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Runoff and sediment yields in semiarid areas have often been reported to be scale-dependent. However, scale relationships in these areas have been poorly quantified using empirical data. Commonly, previous empirical works on scale issues have focused on the effect of the size of the contributing area for a particular scale (e.g. catchments of different sizes), or the scale effect has been confounded with other co-varying controlling factors (mainly vegetation cover). In this study, we established quantitative scale relationships for runoff and sediment yields measured during 4 years at several nested scale levels (plot, microcatchment and catchment), covering a range of contributing-area sizes from 16 m2 to 20 ha, and without major differences in vegetation cover between the scales analyzed. From finer to coarser scales, rainfall thresholds for runoff production increased, runoff frequency decreased, and unit-area runoff decreased. These relationships were well fitted to power law relations with exponents close to −1/2. The decrease in runoff with the coarsening of scale was attributed to the increase in the opportunity for runoff infiltration in vegetation patches along the slope and to other properties appearing at the catchment scale, such as runoff infiltration through limestone fractures and transmission losses through the channel bed. Sediment yield was very low at the plot scale and negligible at the catchment scales. The definition and quantification of the scale-dependent variation in runoff yield obtained in this study clearly show the important effect of scale on semiarid hydrology regardless of the influence of vegetation cover variations and could be used to improve the hydrological modeling and water management of semiarid areas., Depto. de Biodiversidad, Ecología y Evolución, Fac. de Ciencias Biológicas, TRUE, pub

Questions: Knowledge of how extreme drought events induce plant dieback and, eventually, plant mortality, may improve our forecasting of ecosystem change according to future climate projections, especially in Mediterranean drylands. In them, shrublands are the main vegetation communities in transition areas from a subhumid to semi-arid climate. This study analyzed differences in plant dieback after an unusual drought in 2014 and identified their main underlying factors in relation to three groups of explanatory variables: water availability, soil properties and vegetation structure attributes. Location: Four Mediterranean shrublands along a climatic gradient in SE Spain. Methods: At each experimental field site, we sampled a pool of vegetation structure characteristics, soil depth and soil surface properties, and we also determined water availability by continuously monitoring soil moisture and the microclimate conditions. Results: The climatic analysis showed that there was an extreme drought event in 2014, which was below the first percentile of the driest years. Under such conditions, vegetation dieback occurred at all the study sites. However, plant dieback differed between sites and plant biotypes. Subshrubs were the main affected biotype, with diebacks close to 60% at the driest sites, and up to 40% dieback for shrubs depending on their vertical development. Relative extractable water and bare soil surface cover were the best explanatory variables of plant community dieback but changed between plant biotypes. Vegetation structure variables related to plant vertical development (leaf area index [LAI], plant height, phytovolume) were significant explanatory variables of plant dieback in shrubs, subshrubs and grasses. Consecutive dry days fitted the best model to explain subshrub dieback. Conclusions: We found that rainfall pattern rather than total annual rainfall was the climatic factor that best determined water availability for plants in Mediterranean dryland

Tree transpiration considerably contributes to evaporative fluxes to the atmosphere in terrestrial ecosystems. Accurate transpiration quantification provides relevant information about forest water use and may benefit adaptive forest management, especially in a global change context. Tree transpiration can be measured by several methods, and sap flow measurements are one of the most valued. However, species-specific validations of these techniques are required to avoid undesirable bias. This is especially relevant in species with low transpiration rates where errors may be relevant, such as Aleppo pine trees (Pinus halepensis Mill.). Moreover, another significant source of uncertainty in sap flow measurements is probe misalignment. Hence, the aim of this study was to correlate transpiration rates estimated by sap flow probes using the heat ratio method (THRM) and load cells to independently monitor water transpiration in juvenile Aleppo pine trees. The corrections to improve transpiration measures, including misalignment correction, were applied to THRM results to test if the accuracy of results improved. These measurements were recorded in greenhouse under controlled conditions to implement different environmental conditions. The environmental variables that ruled the experiment, mainly vapour pressure deficit and soil water availability, spanned in a wide range of values. The results showed an accurate linear correspondence between TOBS and THRM for low and medium values, but moderate underestimations at high transpiration rates were observed. These underestimations were partly removed when applying probe misalignment correction. This study supports the notion that HRM offers accurate Aleppo pine transpiration estimations with low and medium values under a variety of abiotic conditions, which also has implications for HRM application in other isohydric species. The results also support the interest in the use of probe misalignment correction to estimate transpirat

