Green wood is not dimensionally stable. As the humidity changes it can shrink and swell to a greater extent than seasoned wood. Wood that has been kiln dried will have been taken down to a moisture content of 5-7%. Wood that has dried out in the open or under a roof will reach around 12%. The lower the moisture content, the better-the wood will not soak up the humidity and swell as much. There will always be some amount of swelling and shrinking if the wood, and products made from wood, are exposed to changes in humidity that come with the weather and seasons.
A 2x4 is a good practice piece, but studs are usually produced from pine and spruce. These are softwoods (conifers). Hardwoods (deciduous
trees) such as oak, maple, and pecan offer much greater strength but you will find they take a bit more effort to work by hand. You'll appreciate that stone when you work with hardwoods!
Pallets are often made with hardwoods. I see oak and maple all the time. It's a source of material to practice with for little or no cost.
I know of a guy who makes benches from driftwood. He does not put in the effort of chiseling mortises and tenons. The sticks are cut and screwed together. Interesting to look at, but they look a little rickety. He gets a couple hundred bucks each as I recall. If one were to take the time to build it with superior workmanship, I suspect a premium price could be commanded.