Structure of vegetation patches in northwestern Patagonia, Argentina

Vegetation of arid and semiarid environments has in general a patchy distribution. Our objective was to (a) determine several qualitative and quantitative analytical characteristics of vegetation patches in an arid zone of Patagonia, Argentina, and (b) investigate relationships between them. Annual precipitation in this area was 200 mm during 1999-2005. Eight transects involving ten patches each were studied within a 15 x 15 km area. Mean (+/- 1 SE) values (n = 80) in the vegetation patches were 315 +/- A 25 and 207 +/- A 16 cm for the greatest and lowest patch diameter, respectively; 23 +/- A 2 cm for mound height; 113 +/- A 12 cm for maximum vegetation height; and 170 +/- A 18 cm and 58 +/- A 2% for distance to the next vegetation patch and vegetation patch cover within a transect, respectively. Correlations between greatest and lowest diameters, mound height, maximum plant height and distance to the closest vegetation patch were all significant (P < 0.01; n = 80). In all vegetation patches, the greatest and lowest frequencies were found for the grass Stipa spp. (71.2%) and the shrub Grindelia chiloensis (Cornel.) Cabrera (12.5%). Stipa spp. and the shrub Atriplex lampa (Moq.) D. Dietr. showed the highest simultaneous frequency (50%). A reasonable association among species (> 45%) was found for Stipa spp., Atriplex lampa and the shrubs Larrea divaricata Cav., Lycium chilense Miers ex Bertero and Junellia ligustrina (Lag.) Moldenke. Larrea divaricata and Atriplex lampa contributed more than 84% of the total patch standing crop (5,777 +/- A 435 g). Average patch size and specific diversity were 5.93 +/- A 0.33 m(2) and 1.31 +/- A 0.11, respectively. Aboveground standing crop of the two dominant shrubs decreased as plant species diversity increased (P < 0.05). Conservation of vegetation patches is crucial to prevent increased soil erosion and desertification in the study ecological system.