Short answers, pulled from the story.
Alders are trees of the genus Alnus in the birch family Betulaceae. The genus includes about 35 species of monoecious trees and shrubs distributed throughout the north temperate zone, with a few species reaching into Central America and the northern and southern Andes.
Alders differ from birches, the genus Betula in the same family, because alder female catkins are woody and do not disintegrate at maturity. Instead they open to release seeds in a manner similar to many conifer cones, while birch catkins fall apart.
Alder has a symbiotic relationship with Frankia alni, a nitrogen-fixing bacterium that lives in root nodules as large as a human fist. The bacterium absorbs nitrogen from the air and supplies it to the tree, while the tree provides sugars made through photosynthesis. Red alder stands can supply between 130 and 320 kilograms per hectare of nitrogen to the soil each year.
The largest alders are red alder, Alnus rubra, on the west coast of North America, and black alder, Alnus glutinosa, native to most of Europe, both reaching over 30 metres. The green alder, Alnus alnobetula, is rarely more than a 5 metre shrub.
Alder is stable when immersed and has been used for millennia as pilings for piers and wharves, including most of the foundation of Venice. Electric guitars, most notably those made by the Fender Musical Instruments Corporation, have used alder bodies since the 1950s, and the wood also serves furniture, cabinets, leather tanning, and smoking foods such as salmon and coffee.
Alder bark contains the anti-inflammatory salicin, which is metabolized into salicylic acid in the body. Red alder also contains betulin and lupeol, compounds shown in clinical studies to be effective against a variety of tumors, and some Native American cultures use red alder bark to treat skin irritations, lymphatic disorders, and tuberculosis.
The oldest fossil pollen identifiable as Alnus comes from northern Bohemia and dates to the late Paleocene, around 58 million years ago. Later fossil species span the Ypresian, Oligocene, and Miocene across regions including Oregon, the Okanagan Highlands, and western North America.