Cushion plant community data from an Andean alpine ecosystem in Venezuela

Site These data were collected in the northern-most reaches of the Andes Mountains in the Piedras Blancas páramo, Sierra de La Culata National Park, Venezuela, during January-March 2014 (dry season). Sites were located in the high Andean páramo (superpáramo), between 4250m-4350m, with slopes of 15-25 degrees, and on northeast facing aspects. We sampled three sites in a 5 km area: Rio Azul (8.8866, -70.8685), Avenida (8.8847, -70.8666), and Gloria (8.8928, -70.8714). Life in this region is subject to harsh and dynamic tropical alpine conditions.This region is the driest in Venezuela’s high alpine, with an annual precipitation of 860 mm (Pico El Águila weather station, 4,118 m). The soils at the study sites are coarse, shallow and subject to constant disturbance by needle-ice formation due to frequent freeze-thaw cycles in the soil. The plant community in our study sites sparsely covers the landscape (generally less than 50% cover) and the vegetation is highly clustered. This cover is partitioned into two strata, one dominated by giant rosettes and shrubs, and the other consisting of cushions, disperse grasses, forbs, cushions, and acaulescent rosettes. We studied the two most abundant cushion species in the area, Azorella julianii and Arenaria musciformis. A. julianii is commonly found in the northern Andes from Ecuador to Venezuela (3500-4700 m), while A. musciformis is present in Colombia and Venezuela (3500 and 4300 m). Soil Methods Measurements of soil relative water content (SWC) and organic matter (SOM) were performed by taking soil samples from 5 to 10 cm below the soil surface near midday on a clear day of the dry season (March 2014). One sample was taken from each of eight different haphazardly chosen cushions of Arenaria and Azorella and eight open sites away from the cushions influence (but always within 2-4 m from each replicate pair). SWC measurements were made by calculating the percentage of water in the sample through differences between wet and dry weights. SOM content determinations were obtained using the weight difference between a sample of dry soil, and the remnants of the sample after heating to 400 o C for 4 hours. For measuring shallow soil temperatures within each cushion species, we haphazardly chose three individuals with more than 30 cm in diameter and placed one replicate Onset HOBO TidbiT v2 sensor within each. Sensors were buried 2 cm below the leaf surface at the center of each cushion. Then, we haphazardly sampled three nearby open soil sites away from the direct influence of the cushions and placed one replicate temperature sensor in each, 2 cm below the soil surface. These nine sensors recorded temperatures every 15 minutes during five consecutive clear days between the 27 and 31st of March 2014. From each sensor´s data we calculated the mean, maximum and minimum temperatures and averaged them between the replicate sensors in each microhabitat (inside Azorella and Arenaria and in open areas). Vegetation Sampling Methods At each of the three study sites we haphazardly selected 35 individual cushions of each species within a 200 x 200 m area. For each individual, we placed a wire ring on the cushion and recorded the presence and number of individuals of all vascular plant species within. We defined individuals as ramets, as many species in the páramo are interconnected underground. Then, we randomly sampled open substrate 1 m from each cushion (in 35 paired samples for each cushion species) and measured richness and density within the wire rings in the same way we did in the cushion. In the case of Azorella, there were many cushions that exceeded a mean diameter of 50 cm. However, in the case of Arenaria, the majority of individuals in the population were smaller than that. Hence, we used a 40 cm diameter flexible sampling ring for Azorella (and the paired samples outside) and a 20 cm diameter ring for Arenaria (and paired samples). The area of the rings in each case was then used to calculate plant density for each species. We only sampled individual cushions with a minimum length in one dimension of at least 40 cm for Azorella and at least 20 cm for Arenaria