, Volume 145, Issue 1, pp 93-105

Factors affecting the growth of Eucalyptus delegatensis seedlings in inhibitory forest and grassland soils

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Abstract

In many highland forests of Eucalyptus delegatensis in Tasmania the establishment and healthy growth of eucalypts is promoted and maintained by fire. In the absence of fire, secondary succession from eucalypt forest to rainforest occurs, during which the eucalypts decline and die prematurely. On sites that are prone to radiation frost severe reduction or removal of a tree canopy allows a sward of tussock grasses to develop, in competition with which seedlings of eucalypts decline in growth and a high proportion dies.

Factors of the soil that could contribute to these phenomena were investigated by means of pot experiments that used soils from:

    o
  1. a secondary succession of vegetative types from recently burned healthy eucalypt forest to unburned mature rainforest: this encompassed a sequence of decline and death of the eucalypt trees;

  2. soil from old grassland in which eucalypt seedlings were exhibiting severe growth check and mortality;

  3. from beneath individual trees of several species growing on old grassland.

Growth of seedlings in untreated pot soil reflected closely the condition of eucalypt trees in the field in that growth declined through the successional sequence to rainforest; it was very poor in soil from old grassland; and it varied markedly among soils from beneath different tree species and phases of the grassland. Mycorrhizal development on the pot seedlings differed among soils in both forms and associated fungal types.

Poor growth was overcome only partially by either addition of N and P fertilisers or by partial sterilisation of soil by using steam or chemicals. Inoculation of inhibitory soil from both secondary rainforest and old grassland with 10% to 20% of soil from a healthy eucalypt stand overcame inhibition completely in each case. It is concluded that changes in soil microbiological factors, and specifically in mycorrhizal associations, that accompany changes in vegetative components of the eucalypt stands could be the principal cause of both dieback of older trees and growth check of seedlings.