GROWTH AND COMPETITION BETWEEN SEEDLINGS OF PROTEA-REPENS (L) L AND THE ALIEN INVASIVE, ACACIA-SALIGNA (LABILL) WENDL IN RELATION TO NUTRIENT AVAILABILITY

Seedlings of Acacia saligna (Labill.) Wendl., an alien invasive shrub of coastal fynbos, and Protea repens (L.) L., an indigenous shrub, were grown in pots, as single seedlings and as a mixture. These were amended with one of a range of treatments representing a broad gradient in nutrient availability. When grown in isolation, both species displayed increases in dry mass, and leaf nitrogen and phosphorus contents in response to increasing levels of nutrient availability. A. saligna however, exhibited a greater response than P. repens, particularly at high levels of nutrient availability. No significant differences in root/shoot ratio were found in response to variation in nutrient availability, but A. saligna consistently allocated a greater proportion of its total dry mass to below-ground growth. When grown as a mixture, P. repens exhibited reduced growth at high levels of nutrient availability in response to competition with A. saligna, which totally overtopped the indigenous species. In root observation chambers, the depth penetration of the soil by the tap root of seedlings of A. saligna after germination was approximately twice as rapid as that of P. repens. Seed nitrogen and phosphorus contents were significantly linearly related to seed size, except the phosphorus content of A. saligna. The relatively high absolute growth rate of A. saligna seedlings, at practically all levels of nutrient availability found in the fynbos, appears to be one of the key factors which enables them to compete successfully with P. repens seedlings.