The share of land plants forming mycorrhiza

The slogan "ninety per cent of plants form mycorrhiza" is a useful shorthand, but it is not the result of a single global count. A 2018 synthesis separates the different strategies: about 72 per cent of plant species have arbuscular mycorrhiza, about 2 per cent ectomycorrhiza, 1.5 per cent ericoid mycorrhiza, and close to 10 per cent are orchids dependent on fungal partners. The sum depends on classification and rounding.

A share of species is not the same as a share of individuals, of land area or of biomass. One abundant grass species and a rare tree each weigh one species in such a tally. Some plants change the type of relationship depending on habitat, age or the availability of a partner, and for poorly studied lineages the status may remain uncertain. "Forms mycorrhiza" describes a capacity and a typical strategy, not a guarantee that every root is colonised.

Mycorrhiza is also not a uniform service. The fungus may improve access to phosphorus, nitrogen and water, and the plant passes carbon to it. The balance depends on the soil and the conditions: where nutrients are highly available the cost of maintaining a partner may exceed the benefit, and the relationship shifts along a continuum of cooperation. The popular percentage does not say whether a particular mycorrhizal preparation will work in a pot, or which fungal species will be best for a given crop.

The value of the statistic lies in changing the perspective. A root without fungal partners is not the default state of most land plants, and soil is part of a plant's biological infrastructure. Ninety per cent remains only a historical shorthand in the article; the current panel shows 72% arbuscular mycorrhiza and separates the other types and remainder. Where precision is needed, the type of mycorrhiza and the basis of the percentage should be given instead of treating a rounded slogan as an exact measurement.

One headline conceals several biologies

Brundrett and Tedersoo’s synthesis separates categories instead of repeating the familiar “about 90%.” In its rounded classification, 72% of vascular plant species are arbuscular mycorrhizal, 10% orchid mycorrhizal, 2% ectomycorrhizal and 1.5% ericoid mycorrhizal. About 8% are completely non-mycorrhizal and 7% have an inconsistent non-mycorrhizal–arbuscular status.

These are shares of species, not counts of individuals, vegetation cover or biomass. The values are rounded working categories. After the displayed mycorrhizal types are subtracted, the chart has a 14.5% remainder; the publication’s rounded non-mycorrhizal 8% and inconsistent 7% categories add to 15%. The half-point difference is rounding, not another biological category.

How to read the composition bar

The chart does not try to recreate the “90%” slogan as one field. It separates 72% arbuscular, 10% orchid, 2% ectomycorrhizal and 1.5% ericoid mycorrhiza, leaving the other 14.5% as the remainder. Published shares are rounded, so the sum of the descriptive categories can differ by half a percentage point.