I'm a little scared to say that I agree with Xeberus to some degree. The fact is, is that if you're feeling serious pain, then you're doing more damage than your body is going to be able to heal. If pain automatically equals gain, then every martial artist would be breaking their limbs as often as they can so that they heal back oh so much stronger. But it doesn't work like that. But along those lines, I also agree with Squirrelfighter - there needs to be a certain degree of actual pain. I think, honestly, it's a case of semantics. Maybe what one person considers "discomfort" to someone else is "mild pain", or vice versa.
Although it's kind of a nitpick argument, the fact of the matter is that minor damage does equal growth. That's the way bones are strengthened, or as you technical folk say, "trebuculi is developed"(?). If that particular bone isn't damaged, your body doesn't recognize the fact that it needs to be reinforced. I actually read recently (you're going to laugh at this) in a 'zombie apocalypse' discussion that one of the reasons a zombie apoc' wouldn't last, is because zombies don't feel pain, and because they don't feel pain their body will never recognize damage and therefore will never regenerate. Anyway, my point is, is that if you don't take some kind of damage (even just mild damage) your body doesn't realize it and reinforce it.
To the original poster however, I suggest NOT kicking a tree, even with guards on. To one of the above posters, sorry, no - your bones generally aren't hardned by the fact that something is immobile, they're moreso hardened by the fact that something is hard. Look, I come from a "traditional" background, and I at least think that we're considered to have some of the most crazy conditioning ideas. But I still recognize that hitting a part of my body on something that is hard AND HAS NO GIVE is loony and dangerous. Shaolin monks, to harden their fists and palms, hit their hands on bags of various material, building up from mung beans to steel shot or rocks. BUT, Shaolin monks (and those of us who follow some of their training methods) recognize the fact that hitting something hard and immobile can be unforgiving, and that's why we still hit a bag of whatever material we hit, that way the material has room to move and shift. I have a wooden post in my backyard that I pound my forearms on to toughen them, but I have this wooden post placed in such a way so that it gives at least a little bit because I don't want to break my gosh darn arm.
Think of shin hardening like a plank of wood that can regenerate. If you smash that plank of wood against a brick wall, it's going to splinter and break in half all at once, which will take far longer for it to regenerate. But imagine, that if you did small damage over time to that plank of wood, and over time, that plank of wood healed that damage.
Anyway, really, op, I hope you just kick a bag often. That's all you really need. A boxer need not condition his hands, because just the punching of a bag will over time toughen his hands and knuckles. Or relate to a real world physical labor job; someone who is forced to continually switch from a crouching or kneeling position to a standing position need not work to strengthen his legs, because after having done the job enough, his legs will be strengthened. I use to work this janitor job where I was continually carrying these 60-80lb trash cans out to the dumpster one by one, but not once did I feel the need to lift weights to strengthen myself for that job, because just the act of doing the job, and doing the work, eventually took care of the problem.
Ehhh. Hope I made some kind of sense.