Hi Sam,
I have spent some time wondering about this topic and the complexity of the Frankia species. Our question too was whether the ubiquitous Alders would supply a suitable source of bacteria for seaberries.
The simple answer is that they will not.
Alnus have symbiosis with cluster 1 strains of
Frankia, and the plant species you and I are interested in, use cluster 3 strains. Some symbiosis of alder is seen by cluster 3 but this seems to be an artificial laboratory event.
The next question is “does it matter?” Both cluster 1 and cluster 3 are widespread, or as the ecologist say “cosmopolitan” in their distribution. Unlike cluster 2, cluster 1 and 3 are widely distributed in soil and probably grow extensively in the rhizosphere of non actinozhizal plants as free living bacteria. Cluster 1 is found under birch forest for example. The ecologic preferences of the strains is complex, and understanding seems incomplete. Some Alaskan have ecologist suggest that the strains may vary greatly even for alder species within the state and vary geographically. I can’t find a citation for this, which seems a little against the prevailing viewpoint.
We have been using donor plants obtained from outside for the seaberries with an initial closely approximated growth under nursery conditions before out planting of starts. This helps survival but may have nothing to do with Frankia colonization. If you had space and energy, it would be interesting to see what happened with
Frankia capture in your region after direct seeding.
Good luck with your plantings and a great resource for all things
Frankia is in the link below.
http://web.uconn.edu/mcbstaff/benson/Frankia/FrankiaHome.htm