Fig. 2. The richness of understory plant species declines with increasing density of overstory pine trees, on the Kaibab Plateau in northern Arizona (after Laughlin and Grace 2006). Typical historic conditions would have had about 35 species per 0.1-ha plot, but with changing land use and fire regimes, the most common species richness has declined to 25 to 30 species per 0.1-ha plot.