I think the browning was caused by too much sun and water stress. Now it has better established roots and more shade and it’s more resilient. Maybe the forking did some damage. You can top dress with compost, no need to fork it in, and definitely mulch the tree (but not right up against the trunk).

I planted a dogwood 7 years ago in my small front row-house patio, and although it flowers nicely, every summer its leaves would start to brown at the margins very early -- as early as June - and would be more than 50% browned by August. Identical trees on the same block seemed to do just fine. It didn't appear to be any sort of fungus or disease -- just browning. I was getting to the point of considering removing it. Lo and behold, it did perfectly this summer! Leaves still almost entirely green. And I did NOTHING at all - did not even water once or mulch or anything. Theories:

1) The soil is very poorly drained and even watering just a few times like I did last summer drowned the roots and led to the browning?
2) In previous summers, I worked the soil to turn in some compost and get rid of weeds in the spring. I tried to avoid the dogwood root radius, but maybe I was actually harming the roots?
3) Gas leak. Maybe there was a gas leak turning the leaves brown? There is a tree in the tree box right in front of our house that also turned brown way early in the season, but did well this year. Maybe a gas leak that got fixed was harming both trees?
4) Additional shade. Maybe the additional shade from the quickly growing tree in the tree box made the dogwood happier?