What are Mother Trees and Master Trees?
‘Mother’ and ‘Master’ trees are two completely different ideas from separate branches of ecology, which both demonstrate the vital role large trees play in our landscape.
Mother Trees
How do trees behave?
Some of the questions which come up in modern forestry science might surprise you:
- Do trees have a sense of self-identity?
- Do they feed their offspring?
- Do they warn each other of danger?
The answer to all of the above questions is either ‘probably’ or ‘yes’, but you might already be familiar with the way it works – the mycorrhizal network in the soil. If you’re not sure what that is, or need a refresher, let’s break it down and explain why it’s better than a regular root.
What are mycorrhizae?
Mycorrhizae are a special kind of fungi with a name which is very difficult to spell. They grow on the surface of plant roots and beyond them through gaps in the soil. Their structure is very similar to a root – branching and hairy, but they can grow much more quickly, averaging many centimetres per day.
Mycorrhizae have a symbiotic relationship with plants – the fungi take up nutrients from the soil and carry it to the plants, while the plants exude carbon created from photosynthesis*. But these fungi don’t just help individual plants – as a branching network which spreads through the soil, they connect different trees, too.
It turns out that this tree-to-tree network can be really useful – scientists injected one tree with radioactive carbon and discovered that it was shared through the mycorrhizal network, into other trees. Carbon is an essential building block of tree cells, so this really helps speed up growth. Through methods like this, they’ve found that deciduous trees share carbon with coniferous trees in summer, while in autumn, as their leaves fall, the process reverses. It’s enough to make you want to buy a tie-dye t-shirt and live in a yurt.
*For the sake of readability, I’m using ‘carbon’ here to stand in for ‘photosynthates’, which includes sugars or other substances made by photosythesis. They contain carbon.
What are ‘Mother Trees’?
The concept of a ‘mother tree’ was coined by the researcher Dr Suzanne Simard – she found that, rather than a random network below the ground, the mycorrhizae are arranged into a structured system. As a forest ages, the network becomes more complex, with more connections, and the species of fungus can change, too. The older a tree, the more interconnected its mycorrhizae are likely to be, so that it forms a sort of hub (see image below). The structure is like a real map, with large cities (mother trees) which have branching roads (mycorrhizae) travelling out to the more poorly connected towns (smaller trees).
Why does this matter to us? Well, seedlings attached via mycorrhizae to a mother tree are more likely to survive the first two years of life, as the older tree helps them along, for example, by sending extra carbon through the network. Effectively the parent is ‘feeding’ its offspring, although it may also do this for seedlings of other parent trees and other species. Mother trees also retain older genetic material, not influenced by later shifts in climate (which affects epigenetics) and may harbour colonies of lichen or other organisms, acting as islands of biodiversity.
Why do trees have a sense of self-identity?
You can see how it might be beneficial, then, for a tree to have a sense of self and recognise its offspring. It turns out that trees exude identifying substances from their roots which allow them to do just this. Perhaps these substances are sent along the networks?
Rice plants are more likely to invest resources in root growth when they’re planted next to unrelated neighbours. That means they can recognise their close relatives or ‘kin’ – this has implications for farmers wishing to improve their yields.
How do plants warn their neighbours of an attack?
And what about warning other plants that danger is on the way? Well, plants use certain chemical signals within their bodies to signal that an organ such as a leaf or stem is under attack by a herbivore like an aphid. The plant can then respond in a way which protects itself from attack, such as by producing ‘volatile repellent compounds’.
Scientists have discovered that the warning signals can not only pass through mycorrhizal networks, but also trigger defence responses in neighbouring plants. In this way, an attack on one plant could trigger a defence response across a whole group of plants.
Most of the research into forest networks has been carried out in coniferous woodland.
Image (c) Chris D'Agorne
Why do Mother Trees matter?
OK, so what have we learned about mycorrhizae?
- Older trees have a more complex web of connections
- Connections can carry carbon to young seedlings, helping them survive
- Mycorrhizae are also used to share resources between older trees
- Trees may be able to identify relatives through the network
- The network can be used to help neighbours fend off an attack
The more connections, the better the trees’ ability to perform each of these tasks, so mother trees play a crucial role in the forest. However, it’s worth noting that one research paper found larger trees were less likely to pass carbon down to seedlings.
Is the Mother Tree theory accurate?
To be honest, it’s early days for this research – modelling networks is hard, and mapping underground tendrils of fungus is even harder.
If the Mother Tree hypothesis is entirely correct, then it has huge implications, but some aspects of it aren’t always supported by research (like how larger trees don’t always more carbon). The idea that hubs develop in complexity over time seems to be pretty reliable, though, and carbon is certainly exchanged over these networks.
Master Trees
Where do you find the UK’s most secretive butterflies?
The first thing you’ll hear when most people talk about Hairstreak butterflies is how hard it is to spot one. We have quite a few species in the UK – Purple, Brown, Green, Black, White Letter, Red Herring… OK maybe not that last one. The Hairstreak that gets most people excited is the Purple – you’ll hear descriptions of them flying around the top of Oak trees, their shimmering iridescent wings glittering ‘like a handful of coins’ suspended in the air. The problem with Hairstreaks is that most of them don’t come down to earth all that frequently.
