HABITAT STRUCTURE AND VEGETATION RELATIONSHIPS IN CENTRAL ARGENTINA SALT-MARSH LANDSCAPES

Relationships between habitat structure and spatial variations in vege tation composition were determined in catenas of central Argentina sal t marsh landscapes. Vegetation was classified following a multi-techni que strategy. An analysis of species distributions along an environmen tal gradient was made and a redundancy analysis was used to relate the environmental variables to vegetation data. The spatial covariation w as evaluated through fractal analysis. The vegetation can be divided i nto four discrete noda that correspond to different topographic positi ons (summit, backslope, footslope and toeslope, respectively): nodum o f Stipa trichotoma + St. tenuissima, nodum of Distichlis scoparia, nod um of Distichlis spicata and nodum of Spartina densiflora. Each of the se nodum is characterized by a definite combination of floristic group s. Symmetric aggregation of vegetation borders was observed in all thr ee sites. The existence of vegetation discontinuities along the catena s depended largely on water table depth and salinity which in turn con trolled the edaphic salinity. The arrangement of sites by multivariate analysis reflected the influence of a complex gradient representing h alomorphic and hydromorphic factors. beta-diversity was associated wit h 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 specie s have different ecological amplitudes overlapping along the gradient. The main operational factors associated with the floristic difference s 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 th at the soil is flooded during the rainy season (summer). The fractal d imension was close to 2, implying weak spatial dependence. Fractal dim ension varied as a function of scale. The fractogram only revealed a s ignificant spatial dependence on the summit. This spatial dependence w as associated with shea distances of gradient showing that the organiz ational pattern of Distichlis spp.- Spartina was related to combinatio ns of underlying environmental factors rather than to a specific posit ion in the catena. The catenas are highly structured spatially, floris tic compositions are inextricably linked to this structure. Habitat co mplexity may directly affect associated vegetation by regulating the h ydrohalomorphic conditions in the aerated layer of the soils. The rela tionships of the habitat-floristic groups are not simple, hydromorphis m interacts in a complex way with halomorphism.