Rate of Growth of Banksia grandis Willd. (Proteaceae) in Western Australian Forest

Abstract

Banksia grandis is a major understorey component of the jarrah forest and is a refuge of the soil fungus Phytophthora cinnamomi which kills many native plant species and threatens timber production. This paper presents data on the rate of growth of the lignotuber, shoot and stem of B. grandis. Length of the seedling lignotuber increased 0.23 cm year-1 over 3 years, that of saplings and trees increased 0.44 cm yea-1. The period for a seedling to attain 1.3 m height was calculated as c. 15 years. Short-term diameter increment of trees averaged 0.19-0.27 cm year-1, indicating that diameter at breast height of 20 cm should be attained in c. 80-100 years; this estimate is also supported by direct and indirect long- term data. Watering, shading and spraying with insecticide had no significant influence on height growth of saplings. Four years after fire, sapling coppice from large lignotubers nearly attained the height of the plant before the fire. Trees, but not saplings, grew faster after logging of forest. The significance of these growth rates is discussed in relation to two methods (mechanical removal, moderate-intensity fire) proposed to reduce the abundance of B. grandis in the jarrah forest.