In spite of the relative importance of groundwater in costal dune systems, studies concerning the responses of vegetation to ground water (GW) availability variations, particularly in Mediterranean regions, are scarce. Thus, the main purpose of this study is to compare the responses of co-occurring species possessing different functional traits, to changes in GW levels (i.e. the lowering of GW levels) in a sand dune ecosystem. For that, five sites were established within a 1 km² area in a meso-mediterranean sand dune ecosystem dominated by a <i>Pinus pinaster</i> forest. Due to natural topographic variability and anthropogenic GW exploitation, substantial variability in depth to GW between sites was found. Under these conditions it was possible to identify the degree of usage and dependence on GW of different plant species (two deep-rooted trees, a drought adapted shrub, a phreatophyte and a non-native woody invader) and how GW dependence varied seasonally and between the heterogeneous sites. Results indicated that the plant species had differential responses to changes in GW depth according to specific functional traits (i.e. rooting depth, leaf morphology, and water use strategy). Species comparison revealed that variability in pre-dawn water potential (&#936;_pre) and bulk leaf &delta;¹³C was related to site differences in GW use in the deep-rooted (<i>Pinus pinaster, Myrica faya</i>) and phreatophyte (<i>Salix repens</i>) species. However, such variation was more evident during spring than during summer drought. The exotic invader, <i>Acacia longifolia</i>, which does not possess a very deep root system, presented the largest seasonal variability in &#936;_pre and bulk leaf &delta;¹³C. In contrast, the response of <i>Corema album</i>, an endemic understory drought-adapted shrub, seemed to be independent of water availability across seasons and sites. Thus, the susceptibility to lowering of GW due to anthropogenic exploitation, in plant species from sand dunes, is variable, being particularly relevant for deep rooted species and phreatophytes, which seem to depend heavily on access to GW.