Anthropogenic soil degradation affects seed viability in Polylepis australis mountain forests of central Argentina

Abstract

South American Polylepis mountain forests belong to the most endangered forest ecosystem in the world. Reforestation measures have been strongly recommended but may be hampered due to the very low seed germination rates reported for several Polylepis species. In order to determine the causes behind reduced seed germination we analysed seed viability of Polylepis australis trees in the mountains of central Argentina. We picked seeds from seven heterogeneous areas (4–5 well-separated trees per area totaling 29 trees) with high within and between variation in degradation status. At the landscape scale we measured altitude above sea level and forest fragmentation in a radius of 1000 m around seven areas. At the mesohabitat scale we measured vegetation types and amount of soil erosion in a square of m around each study tree. At the microhabitat scale we assessed site characteristics under the tree canopy. Additionally, we measured some individual characteristics of each tree. Average percentage of viable seeds was 23.0±15.7 S.D. and lack of an embryo was the main reason for seeds not being able to germinate. Landscape variables were not related to seed viability. At the mesohabitat scale, a multiple regression procedure showed that seed viability was positively associated with relatively undisturbed soils supporting tussock grasslands (38.7% of variance) and negatively associated with soil erosion (18.8% of the variance). Microhabitat variables and individual Polylepis characteristics were also related to seed viability, but explained less variability than mesohabitat characteristics. In order to improve seed viability, our data suggests that livestock pressure and burning practices should be reduced, as these are the main causes for erosion and other forms of soil destruction. For reforestation purposes, we recommend collecting seeds from undisturbed areas.