase in evaporation from the headwaters. These results confirm that the assessment of future water resources must take into account the past and expected changes in land cover in the catchment headwaters and that land use and cover planning should be integrated in the management of catchment water resources, especially in scarce water environments.

Rainfall interception by a Pinus sylvestris forest patch overgrown in a Mediterranean mountainous abandoned area II. Assessment of the applicability of Gash's analytical model

Abstract A set of 147 rainstorms each exceeding 1 mm bulk rainfall was used to validate the Gash analytical model of rainfall interception in a Pinus sylvestris forest patch overgrown in an abandoned Mediterranean mountain area. The results seems to indicate that the Gash analytical model is sufficiently robust to be applicable in Mediterranean mountain conditions for obtaining long-term total interception losses, and that prediction errors are in the same range as those obtained in earlier validations of the model. In contrast, there is a marked difference between observed and predicted values comparing individual events.

Response of Scots pine (Pinus sylvestris L.) and pubescent oak (Quercus pubescens Willd.) to soil and atmospheric water deficits under Mediterranean mountain climate

The physiological responses to water deficits of Scots pine (Pinus sylvestris L.) and pubescent oak (Quercus pubescens Willd.) were studied under Mediterranean mountain climate. Minimum leaf water potentials were −3.2 MPa for oak and −2.1 MPa for pine, with higher predawn values for pubescent oak. Relative sap flow declined in both species when vapour pressure deficit (D) went above ca. 1.2 kPa, but stomatal control was stronger for pine during the 2003 summer drought. P. sylvestris plant hydraulic conductance on a half-total leaf area basis (kL,s−1) was 1.2–2.6 times higher than the values shown by Q. pubescens, and it showed a considerably steeper decrease during summer. Leaf-level gas exchange was positively related to kL,s−1 in both species. Scots pine was more vulnerable to xylem embolism and closed stomata to prevent substantial conductivity losses. The results of this study confirm that pubescent oak is more resistant to extreme drought events.RésuméLes réponses physiologiques aux déficits hydriques du pin sylvestre (Pinus sylvestris L.) et du chêne pubescent (Quercus pubescens Wild.) ont été étudiées sous climat montagnard méditerranéen. Le potentiel hydrique foliaire minimum atteint a été de −3,2 MPa pour le chêne et de −2,1 MPa pour le pin, avec des valeurs de potentiel de base plus élevées pour le chêne pubescent. Un relatif déclin du flux de sève a été observé chez les deux espèces lorsque le déficit de vapeur d’eau dépassait 1,2 kPa, mais le contrôle stomatique a été plus fort chez le pin pendant la sécheresse de l’été 2003. La conductance hydraulique des pins, ramenée à la surface foliaire (kL, s−1) a été de 1,2 à 2,6 fois plus élevée que celle de Quercus pubescens et a présenté une rapide décroissance pendant l’été. Le niveau des échanges gazeux foliaires était positivement corrélé à kL, s−1 chez les deux espèces. Le pin sylvestre est plus vulnérable à l’embolie du xylème et a de ce fait fermé ses stomates pour empêcher une perte substantielle de conductivité hydraulique. Les résultats de cette étude confirment que le chêne pubescent est plus résistant aux épisodes d’extrême sécheresse que le pin sylvestre.

Hydrological functioning of mediterranean mountain basins in Vallcebre, Catalonia: Some challenges for hydrological modelling

The Vallcebre research catchments are located in the south-eastern Pyrenees, in an area of diverse land use and varying levels of degradation, including forested hillslopes, abandoned agricultural terraces and badland areas. Outside the badlands, the hydrological response is controlled by saturation mechanisms. Between September and June the spatial patterns of saturated areas and soil moisture are determined by subsurface flow, modified by the premature saturation of the inner parts of agricultural terraces, and the negative soil moisture anomalies induced by forest patches overgrown in grassland areas. During summer, this behaviour ceases because of soil moisture depletion and badland surfaces are the only hydrologically active areas, producing excess runoff in response to the intense rainstorms. During the beginning and the ending of the wet season, the saturation of active areas shows a hysteretic behaviour that breaks the linearity between the mean water reserve of the basin and the relative saturated area.

Small basin response in a Mediterranean mountainous abandoned farming area: Research design and preliminary results

Abstract A 36 ha basin named Cal Parisa has been studied since early 1988 in the eastern Pyrenees (Spain). This basin was selected as a representative of extensive areas on clayey bedrock formerly used for farming, nowadays almost abandoned, where badlands are of great importance as far as sediment production is concerned. As we are interested in the hydrological behaviour of terraced areas, and badland areas could strongly modify the hydrological and sediment response, the basin is devoid of badlands but has some bedrock scars and small gullies. Severe geoecological disturbances were produced in it by agricultural land use in the past: the basin was deforested, topography was terraced, and the drainage net was increased by ditching. Monitoring design is based on two measuring stations, one at the main outlet and the other in a 16 ha sub-basin, and two sampling stations at the outlets of the two main sub-basins; a weather station and four rain recorders. Preliminary data show a hydrological response fully controlled by antecedent moisture conditions, suggesting that soils have high infiltration capacities and that runoff is generated by saturation overland flow; delayed flow being less significant. Stream water samples show relatively high dissolved solids concentrations, dominated by calcium bicarbonate, and suspended sediments concentrations about ten times less than dissolved ones, as a consequence of the strong sediment conveyance discontinuities observed in the field.