Pine Invasions in the Southern Hemisphere: Determinants of Spread and Invadability

Many factors interact to determine whether an introduced organism will become invasive. The widespread planting of many Pinus species at various densities and in many habitats at different times in the Southem Hemisphere (well outside their natural range) provides a unique opportunity to identify the determinants of invasive success for a large group of related organisms. At least sixteen pine species have spread from planting sites to invade natural or semi-natural vegetation. Species with adventive populations over fairly large areas are P. contorta Dougl., P. halepensis Mill,, P. nigra Arnold, P. patula Schiede & Deppe, P. pinaster Ait., P. ponderosa Laws., P. radiata D. Don and P. sylvestris L. This paper explores the biological profiles of successful invaders, the inherent and induced susceptibility of different vegetation types to invasion, patterns of invasion, the role of disturbance in initiating and sustaining invasions, the role of time, and the various functions of the resident biota in the receiving habitat in mediating the outcome of an introduction. This information, with evidence from contemporary studies of pine dynamics in the Northem Hemisphere and reconstructions of pine migrations in the early Holocene, shows that: (1) Most of the important invaders share a suite of biological traits that facilitate long-distance dispersal by wind, and allow invading populations to persist in habitats subjected to frequent disturbance. (2) The extent of invasion for widely planted species is positively correlated with the residence period. (3) Ground-cover categories can be ranked according to their vulnerability to invasion as follows: forest < shrubland < grassland << dunes < bare ground. (4) Disturbance regimes and the resident biota have a marked (and complex) influence on invadability. This information is used to predict future trends in pine invasions