Seedling dynamics in an environmental gradient of Andean Patagonia, Argentina

LOPEZ-BERNAL PABLO; URRETAVIZCAYA M. F.; DEFOSSÉ G.E.

From seed germination to young plants: ecology, growth and environmental influences

NOVA PUBLISHERS

Lugar: New York; Año: 2013; p. 189 - 210

In this chapter, we analyze the most important environmental and biotic factors that influence seedling dynamics of three of the most conspicuous species grown in a forest-steppe gradient of Patagonia in Argentina. In a short distance of about 80 km in this gradient, vegetation abruptly changes from pure Nothofagus pumilio forests (grown from the timberline down to altitudes of about 1,200 m a.s.l., with 2,000 to 3,000 mm/yr of precipitation), to mixed Austrocedrus chilensis-Nothofagus dombeyi forests and pure A. chilensis forests towards the east, and isolated A. chilensis forests in the ecotone with the steppe (at altitudes from 900 to 500 m a.s.l. and 1,200 to 500 mm/yr of precipitation), and then to a grassland steppe zone, located at about 400 to 600 m a.s.l., and in which precipitation barely reaches 300 to 400 mm/yr. Although precipitation values greatly differ, all areas share the same Mediterranean climate, with rains concentrated during late fall and winter followed by a marked dry spring and summer period. The most important native species in this gradient are the trees, Nothofagus pumilio (lenga), in the higher slopes of the Andes, Austrocedrus chilensis (cypress or ciprés de la cordillera) at mid slope and in the forest-steppe ecotone, and the grass Festuca pallecens (coirón blanco), in the steppe zone. All three species reproduce by seed, and form transient soil seed banks.In the case of Festuca pallescens this bank is annually replenished, while for A. chilensis and N. pumilio seed production occurs during favorable periods every that may vary from two to several years. Among the environmental factors that affect seedling dynamics, water stress during the summer dry period appear to be of paramount importance for Festuca and A. chilensis, and less important for N. pumilio. For the latter species, and after germination, light availability appears to be crucial for further sapling growth. Right after germination, young Festuca or A. chilensis seedlings appear to grow better under protected microsites provided by either an adult Festuca plant or a nurse shrub in the case of A. chilensis. This facilitation effects seems to change as seedlings become older, shifting this interaction to competition as secondary succession progresses. Apart from the environmental conditions, seedling dynamics of all three species are also affected by natural (fires, earthquakes, avalanches, falling out of senescent trees in the case of N. pumilio) and anthropogenic disturbances (fire, grazing, browsing, and logging. Although the inferences made here about seedling dynamics may be valid for the whole area of distribution of A. chilensis and F. pallescens, for N. pumilio, by instance, will be constrained to continental Patagonia (excluding Tierra del Fuego), in which Mediterranean climatic conditions prevail.