Question: Is it proper to decorate a shul with trees?
Answer: The Rema (Orach Chayim 494:2) rules that common custom is to have grass spread in shul on Shavuot to recall and symbolize the Divine Revelation of the Torah on Mount Sinai. The Mishnah Berurah (494:9, 10) cites the Magen Avraham who mentions a custom to place trees in shul to symbolize the fact that, according to tradition, fruit of trees are judged on Shavuot. The Mishnah Berurah, however, also cites the Vilna Gaon who opposed this custom, arguing that it mimicked the gentile practice of using trees as decorations on Christmas.
What about grass? Was the Vilna Gaon opposed to that as well? Rav Shlomo Zalman Auerbach (Responsa V’aleihu Lo Yibol, p.184 siman 289) argued that the Vilna Goan only opposed decorating shul with trees. This would make the Vilna Gaon consistent with the Rema who only mentions the custom to decorate shul with grass, not with trees.
The Vilna Gaon’s position may be based on the biblical prohibition against placing trees in the Beit Hamikdash. Since a shul is considered a mikdash me’at, the Vilna Gaon extended the prohibition to shuls as well.