They are ordinary (epiphytic) ferns, but not likely sword ferns.

Try researching epiphyte and polypody and ferns. I do see one reference where existing ferns of this type are attached to an appropriate tree or piece of wood with fishing line. Would probably be more successful to use a branch that already has moss or lichens on it, giving the fern roots something to grab besides smooth bark.

Since I live in a clearing in the woods I find them growing on fir cones near the edges of our backyard. As long as I move the wood they're attached & put them in a moist, shady spot they thrive. If you remove the wood they don't live.

Most likely licorice fern, Polypodium glycyrrhiza. A very common plant in moist, NW woodland environments - in my neck of the woods they grow on trees everywhere. These are often sold by area nurseries and garden centers, especially those that focus on native plants.

They can be used as a ground cover...they form good sized colonies. But if you want to grow them in a tree, you need a good mossy tree or decomposing trunk to do so.

Here is a link that might be useful: licorice fern

The licorice ferns normally grow on Acer macrophyllum, aka Oregon Big Leaf maple. Not so common on anything else.
Mike

Bigleaf maple gives off nutrients that make it a good host for ferns, mosses etc.

There are several species of native polypodies, with there having been a reinterpretation in recent years. I would want to use an up-to-date manual or other botanical reference before naming a particular example to the species level.

Almost definitely licorice ferns. I remember breaking and tasting the roots as a kid. It's strange how much they taste like aniseed.

I purchased a rotting stump with licorice ferns and moss growing on it at the Hardy Plant Society sale a couple years ago. :)

Just noticed one of these growing on my Eastern Black Walnut (Juglans nigra). Interesting.

Licorice fern is most common in Big-leaf maple, but I have seen it in a few other trees. I'm not sure if its the same, but research done in the tropics to figure out why some trees have huge loads of epiphytes while others are basically bare hints that more than likely, the ph of the bark has a lot to do with it. This makes sense for mosses especially, which are generally sensitive to that.

I have had success getting pieces of licorice root rhizomes to "root" onto pieces of fir lumber using fishing line, and also by stapling them in place, using a staplegun designed with a raised notch where the wire is supposed to fit. This can be a little more tricky if the rhizome is thicker than the raised part of the staple, or if you miss. Those little rhizomes are brittle.

Anyway, to do this you need to find the licorice fern and the moss it is growing in. If there is enough moss, put some of that down on to the wood first, then lay the fern rhizome on top. Wrap with wire or carefully staple, making sure that the rhizome is firmly in contact with the substrate. I've thought about trying a glue gun, but haven't actually tried it. So long as it doesn't actually burn the rhizome too much, that might work well.

Remember that licorice fern has a reverse dormancy- it is green and active in winter, but usually dried out and goes dormant in summer. This is especially true when it is trying to reestablish itself as when you try and attach it to the wood, so expect that it will go dormant in early summer, late June or so, when things start to dry out. This is natural, and necessary for this plant. Give it time and patience.

Alternately, in another month or two, in March I think? the leaves will start to produce their spores. If you have a tree with suitable moss already growing on it, you can take a walk in the woods and bring home a frond or two with ripe spores (shake it over a piece of paper to see if you get a dusting of spores, that means it's ripe). Rub these into the moss wherever you want the ferns, and like with seeds, say a little prayer to get them growing. My guess is like many ferns, they need a spot where water puddles just a little bit, like a crotch or a leaf node with a little recession filled with moss, even if its basically just a few drops of water. The gametophytes can swim around in that water to find each other and accomplish the "birth" of the fernlet.

I've been trying for years to get these growing on my apple tree, but the darn squirrels keep knocking them off. In the ground is slightly easier, just make sure you have an organic soil, and good to excellent drainage.