Carbon sequestration and water yield tradeoffs following restoration of abandoned agricultural lands in Mediterranean mountains.

Abandoned cropland areas have the potential to contribute to climate change mitigation through natural revegetation and afforestation programs. These programs increase above and belowground carbon sequestration by expanding forest cover. However, this potential to mitigate climate change often involves tradeoffs between carbon sequestration and water availability. Particularly in a water limited environments such as the Mediterranean region, any loss of recharge to groundwater or streamflow can have critical societal consequences. In this study, we used an ecohydrologic model, Regional Hydro-Ecological Simulation System (RHESSys), to quantify these tradeoffs for land management plans in abandoned cropland areas in Mediterranean mountains. Changes to Net Ecosystem Production (NEP), water yield and Water-Use Efficiency (WUE) under different land management and climate scenarios were estimated for Arnás, a catchment with similar geology, vegetation and climate to many of the locations targeted for land abandonment restoration in the Spanish Pyrenees. Results showed significant changes to both carbon and water fluxes related to land management, while changes related to a warming scenario were not significant. Afforestation scenarios showed the highest average annual carbon sequestration rates (112 g C·m-2·yr-1) but were also associated with the lowest water yield (runoff coefficient of 26%) and water use efficiency (1.4 g C·mm-1) compared to natural revegetation (-27 g C·m-2·yr-1, 50%, 1.7 g C·mm-1 respectively). Under both restoration scenarios, results showed that the catchment ecosystem is a carbon sink during mid-February to July, coinciding with peak monthly transpiration and WUE, while during the rest of the year the catchment ecosystem is a carbon source. These results contribute to understanding carbon and water tradeoffs in Mediterranean mountains and can help adapt restoration plans to address both carbon sequestration and water management objectives.

20-Years of hindsight into hydrological dynamics of a mountain forest catchment in the Central Spanish Pyrenees.

Mediterranean mountain forests play a significant role in hydrological regulation. In this study, hydrological dynamics was examined at different temporal scales in a small mountain forest catchment in the Central Spanish Pyrenees (San Salvador), based on a 20-year dataset (1999-2019). Mean annual runoff coefficient is 0.21, and ranged from 0.02 to 0.58. The catchment has a bi-modal hydrological behavior with two hydrological periods: a dry-period between July and December, and a wet-period between January and June. During the study period, only 108 floods were recorded, suggesting a low responsiveness of the catchment, with a high variable response. Spearman correlation analysis and stepwise multivariate regression suggest that the hydrological response in the San Salvador catchment is mainly depending on water table, with antecedent moisture conditions and rainfall depth as secondary factors. Seasonal differences were also observed: during dry season, the response was mainly related to rainfall depth and rainfall intensity; in contrast in wet season, the response was mainly related to antecedent conditions (previous rainfall and base flow). Thus, the already challenging water resources management in the Mediterranean basin is magnified by the key function of forests as natural modulators of water cycle. Consequently, the study of natural forested catchments is needed and long-datasets have to be analysed to understand the role of natural Mediterranean forest in the hydrological dynamics and its evolution and adaptation in a context of Global Change.

Effects of active and passive land use management after cropland abandonment on water and vegetation dynamics in the Central Spanish Pyrenees.

The Mediterranean mountains have been subject to significant land abandonment process during the second half of the 20th century. The subsequent natural revegetation following abandonment in rural areas has been widely documented to have substantial implications on the hydrological cycle and the vegetation. The Spanish Pyrenees are one of the most affected areas by these land transformations which could threaten their importance for water supply and agricultural activities in the downstream lowland areas. Land managers as well as scientists around the world have taken different positions on how to deal with these land use changes. Some are in favor of active management (AM) (i.e. density reduction) while others are supporting passive management (PM) (letting the process of revegetation continue). This study aims to investigate the implication of AM and PM on hydrological and vegetation dynamics under different climate trajectories in a representative abandoned cropland catchment in the Central Spanish Pyrenees. A coupled ecohydrologic model is used to estimate the post management response of streamflow (STR), evapotranspiration (ET), soil saturation deficit (SD) and plant carbon (PC) following shrub clearing. Clearing increased annual STR by 16%, while ET and SD decreased by around -9% and -6% respectively during the first year after management with changes to monthly flows. These changes to water regimes may be even higher in wetter years. Over a 10-years period of vegetation recovery annual STR increased between 7.1% and 24.2%, while annual ET and SD decreased between -2.6% to -8.7% and -2.7% to -6% respectively due to shrub clearing, with the highest changes occurring in the first three years of AM. On the effect of climate change, our results show that a 2 °C increase in temperature could reduce AM effects on water regimes and accelerate the recovery of PC given averaged rainfall conditions.

Clearing shrubland and extensive livestock farming: Active prevention to control wildfires in the Mediterranean mountains.

Forest fires are one of the main environmental problems in Mediterranean environments and different fire prevention policies have been applied: livestock grazing, prescribed fires and a combination of both. However, none present satisfactory results. In that context, in 1986 the Regional Government of La Rioja started the Plan for Shrub Clearing (PSC), combining shrub clearings and livestock grazing to control fires and improve the land management of abandoned mountain areas. Our study aims to analyse the effects of shrub clearings on forest fires in La Rioja and to compare the main results with those observed in Spain in the last 30 years. We apply an interdisciplinary methodology based on the analysis of the evolution of cleared areas, the evolution of wildfires in La Rioja and Spain, mapping land use and land cover changes, and quantifying the combustibility. Results obtained in La Rioja are very positive compared with the evolution of Spain, both in the reduction of the number of fires and the burned areas. Decreases in the combustible material, fuel load and biomass and in the occurrence of fires (>1 ha) were observed. In addition, clearing shrubland and extensive livestock provided other environmental benefits (i.e. mosaic landscapes, ecosystem services). Finally, this study suggested that they are good and sustainable techniques to prevent and control wildfires and they could be used as a land management strategy in other Mediterranean areas.