Habitat structure and vegetation relationships in central Argentina salt marsh landscapes

Abstract

Relationships between habitat structure and spatial variations in vegetation composition were determined in catenas of central Argentina salt marsh landscapes. Vegetation was classified following a multi-technique strategy. An analysis of species distributions along an environmental gradient was made and a redundancy analysis was used to relate the environmental variables to vegetation data. The spatial covariation was evaluated through fractal analysis. The vegetation can be divided into four discrete noda that correspond to different topographic positions (summit, backslope, footslope and toeslope, respectively): nodum of Stipa trichotoma + St. tenuissima, nodum of Distichlis scoparia, nodum of Distichlis spicata and nodum of Spartina densiflora. Each of these nodum is characterized by a definite combination of floristic groups. Symmetric aggregation of vegetation borders was observed in all three sites. The existence of vegetation discontinuities along the catenas depended largely on water table depth and salinity which in turn controlled the edaphic salinity. The arrangement of sites by multivariate analysis reflected the influence of a complex gradient representing halomorphic and hydromorphic factors. β-diversity was associated with abrupt changes in the physical structure of the catena for a reduced spatial dimension (300-m scale). Absolute diversity and evenness were higher on the summit and declined progressively toward the toeslope. The rate of change was higher on the backslope and the dominant species have different ecological amplitudes overlapping along the gradient. The main operational factors associated with the floristic differences are: (i) the variations in the chemical composition and the seasonal dynamics of the soil solution in the aerated layer of soils, (ii) the salinity and dynamics of groundwater, and (iii) the length of time that the soil is flooded during the rainy season (summer). The fractal dimension was close to 2, implying weak spatial dependence. Fractal dimension varied as a function of scale. The fractogram only revealed a significant spatial dependence on the summit. This spatial dependence was associated with short distances of gradient showing that the organizational pattern of Distichlis spp.- Spartina was related to combinations of underlying environmental factors rather than to a specific position in the catena. The catenas are highly structured spatially, floristic compositions are inextricably linked to this structure. Habitat complexity may directly affect associated vegetation by regulating the hydrohalomorphic conditions in the aerated layer of the soils. The relationships of the habitat-floristic groups are not simple, hydromorphism interacts in a complex way with halomorphism.