One drop after another of the Edreaf Essence was absorbed by Rezen. The essence was not only absorbed by his body but also by his soul and elixir field
The body and soul are connected, when increasing one's talent, it was better to target both so that one aspect will not drag down the other aspect
There are mages that are talented when using their physical bodies but are untalented when using their soul and vice versa
Since the essence of the Edreaf Tree does not specifically target and boost a single aspect and gave an overall increase in talent, both Rezen's body and soul are benefiting
His physique was elevated by another level, his elixir field expanded, and his soul was refined and became sturdier
He was receiving great benefits just by absorbing the Essence around him. At first, his absorption speed was slow but the more essence he absorbed, the faster the absorption rate becomes. That is proof that Rezen's talent is really increasing