Purple Hairstreak butterflies spend most of their time in the canopy.
Image (c) David Chew
Where do Hairstreaks and Emperors lay eggs?
High canopies are not just important for Hairstreaks – Purple Emperors are similar in that they spend much of their lives at the top of Oak trees. But, like Brown Hairstreaks (which tend to favour Ash), they’re a little bit less fussy about the choice of tree and will happily flit across other species.
Some Hairstreaks even stay up in the canopy to breed. White Letter Hairstreaks (so-named because of the streak on the outside of their wing) lay eggs on the branches of their chosen tree canopy – the Elm, while the Oak serves the same purpose for Purple Hairstreaks.
However, a lot of Hairstreaks and the Purple Emperor come down from the upper canopy to breed, with Emperors laying on Willow, while Black and Brown Hairstreaks lay on fresh Blackthorn foliage. Green Hairstreaks are renegades, tending to avoid canopies altogether.
Purple Emperors often visit the ground to find salt in the morning.
Image (c) Nick D'Agorne
How can rewilding help butterflies?
Purple Emperors are famously the species of butterfly which rebounded at Knepp- the UK’s most popular rewilding project, defying a nationwide slump. With Willow allowed to rebound and old growth Oak trees still remaining in place, this was a perfect habitat for them. Rewilding could also benefit Brown Hairstreaks, which suffer population declines when Blackthorn hedges are cut back hard every year.
What are Master Trees?
So what are ‘Master trees’? This is the term used to describe high canopy trees (which are generally old) or clumps of medium-sized trees which form a habitat for canopy-dwelling butterflies.
- Hairstreak and Purple Emperor butterflies usually feed on aphid secretions – ‘honeydew’ in tree canopies
- Their caterpillars may feed in the canopy, or the eggs are laid on trees and bushes lower down
- These butterflies often spend most of the day in the canopy
What do Master Trees and Mother Trees mean for land managers?
Rather than felling all the trees in an area, or all the old trees (both these methods are commonly practised in forestry), leave some old trees standing. These trees act as hubs in the mycorrhizal network, potentially speeding up the regeneration process by sending extra resources to seedlings. In the UK, certain trees are also valuable habitats for tree-dwelling butterflies – especially Ash, Elm and Oak.
Willow from 15-20 years of age is also thought necessary for Purple Emperors to breed, so consider leaving some fully grown trees if you are coppicing Willow near an old Oak tree.
I found these sources of information useful when researching this article:
Alaux, P. L., Naveau, F., Declerck, S., & Cranenbrouck, S. (2020). Common mycorrhizal network induced JA/ET genes expression in healthy potato plants connected to potato plants infected by Phytophthora infestans. Frontiers in plant science, 11, 602.
Baluška, F., & Mancuso, S. (2021). Individuality, self and sociality of vascular plants. Philosophical Transactions of the Royal Society B, 376(1821), 20190760.
Beiler, K. J., Durall, D. M., Simard, S. W., Maxwell, S. A., & Kretzer, A. M. (2010). Architecture of the wood‐wide web: Rhizopogon spp. genets link multiple Douglas‐fir cohorts. New Phytologist, 185(2), 543-553.
Born, N.A.D, Warren, M.S. (2000) Species Action Plan. Purple Emperor Apatura iris. Butterfly Conservation.
Butterfly Conservation Pamphlet – Life cycles of Purple Hairstreak and White-letter Hairstreak
Gilbert, L., & Johnson, D. (2017). Plant–plant communication through common mycorrhizal networks. In Advances in Botanical Research (Vol. 82, pp. 83-97). Academic Press.
Gorzelak, M. A., Asay, A. K., Pickles, B. J., & Simard, S. W. (2015). Inter-plant communication through mycorrhizal networks mediates complex adaptive behaviour in plant communities. AoB plants, 7.
Newland, D.E., (2006) Behaviour of Purple Hairstreak butterflies in the canopy of oak trees at Sheringham Park, Norfolk
Simard, S. W. (2018). Mycorrhizal networks facilitate tree communication, learning, and memory. In Memory and learning in plants (pp. 191-213). Springer, Cham.
Staley, J. T., Botham, M. S., Amy, S. R., Hulmes, S., & Pywell, R. F. (2018). Experimental evidence for optimal hedgerow cutting regimes for Brown hairstreak butterflies. Insect Conservation and Diversity, 11(2), 213-218.
Teste, F. P., Simard, S. W., Durall, D. M., Guy, R. D., Jones, M. D., & Schoonmaker, A. L. (2009). Access to mycorrhizal networks and roots of trees: importance for seedling survival and resource transfer. Ecology, 90(10), 2808-2822.
Van Dorp, C. H., Simard, S. W., & Durall, D. M. (2020). Resilience of Rhizopogon-Douglas-fir mycorrhizal networks 25 years after selective logging. Mycorrhiza, 30(4), 467-474.
Wild Oxfordshire Fact Sheet – Brown Hairstreak Thecla betula
Wipf, D., Krajinski, F., van Tuinen, D., Recorbet, G., & Courty, P. E. (2019). Trading on the arbuscular mycorrhiza market: from arbuscules to common mycorrhizal networks. New Phytologist, 223(3), 1127-1142.
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Chris D'Agorne MSc - Founder / How to Rewild
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