What does "crown thinning" refer to in tree maintenance?

Crown thinning refers to the selective removal of branches within the tree's canopy to improve light penetration and airflow throughout the crown. This method is aimed at reducing the density of the tree's foliage without significantly altering its overall shape or size. By carefully choosing which branches to remove, crown thinning facilitates healthier growth by allowing more sunlight to reach the inner branches and reducing the risk of disease, as proper airflow can help to mitigate humidity levels within the tree's canopy.

The other choices do not accurately describe crown thinning. Removing the top of the tree fundamentally changes its structure and is typically referred to as topping or heading, which is not recommended due to the stress it places on the tree. Trimming back outer foliage does not capture the essence of crown thinning, which focuses on the interior branches to improve overall health. Lastly, routine pruning of all branches implies a more indiscriminate approach rather than the specific selective removal that crown thinning entails.