Water availability in sand dunes and its implications for the distribution of Artemisia monosperma

Plants in many sand dune systems experience arid-like conditions due to low water capacity and high hydraulic conductivity of sand grain medium. Here, water availability was studied in coastal sand dunes in a Mediterranean climate on which the dominant species is a desert dwarf shrub, Artemisia monosperma. Special emphasis was put on landscape heterogeneity and contrasts between dune stabilisation states and microhabitats, conditions after rain events and in the dry season and catenary profiles. During the approximately 150 day-long dry season, as well as during multiple dry spells in the rainy season, soil water content at 50 cm depth was below permanent wilting point in both mobile and stabilised dunes. The open matrix between the A. monosperma dwarf shrubs was characterised by higher hydraulic conductivity and soil water content compared with soil under A. monosperma dwarf shrubs, and played an important role in supporting the dwarf shrubs. In the mobile dunes, in which there is greater open matrix area, this has resulted in higher soil water content in the dunes that increases into the adjacent interdunce, while the opposite was found in the stabilised dunes. The dominance of A. monosperma is thus related to its ability to utilise water from the open matrix, and its distribution is driven by source-sink relations between the open matrix and A. monosperma dwarf shrubs, respectively. In stabilised dunes, A. monosperma dwarf shrubs increase in size and total percentage cover, reducing water-contributing open matrix area and probably increasing competition over water. As a result, the spatial distribution pattern of A. monosperma dwarf shrubs is regular, maintaining a certain degree of open matrix to support the living dwarf shrubs, which theoretically cannot achieve a full coverage of the dune surface in this Mediterranean system.