The factors that mediate tree invasions have been studied mainly for pines (Richardson et al., 1994; Higgins et al., 1999; Richardson et al., 2000b) in Zimbabwe (Tafangombe, 2001) and elsewhere (Rejmánek, 1996; Rejmánek and Richardson, 1996). The extent of invasions can be explained by a model that incorporates:
species attributes (small seeds with wings, short juvenile period, short interval between large seed crops, ability to survive moderate browsing);
residence time usually more than 50 years;
extent of planting i.e. must be planted in large areas;
ground cover characteristics: usually sparsely vegetated ground is prone;
locality (altitude and latitude);
Interaction with receiving ecosystem elements.
The species attributes of a successful invader, such as a short juvenile period and a short interval between large seed crops, imply early and consistent reproduction. A small mean seed mass is associated with other important phenomena, such as: large number of seeds produced, better dispersal, high initial germinability and shorter chilling period needed to overcome dormancy (Rejmánek and Richardson, 1996). Resident biota or mutualism (Richardson et al., 2000b) involves animal mediated pollination and seed dispersal, and symbioses between plant roots and microbiota (fungi, bacteria, etc.) to facilitate invasions. Acacias have been known to nodulate promiscuously with local bacteria and have mostly benefited from the resident biota.
Most of the invasive species in Zimbabwe and South Africa were introduced more than 100 years ago and they are growing in geographic environments (climatic, latitude, altitude) similar to their origin. They also have short juvenile periods, produce small seeds and have a short period between large seed crops, all important factors for successful invasion.