The answer to 'tree thin grass' is that thinning grass under a tree is a common problem caused by the tree's roots and shade. Trees can block sunlight and absorb moisture and nutrients, leaving the grass struggling to grow. Symptoms include bare or patchy areas, stunted grass growth, and a gradual thinning over time.
The best solutions depend on the tree and grass type, but options include pruning the tree to allow more light, aerating the soil, overseeding with shade-tolerant grass varieties, and applying fertilizer and water to help the grass thrive. In severe cases, you may need to consider removing the tree or replacing the grass with a groundcover better suited for the conditions.