Sorry to hear about the death of your tree Oldtimer. It's a significant event for sure. After you get to know a tree like a friend it's tough to lose it.
I could go on and on about the benefits of tree climbing
As far as decompression goes here are some aspects of climbing that contribute to this:
1. Focus and emotional relaxation
A friend asked me if I meditate when I'm in a tree. I don't intentionally but the focus required in climbing creates a related conciousness experience. Each deliberate step in the tree requires relaxed and focused mental attention. Doing this for 2-3 hours or more moves the climber into an invigorated yet calm mental space. I guess this is an important part of what is often called "tree time".
2. Fear/release of fear or processing fear
I don't have as much fear response as I did in my early climbs but fear is always there in some form. Working through this fear in the tree has a tremendous positive effect. I have no idea what's going on on a neuro/emotional level but I think it has something to do with the rewards of working through an internal obstacle. There is a release effect which is very powerful. This is not the same as dealing with panic. Panic is out of control and is not useful in a tree
3. Reconnecting with nature
Many of us suffer from thinking that we are separate from nature. Being in a tree for more than ten minutes starts to break this idea down. Being in a tree for a couple of hours helps you connect with your true self.
4. Healthy physical exertion
One of the things I love about tree climbing is that you can pretty much climb no matter how strong or not so strong you're feeling. Go slow or push it 'til your muscles beg for mercy. Either way you're creating benefit for all of your major physical systems. It plays a big part in the decompression effect.
-moss