Interesting! Now that you mention it, I see Pleco does list 小核桃 as "hickory; hickory nut; pecan" and calls it a variant of 山核桃. Useful to know it's probably not an exact synonym actually. (I have to wonder how often Chinese-English dictionaries call one word a "variant" of another without mentioning that there are actually differences in meaning... or might be, depending on region etc...)