Mountains are among the most important ecosystems for the supply of ecosystem services (ES), which are valued by mountain inhabitants as well as by people living in lowlands and mountain visitors [1
]. For example, they are known as “water towers” because of their strong enrollment in the hydrological cycle, namely their important role in storage and gradual release of water [3
]. They are essential refuges for biodiversity and hold a remarkable diversity of landscapes and cultural heritage. However, many mountain areas across Europe are undergoing rapid socio-ecological transitions following the decline of traditional land management systems [4
]. Extensive land abandonment reflects the decline in importance of provisioning services (food, fodder, wood) from local ecosystems explored through traditional farming due to external inputs of commodities and demographic changes. Generally, this has resulted in increased landscape vulnerability to wildfires, invasive species, and other hazardous events, although positive effects can also be pointed out, such as the decrease in soil erosion and the reduction in agricultural diffuse pollution [5
]. Satellite-based products have been used to provide information about the area of farmland abandonment in Europe, estimated to be up to 7.6 Mha from 2001 to 2012, in part occurring in mountains [7
]. Future scenarios for mountains of northern Portugal at the transition between Atlantic and Mediterranean climates suggest a trend of declining agricultural activities to be replaced by forestry as the major source of income [8
Mountains are often classified as protected areas, holding an essential role in the conservation of natural and cultural heritage and in the supply of ecosystem services. Tourism activities, especially those based on agro-tourism, eco-tourism, and wildlife (in particular symbolic species) in protected mountain areas, has been increasing in the last decades [9
]. Several studies have shown that existing networks of protected areas supply considerable quantities of ecosystem services. Castro et al. [10
] reported that two semiarid mountains in Spain contribute to safeguard both biodiversity and ecosystem services, in particular carbon stocks and groundwater recharge. Ecosystem functional types (EFTs), computed from satellite products such as those derived from MODIS (Moderate Resolution Imaging Spectroradiometer), may support the analysis of current protected areas under the new conservation paradigm that considers ES conservation targets to achieve biodiversity conservation, as evaluated by Cabello et al. [11
] in Portugal and Spain. Recently, an extensive list of ecosystem functions that can be evaluated by satellite remote sensing products was outlined by Pettorelli et al. [12
] with the aim of fostering the efficient monitoring of biodiversity and ecosystem services.
Earth observations, namely satellite products, have been used to support ES analysis for land management and policy options [13
]. Recently, efforts are being developed to select essential ecosystem services variables (EESVs) under the GEO BON (Group on Earth Observations Biodiversity Observation Network) initiative [15
]. Earth observations (EO) are at the genesis of the concept of essential variables, which arise to characterize changes in the systems, and have focused primarily on climate, followed by the ocean and biodiversity [16
], and finally on ecosystem services [15
] and Sustainable Development Goals (SDG) [17
]. Traditionally, satellite-based products provide information about the biophysical components of ecosystem services supply, namely the processes and functions that sustain human well-being, such as productivity, evapotranspiration, and temperature regulation [14
]. The demand side of ES is often very difficult to monitor from space, although some attempts (with limitations) can be made using satellite products to estimate, for instance, population density and human settlements [15
]. In addition, the mapping and analysis of the full range of ecosystem services is currently beyond the capabilities of remote sensing alone and should thus rely on complementary information from other sources, such as statistics and in situ data [13
In this context, the objective of this study is to illustrate how satellite products, combined with other sources of information (in situ measurements, statistical data), can be used to monitor the state and trends of several categories of ecosystem services (provisioning, regulating, and cultural). This study is intended as a contribution to the development of EESVs, previously outlined by Cord et al. [15
], and a contribution to the characterization of ecosystem services supply in mountain areas using satellite products, either through direct interpretation of the products themselves or as input variables to inform ecological models. To our knowledge, this is the first study assessing one ecosystem service per category using different sources of satellite products in a protected mountain area. The provision of reared animals, the regulation of water flows, and the cultural service of supporting the conservation of a narrow endemic and iconic plant species (Iris boissieri
) were evaluated following guidelines suggested by the CICES (Common International Classification of Ecosystem Services). Satellite-based products (grassland cover with Sentinel-2 and Landsat, Sentinel-1 and ESA CCI soil moisture, and Ecosystem Functional Attributes (EFAs) from MODIS) were computed and analyzed together with in situ (presence data on Iris boissieri
, field measurements of soil moisture, grassland occurrence data) and statistical sources (precipitation, temperature, number of cattle heads, burnt area) to obtain ES estimates. This study partially builds on satellite products and ecological models previously developed by the authors under the H2020 project Ecopotential (www.ecopotential-project.eu/
), extending their application to the assessment of ecosystem services. Finally, the monitoring of ecosystem services using satellite-based products is discussed under the scope of EESV development and sustainable management of mountain areas.
Three ecosystem services were evaluated in Peneda-Gerês National Park, a protected mountain range in northern Portugal. Provision of reared animals, regulation of water flows, and cultural services through habitat suitability for an endangered and iconic plant species were analyzed using satellite-based products together with in situ and statistical data.
Grassland cover based on Sentinel-2 (2016) and Landsat-7 (2002) images, together with statistical data on livestock, revealed that the provision of reared animals is decreasing in PGNP, although the grassland area has stabilized since 2002 after a decline since the 1980s. Accordingly, the grazing livestock density index has decreased since 2002 in PGNP; however, in one parish, it has increased, showing a tendency toward specialized livestock farming systems together with generalized land abandonment in the park.
Sentinel-1 soil moisture retrievals were used for mapping the current state of water regulation in PGNP at very high spatial resolution, and ESA CCI soil moisture with lower spatial resolution were used to evaluate the trend since 2000. The provision of the water regulation service remained stable, and there was a strong relationship with interannual precipitation pattern. The very high spatial resolution of Sentinel-1 soil moisture allows local targeting actions to cope with natural hazards, such as drought and fire, particularly under future consequences of climate change.
The cultural service of preserving the habitat of Iris boissieri, an iconic species of this National Park, was computed based on ecosystem functional attributes derived from MODIS. Predicted suitable areas for Iris boissieri were modeled and yearly projected since the year 2001, which showed interannual fluctuations of higher and lower PSA values with possible influence of burnt areas and precipitation. The model based on satellite predictors allowed the spatial and temporal monitoring of PSA for this iconic species as an adequate indicator of the stability of habitats for future conservation and consequently for the provision of a cultural service.
We believe that this study can be replicated in other mountain areas to assess the supply of these three ecosystem services considering that satellite products used here are global and freely available, and the codes to run the models are open. Notwithstanding, local data is important, especially to validate satellite products and to calibrate/validate the models. Overall, satellite-based products were shown to be efficient indicators to support the current state and trend analysis of ecosystem services in a protected mountain range, in particular for increasing the spatial and temporal accuracy of assessment and monitoring. This study contributes to innovative perspectives on the use of satellite-based products on ecosystem services assessments, with relevance for monitoring efforts and in particular for the ongoing definition of essential ecosystem service variables.