I'm not sure what your question is.  Do you wish to save these trees? All of them? What kind of alder (there are native alders - speckled alder, green alder - and invasive alders - European alder)? What is the site like where they are planted? Are they a valuable tree to you? Do you wish to know what happened to them? Have they been struggling for several years or did this just happen?

This link might be useful:
https://hort.extension.wisc.edu/articles/shade-trees-disorder-decline-dieback-or-early-senescence/