​A gravel bed tree nursery is a method used by growers to promote a higher success rate of tree survival as well as faster growth once the tree gets planted in the ground. The bed is filled with 3/8″ gravel with 10% sand mixed in to allow for ample drainage and a lot of space for the roots to spread out. The main purpose of the gravel bed is to not promote growth in what we can see above ground, but to promote an explosion of growth below ground. When using gravel as a growing substrate it allows for tiny fibrous roots to grow in the root mass. These roots play a huge role in the wellbeing of the tree in years to come. 
The fibrous root system of trees are classified as an abundance of horizontal growing lateral roots that help stabilize the tree. Picture a mat of small roots widely spread and running horizontally close to the soil surface. Since there are many fibrous roots out in all directions they cling to soil particles and help control soil erosion. ​
Since gravel is in relatively large pieces when the time comes to transplant the trees into their permanent home in the ground the gravel falls away from the roots leaving the small, but important, fibrous roots intact.  
One of the big advantages of a gravel bed is when you go to purchase the trees you buy bare root trees instead of trees planted in containers. This saves on a lot of money as well because bare root trees are a less expensive option than container trees are and you can get larger, more developed trees for less money.