Influence of Topography on the Colonization of Subalpine Grasslands by the Thorny Cushion Dwarf Echinospartum horridum

In this study, we analyzed the effect of topography, particularly slope, on the expansion rates and population dynamics of Echinospartum horridum at small spatial and temporal scales in the grassland communities of Ordesa-Monte Perdido National Park (OMPNP) (NE Spain). E. horridum is a thorny cushion dwarf, endemic of Spain and south of France, forming dense mono-specific large patches difficult to be penetrated by other plants and large herbivores once it is established. Between 2005 and 2007, we collected demographic parameters from 300 marked plant; of E. horridum that were distributed in the center and the edge of four patches. At the patch edge, two of the four patches had a high slope (>= 10 degrees) and two had a low slope (< 10 degrees), whereas at the patch centers the slope was high (18 degrees-30 degrees). To calculate invasion speed, we use aerial photographs from 1981 and 2003. Plants in the center of patches had lower growth rates, fewer flowers, and higher crown death rates than did le plants at edge of patches. Slope influenced significantly invasion rates: the speed of expansion was lowest on gentle slopes, probably because of competition with grass. The speed of diffusion from 1981 to 2003 varied from 2.09 m yr(-1) on the steep slopes to 1.93 m yr(-1) on the shallow slope. Plant dynamics at patch edges suggest that colonization by E. horridum will continue in the grasslands of the OMPNP.