The increase of carbon dioxide (CO2) in the atmosphere is one of the main causes of global warming [1]. Under the United Nations Framework Convention on Climate Change (UNFCCC 2015), some national governments revised their environmental policies to reduce the emission of GHG by controlling the consumption of fossil fuels and by encouraging consumers to use renewable energies instead. Following the Paris Agreement and the United Nations 2030 Agenda for Sustainable Development, the GHG mitigation strategy aims to maintain the global average rise of the temperature bellow 2º C [2].
The aboveground biomass (AGB) of forests is an indicator of productivity, carbon stock and sequestration caused by land use and land cover (LULC) and climate change in forest ecosystems [3, 4]. Thus, its precise measurement is important for assessing the carbon budget of terrestrial biomes [5, 6]. The most accurate method for measuring AGB is through ground measurements which are very costly and difficult to implement, especially, if the study area is very wide [7]. Remote sensing methods are one credible and advantageous alternative to estimate AGB [3]. These can use free open imagery data and be combined with several methods, such as machine learning and others, to provide accurate AGB estimates [8, 9]. AGB can also be estimated from existing LULC data using simpler models [10, 11].
Forest conversions by forestry, agriculture and anthropogenic LULC changes have direct impacts in climate regulation by altering atmospheric CO2 concentrations [12, 13]. Measuring spatiotemporal distributions of terrestrial carbon stocks subject to LULC changes is key to Greenhouse Gas (GHG) estimates and mitigation [14]. Estimates of LULC changes are responsible for 12.5% of global carbon emitted by human activities [5]. Particularly, the agriculture and forestry sectors play a major role in the GHG mitigation strategy [15]. Studies about LULC changes and its impacts on ecosystem services (ES) contribute with helpful information in defining effective sustainable policies [10, 16]. These studies may provide relevant information to engage stakeholders in the decision processes involving ES approaches [17].
Over the past decades, studies on ES have been quite intensive [18–21] regardless the nature of their functions: provisioning, regulation, supporting or cultural (MEA 2005). The integration of these assessments with scenarios of LULC enables the quantification of the impacts of different land use policies and trade-offs on ES [10, 23]. Geographic Information Systems (GIS) tools and spatiotemporal analysis are usually used alongside the scenarios projection in ES assessments [20, 24].
The carbon storage and sequestration by forests is a complex regulation ES (MEA 2005) and is strongly influenced by internal conditions, such as plant species, phenology, density of the settlement and the landscape structure [25]. It is also influenced by external conditions like human activities set out by LULC management [26]. The inclusion of these conditions in spatially explicit ES approaches is relevant for designing effective strategies to mitigate climate change through the reduction of CO2 emissions [27]. The incorporation of development scenarios make ES assessments useful in a science-policy interface perspective [28, 29]. These scenarios can be expected at regional and national scales to support the relationship between sustainable development and global environmental changes [30].
Valuation processes are crucial for the decision makers´ perspective in management actions [31]. The valuation methods consist, in a broad sense, to “assigning importance” to what should represent the diversity of the dimensions of nature values aiming long-term sustainable strategies that evaluate the trade-offs between nature and human well-being [32]. Economic valuation techniques bring a monetary perspective to ES studies and provide information that may help organizations to define policies for effective management of resources, particularly, over the LULC sector (Daily et al, 2013).
Carbon stocks’ assessments based on LULC changes have been carried out worldwide at local and national levels [33]. Fernandes et al. (2020) assessed and valued carbon sequestration for a semiarid region in Brazil using scenarios. Leh et al. (2013) modelled several ES including carbon sequestration based on land cover changes for two countries in West Africa. In a comprehensive review on forest models of sustainable land use management, Mäkelä et al. (2012) show a spatial relation between forest resources and their contribution to the carbon dynamic cycles. Under a European perspective, some studies highlight the vulnerability of forest ecosystems to land use and climate changes [37–39]. In this context, Sil et al. (2017) have analysed carbon sequestration and storage dynamics in a mountain landscape based on land cover changes in Portugal. Additional studies were also carried out in other parts of the country [41–44].
In Portugal, the GHG strategies are defined by the National Low-Carbon Roadmap (APA, 2012), which aims to implement a low carbon economy by increasing the consumption of the renewables sources rather than fossil fuel. Another important instrument is the National Forest Strategy (PCM, 2015), which stands for the development of the forest sector at social-economic and environmental levels. Alongside with this strategy, it is also important to mention the Common Agricultural Policy (CAP) that supports the economic viability of rural communities through rural development measures. Landscape planning from CAP assumes that the support for sustainable and climate-friendly land use must include the development of forest areas and sustainable forest management. Moreover, agricultural areas fall within existing policy instruments with impact on the forest sector generating relevant benefits for climate change mitigation, such as increasing soil carbon and improving soil health (Rosenstock et al. 2019). Therefore, the forestry measures to be implemented in Portugal through the European Agricultural Fund for Rural Development (EAFRD) should contribute to the implementation of the forestry strategy for the EU (PCM, 2015).
Despite the existence of several policy instruments for GHG mitigation and studies, a national assessment of the carbon storage and sequestration based on LULC using a scenario approach is still missing for Portugal. This paper aims at filling this gap and proposes a combined approach of GIS and ES modelling tools to measure the AGB and study the impact of future scenarios on carbon storage and sequestration and trade-offs. In our analysis, we include different forest classes to estimate expected trends of carbon variation according to three different land use scenarios by 2030. Results are expected to provide new insights for national authorities acting on GHG mitigation strategies within the existing Sustainable Development Goals (SDGs).