Toadstool time
The autumn rain brings a hidden world into view (6 minute read)
There’s a heavy rain warning for the mountains north of Wellington, and in the city it’s been raining steadily for the last 24 hours. But when you have a dog bred from Scottish sheepdogs, there’s no such thing as bad weather. It doesn’t matter how windy, wet or cold it gets, Donna still wants her walk. In the five years I’ve owned her, and the five years before that when I walked her three times a week, I’ve only once seen her troubled by the weather. That was in a thunderstorm with a downpour so heavy it caused flooding down the hill from the park where we were walking.
There are compensations for the rain, though. The fair-weather dog walkers stay home, which means it’s easier for me to give Donna the space from other dogs that she needs. So, I pull on my waterproofs, clip Donna into her harness, and head for Khandallah Park. It’s a favourite place for me, but one where there are usually too many dogs for Donna. We encounter only one other dog, and with me holding a treat in front of her nose she walks past without paying it any attention. I’m glad of the small victory.
There’s something else about the rain, too. As it has soaked into the soil, it has signalled to something below the surface, saying that it’s time to show itself. It’s toadstool time.
The term toadstool refers to the aboveground parts of certain fungi. Like the term mushroom, it’s imprecise. Some people use toadstool to refer to those that are poisonous, while others use either toadstool or mushroom for all of them. To be honest, I seldom use the word toadstool and normally use mushroom for all, but I couldn’t resist the alliteration.
Fungi used to be classified as plants, but by late the 1970s scientists had realised fungi had more in common with some animals. They are now classified as a separate kingdom, alongside plants, animals and various microbe groups. The fungal kingdom is far more diverse than the mushrooms we are familiar with. It includes the yeasts which ferment our bread and beer and the moulds which makes blue cheese blue. There are fungi which take over the bodies of insects, some of which were used by Māori as a pigment for tā moko, the traditional tattoo. There are fungi which cause unpleasant infections such as ringworm and athlete’s foot, fungi whose spores contribute to allergies and respiratory disease and even some which can cause life-threatening brain infections. There are fungi which produce antibiotics, spoil food, break down toxic pollutants, destroy crops, control pests and decay our houses.
Many of these deserve an article to themselves, but they aren’t the fungi I spotted on my wet walk with Donna. As we headed up the hill, I was watching out for other people with dogs, reminding Donna not to pull and trying to identify ferns, which left little attention for anything else. But then we passed a mushroom so conspicuous that I couldn’t fail to notice it. Amongst the brown of the leaf litter and the green of the ferns, it stood out in red and white. It’s so distinctive that everyone recognises it, the mushroom commonly known as the fly agaric.
The mushroom that we see, though, is only a small part of the story. The part which lives underground is far more important, and far more complex.
When we look at a mushroom, whether it’s the red and white fly agaric or the button mushroom we buy at the supermarket, we are looking at a structure that the fungus produces as part of its reproduction. A typical mushroom has a stalk, known as the stipe, and a cap which sits on top. From the underside of the cap hang numerous gills, appearing as thin blades radiating out from the centre1. The gills produce dust-like spores, which are the equivalent of seeds in flowering plants, but are simpler in structure.
However, when the spores grow, they don’t begin forming new mushrooms. Instead, they grow tiny threads. This is the form in which fungi spend most of their life cycle – all fungi, not just the ones which form mushrooms. Although we seldom see them, these fungal threads are all around us. They are in the soil, in decaying leaves and wood, in food which is spoiling and in the leaves of plants infected with fungal diseases like myrtle rust. They are even in us when we are infected with fungal diseases.
However, of all the places they are found, one of the most important is also one of the most surprising. When I first learned about it, I looked at the plants growing around me as if I’d never seen them before. The knowledge shook the foundations of my understanding of what defined a plant.
The vast majority of the plants we see around us do not live in isolation, as individual organisms. Although we seldom see any sign of it, the plant roots grow entangled with certain fungi. The fungi and plants may or may not be able to live independently of one another, depending on the species. But since both the fungi and plants benefit from the interaction, they do usually live together.
A plant root growing in association with a fungus is called mycorrhiza – from myco meaning fungal and rhiza meaning root in ancient Greek. Mycorrhizae are thought to go back to the earliest plants which grew on land, more than 475 million years ago. Although they don’t occur with mosses, they do occur with other very old plant forms such as liverworts. I’ve seen different estimates for the exact number of plants which form mycorrhizae, but one comprehensive review suggested that the figure is around 80% of plant species. A notable exception is plants belonging to the brassica family. This family includes familiar vegetables such as cabbage, broccoli and cress, but there are well over 3000 species found around the world.