El acuífero del Ventós es un pequeño acuífero próximo a la ciudad de Alicante cuyas aguas se destinan exclusivamente al abastecimiento de la localidad de Agost. Sus recursos subterráneos son, por tanto, de especial importancia para esta población. Puesto que estos recursos son fuertemente dependientes de la precipitación, y ante el actual escenario de cambio climático, se ha realizado un estudio con el objeto de comprobar si se han producido cambios y tendencias en la precipitación anual, así como en el número de días de lluvia, episodios lluviosos y episodios lluviosos eficaces frente a la recarga durante los últimos 44 años. Los resultados obtenidos indican que el valor de precipitación anual media es de 282 mm, y a pesar de observarse una ligera tendencia positiva, esta no es estadísticamente significativa. Tampoco la disminución del número de días y episodios lluviosos tienen significación estadística. Los episodios superiores a 20 mm, considerados eficaces frente a la recarga, no reflejan tendencia alguna, por lo que, en principio, se puede concluir que no hay evidencias de un cambio significativo en la precipitación anual sobre el acuífero del Ventós, y especialmente en lo que respecta a precipitación que genera recarga.

Questions: Knowledge of how extreme drought events induce plant dieback and, eventually, plant mortality, may improve our forecasting of ecosystem change ac-cording to future climate projections, especially in Mediterranean drylands. In them, shrublands are the main vegetation communities in transition areas from a subhu-mid to semi- arid climate. This study analyzed differences in plant dieback after an unusual drought in 2014 and identified their main underlying factors in relation to three groups of explanatory variables: water availability, soil properties and vegeta -tion structure attributes.Location: Four Mediterranean shrublands along a climatic gradient in SE Spain.Methods: At each experimental field site, we sampled a pool of vegetation structure characteristics, soil depth and soil surface properties, and we also determined water availability by continuously monitoring soil moisture and the microclimate conditions.Results: The climatic analysis showed that there was an extreme drought event in 2014, which was below the first percentile of the driest years. Under such conditions, vegetation dieback occurred at all the study sites. However, plant dieback differed between sites and plant biotypes. Subshrubs were the main affected biotype, with diebacks close to 60% at the driest sites, and up to 40% dieback for shrubs depend-ing on their vertical development. Relative extractable water and bare soil surface cover were the best explanatory variables of plant community dieback but changed between plant biotypes. Vegetation structure variables related to plant vertical devel -opment (leaf area index [LAI], plant height, phytovolume) were significant explanatory variables of plant dieback in shrubs, subshrubs and grasses. Consecutive dry days fit-ted the best model to explain subshrub dieback.Conclusions: We found that rainfall pattern rather than total annual rainfall was the climatic factor that best determined water availability for plants in Mediterranean dr

Esta tesis doctoral aborda el estudio del transporte de agua y la ecohidrología de distintas especies que componen los ecosistemas forestales de la cuenca mediterránea. El objetivo general consiste en evaluar su respuesta a las sequías y la aridez como aproximación para entender los posibles efectos del cambio climático en estos ecosistemas. En concreto, se analiza el rol de distintas estrategias de uso de agua (isohídrica vs. anisohídrica), la influencia del decaimiento inducido por sequía, y la capacidad de aclimatación de rasgos funcionales que determinan el uso de agua. Para ello, en primer lugar, se realiza una revisión del estado del arte actual sobre el balance hídrico en bosques de Pinus halepensis y Quercus ilex, así como su respuesta a la variabilidad climática y a las condiciones previstas por distintos modelos de cambio climático. En segundo lugar, se analiza la transpiración en individuos de Pinus halepensis y Quercus coccifera en climas mediterráneos secos y subhúmedos, evaluando su sensibilidad a los cambios en la humedad del suelo y la demanda hídrica de la atmósfera, así como su contribución a la evapotranspiración y al balance hídrico del sistema. A continuación, se realiza un seguimiento estacional de variables hidrológicas y fisiológicas en individuos de Pinus halepensis y Quercus ilex en un bosque mixto de ambas especies, analizando su respuesta a las variaciones de humedad del suelo y demanda hídrica de la atmósfera, e incluyendo individuos sanos y decaídos de Quercus ilex. Por último, se estudia cómo varían 11 rasgos funcionales que controlan el uso de agua en 6 poblaciones de Fagus sylvatica localizadas en un amplio gradiente de temperatura y precipitación, desde la zona más árida y mediterránea de su distribución geográfica, hasta su límite más húmedo y frío en la región de los Alpes franceses.

