the mishanah at 34b says that one may not climb on a tree on shabbos because one may lop off a branch of a tree, one answer the gemara gives is "gezayrah shema yiktom" -- it may break off and Rashi, ibid, explains that is because the breaker can be ovayr the melachah of kotzayr.
The de'oraysah melachah of kotzayr is when one requires the branch he is cutting off. In our case, certainly the breaking off is inadvertent.
How can we reconcile this?
thanks
yerachmiel bratt
If the fear is that one might break off a branch inadvertently, then your question is correct -- the Gezeirah not to climb a tree would be a Gezeirah l'Gezeirah. However, the fear is that one might see a branch which he could use as a staff for walking or for prodding his animal, etc., and break it off advertently, as Rashi writes in Beitzah (36b, DH Shema Yachtoch Zemorah).
Be well!