The answer to your question is no. North America is a huge region. Even California is vast and varied, e.g., we have miner's lettuce at low elevations, but not higher up. For a given subregion, e.g., low altitudes in the Transverse Ranges of California, it's fairly easy to learn enough to identify a few trail snacks that might (or might not) be available. All these will typically do is allow you to have some fun and vary your diet of backpacking food with the occasional nibble of greens. But given this level of knowledge, you are very unlikely to find enough plant food in a day to satisfy your appetite. Sure, you might get lucky if, e.g., blackberries are in season or something, but that's not a high-probability thing that you can expect throughout North America.
In an emergency situation, exposure, dehydration, and falls are the big things that could cause you to die, unless you're lost in the wilderness for weeks. In nearly all cases that a search and rescue team responds to, the optimal thing for the victim to do is to stay put. Moving around makes it harder for S&R to find you, which ends up lengthening the amount of time until you're rescued. That means that roaming around and foraging for food is almost certainly a mistake.