I can not stress this enough and I'm sure I have tons of back on this statement. DO NOT USE HARMFUL/LETHAL DEVICES TO SECURE YOUR PLOT!! You know how the cops look at that? "These marijuana growers use bear traps, sawed off shotguns with tripwires, etc to protect their crop" That looks really bad on the cannabis community. Don't do it. There's a thing called camouflage. Use the native brush to your ability. I am aware that cannabis doesn't blend in with 90% of forests. Sh*t happens.
I've heard of people use infrared systems that take pictures and/or start recording video when an object of particular size/temperature enters the vicinity. It won't help too much theft-wise but you can see if there's cops scoping your plot so you know just to lay low and forget about it, its gone. Some people use alarms connected to tripwires. These cause great amounts of attention but if you set them low enough to scare the person, they'll run. But please follow the Golden Outdoor Rule. Be creative and be safe.