It looks as though you have put extra soil and a garden around the base of the tree.  I cannot see the root flair.  May we have a better photo of the base of the tree and surrounding area.

Hopefully this picture will help you see what you need.

I still can't see the base of your tree.  All I can see are the plants around it.  Please clear away the plants and give me a close up of where the trunk meets the soil.

Here are several pics from different angles.  Let me know if you need something more.  Thanks,

Thank you for the new photos.  Your tree should have a flair at the base.  Instead yours is straight up and down like a telephone pole.  That is an indication that it was planted too deep.  Now, maybe it wasn't too deep initially but the extra soil piled up around the base for the flower bed has caused the same outcome.  When that happens, stem girdling roots can form. They go in a circle and as they get larger they cut off the flow of nutrients up and down the trunk.  That is reflected in the sparse foliage in the canopy and browning.You can pull the dirt away and see if you find these type of roots.  They can be cut away without damaging the tree. https://apps.extension.umn.edu/garden/diagnose/plant/deciduous/linden/branchesthin.html   Or, you might want to have a certified arborist evaluate the tree and make corrective actions.  Go to this site and click on Find An Arborist.  Put in your zip code when prompted and a list of certified arborists in your area will appear: https://www.treesaregood.org/

Thanks for the advice.  This spring (April) I pulled back soil at the base of the tree.  However, shortly after flowering in May, the leaves seem to be blighted.  I noticed it on other similar trees in the community.  Do you have any suggestions?

I am sorry but I can't open your zip photo.  Please send again but make sure it is open.

You did not mention the variety of tree you have but I am guessing it is a crab apple, ornamental or otherwise.  Also, as you are seeing it on other trees in your area, it is probably apple scab.  It is very wide spread this year due to the wet spring.  Nothing can be done now except for sanitation.  Clean up and destroy all leaves as they fall.  In the early spring, at green tip time, spray the appropriate fungicide.  https://extension.umn.edu/plant-diseases/apple-scab
Also, you did mention pulling back the soil and I can see the stem girdling roots.  Just removing the soil will not solve the problem.  They need to be removed.  Either do it yourself or have a certified arborist do so.  The problem will only get worse as they get larger.