Quantitative Estimates of Mycorrhiza Populations in Radiata Pine Forests

Abstract

A description is given of a technique for assaying the mycorrhizas found in a forest soil using a wet sieving method with two B.S.S. standard sieves of mesh size 0.066 and 0.0197 inches. Sampling is restricted to the top six inches of soil, from which the samples are collected with a corer. After extraction from the soil cores, the mycorrhizas are counted on a 10-inch square grid subdivided into numbered, one-inch squares that are further subdivided into 0.1-inch squares. A travelling stereo-zoom binocular microscope is used to examine and count the mycorrhizas on randomly-sampled 1-inch squares. It is estimated that there are about 580kg/acre of mycorrhizas in the top six inches of a good quality radiata (P. radiata D. Don) forest soil, and that approximately 55.2 per cent of them are living. A proposal is made that the results of future mycorrhiza assays include an ‘attrition ratio’, (ratio of inactive mycorrhizas to the total number seen in the sample). Examples are given of the use of this expression in relation to variation in site productivity. The attrition ratio was found to be higher on poor quality sites than on the more productive ones. In contrast, more mycorrhizas were produced on the former than on the latter.