I think the word "assessing" refers to "human studies". They searched all the human studies (of clinical trials) which assess the effect of Crataegus.

Both. First people did studies "assessing" meaning "measuring and analyzing". Then we searched for these studies so we could review them. So the assessing was done by other scientists, but we searched for the studies.

Good question. Both are possible from the way in which the sentence is structured. From the context, I suspect your second option is correct. It would have been clearer if the writer had written: "...in English, which assess (or assessed) the effect of Crataegus"