However, here are some options you could use, depending on the context:
Formal/Descriptive:
* Grove - This is a common term for a small group of trees, especially when they are of a single species.
* Stand - This term is often used for a larger group of trees, especially when they are growing closely together.
* Arbor - This term is typically used for a group of trees that are planted for shade or decoration.
* Canopy - This focuses on the overall effect of the trees, emphasizing their interconnectedness.
Informal/Descriptive:
* Cluster
* Collection
* Assembly
Figurative/Metaphorical:
* Green umbrella - This emphasizes the protective quality of the shade.
* Sun-shield - This highlights the trees' function of blocking the sun.
Ultimately, the best collective noun for shade trees will depend on the specific context and the desired effect.