Are the clump of red berries found on red sumac poisonous?

i have heard that if they point up, they are ok, and if they point down, they are not. this is not backed by any scientific evidence whatsoever.

They are definitely poisonous for pets, but it seems like a human would have to consume a lot to feel any ill effects.

This is a longish answer where I deal with the question of poisonous fruits in general.

(Sorry, I did not realize carrage-return meant post). I think it important to say at the outset that common names for plants are inexact. What you call 'red sumac' whereever you are may be different from what someone somewhere else calls red sumac. It is not impossible (though in this case unlikely) that the answer will be different for the two different plants. I assume you are asking from the midwet/east and you mean Rhus glabra, which has cones of downy red berries. (So places like Ohio, Michigan, Ontario, etc.) If that is the case, then not poisonous (see, e.g. this from the Ohio Dept of Natural Resources (http://www.ohiodnr.com/news/aug01/0828edibles.htm) where they tell you how to make sumac lemonade. (more)

The general rule is not to eat wild things unless you know they are safe, but remember that fleshy berrires are designed to be eaten by some sort of animal or bird: the fleshy part does no good at all to the plant or the germinating seed. It serves purely as a reward to the animal which serves to disperse the seeds. Dispersal can happen in two ways: the seed can be eaten around or spat out (as we do with peach pits and watermelon seeds) or the seed can be ingested and pass through the gut. That second method is way more common, and is usually more benficial to the plant: the action of the gut softens the seed so it can germinate more easily, and the seed comes out cleaned of pests (by gut acids) and wrapped in a fertilizer package!

Now the plant has an interest (if I can anthropomophize a bit -- this is a shorthand for saying natural selection selects variants that fit the criteria) in (1) its fruits being eaten by animals that disperse it effectively, and (2) its seeds NOT being digested. So there is some evolutionary pressure to make the fleshy part tasty to effective dispersers and bad tasting to ineffective one. And there is a stronger evolutionary pressure to make the seeds poisonous. And there is pressure to make the flesshy part have a laxative effect (to speed the seed passing through the gut). I do not know of cases where the fleshy part is strongly poisonous (as opposed to bad tasting or likely to cause a tummy ache). I know lots of cases where the seeds, if digested, and very poinsonous. Apple and pear seeds, for example, contain a cyanide compound. Humans do not produce enzymes which free the cyanide, but many bacteria, including some that live in human guts, do possess that enzyme. If you have such a bacteria in your gut, as few as a dozen chewed up apple or pear seeds can kill you. Generally, it is a bad idea to chew of fruits and berries.

As to Sumac specifically, that is the common name typically given to some members of the Rhus genus. That geneus includes poison oak/ivy/sumac, and many non-poisonous plants. The poisonous plants has an irritating oil in their leaves and bark which cases severe dermatitis (rash). The poison varieties more commonly have white berries (poison sumac does, as does poison oak and I think poison ivy). The many other Rhus's have yellow or red (tytpically) berries which are tart and generally edible. (The poison Rhus's are now generally placed in the Toxicodendron genus, but that is for marketing rather than botanical reasons.) Native Americans used the non-poisonous Rhus's, as the link says, for refreshing drinks (not only on Ohio but in California, where I am much more familiar with the Rhus's.

Note finally the laxative thing: too many berries will often give you a tummy ache, and have the usual laxative results.

Slight correction: I said "Generally, it is a bad idea to chew of fruits and berries." I meant "Generally, it is a bad idea to chew of fruits and berry seeds."

WOW!! What a fantastic answer! Thanks so much! And, yes, these are lovely red sumac in northern Illinois. I just wanted to use the berries in dry flower arrangements. Thanks again!