This work explains the effect of microtopography on the spatio-temporal gradient of topsoil moisture in a first-order stream in a forested mountainous area of northern Spain. This gradient was also related with the availability of suitable microsites for a forest riparian sedge (Carex remota).
Topsoil moisture, presence of C. remota and height and distance from the stream edges were measured in 385 points along 35 transects perpendicular to the stream. Soil moisture measurements were repeated in three different dates.
Topsoil moisture showed a sigmoid trend that defined the limits of a wet riparian zone at 125m of distance from and 055m in elevation above stream banks. Our riparian zone was narrower than other studies because of the steep slopes (25%) of the mountainous area studied. Elevation above stream banks was more influential than distance in defining the limits of the riparian zone.
In the riparian zone, values of soil moisture were high and constant even at the end of a dry period due to the continuous water flow. In the adjacent upland forest, soil moisture varied according to rainfall. These high and constant soil moisture values defined the suitable microsites for C. remota.