Some groups of plants are much more dependent on their fungal partners than others. The most extreme example is the orchid family. The seeds of orchids are incapable of germinating and growing unless the right fungi are present. Around 200 orchid species, including several from New Zealand, remain entirely dependent on fungi for their entire life cycle. These orchids lack green leaves and the ability to produce their own food from sunlight. Instead, they receive all their nutrition from the fungi, meaning that they are considered parasites on the fungi. However, the relationship isn’t straightforward, because the fungi themselves may be parasitic on other plants, or have other relationships which haven’t yet been untangled.
As well as plants which are parasitic on fungi, there are also fungi which are considered parasitic on plant roots, because they harm the plants in taking nutrition from them. However, when you look around at a forest or a field, you are mostly looking at plants growing in a mutually beneficial relationship with fungi. Plants can make food in the form of sugars using energy from the sun, and supply this to the fungi. In turn, the fungi are more efficient than plant roots at absorbing nutrients from the soil, particularly phosphorus, and some of these nutrients are passed on to the plant. There are also thought to be other benefits of mycorrhizae, such as enabling plants to better resist fungal diseases, and improving the structure of soil.
Some of the claims made about mycorrhizae are not supported by evidence. It’s clear that plants and fungi are sharing nutrients with one another. However, it’s also clear that the relationships aren’t always equally beneficial. Claims that plants are communicating and sharing resources via mycorrhizae have been overstated. Nonetheless, mycorrhizae are important, and they are poorly-studied in comparison to their importance.
The reason they are poorly studied can probably be summed up with the old adage: out of sight, out of mind. The majority of the world’s plants form a relatively inconspicuous kind of mycorrhiza with a single group of fungi which has only around 230 species. But one kind of mycorrhiza does demand attention. It’s found in only 2% of plant species, but most of those species are large trees which are important in many landscapes. It also involves an extraordinary diversity of fungal species, around 20,000 of them. Many of these fungi form conspicuous structures for their reproduction, like the brightly-coloured fly agaric I saw on my walk.
The fly agaric belongs to a fungal group known as Amanita, most of which form mycorrhizae with trees. Amanita is a worldwide group, and important for many reasons, not least because it contains some of the most prized edible mushrooms, as well as the deadliest, the death cap. I won’t get into that here, because I recently interviewed an Amanita expert and I’m working on an article on that topic – look out for it in late April or early May. What I wanted to mention here is that New Zealand has a number of native Amanita species which form mycorrhizae with native trees such as beech and mānuka. The fly agaric, though, is a northern hemisphere species, which begs the question of why I spotted it on my walk in Khandallah Park, which is largely native forest.
A close look at the forest floor reveals the answer. Among the fallen leaves are pine needles, and above me are the branches of massive, old pines. These date back many decades, almost certainly to the time when the reserve was much smaller and surrounded by farmland. The native forest has regrown around them, and there are few, if any, pine seedlings, because they don’t grow in the shade. The fly agaric commonly forms mycorrhizae with pine trees, and most of the time when you see it, there’s a pine tree not far away.
But, in places, it’s possible to spot fly agarics far from any pines. Have the pine roots travelled further than I realise? Or is the fly agaric forming mycorrhizae with some of the native plants in Khandallah Park?
I wanted to look more closely at this possibility, because I could remember that there was research underway a few years back. When I looked at some of the papers, I learned the fly agaric is one of few introduced fungi to be found in native forest, forming mycorrhizae with native trees. However, it isn’t usually found with the kinds of trees found in Khandallah Park. In fact, the trees in Khandallah Park almost all form the inconspicuous type of mycorrhizae, the ones which don’t produce showy mushrooms. This means that all the fly agarics I saw were probably on pine roots.
In other parts of New Zealand, this may not always be the case. Fly agaric can now be found in native beech forest, on beech tree roots. In Australia and South America, it has also moved onto their native beech species, as well as other native trees.
The fly agaric illustrates some of the challenges we face when assessing and managing the risks of new species introduced to an area. It’s unlikely that the fly agaric was deliberately introduced. It was first reported in 1937 and probably sneaked in with imported plants in the days when rules were much less strict. It was decades before it was found associated with native plants, partly because it may have taken that long to spread to them, and partly because it’s hard to tell what it’s growing with. If there are many trees around, we may not know which ones are associated with the fungi.
If it’s in native forest in Khandallah Park, but only associated with old pine trees, does that matter? It might be obvious, but it’s probably not doing any harm, since the pine trees will probably die out eventually, and with them, their associated fungi. It may be more of a problem in native beech forest though, since there are a number of species related to fly agaric which are native to New Zealand and found in native beech forest. These species could be at risk if they were displaced by fly agaric. However, this would be difficult to demonstrate and even harder to do anything about if there was a problem. Most of the time, all of the action is below ground, and we have enough to worry about with introduced species we can see. Still, I’ll now be paying closer attention.
There are other forms, such as puffballs, but I’m just focusing on the typical mushrooms here.
Very informative, thanks. Shrooms and fungi are truly fascinating - their influence on us and our ecosystems is profound.