When you post more than one ort two images, it would be nice if you could number them as this makes it a lot easier for a reviewer to refer to them, rather than having to "flip back and forth" to ensure that they still have the correct image for discussion.
1st one) I am not sure what the subject is here, but this image does nothing for me. Maybe more emphasis on what looks like a very interesting sky and less on the "rock", or the rock from a different angle - don't know.
2nd one) This one has some potential, but it is quite dark (sunset/rise?). My first reaction was that it is about 1/2 to 1 stop underexposed. Your meter is possibly compensating for the amount of sky in the image - see later discussion.
3rd one) I find this one a little "cluttered". If the tree is supposed to be the subject (my assumption), then cluttering up the background with the rocks doesn't do anything for me and is in fact, very distracting.
4th one) Maybe the best of the bunch. It is still quite dark (underexposed?)and the colours are very muted - maybe that is what you were going for? I would have also tried to bring out more detail in the sky in post-processing as there seems to be some "structure" there that is currently "hidden". also, in this image, there is a smaller tree on the right side of the main tree that starts just about where the "foliage" starts. I am assuming it is a tree in the background, but the fact that it sticks onto the main tree is distracting. I would have moved a bit to the left or right to ensure that the entire trunk of the main tree was free of any "additions"
When you are photographing subjects against a sky, regardless of its condition, your camera's meter may be fooled intro thinking that the scene is brighter than it really is, so it stops down your aperture and you end up with a subject (the Joshua trees) that is underexposed and thus darker than it should be. If you compare the 3rd and last one, you can see this situation. In the third, your meter sets up a proper (more or less) exposure because it is reading off the sunlit rocks behind the tree and there is not as much sky in it as there is in the 3rd one (what exposure mode are you using?). Thus the Joshua tree is better exposed in the 3rd one than in the 4th one and so the "green" of the tree shows up much better. A lot of these "differences in exposure can often be adjusted in post-processing (PP), but not always. You may want to investigate things like "exposure compensation" for situations like these...and don't be afraid to blow out the sky if that is what it takes to get a good picture of your main subject. Alternatively, you can use spot metering and take a reading off the trees and the ground, rather than let your meter "read everything" and expose some parts properly and others not so properly. Regardless, you will find that in mixed conditions such as these, you may have to experiment a bit to get a decent exposure.
Do you shoot raw and do you shoot to the right? Just asking - there are advantages and disadvantages to both and not everyone agrees with the "approach" of shooting to the right or ETTR as it is sometimes referred to (Expose To The Right).
I like your images. Of course, I'm a little nutty about JTNP and have been thee zillions of times. I can see a piece of it from my street. I have to say the last one doesn't look real to me. Too much post-production? Number two is my favorite.