**4. Discussion**

The first step in inferring human disturbances in vegetation was to generate an accurate object-based map of the study area. The high accuracies obtained in the segmentations, evaluated with the ED2 index, facilitated the classifications of the shrubs, obtaining similar accuracies to previous studies in which other species of shrubs with OBIA were detected [21,62]. In the classification step, the segments of *Z. lotus* and bare soil with sparse vegetation showed high separability, using features related to their brightness and texture as a consequence of clear spectral differences between vegetation and bare soil [63]. According to [64] and [52], the best spectral-related features for discriminating between vegetation and bare soil with sparse vegetation were Brightness, and the GLCM family of features, which present high separability values in well-differentiated classes, such as vegetation and soil [52,65,66]. The worst features to discriminate (i.e., lowest separability) were the geometry-related ones (e.g., Roundness, Area). This is contrary to previous studies, in which Roundness has been suggested as a potential feature to discriminate between rounded shrubs and bare soil [21,66,67]. This discrepancy with previous studies could be explained by the high heterogeneity in the vegetation form that can present after disturbance [5]. However, care must be taken interpreting these results, since shadows generated by large individuals may result in an overestimation of shrub cover area [68,69], and individuals growing together may be underestimated as a result of appearing in the images as one [62].

The spatial distribution of sand extraction areas was unrelated to the topography of the area but was related to the location of older roads and tracks. This suggests that sand extractions were preferentially located to previous sand extractions in an effort to minimize labor costs. This observation is consistent with a previous study on sand extraction in the region (e.g., [30]). According to [70], the negative effect on shrubs by sand mining was shown in the low values of AMD calculated in the period 1956–1977. This reduction in population cover area could be related to sand mining during the 1960s and 1970s [30], which might confirm the positive effect that sands have on the health of *Z. lotus* shrubs in the study area [31] and in other areas of North Africa [71,72].

The lower value of the AMD between shrubs with reduced cover in the period 1977–1984 to the coastline suggested by the ARD that this reduction could be related to a decrease in the freshwater table and the intrusion of seawater into the aquifer [35,73]. The smallest shrubs were the most affected, which can be related to difficulties of access to groundwater due to their smaller roots compared to larger and more developed individuals [28,74]. These results agree with previous studies evaluating the negative effect that seawater intrusion has on vegetated communities and groundwater-dependent ecosystems (e.g., [75,76]).

In addition, the results of this work could be affected by other natural conditions or affections, for example, shrubs could be affected by herbivory [5,77], climate change [78], or uncontrolled use of pesticides [79]. We argue that in order to better understand the results obtained in this work, it is necessary to complement remote sensing techniques with in situ works. For example, complementing the results obtained with the presence–absence of isotopes and relating them with the seawater intrusion [80,81] could provide a better understanding of how this phreatophytic community responds to anthropic perturbations over time.

Although 742 *Z. lotus* individuals were lost during the study period, their average size and the round shape of the shrubs were higher and bigger at the end than at the beginning of the period. However, the variability of these characteristics also increased over time, which means that a greater variety of shapes and sizes was observed in the population. This could be explained by the 1987 declaration regarding the Cabo de Gata-Níjar Natural Park, where the study area is located. This protection, in addition to a slow recovery of the aquifer after undergoing seawater intrusion between 1977 and 1984, could have contributed to a slow but continued development in time by adults, which might have better access to fresh water from the aquifer due to a more developed root system [82] up to a length of 60 m [83]. Furthermore, the fact that the largest shrubs were the most developed in time supports the longevity character of this species through longevity [27], which is an important strategy for its survival in the Mediterranean region [26]. This, together with the anthropic pressure on the system, may explain the development of adult individuals, but the lack of recruitment of juveniles, as observed by [27] not only in this area but also in other regions in SE Spain.
