When it comes to trees:
Trees need water. Not too much, not too little, but just right.
Trees need appropriate temperature. Not too hot, not too cold, but just right.
Trees need to prune. Not too much, not too little, but just right.
If you are looking for some cozy winter reading, you can check out our publication about Tree and Shrub Problems in Kansas.
In addition, I just came across a great publication from University of Kentucky that discusses tree and shrub decline. The information is similar to parts of the Tree and Shrub Problems in Kansas book, but sometimes it is helpful to read information from a new source since everyone presents information in a slightly different way. The reference is Stress and Decline in Woody Plants.