Good Morning Anne, 

Thank you for contacting the U of M Extension Service. 

I am wondering if the tree was watered during the previous year's drought: https://extension.umn.edu/natural-resources-news/drought-minnesotas-woodlands

Trees are slow to react to drought conditions.  

Here is a link about pruning: https://extension.umn.edu/planting-and-growing-guides/pruning-trees-and-shrubs

You did not indicate if you inspected the trunk and the leaves and branches for signs of disease or pests. 

It is always good to have some wood chip mulch around the trunk of the tree, a few inches away from the trunk, about 3" deep. The wood mulch holds the moisture, protects the roots and keeps them cool.   Have you done any changes to the soil around the tree? The tree will have to compete with anything you've planted under it. 

Here is a link to finding a tree care professional: https://extension.umn.edu/planting-and-growing-guides/how-hire-tree-care-professional

Good Luck!