The deciduousness of most tree species is a significant difference from the tropical rain forest. Many evergreen tree species of the rain forest become deciduous in this zone. Growing conditions are not so optimal, thus the tree canopy is lower (10-30m) than in the tropical rain forest and the trees less dense where drought is more extreme. The undergrowth is often dense and tangled because of greater light penetration. Lianas are much less common than in the rain forest, not such an important growth form where light is less limiting and also perhaps highly susceptible to desiccation. Drought-resistant epiphytes (orchids, bromeliads and cacti) may be abundant. The trees have thicker, more ridged, bark; deeper roots without buttresses; much more variable leaves, including many compound-leaved legumes; and more species with thorns.