Experiments on the Ecological Significance of Phycomycetous Mycorrhizas

S ummary The mixed rain forest of New Zealand is dominated by conifers and dicotylous trees all of which form phycomycetous mycorrhizas. They may be present only intermittently in the field, but they become conspicuous in seedlings potted in forest soil. The time required to establish infection is variable, and may exceed 1 year. Growth is limited meanwhile by the reserve of phosphorus in the seed. Heating forest soils to temperatures above 55°C had two contrasted effects on the growth of tree seedlings—a small favourable effect associated with a rise in available phosphate measured hy the Truog test, and a larger deleterious effect associated with destruction of potential endophytes. Coprosma robusta (Rubiaceae) was the species principally used for experiments. By adding sufficient soluble phosphate to either heated or unheated soil vigorous non‐mycorrhizal plants were obtained. In the unheated soil there were also mycorrhizal plants which were, if anything, smaller. With less phosphate plants with numerous mycorrhizas were always larger than those with little or no infection; with no added phosphate there was virtually no growth unless mycorrhizas were formed. The relatively slow‐growing Griselinia littoralis differed in that when mycorrhizal it was insensitive to added phosphate. In steamed soil the growth of non‐mycorrhizal plants of both species was directly proportional to the amount of phosphate added. In Coprosma robusta shading to about one‐third of full daylight did not reduce mycorrhizal infection. Chemical analyses are given which demonstrate the low availability of phosphorus in the soils employed, and the greater uptake by mycorrhizal plants. These observ'ations suggest that mycorrhizas are normally essential for the uptake of phosphorus from forest soils, the phycomycetous type being particularly important in the Tropics and Southern Hemisphere, where the ectotrophic type has been little recorded.