In the southwest of the Iberian Peninsula dwarf rush communities belong to the class Isoeto-Nanojuncetea and constitute the ephemeral wetland vegetation. The hydrological conditions are determined by the Mediterranean precipitation regime and therefore vary with the seasons. Previous studies found this vegetation type to be highly dynamic in time and space, but until now drivers of this dynamic are not well known. Nonetheless, this dynamic should be considered in evaluating the current state of dwarf rush communities protected by the EU Habitats Directive.

For that reason we aim a) to describe and to clarify the influence of the temperature sums and soil water balance on the seasonal dynamic and the phenological development and b) to decide whether separate phenological or ecological phases exist. Vegetation was surveyed in permanent plots and selected environmental parameters were measured in the study region Campo de Gibraltar (Spain) in spring 2008. Multivariate ordination, variation partitioning, calculation of turnover rates and examination of the flowering phases were employed to characterise the vegetation dynamics and to separate the influence of temperature sums and the soil water balance on phenological development.

We show that the variability of ephemeral dwarf rush communities was high and equally partitioned in time and space. Temperature sums and soil water balance were the main drivers of phenological development and seasonal species turnover. Species turnover was largest when dry periods occurred. A differentiated examination of the flowering phases of Radiola linoides and Solenopsis laurentia revealed a considerable time lag of 10 days correlated to differences in the soil moisture content at the growing sites. These findings suggest that soil moisture is an important driver of the temporal dynamics. The parallel temporal development of the vegetation in the permanent plots indicates that no independent ecophases exist. Overlapping flowering phases of species of the Isoëto-Nanojuncetea and Helianthemetea (rock rose communities) support earlier findings that the temporal replacement of Isoëto-Nanojuncetea species by Helianthemetea species is marked by a gradual turnover rather than an abrupt shift.

Open access: Ecologia_mediterranea-2011-37_1