Can I place a Belly Pad on a tree and practice some body kicks, or it is not recommended?
For example, the following:
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Yes, you can attach a belly pad to a tree and kick it, but I wouldn't. Bear in mind that, unless it's a very thin tree, it is likely to offer more resistance than you would get hitting a human being (or suitable substitute). This will increase the chance of injuring your leg.
Rather than attaching a pad to the trunk of a tree, I would suggest hanging a punch bag from a branch instead. The advantage of this is that you can practice moving around the bag as well as strikes other than kicks. You also reduce the risk of hitting the trunk instead of the pad.
I agree, there's more of a chance of injury, since there's no 'give'. However, when you've been doing martial arts for awhile and your body is used to it you may move to toughening up by hitting the tree bare. It may sound like a contradiction, but it's not once you learn your body control that comes to many through years of practice. You'd have to work yourself up to that or you may pay for it long-term. I recommend using a bag or dummy to practice on. Plus, seeing them move, somewhat tells you how much force you put out in your moves;)