One of the results of the natural process that may be most difficult to duplicate at the present stage of plant succession is fire's role in making openings in the crown canopy. When periodic light fires burn through the forest, young white fir are killed while they are still part of the understory level of vegetation and fuels. Fairly mild burning conditions can still accomplish this and leave a mosaic of openings in the crown. These openings in turn allow sunlight to reach the forest floor and permit growth of sequoia seedlings, shrubs, and herbaceous plants that require substantial sunlight. Since fires have been suppressed, however, certain fir trees - perhaps not yet many - have continued to grow in height and size in a way that (1) they have become part of the crown canopy, or at least the lower levels of this canopy, and hence fire moving into this canopy can more readily threaten crowns of other trees, including giant sequoia; and (2) they are much less likely to be killed by moderate fires because the trees are much larger and have much thicker bark.