Satellite images reveal how Andalusia’s forests have changed over the past three decades

The first Landsat series satellite was launched by NASA in the summer of 1972 with the aim of tracking changes in the Earth's surface. It was followed by 8 more. The last of them, put into orbit in 2021, adds to the extensive list of these systems currently hurdling through space in search of key information for planetary management.

This satellite mission, which has already been surveilling the planet for more than half a century, has become the raw material of the Terrestrial Ecology research group at the University of Cordoba, which has processed the images offered by this satellite for the last 28 years (1994-2021) with the aim of analyzing how Andalusian forests have evolved in the face of aridity over the last three decades. Drawing on these images, capable of capturing information in bands of the electromagnetic spectrum not visible to the human eye, the team examined phenological changes in the main Mediterranean forest species, including the holm oak, cork oak, and different species of pine, eucalyptus, olive and chestnut. 

A ‘greener’ Andalusia than 30 years ago

One of the work's main conclusions is that Andalusia’s forest landscapes, in general terms, are now greener than they were three decades ago. In other words, Andalusia now has more forest mass than it had at the beginning of the 90s, although this growth was especially marked during the first years (1994-2005). 

The reasons for this increase are several and complex, explained Rafael Villar, principal investigator of the group that carried out the work. The abandonment of fields after the rural exodus, plants' own adaptation to adverse climatic conditions (such as Pinus halepensis and Quercus ilex), the effects of atmospheric CO2 fertilization, forest management, and changes in conservation policies could be some of them.  

Despite this, as another of the participating researchers, Cristina Acosta, stressed, it is important to highlight that "this trend towards greening is the result of an average of all of Andalusia over time." In fact, this increase in greenery has not been as pronounced in the region's most arid areas, such as Almería, where Pinus sylvestris has shown a more moderate response, due to the scarcity of rainfall.

The work also points to the effect of aridity on the growing season of trees. Although this has been shortened in some of the species studied, such as the olive tree and eucalyptus, its effect is even more pronounced in the wild pine, a tree that is particularly sensitive to scant rainfall, and that reduces its growth period as an adaptation mechanism, depending on the start of the first autumn rains and summer's early arrival. 

The importance of remote sensing

The study, in the words of Pablo Salazar, a researcher in the Department of Botany, Ecology and Plant Physiology, underscores the importance of remote sensing as a key tool in species management in a context of global change, especially to monitor large areas in a way complementing field work, which makes it a more cost-effective, efficient and a faster option when it comes to obtaining results on a large scale. 
In this way, the methodology makes it possible to continuously evaluate the evolution of forests, detect phenomena like decay early, observe forests’ capacities as carbon sinks, and analyze how climate change affects vegetation.