these are each sequoia seeds >> reporter: dean shenk has been a ranger in yosemite for 40 years. he educates the public about the park's iconic giants. since the rim fire began, shenk has answered a lot of questions about how the sequoias cope with fire. like rain, you want a little bit of rain but you don't want too much. this is too much fire. lawye >> reporter: shenk says the mighty trees have their own defense mechanisms against fire including a natural flame retardant in their thick bark, which do a pretty good job of defending them from most fires. >> it's rare to find a giant sequoia without a fire scar. the bark at the base of a giant sequoia can be four feet thick but as it goes up 40 feet it can thin to two. >> reporter: at a nearby stand of young sequoias, shenk explained how a prescribed burn- - a low-intensity fire set and controlled by park staff which have been standard practice in the park for 40 years-- actually helps the trees reproduce. >> we're looking at a grove of sequoias about 14 years old and all of them owe their existence to a prescribed burn we had her