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.

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.

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.

Woody encroachment and soil carbon stocks in subalpine areas in the Central Spanish Pyrenees.

Woody encroachment has been an ongoing process in the subalpine belt of Mediterranean mountains, after land abandonment, the disappearance of the transhumant system and the decrease of the livestock number. The main objectives of this study were: (i) to identify land use/land cover (LULC) changes from 1956 to 2015, and (ii) to investigate the effects of LULC changes in physical and chemical soil properties and soil organic carbon (SOC) and nitrogen (N) stocks. It is hypothesized that woody encroachment in the subalpine belt may lead to significant changes in soil properties, and will generate an increase in the SOC stocks. A land use gradient was identified in the subalpine belt of the Central Spanish Pyrenees: (i) subalpine grasslands, (ii) shrublands, (iii) young forests, and (iv) old forests. Mineral soil samples were collected every 10 cm, down to 40 cm, at three points per each LULC and a total of 48 samples were analyzed. The results showed that (i) woody encroachment has occurred from 1956 to 2015 due to the expansion of coniferous forests and shrublands (at the expense of grasslands), (ii) land cover and soil depth had significant effects on soil properties (except for pH), being larger in the uppermost 0-10 cm depth, (iii) SOC and N contents and stocks were higher in the grassland sites, and (iv) the woody encroachment process initially produced a decrease in the SOC stocks (shrublands), but no differences were observed considering the complete soil profile between grasslands and young and old forests. Further studies, describing SOC stabilization and quantifying above-ground carbon (shrub and tree biomass) are required.

The footprint of marginal agriculture in the Mediterranean mountain landscape: An analysis of the Central Spanish Pyrenees.

Agriculture forms an essential part of the mountains of the Mediterranean. For centuries, large areas were cultivated to feed the local population, with highly marginal slopes being tilled at times of heavy demographic pressure, using the shifting agriculture system. A great deal of agricultural land was abandoned during the 20th century, giving rise to secondary succession processes that tend to eliminate the agricultural footprint. However, revegetation is a highly complex process leading to areas with dense, well-structured plant cover, and other open areas of scrubland. This article studies the role of traditional agriculture in the deterioration of the landscape. By using experimental plots in the Central Pyrenees to reproduce traditional agriculture and abandonment, maps of field types, and current uses and ground cover, it could be confirmed that shifting agriculture has caused very heavy soil loss, which explains the deterioration of the landscape on several slopes. Burning scrub and adding the ash to the soil as a fertilizer did not greatly help to improve soil quality, but caused high rates of erosion and a very slow process of regrowth. The average data obtained from the shifting experimental plots recorded losses of 1356kgha-1years-1, 1.6 times more than the plot of fertilized cereal, and 8.2 times more than the dense scrub plot. Following abandonment, losses in the shifting agriculture plot were almost three times higher than the abandoned sloping field plot. Traditional shifting agriculture in the Pyrenees is the main cause of the deterioration of the landscape 50-70years after agriculture ceased.

How do soil organic carbon stocks change after cropland abandonment in Mediterranean humid mountain areas?

The effects of land use changes on soil carbon stocks are a matter of concern stated in international policy agendas on the mitigation of greenhouse emissions. Afforestation is increasingly viewed as an environmental restorative land use change prescription and is considered one of the most efficient carbon sequestration strategies currently available. Given the large quantity of CO2 that soils release annually, it is important to understand disturbances in vegetation and soil resulting from land use changes. The main objective of this study is to assess the effects of land abandonment, land use change and afforestation practices on soil organic carbon (SOC) dynamics. For this aim, five different land covers (bare soil, permanent pastureland, secondary succession, Pinus sylvestris (PS) and Pinus nigra (PN) afforestation), in the Central Spanish Pyrenees, were analysed. SOC dynamics have been studied in the bulk soil, and in the fractions separated according to two methodologies: (i) aggregate size distribution, and (ii) density fractionation, and rates of carbon mineralization have been determined by measuring CO2 evolution using an automated respirometer. The results showed that: (i) SOC contents were higher in the PN sites in the topsoil (10cm), (ii) when all the profiles were considered no significant differences were observed between pastureland and PN, (iii) SOC accumulation under secondary succession is a slow process, and (iv) pastureland should also be considered due to the relative importance in SOC stocks. The first step of SOC stabilization after afforestation is the formation of macro-aggregates promoted by large inputs of SOC, with a high contribution of labile organic matter. However, our respiration experiments did not show evidence of SOC stabilization. SOC mineralization was higher in the top layers and values decreased with depth. These results gain insights into which type of land management is most appropriate after land abandonment for SOC.

RESEARCH: Managing Mountainous Degraded Landscapes After Farmland Abandonment in the Central Spanish Pyrenees

/ Plant succession and pasture resources have been studied in abandoned fields of the central Spanish Pyrenees, in an environment severely affected by strong demographic pressure in the past. Several hydromorphological features (runoff and sediment yield) were also analyzed for different environments of the abandoned fields, in order to forecast the effects of their reclamation and transformation into areas for livestock use. The availability and accessibility of pastures as well as soil and water conservation is related to the process of colonization of Genista scorpius. Under a dense shrub cover both runoff and sediment yield are strongly controlled. As the shrub cover becomes open, sediment yield and runoff increase greatly. A dense herbaceous cover yields high runoff coefficients but moderate soil losses. From the results obtained, the possibility of abandoned field reclamation by means of selective clearing of scrub is discussed.KEY WORDS: Abandoned fields; Plant succession; Degraded environments; Soil erosion; Runoff; Spanish Pyrenees