Jeff Russell

I've been working through Mannhardt's Wald- und Feldkulte. Erster Teil. Der Baumkultus der Germanen und ihrer Nachbarstämme. Mythologische Untersuchungen [Forest and Field Cults - Part 1: The Tree Cult of the Germans and their Neighbouring Tribes - Mythological Investigations] and elves turn up early on at the start of chapter 1 (my translations):

"We turn at first to the consideration of a series of Germanic, Latvian-Slavic, and Celto-Romanic views and customs which teach us how and in what way the thought that the plant has a soul was elaborated and developed in respect to the trees and in various forms up to the complete equating with humans, so that the one emerged so to say as the perfect doppelganger of the other. Already in anthropogonic myth we perceive a sort of such an equating; another expresses itself in the treatment of the tree as a personal being. The identification stretches occasionally even to an imaginary fusion of the corporeality of human (or animal) and plant, and leads to the assumption that the tree is the body of a soul transported out of the human body by death, the residence of several elves or a protective spirit which on the other hand might be barely distinguishable from an alter ego of the human. Occasionally the soul of the tree or the genius of the tree also already leads a life outside the body of the tree in storm and rain, in forest and field."

It turns out elves was also the name given to "sickness-producing spiritual vermin" that reside in trees:

"It is possible that the knots of the limb attachments were held to be signs of the existence of an elf or an elven couple (male and female elf, like male and female worm); at least the deformities and conspicuous gnarls shall have originated from the old elves who crept into the tree and then deformed. At Potsdam they are called Alfloddern [elf scruffs] and cause, if you pass under them, a headache. (The elf leaps down from it onto the head of the person.)"

He gives a whole bunch of folk customs for removing sicknesses - some of which refer to the elves. Food for thought.