And sure enough, there she was, a cat in a tree:

As might be expected, Kitty was none too happy about her predicament, and she was being pretty vocal. Unfortunately, kitty was pretty far up in the tree, maybe 12-15 feet? But, the tree was quite close to the fence, which is pretty tall but has a rail running across the top:

I could just reach up with 1 arm to the kitty. Kitty's kind of freaked out, but glad to see me. I ended up petting the cat for a long time, trying to get her to calm down, to earn her trust. I'm not sure how I can get her down to the ground -- it will be hard enough just to get myself down from atop this fence, let alone get down with a freaked out cat. But I'm thinking that if I get the cat, some passer-by with some courage will take the cat from me.
So I try to get the cat with the 1 arm, but, alas, Kitty is having none of it. I tried to grab her under the belly and lift, but she dug her claws into the tree like her life depended on it. Passers-by were stopping occasionally to look at the spectacle, and to give me advice in spanish that for the most part I couldn't understand.
I stood on the fence about 8 or 10 minutes, a long time to be perched up 7 or 8 feet off the ground on a teeny railing. I could have used force and just ripped the cat from the tree, but lord knows what the cat would have done to me, and for sure having a snarling scratching cat in my hand would dramatically increase my chances of falling off of my precarious perch.
So, heroism was not to be. Kitty, I tried, but you're on your own. Before I lowered myself down, which wasn't so easy, I took one picture of the kitty from up on the fence. Here she is, looking at me, thinking, oh hell, where are you going?

Later in the day I did worry about the cat. And today, I went back to the park to see if she was still there, or if I could find her. And, good news, I did find her, out of the tree, hanging around on the lawn. She did look a little scared, but I hung around and pet her for a long time. It was sweet.