Acer monspessulanum (Montpellier Maple) is an important deciduous tree species native to the Mediterranean region. It is largely distributed in the southern part of western Europe; however, it is geographically less present in north Africa and western Asia. The effects of the most significant environmental variables for its habitat suitability, and climate change, are unclear in terms of the future changes to its distribution. The objective of the present study was to model the current and future geographical potential distribution of the Montpellier Maple in the Mediterranean basin and West Asia using maximum entropy modeling software (MaxEnt). The value of the Area Under the Curve (AUC) of MaxEnt was used to analyze the model’s performance. More than 5800 well-distributed presence points, elevation, slope, aspect, topographic wetness index (TWI), natural vegetation characteristics from MODIS products, and 19 bioclimatic variables were used to conduct the study. Regarding the projections of the species distribution under climate change, 17 global climatic models were used under two RCP scenarios (4.5 and 8.5) for the 2040–2060 and the 2060–2080 time periods. The results show that temperature seasonality (40% contribution to the model), elevation (33.5%), mean annual temperature (6.9%), mean annual precipitation (6.2%), and max temperature of the warmest month (4.5%) were identified as the primary factors that accounted for the current distribution of the Montpellier Maple. Under the climate change scenarios, MaxEnt predicts a large decrease in the species suitability area, with a shift towards the southwestern regions of the species distribution, especially to the mountainous zones of the Moroccan Atlas. Our results show that climate largely limits the distribution of the Montpellier Maple in the Mediterranean basin, as its change in the future is expected to significantly reduce the suitable area by more than 99% from the historical climate conditions, to reach on

Coastal dune landscapes are subject to morphological and ecological changes. In many parts of the world, coastal dunes are under severe pressure. The present study illustrates an integrated remote sensing and Geographical Information System (GIS) approach, i.e., geospatial techniques for assessing land-cover dynamics in Zouaraa coastal dunes, located in northwest Tunisia. As a main result, the analysis of the situation in the past six decades indicates that the dune area showed a decreasing trend with up to 31% (i.e., 6198 ha) in favour of forest area, which has increased by up to 6485 ha. The geo-spatial analysis revealed that restoration works have positively contributed to stabilize coastal dune systems with a substantial increase in vegetation cover. An increase in drought frequency and intensity was detected during the 1952-2017 period using the SPEI index, which enhanced the vegetation activity and growth in the study area. The SPEI significantly correlated with vegetation greenness on the 12- and 24-months’ time scales. The croplands, water and buildings in the study area have increased respectively by 6% (i.e., 1256 ha), 13% (i.e., 3073 ha) and 3% (i.e., 719 ha). In contrast, land cover like shrub and bare soil has decreased respectively by 13% (i.e., 3073 ha) and 2% (i.e., 1831 ha) during the same period. Furthermore, this study highlights the importance of the revegetation techniques undertaken for conserving coastal dune systems. The findings of this study allow land-use planning decision makers to manage and improve situations in similar coastal regions.

Adaptive forest management (AFM) is an urgent need because of the uncertainty regarding how changes in the climate will affect the structure, composition and function of forests during the next decades. Current research initiatives for the long-term monitoring of impacts of silviculture are scattered and not integrated into research networks, with the consequent losses of opportunities and capacity for action. To increase the scientific and practical impacts of these experiences, it is necessary to establish logical frameworks that harmonize the information and help us to define the most appropriate treatments. In this context, a number of research groups in Spain have produced research achievements and know-how during the last decades that can allow for the improvement in AFM. These groups address the issue of AFM from different fields, such as ecophysiology, ecohydrology and forest ecology, thus resulting in valuable but dispersed expertise. The main objective of this work is to introduce a comprehensive strategy aimed to study the implementation of AFM in Spain. As a first step, a network of 34 experimental sites managed by 14 different research groups is proposed and justified. As a second step, the most important AFM impacts on Mediterranean pines, as one of the most extended natural and planted forest types in Spain, are presented. Finally, open questions dealing with key aspects when attempting to implement an AFM framework are discussed. This study is expected to contribute to better outlining the procedures and steps needed to implement regional frameworks for AFM.

