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 G. robusta, a non-gregarious timber species of subtropical rain forest, grow poorly in S. 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 only nutrient solution remained healthy. It was concluded that G. robusta fails to regenerate in G. robusta plantations because of some watertransferable factor associated with the rhizosphere of this species, in which an antagonistic microflora may be involved. The killing of seedlings by parent trees of the same species may explain the maintenance of floristic diversity in complex tropical rain forests. Commercial production of these species may be possible only in polycultures. KEYWORDS: Allelopathy \ plant ecology \ Grevillea robusta \ Regeneration \ natural \ non gregarious rain forest species

Key words

• allelopathy
• botanical composition
• coffee
• ecology
• forest inventories
• forest trees
• forests
• leachates
• leaves
• microbial flora
• nutrient solutions
• nutrients
• plant ecology
• plantations
• production
• rain forests
• regeneration
• rhizosphere
• roots
• seedlings
• soil
• species diversity
• species richness
• subtropics
• symptoms
• trees
• trenching
• tropical rain forests
• tropics
• woody plants