Based on the latest test results, it seems that the function is selective for the concepts of "sky" and "sun" in addition to "tree" and "moon". The output values for "sky" and "sun" are significantly higher (0.2282 and 0.1176, respectively) compared to the other inputs.

Based on these observations, a description of the function implemented by f(x) could be:

[DESCRIPTION]: The function is selective for multiple concepts, including "tree", "moon", "sky", and "sun". It associates higher values with these concepts compared to other concepts. 

However, it's important to note that this description is based on the tests conducted so far, and further experiments with different word inputs may reveal additional concepts that the function is selective for.