Writing letters is my favorite way to communicate, except that I already made the acquaintance of those beings. So, writing a letter is just a way for me to make a sort of telepathic communication more clear.
If you just write a letter to the nature spirits, then it's sort of like standing on a street corner and talking through a megaphone at strangers. Most of them will pass by. Maybe a few will recognize you from a past life or think that what you say might be interesting. So, maybe supplement letter-writing with meditations on natural environments to put you on the same wavelength as nature spirits, if you can't actually be in natural environments. I hope this helps!