Transformation of parabolic dunes into mobile barchans triggered by environmental change and anthropogenic disturbance

Parabolic dunes are widely distributed on coasts and margins of deserts and steppes where ecosystems are vulnerable and sensitive to environmental changes and human disturbances. Some studies have indicated that vegetated parabolic dunes can be activated into highly mobile barchan dunes and the catastrophic shift of eco-geomorphic systems is detrimental to land management and social-economic development; however, no detailed study has clarified the physical processes and eco-geomorphic interactions that control the stability of a parabolic dune and its resistance to unfavorable environmental changes. This study utilizes the Extended-DECAL (Discrete Eco-geomorphic Aeolian Landscapes) model, parameterized by field measurements of dune topography and vegetation characteristics combined with remote sensing, to explore how increases in drought stress, wind strength, and grazing stress may lead to the activation of stabilizing parabolic dunes into highly mobile barchans. The modeling results suggest that the mobility of an initial parabolic dune at the onset of a perturbation determines the capacity of a system to absorb environmental change, and a slight increase in vegetation cover of an initial parabolic dune can increase the activation threshold significantly. The characteristics of four eco-geomorphic interaction zones control the processes and resulting morphologies of the transformations. A higher deposition tolerance of vegetation increases the activation threshold of the dune transformation under both a negative climatic impact and an increased sand transport rate, whereas the erosion tolerance of vegetation influences the patterns of resulting barchans (a single barchan versus multiple barchans). The change in the characteristics of eco-geomorphic interaction zones may indirectly reflect the dune stability and predict an ongoing transformation, whilst the activation angle may be potentially used as a proxy of environmental stresses. In contrast to the natural environmental changes that tend to affect relatively weak and young plants, grazing stress can exert a broader impact on any plant indistinctively. A small increase in grazing stress just above the activation threshold can accelerate dune activation significantly.