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Journal of Arid Environments (1999) 41: 401–410 Article No. jare.1999.0492 Available online at on

Plant community patterns in a gypsum area of NE Spain. I. Interactions with topographic factors and soil erosion

JoaquıH n Guerrero-Campo* A, Francisco Alberto-, John Hodgson?, Jose Ma. GarcıH a-Ruiz* & Gabriel Montserrat-Martı´ * H *Instituto Pirenaico de EcologıH a(C.S.I.C.), Aptdo. 202, 50080 Zaragoza, Spain -Estacion Experimental de Aula Dei (C.S.I.C.), Aptdo. 202, H 50080 Zaragoza, Spain ?The NERC Unit of Comparative Plant Ecology, Department of Animal and Plant Sciences, The University, Sheffield S10 2TN, U.K.
(Received 15 May 1998, accepted 5 January 1999) The aim of this paper is to explain how topographical factors and soil erosion affect plantcommunity patterns in the gypsum hills of the Middle Ebro Basin (NE Spain). Both the altitudinal location on the hill and, secondarily, the aspect determine a regular distribution pattern of the plant communities, which is closely related to soil and erosion features, as well as to some plant attributes. Tall perennial grasses dominate the valley bottoms, and scrub vegetation the hillsides andhilltops. Water availability, soil depth and particle movement down the slope are the main factors that could explain this segregation. 1999 Academic Press Keywords: topography; Mediterranean vegetation; gypsum hills; semi-arid climate; soil depth

Introduction The special physico-chemical properties of gypsum are the main factors responsible for the particular landscape and singular vegetation ofhypergypsic areas (Herrero et al., 1992). Gypsum is the most extensive rock substratum in the Middle Ebro Basin (in NE Spain), occupying 12,000 km (Macau & Riba, 1966). It conditions the landscape geomorphology, which is characterized by low rounded hills and flat-bottomed valleys (Ibanez & Mensua, 1976; Herrero et al., 1992), as well as the existence of a specialized H  flora including a numberof endemic plant species (Braun-Blanquet & Bolos, 1957; ` Rivas-MartıH nez & Costa, 1969; Parsons, 1976).

A(E-mail: 0140}1963/99/040401#10 $30.00/0 1999 Academic Press



Plant community distribution patterns have been little studied in gypsum areas, and even less is known about the effects of topography, soil and erosion on thesepatterns (see, e.g., Campbell & Campbell, 1938; Meyer & GarcıH a-Moya, 1989; Meyer et al., 1992). This lack of knowledge also affects the studied area, in spite of the well established supposition that the development of a denser vegetation cover would reduce the amount of salts drained into the Ebro River, thus diminishing the social and economic consequences of damage through salinization (Alberto &Lebron, 1984; H Navas, 1991). In semi-arid climates the availability of water is a key factor determining vegetation composition and patterns of plant distribution. The spatial availability of water in the soil is mostly controlled by topographic factors such as the aspect and the position on the hill slope (Montana, 1990; Kutiel, 1992). The amount of radiation is greater in  sunny than in shadyaspects, while soil and air moisture follows the opposite pattern. These reasons can explain the heterogeneous distribution of vegetation in sunny and shady aspects, and in the upper and lower part of the hill slopes (Dargie, 1987; Monson et al., 1992). Soil erosion increases the level of stress that plants suffer by reducing soil depth and, thus, water and nutrient availability, whilesedimentation may produce the opposite effect. Furthermore, both erosion and sedimentation can also be considered as disturbance factors (any force that destroys live plant biomass; Grime, 1979). These ideas make it possible to relate plant morphological and functional attributes to the erosion}sedimentation process and topographical location (Guardia, 1995; GuerreroCampo, 1998). Functional strategies in...
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