Distribution of the nurse species Pycnophyllum molle J. Rémy and P. weberbaueri Muschl. in the Andean dry puna (Arequipa district-Southern Peru): role of topographic/soil variability and disturbance regime

The dry Puna is the widest pastoral ecosystem of the tropical alpine Andes, characterized by harsh environmental conditions (long and intense drought stress periods and un-fertile soils) and grazed by wild and domestic camelids. In these conditions, facilitation is of key importance in plant diversity conservation. Indeed, facilitation is a positive plant-plant interaction by which the so called nurse species provide environmental amelioration of harsh conditions and/or refuge to other plants (beneficiary species), which otherwise might fail to establish. The research aims were to understand which ecological variables affect the distribution of the potential nurse cushion species Pycnophyllum molle J. Remy and P. weberbaueri Muschl., and if these species are affected by grazing disturbance. The study area (4000-4900 m a.s.l.) is located in the southern Peruvian Andes. Data of species cover, topographic and soil features, besides type of disturbance were collected along transects. We used canonical redundancy analysis to understand the relations between the cover of the two Pycnophyllum species and the above mentioned constraining variables. Results indicate that both the Pycnophyllum species grow on sandy loam, moderately acid soils, with low organic matter and very poor nitrogen content, and avoid high disturbance intensities. molle is more sensitive than P weberbaueri to disturbance, and grows on relatively more fertile soils, also at higher altitudes and on steeper slopes characterized by greater rockiness.