Abstract:

Commercial forestry is carried out with a reduced set of species, with known silvicultural and industrial managements. Thus, these species are frequently planted far away from their natural habitats and introduced in new environments, creating a risk for starting invasion processes in natural or seminatural systems adjacent to the plantations. In NW Patagonia, commercial forestry with exotic species is recent (< 50 years), and presently only 70000 hectares have been planted of an area estimated to have 2 million hectares suitable for tree planting. The rate of afforestation is increasing in the last decade. The most commonly planted species is Pinus ponderosa Doug (Laws) (ponderosa pine) (80% of planted surface area), followed by Pseudotsuga menziesii (Mirb) Franco (Douglas fir) and Pinus contorta Dougl. (lodgepole pine). In contiguous areas to plantations, seedling recruitment of these species has been already observed irrespectively if they were covered or not by natural vegetation. This work analyses, at a regional scale, the presence and rate of recruitment of seedlings of these introduced species in two natural ecosystems: steppe areas and woodlands dominated by a native conifer Austrocedrus chilensis (D.Don) Pic. Ser. et Bizzarri (“ciprés de la cordillera”). Our aim was to determine whether these exotic species have initiated an invasion process. Thirty-four plantations that had reached reproductive stage were selected in an area extending from the center of Neuquén Province (39º48’S) to the North of Chubut Province (42º10’S). Natural regeneration, density (nº individuals/ha), distance to plantation, age, height and presence of cones were registered in adjacent areas of seminatural communities. Any symptoms of disturbances (past and current) and vegetation cover (percentage and dominant species) were recorded. In the steppe, lodgepole pine had greater regeneration than ponderosa pine, considering both expansion distance (> 300m vs. 50m) and density (420 vs. 100 indiv/ha). Also, lodgepole seedling recruitment occurred in more sites (63% vs. 36%) and in plantations with younger trees (12 years vs. 18 years). Overall, recruitment of the introduced conifers was greater at steppe sites in which cattle rearing had been reported in the past, typically with a low vegetation cover or where burned. In the native woodland sites, Douglas fir displayed a better capacity to propagate than ponderosa pine. Both species began to produce seedlings at the same age (17-18 years). Nevertheless, distances of recruitment were greater for Douglas fir (140 versus 70 m), as were densities (1500 vs. 250 individuals/ha) and percent sites with recruitment (100% vs. 67%). Open areas and disturbed places (paths, trails or clearings) displayed greater recruitment capacity than undisturbed areas. Indeed, recruitment was absent in sites highly covered and in sites with abundant understorey vegetation. In both studied systems, areas located downwind presented greater recruitment. Results of expansion distance to determine whether these exotic species have initiated an invasion process were compared with distance of advance proposed by Richardson to define invasions. Our results indicate that an incipient invasion of lodgepole pine in steppe and Douglas fir in woodlands dominated by the native conifer is occurring. Lodgepole pine produced seedlings at the earliest age (12 years). Ponderosa pine does not seem to behave like an invader to the studied communities. The information obtained from this study allowed us to identify factors that could facilitate or limit the process of expansion of these species related to propagule pressure, seed predation and disturbances. We propose recommendations on how to follow it and assure that preventive tactics are adopted to reduce the risk for the occurrence of a biological invasion.