Comment by thewayfarer | Hacker News Reader

Comment by thewayfarer | Hacker News Reader

Absolutely fascinating work! However, I'm not entirely sold on the usage of the term "mother tree." While Simard says

> That’s how we came up with the term “mother tree,” because they’re the biggest, oldest trees, and we know that they can nurture their own kin.

the interview doesn't reference any work that specifically states these mother trees will preferentially nurture kin over other species. For example, when she describes the more controlled greenhouse experiment with Douglas firs and Ponderosa pines, an injured mother tree Douglas fir dumped carbon into the network and the ponderosa pines still absorbed it. She may have other work that describes some kind of kin preference, but I don't see it cited and I don't quite understand what the mechanism to make this happen would be.

Rather, (as a layman without any knowledge of the work and published papers around this topic), I see this phenomenon not as the result of "mother trees" but from "farmer fungi." The fungi, because of their large networks and relationships with the trees, become "resource managers" of the forest. The fungi have an incentive to make sure that the trees are healthy and will continue to provide nutrients for the fungi. When younger trees are injured, that is a threat to the fungi's survival, and therefore one possibility is that they have evolved this mechanism that transports resources from older trees to the younger ones to help the younger trees survive. The relationship between these fungi and trees are normally symbiotic, but the older trees "tolerate" (or fail to evolve some immune response to) this mildly pathogenic behavior because it likely benefits its nearby offspring or close kin.

I think the concept of the "mother tree" might be slightly anthropomorphic, assuming that a large, multi-organ plant must be more intelligent and possibly be even more caring than small fungi that must only be able to perform simple functions. In reality, the fungi are the organisms in the best position to evolve this beneficial behavior.

Again, I'm a non-expert with zero knowledge on this topic. If anyone could provide a reference to a free online paper that describes these "mother trees" as preferentially nurturing kin, I could be persuaded. And that would be a very interesting read!

This was the specific quote from the article:

> In later experiments, we’ve been pursuing whether these older trees can recognize kin, whether the seedling that are regenerating around them are of the same kin, whether they’re offspring or not, and whether they can favor those seedlings — and we found that they can. That’s how we came up with the term “mother tree,” because they’re the biggest, oldest trees, and we know that they can nurture their own kin.

It seems that Kevin Beiler was the one who did research about this so that may be a good place to start rather than the author.

But whenever reading something like this quote below, where it's clearly a useful narrative for funding and public support, I find it's always good to be skeptical about how far that analogy extends:

> We’ve got a lot of interest from First Nations groups in British Columbia because this idea of mother trees and the nurturing of new generations very much fits with First Nations’ world view.

These networks also transport signalling and defense molecules, which seem to transmit kin selection information. They also (as you might expect) work better on plants of the same species. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4497361/

>The relatedness of neighbours in mono-specific plant communities can also influence whether MNs will elicit adaptive behavioural changes. For example, foliar nutrition in AMF Ambrosia artisifolia L. improved when it was integrated into an MN with related plants but not conspecific strangers (File et al. 2012). Likewise, in mono-specific pairs of EMF interior Douglas-fir grown in greenhouse conditions, foliar micronutrients were increased in kin compared with strangers grown with older conspecifics (Asay 2013). This appears to be linked with mycorrhizal association of this system as mycorrhizal colonization was also elevated in kin seedlings (File et al. 2012; Asay 2013). These findings reveal that MNs can play an integral role in kin selection, but the exact mechanisms by which they do this are unclear. However, there is strong evidence that biochemical signals derived from mycorrhizas or roots are involved. For example, Semchenko et al. (2014) showed that root exudates carried specific information about the genetic relatedness, population origin and species identity of neighbours, and locally applied exudates triggered different root behaviour responses of neighbours. This included increased root density, achieved through changes in morphology rather than biomass allocation, suggesting the plants limited the energetic cost of their behaviour.

>Because the overwhelming majority of plants are predominantly mycorrhizal in situ, any root exudates involved in kin recognition are likely to be filtered through mycorrhizal fungi. In a recent study using stable-isotope probing, we found that MNs transmitted more carbon from older ‘donor’ Douglas-fir seedlings to the roots of younger kin ‘receiver’ seedlings than to stranger ‘receiver’ seedlings, suggesting a fitness advantage to genetically related neighbours (Pickles et al. unpubl.). ...

