Just as a reminder to recognize those trees:
elm always has doubly serrated (each tooth is in turn toothed with one or more smaller teeth) singular leaves, fish-bone branching leaf veins, and an asymmetric leaf basis. These traits show very well in the pics here. Moreover, the branching pattern is also fish-bone like, more or less all twigs of the same branch in a single plane (helps you to ID them in winter).
http://www.carolinanature.com/trees/ulpa100106.jpghttp://www.carolinanature.com/trees/ulpa100102.jpgPecan has pinnate leaves, whereas the leaves shown in the second pic are singular. When you crush the leaves, they have a typical walnut-like smell (same for hickory and walnuts; for those who don't know the smell of fresh walnut leaves: reminiscent of a dry sherry).
Plum (which this indeed is) has singular, finely serrated leaves, and has at the stalk-leaf transition little round paired glands (barely visible in the enlarged photobucket pic).
When you crush these leaves, or rip a bit of the bark-cambium, you get an acidic almond-like smell, caused by prunasin or a similar compound like amygdalin (toxic but volatile cyanogenic glycosides).
Joachim