A Factor Toxic to Seedlings of the Same Species Associated with Living Roots of the Non-Gregarious Subtropical Rain Forest Tree Grevillea robusta

Abstract

Monocultures of Grevillea robusta, a non-gregarious timber species of subtropical rain forest, grow poorly in south Queensland where the tree occurs naturally. Several other non- gregarious species in the same region also fail in pure plantations. In G. robusta plantations G. robusta does not regenerate, though other rain forest species do. G. robusta regenerates freely, however, in adjacent plantations of the gregarious Araucaria cunninghamii and along the edges of areas of rain forest In plantations of G. robusta the tips of the leaves of seedlings of this species become characteristically blackened and the seedlings die. The same phenomena were reproduced under experimental conditions where light, moisture and presumably mineral nutrients were not limiting. Blackening symptoms and deaths were shown, by trenching experiments and other observations, to follow contact of seedling roots with actively growing roots of older plants of G. robusta. Similar symptoms and deaths were produced in single G. robusta seedlings in sand and soil in pots watered with nutrient solution plus leachates from older G. robusta plants in sand. Control seedlings in pots receiving nutrient solution only remained healthy. G. robusta falls to regenerate in G. robusta plantations because of some water-transferable factor associated with the rhizosphere of 1his species, in which antagonistic microflora may be involved. The killing of seedlings by parent trees of the same species may explain the maintenance of floristic diversify in complex tropical rain forests. Commerical production of these species may be possible only in polycultures.