An additional selection process could be as simple as spatial proximity. Daughter trees are more likely to germinate close to their mother, and the mycorrhizal network can be expected to conserve energy by transporting nutrients no farther than needed.

Besides that, there's other evidence of kin selection in plants, in the form of reduced root competition. http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.12121/p...

But there's a broader point here. You're reading the word "mothering" narrowly, only w/r/t/ the selection of individual organisms. But selection can also take place on the whole plant community. Eugene Odum, widely considered the father of ecology, points this out: https://www.youtube.com/watch?v=6P9V6h2z79w&t=3m15s Plants can't get up and walk away, and polyculture plant communities are healthier than monocultures. So I would argue that even in the hypothetical absence of kin selection, "mothering" could take the form of encouraging a diversity of species in a tree's immediate surroundings, making a favorable local environment for their offspring.

Brilliant and fascinating!

I thought the sentences following your second quotation from the first paper, hypothesizing on the mechanism for kin selection, to be quite interesting:

> This may have been facilitated by the greater mycorrhizal colonization of kin than stranger seedlings (Asay 2013), creating a stronger sink in the MN, an effect also noted in the study by File et al. (2012). The greater colonization of kin seedlings may have arisen from complimentary genetics of the fungal genet and tree genotype (e.g. Rosado et al. 1994a, b). [...]although the mechanism through which the MN elicits the behavior response remains to be resolved.

This genetic mechanism makes sense to me.

The first paper talks about many rich, complex relationships. Two topics I thought were very interesting:

> A fungus can express a mutualism with one plant, while simultaneously exploiting a different plant. Mycoheterotrophic plants are perhaps the most extreme example of this type of exploitation, where a plant acquires all of its carbon by parasitizing fungi through the MN (e.g. Leake 1994; Massicotte et al. 2012). These plants link into the MN of a nearby tree and siphon off photosynthate, enabling them to survive and grow. Importantly this reveals the existence of a mechanism by which plants can acquire nutritional levels of carbon from mycorrhizal fungi. The fitness of all participants in this scenario is increased by the existence of the MN: (i) the mycorrhizal fungus acquires carbon from the tree (or multiple trees) and may use the mycoheterotroph as the staging ground for long-distance exploration and colonization, (ii) the mycoheterotroph acquires carbon from the fungus and (iii) the tree gains access to a wider pool of soil resources, and potentially connection to other trees facilitating the detection of defence signals.

> There is evidence for both tit-for-tat and reciprocal altruism in MNs in forests, both which would be resistant to cheaters (i.e. individuals that benefit without reciprocating). Tit-for-tat, distinct from mutualisms, is evident in bidirectional transfer between paper birch and Douglas-fir (Simard et al. 1997a, b; Philip et al. 2010) and between unrelated Douglas-fir (Teste et al. 2010). This cooperative bidirectional exchange occurs over a period of a few days and appears to be related to the behaviour and possibly fitness of the individuals involved in the network. However, reciprocal altruism, or repeated prisoners dilemma, occurs over longer time periods, and this explanation is more congruent with the highly variable disturbances and hiatus in forests. There is some evidence for reciprocal altruism through the switches in the direction of net carbon transfer between paper birch and Douglas-fir (Philip 2006) or maple and trout lily seedlings (Lerat et al. 2002) in response to differential changes in plant phenology over a period of several months.

Your second paper on kin selection appears to be mainly theoretical on the topic of competition theory, not an observational study of plants or fungi. It makes weak conclusions like "It is reasonable to hypothesize that traits expressed only in the presence of strangers may indicate competition or selfishness, while traits expressed in the presence of kin may indicate cooperation or altruism (Murphy & Dudley 2009; File et al. 2012)" and "It is too soon to know if plant kin recognition responses will demonstrate the breadth that has been found for kin selection in animal behaviour."

> But selection can also take place on the whole plant community.

No doubt.

Crap, wrong link. This is the kin recognition root growth study: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2104794/

PBS's Nature did a whole documentary on plant behavior, which includes the author of that study: https://www.youtube.com/watch?v=CrrSAc-vjG4&t=38m00s

Reply on news.ycombinator.com