You don't have to actually be a certified outdoor expert to convince a kid that you are. My nephew so believes that I know EVERYTHING about the outdoors, that he's taken to saying, "Ask Dana, she'll know." for anything that is outside of a man-made structure. He'll pick up a random leaf, and fully expect that I'll know the name of the plant and use for it. How did I get him so convinced? By taking him on walks and pointing out the tiniest interesting thing I can think of.
"See that gravel? Some birds actually have to eat it! Really, it helps clear out their food somehow." I stored that info since 8th grade science when we dissected a bird. See how I didn't exactly explain why birds eat it? It was enough that I knew it helped them with their food somehow. "This is birch bark - it's good for starting campfires with. Some Indians actually made canoes out of it!" Here's another tip - sound impressed. Like emphasize how it's SOOOO crazy that someone can actually make a canoe out of bark. That heightens the excitement level. Use anything - see it, smell it, touch it, or use it to remind you of a good story. See a mouse? Remember to tell him of the owl you saw last year catching a mouse. See new corn? Tell her about the time you walked through standing dry corn and saw does bedded down.