I am attempting to make a tree but when I try to join the different components together ctrlJ my decimated icospheres revert back to its original shape. Any idea on how I can solve this? I also forgot to mention that I am trying to create a low poly tree and that I am still in the learning stages.
The first image is the tree before joining and the second is the tree after joining
It looks like what is happening is this. On the two green objects of your tree, you added a Decimate modifier, but before joining the objects you did not apply the modifier. On the "trunk" of the tree you do not have a Decimate, so when joining the objects the modifier is removed.
To apply the modifier, in object mode, just click on the apply button on the modifier.