In recent years many experiments have been done with a variety of plants or ecosystems and the results have been variable. Some show sustained increases in growth rate, some transient increases and others show no response at all. Suggestions to explain this variability have mostly focused on nitrogen availability.
This latest paper is a meta-analysis, or combination of results, from earlier work.
What emerges is that plants associated with ectomycorrhizal fungi show sustained increases in plant growth regardless of apparent nitrogen availability, while plants that use arbuscular mycorrhizae need extra nitrogen to ensure enhanced growth rates at elevated CO2.
Even nitrogen-fixing plants failed to match the ectomycorrhizal effect.
Ectomycorrhizae are the fungal species that form a visible sheath around the small roots of many trees, notably pines, and can extend for some metres into the soil.
They are very effective mobilisers of soil nitrogen, exuding enzymes that degrade much of the nitrogen-containing organic matter. This nitrogen is then passed on to the host plant, which returns energy-providing carbohydrates.
Ectomycorrhizae are the fungal species that form a visible sheath around the small roots of many trees, notably pines, and can extend for some metres into the soil.
Arbuscular mycorrhizae actually penetrate into the cellular structure of the roots but only extend millimetres into the soil. They are less demanding of carbohydrate but also much less effective at supplying nitrogen.
This may be partly because of the more rapid decay of litter under arbuscular mycorrhizal plants resulting in higher soil carbon and more retained nitrogen.
On the other hand arbuscular mycorrhizae are very effective at transferring phosphorous, and the paper’s authors admit that most of the trials have been in northern hemisphere temperate sites where nitrogen is limiting.
In tropical forests where phosphorous is often limiting and arbuscular mycorrhizae dominate, it may be a different story.
For this country this effect could be significant. Our dominant plantation species – pine, Douglas fir and eucalypt, but not cypress – are ectomycorrhizal species and can be expected to thrive at higher CO2.
Grasses are generally arbuscular so enhancing pasture growth from rising CO2 will probably need more nitrogen.
Meanwhile, little is known about the mycorrhizal associations of our indigenous forests. Peter Gadgill’s standard reference, Fungi on Trees and Shrubs in NZ (2005), estimated that less than 10% of NZ’s mycorrhizal fungi had been identified.
Our beech, manuka and kanuka certainly have ectomycorrhizae but many trees can host both forms simultaneously.
Thus, rising CO2 levels may well change the relative growth rates of many plants and affect ecosystems, especially forests, in rather unpredictable ways.
Another Science paper was by Swiss scientists (Science 352, p342, 15.4.16). They fed isotopically labelled (13C depleted) CO2 into mature Norwegian spruces and looked at where the labelled carbon turned up.
The mind boggles a little at gassing 40-metre trees but the carbon could be traced through the treated trees into the mycorrhizal fungi and on into neighbouring trees of different species.
One of the recipient trees was a beech, a hardwood or angiosperm, being fed carbon from the gymnosperm spruce.
However, both use the same ectomycorrhizal fungi and one fungal mycelium seemed to be transferring carbon between the trees. Arbuscular under-storey plants and decay fungi did not get labelled. Thus significant amounts of carbon, estimated at 280kg/ha/year appear to move between trees through ectomycorhizzal fungi.
So the message is that there is a lot going on under our feet, especially in forests and I doubt this will be the final word.
• Next year’s NZ Farm Forestry Association conference will feature presentations by two international experts on mycorrhizal fungi. For more information go to www.nzffa.org.nz.