A global remote-sensing assessment of the mobility of coastal dunes

Abstract

Over the last decades, many coastal dunes have lost their natural geomorphologically dynamic character due to spatial expansion of vegetation. This “greening” may not only adversely affect the resilience of dune systems to climate change, but already puts at risk their ecological diversity. While climate has been considered the dominant factor for greening in areas where human activities are limited or rare, on a regional scale over the last century, the role of human interventions has been significant. Whether dune greening is limited to the effects of global warming or is site-specific is largely unknown. Therefore, the aim of this work is to detect and assess the change in vegetation cover over time, at a large number of dunefields worldwide, by using Landsat satellite imagery, and to correlate the identified trends with the trends in the main climatic variables, such as temperature, precipitation and wind speed, in order to decide on the drivers of the change. The Climate Engine App (http://climateengine.org/) has been used to retrieve the multitemporal series of the Normalized Difference Vegetation Index (NDVI; Landsat 4, 5, 7 and 8) and those of the climatic variables for the period 1984 – 2021. This has been done for 186 individual coastal dune sites from different continents: Europe (69); South America (33); North America (31); Africa (20); Oceania (29); and Asia (4). The dune systems are evolving locally under different climatic and environmental conditions, and are subjected to different levels of human pressure. The main results confirm that greening trends are indeed dominant on a global scale (87.1%), as compared to a few dune sites with sand mobilising trends (9.1%), where the aeolian process dominates the stabilisation, and sites with no change (3.8%). Steeper trends are mostly associated with denser average vegetation cover (greener sites) which is typical for the temperate climates. Those sites also exhibit pronounced NDVI seasonality. About 63% of all significant changes show acceleration over the second half of the study period. The other important result is that the vegetation trends show no statistical relationship with the trends of the climatic variables which has been additionally corroborated by a multiple regression analysis. On one hand, possible explanation may be that the current climate has already reached the threshold of being sufficiently warm and wet, so that the dunes are greening anyway. Therefore, further changes in the climatic variables may become irrelevant to the greening. On the other hand, other regional factors (atmospheric pollution, decline in rabbit population) may play a role. Also, management interventions can explain some of the accelerated greening trends, as well as cases of reduced NDVI at other dune sites, despite local climate favouring greening.