>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.