River Birch Trimming April in Minnesota sapping

[mary b]

I think I made a mistake. I had my River Birch which is about 20 feet tall trimmed the second week of April before any budding. Only the lower branches were cut because they were reaching towards down to the ground and it was very hard to mow. I live in Minnesota and it is really sapping, do I need to do anything or watch for anything? I hope I didn't hurt the tree.