The Mediterranean region is one of the most vulnerable regions to climate change. The majority of climate models forecast a rise in temperatures and less rainfall, which have been observed in recent decades. These changes will affect several vegetation properties, especially phenological dynamics and traits, by increasing drought intensity and recurrence. In this climate change context, the present study aims to assess the evolution of vegetation state and its relation with the climate dynamics in the Mediterranean forest region of northeast Tunisia using Land Surface Phenology (LSP) metrics and the vegetation index (NDVI) analysis from 2000 to 2017. To conduct this work, we used precipitation and temperature data from the two closest weather stations and 16-day NDVI composite images from the MODIS satellite source, with 250-m spatial resolution. Three phenological metrics— start of season (SOS), end of season (EOS), and length of season (LOS) — were obtained and compared for different vegetation types. The LSP variation in response to climatic metrics was also analyzed. The results showed that the LSP in our study area changed significantly during the 2000–2017 period, which includes an average 7.8 days delay in the SOS, an average advance in the EOS by 5 days, and LOS shortened by an average 12.8 days. Autumn (Pr_9) and spring (Pr_3 and P3_4) precipitations, as well as maximum temperature (Tx9+10), represent the best climate parameters to explain the changes in LSP. Both the NDVI and SPEI showed a significant high correlation (p < 0.001) on longer time scales. LSP and NDVI proved useful tools for monitoring the vegetation state according to climate for better planning territory purposes.

Increased frequency of severe drought events, coupled with rising air temperatures and vapor pressure deficits (VPD), pose a great threat to Mediterranean forests. Pinus halepensis Mill. is one of the most widespread species in the countries surrounding the Mediterranean basin. Thus, water use of this species plays a critical role in the regions water balance. Studying transpiration patterns and the mechanisms behind stomatal responses to the combined effects of changing VPD and soil moisture can help us improve estimation of forest water use in a changing climate. To improve the estimation of forest evapotranspiration in the Mediterranean basin, the objective of this thesis is to evaluate the transpiration patterns of Pinus halepensis and the role of this species in the soil-water balance under different environmental conditions. Two pine forests in the Turia river basin, eastern Spain, were monitored over a two-year period. The two locations were selected at contrasting altitudes and distances to the sea but within the same hydrological basin, to evaluate if this placement would change the relationship between environmental conditions and the water use of the pines. Sap flow measurements were obtained on a 30-minute interval together with soil moisture measurements and meteorological variables. A soil-water balance was performed on a forest plot-level using an eco-hydrological model in combination with the transpiration data, to assess the contribution of pine transpiration to actual evapotranspiration. Transpiration was dictated by changes in VPD, relative extractable water (REW) and the interaction between these two variables at both sites, indicating that the pines depended on water in the shallow soil layers, and this was restricted during large parts of the year. Except for low winter temperatures having a decreasing effect on transpiration only at the inland site, no significant differences were found in the relationship between environmental drivers and tra

In the climate change context, the severity and duration of drought periods are expected to increase. Forests are known to be extremely sensitive to climate change, especially in the Mediterranean Basin, and particularly in Tunisia, where climate is characterized by dry hot summers which will intensify and prolong due to climate change effects. A better understanding of the mechanisms allowing plants to survive during prolonged drought would help to anticipate expected changes. The present study aimed to analyze the effect of extremely dry years on the mortality and decline of cork oak forests in northwest Tunisia during the 1988–1995 period. Extreme drought years with significant effects on tree growth were registered. Cork oak mortality was recorded for up to 63,622 trees. The study area was largely influenced by the frequency of prolonged drought periods, especially from 1988 to 1995.

Adaptive forest management (AFM) is an urgent need because of the uncertainty regarding how changes in the climate will affect the structure, composition and function of forests during the next decades. Current research initiatives for the long-term monitoring of impacts of silviculture are scattered and not integrated into research networks, with the consequent losses of opportunities and capacity for action. To increase the scientific and practical impacts of these experiences, it is necessary to establish logical frameworks that harmonize the information and help us to define the most appropriate treatments. In this context, a number of research groups in Spain have produced research achievements and know-how during the last decades that can allow for the improvement in AFM. These groups address the issue of AFM from different fields, such as ecophysiology, ecohydrology and forest ecology, thus resulting in valuable but dispersed expertise. The main objective of this work is to introduce a comprehensive strategy aimed to study the implementation of AFM in Spain. As a first step, a network of 34 experimental sites managed by 14 different research groups is proposed and justified. As a second step, the most important AFM impacts on Mediterranean pines, as one of the most extended natural and planted forest types in Spain, are presented. Finally, open questions dealing with key aspects when attempting to implement an AFM framework are discussed. This study is expected to contribute to better outlining the procedures and steps needed to implement regional frameworks for AFM.