Tracking tree growth through satellite soil moisture monitoring: A case study of Pinus halepensis in Spain

Abstract

Abstract In this study, the ability of satellite soil moisture to track tree growth is analyzed. Despite the reticence of using satellite soil moisture data in forest areas, there is evidence that in some cases, good and reliable results have been obtained. Forests cover very important continental areas and have great importance in many natural processes related to water resources, biodiversity and climate. For these reasons, it is interesting to investigate the applicability of remote sensing soil moisture products for new applications in these environments. In the present study, the CCI (Climate Change Initiative, Global Monitoring of Essential Climate Variables, European Space Agency) soil moisture (CCISM) database has been correlated with the tree-growth series of 22 samples of Aleppo pine (Pinus halepensis Mill.) in Spain in different locations and environmental conditions. Aleppo pine is the most widespread pine species in the Mediterranean basin, and therefore, it is highly representative of the bioclimatic conditions in these water-limited environments. The series spans from 1978 to 2016, and the daily CCISM has been correlated with the annual tree-growth anomalies. The daily CCISM has been obtained using a moving window scheme with 1-, 7-, 15- and 30-day averages, starting on October 1st of the previous year and finishing on December 31st of the corresponding year. Another soil moisture product (Lisflood model) and precipitation have also been used in a similar approach to strengthen the assessment analysis. The results obtained show a clear temporal pattern of the relationship between satellite soil moisture and Aleppo pine tree growth, and the influence of soil moisture on tree-growth dynamics increases with reduced water availability. The CCISM was able to detect a bimodal pattern of tree growth with a maximum in May and a secondary peak in autumn. This temporal pattern was much clearer than that obtained using the modeled soil moisture and the precipitation. This study proves that satellite soil moisture is sensitive enough to track the phenology of this forest species. The results obtained demonstrate that satellite soil moisture data could be suitable for use in forest environments and for new applications.