Vegetação de charcos e cursos de água temporários. Estudo da ordem Isoetetalia em Portugal

Abstract

The pioneer ephemeral vegetation of ponds and temporary streams presents a particular floristic composition, adapted to unstable ecological conditions due to the alternation between periods of flood and of total dryness. This amphibian plant species are established in soils flooded during the winter and dry in the early summer. The water depth and inundation period determine the plant communities distribution included in the Isoeto-Nanojuncetea. This class of vegetation presents a holartic distribution and is recognizable, in the area of study, by two orders: Isoetetalia, mediterranean and west-european, with spring and early summer bloom, and Nanocyperetalia, central-european, with late summer and autumm bloom. This study intends to cover only the Isoeto-Nanojuncetea spring communities, which distribution in alliances are not well clarified. It follows the classic syntaxonomical scheme which is subdivided in four alliances: Isoetion, Western Mediterranean dwarves pioneers communites rich in geophytes, submerged until late spring; Menthion cervinae, Mediterranean tall therophyte plant communities with a long inundation period that lasts until summer; Cicendion, Iberian-Atlantic dwarves pioneers communities subjected to periodical flood during winter and early spring; Agrostion pourretii, Mediterranean West Iberian ephemeral graminoid, slightly inundated in small depressions and ponds margins. The syntaxonomical scheme presented is consistent with numerical analysis, resulting in homogenous floristic communities that correspond to syntaxa at association level. The phytosociological approach, in the interpretation and characterization of vegetation, has its last practical example applied to nature management and biodiversity conservation in the habitat-type classification presented in Natura 2000 typology and habitat network. In the Portuguese interpretation (Natura 2000 network management plan), the seasonal wetlands ecosystems are covered by group 3 – Freshwater habitats, sub-group 31 – Standing waters. Aiming to typify the plant communities occuring in the priority habitat “Mediterranean temporary ponds”, about 100 surveys were carried out in 3 different locations in the alentejan district. The characterization is based on several criteria: i) temporal variation along the year in the same biotope, that is, the sucession of plant communities in the same physical area, when the water level decreases with the start of summer; ii) microgeosigmeta composed by a variable number of plant communities ( 2) belonging to more than one alliance of the order Isoetetalia (class Isoeto-Nanojuncetea); and iii) the coexistence oftwo Isoetes species in the same pond, following the moisture gradient by a transect from the bottom to the edge of the pond: Isoetes setaceum (Menthion cervinae) – Isoetes histrix (Isoetion). Generally, three vegetation belts can be distinguished as a microcatenal repetitive pattern: at the bottom of the deepest depressions, where moisture persists almost all year long, occur perennial plant communities of Phragmito- Magnocaricetea. The ephemeral vegetation of Isoeto-Nanojuncetea becomes dominant towards pond edges. In the second vegetation belt, pond topographic depression with soil impervious layer engenders a long hydrophase, creating good conditions for Isoeto setacei-Eryngietum corniculati association. In sandier soils with short flood periods, Junco capitati-Isoetetum histricis and Agrostion pourretii plant communities can be distinguished. These complexes of vegetation are very sensible to the environmental and antropic disturbance, become bioindicators with high value for diagnose and conservation.