Our property has a ton of speckled / tag alders growing everywhere. They are multistemmed shrubs that form big thickets. From what I read, these fix quite a bit of nitrogen. I'm thinking about planting some fruittrees in that area and am wondering if anyone has any ideas of strategies to use in order to take advantage of the nitrogen fixing ability of the alders. What do you think of removing some of the alders and planting fruit trees in their place, while keeping other nearby alders to provide nitrogen to the fruit trees? How far away do you think an alder should grow from a fruit tree? Should I prune the suckers of the alders to make them grow in more of a tree form (which I've read they can do but I have not tried)? Any ideas are appreciated, especially if you have done something similar with alders. Thanks.
Alders are great! I don't know speckled alder but I'll give you some examples I know of with alders generally. A citrus orchard where the guy planted oldhamii bamboo as a nice dense windbreak, but a lot failed and he ended up pulling out the bamboo on 3 sides and replanting with alders. All the citrus trees in the area of the alder rootzone are producing at almost three times the production of the orchard average! By contrast, the trees nearest the bamboo windbreak are the lowest producing part of the crop.
Another example almost as a contrast, a friend of mine with an apple orchard has alder windbreaks in his block. He is getting to the point of hating the alders because the apples closest to the windbreaks grow twice as fast as the rest and he spends a lot of time pruning, these trees produce less fruit but only because they are so hyper-vigorous.
Another example, I guy I know planted alnus acuminata with feijoas immediately beside them, I'm talking 1-2 feet away from the alder trunks. Production was phenomenal on the feijoas for years, but last time I spoke to him he was starting to cut out the alders as they were shading out the feijoas too much. But the alders were at least 30 feet high by this stage.
There is also anecdotal evidence that alders prevent a lot of phytophera root rots in fruit trees (although there is an alder-specific strain of phytophera which kills alders).
I've planted about 40,000 alders, mainly Alnus jorullensis and acuminata, but also significant numbers of A. cremastogyne, nitida, cordata and rubra. My interest is primarily shelter and N-fixation. My observation is that shading might be the biggest factor with them, they are dense enough to shade out grass completely. This suits me as it allows native rainforest trees to establish in the shade, but not ideal for food production. Options are heavy pruning to allow light in, or regular coppicing or pollarding (which is great for mulch production anyway). In my climate excessive light can be a problem in summer and a lot of plants do better in light shade than full sun. Alders need a lot of water, so if you are in a water deficit area in summer be careful about competition.
It's hard to make specific recommendations without knowing your climate and the light requirements of the crops you want to grow. But on the whole I believe alders are one of the most under-appreciated plants in the world and it would be a rare situation where they would have no value.