Great question, but I don't know the answer. There are a couple people I can ask, more soon.
I spoke with three colleagues in the Department of Natural Resources (Fahey, Yavitt, Goebel). All were of the same opinion that there was no need to treat the seeds in any special manner before planting because in the moist soils where alder usually grows, the tree roots will find the bacteria they need. The nodulation isn't essential for tree growth, so even if they don't immediately find the bacteria the tree can survive.
If you wanted to plant the alder on drier sites (which may not be a good idea), you could start the seeds in soil from a moist area likely to have the bacteria, and then transplant the seedlings.
I hope this helps. Let me know if you need more.
thanks Peter. So it sounds like I can start them and
1) they would thrive but not fix nitrogen
2) the bacteria Alders host is so widespread it is likely to be in any wet-ish habitat even if recently disturbed
3) I could find some Alders and later introduce nitrogen fixing bacteria with a arbored hole to the root area.
4) All of the above.
Apparently the bacteria are common in soils that might normally hold alder. Also, there doesn't seem to be an effective way to inoculate the seed, though I believe that happens for other plant species.
My take-away was there wasn't a viable way to introduce the bacteria, that the bacteria probably existed in the soils if the soils were suitable for alder, and that the absence of bacteria (if that was the case) wasn't significant.
Thanks for all the good questions.
Thanks for the reminder about transferring some soil from a place where mature plants are thriving & using it to inoculate the new planting.
Finding this discussion, I researched the native range of Alders and found the only one native to the East Coast, Alnus maritima, does not range north of Delaware. I am curious if the composition of the wood (chemically) would be similar to the other birches, as they are in the Betulae family. Asking because alder wood chips are a good substrate for some mushroom species.
Even though their natural range might not include Upstate NY it seems possible that they could be
cultivated there in the age of climate change.
I don't know about the chemical properties of alder wood, but I did find a couple that are native to the Northeast. Speckled alder (Alnus incana) and hazel alder (Alnus serrulata) are both native to portions of New York State and New England. Speckled alder is actually a pretty frequent wetland shrub, here in northern New Hampshire, where it forms thickets that are an outdoorsman's nightmare.
What I found intriging about the Alders is that they are nitrogen fixing and tolerable of a high water table. Could be good for erosion control.
While perhaps not nitrogen-fixing, I find Carpinus caroliniana, variously called muscle wood, blue beech, and Ironwood around here, not to be confused with hophornbeam, Ostrya virginiana, a beautiful small tree naturally occurring and my wet areas happily forming colonies along stream Banks which, I assume, also do the job of holding soils well. Although perhaps little loved by Foresters, it was found a very useful tree in earlier times for it's extremely dense hardwood and is also very ornamental. Personally, I welcome it wherever it chooses to grow.
I looked this tree up and yes it does seem like a potential candidate for the riparian location in mind. Someone once mentioned to me that they had "Ironwood" and it was a "real pest". Their Ironwood does not seem to be this tree.
Thanks Eli for the great information. Two new species for me to investigate. ☺