The savanna portion of the cerrado is heterogeneous in terms of canopy cover. Goodland (1971) divided the cerrado into four categories ranging from least to most canopy cover; campo sujo (herbaceous layer with occasional small trees about 3 m tall), campo cerrado (slightly higher density of trees about 4 m tall on average), cerrado sensu stricto (orchard-like vegetation with trees about 6 m high) and cerradao (canopy cover near 50% with general height 9 m). Around 800 species of trees are probably found in the cerrado. The most diverse families in the cerrado in terms of tree species include the Leguminosae (153), Malpighiaceae (46), Myrtaceae (43), Melastomataceae (32) and Rubiaceae (30). Much of the cerrado is dominated by the Vochysiaceae (23 species in the cerrado) due to the abundance of three species in the genus Qualea. The herbaceous layer usually reaches about 60 cm in height and is composed mainly of the Poaceae, Cyperaceae, Leguminosae, Compositae, Myrtaceae and Rubiaceae families. Much of the vegetation in the gallery forests is similar to nearby rainforest, however there are some endemic species found only in the cerrado gallery forests. Soil fertility, fire regime and hydrology are thought to be most influential in determining cerrado vegetation. Cerrado soils are always well drained and most are oxisols with low pH and low calcium and magnesium. The amount of potassium, nitrogen and phosphorus has been found to be positively correlated with tree trunk basal area in cerrado habitats. Fire is an important process in maintaining and shaping the cerrado landscape much like other grasslands and savannas. Many plants in the cerrado are fire adapted exhibiting characters like thick corky bark to withstand the heat.