What is the name of the movie from the 1940's where a little girl tries to save an ancient tree in which she believes fairies live. At the end of the movie a very old faerie is seen in the tree telling the fairy children that humans are not a myth, but for real. It was on t.v. several years ago, but I only caught the last half hour. I think she chained herself to the tree to keep it from being cut down. I thought it was Margaret O'brien, but it doesn't show up in her film biography.

Three Wise Fools (1946) starring Margaret O'Brien
IMDb synopsis:It has been 40 years since Richard, James and Theodore insulted The O'Monahan and he put a vexing blessing on them. All three have obtained their dreams of grandeur, but they all live in a single house as bachelors. Into this grand house comes 7-year-old Sheila with a request that they become her guardians. They do not want her and let her leave until they find that they need the property that Sheila has inherited. They then take her and O'Davern into the house with the expressed intention of relieving Sheila of her property so that they can donate it to the University in their name and buy remembrance in the future. However, Sheila cannot sell the property because of the Little People.

Three Wise Fools (1946) - IMDb

You can read a more-complete synopsis, which begins with the scene you mention, at TCM's website:
http://www.tcm.com/tcmdb/title/2739/Three-Wise-Fools/