Few forest trees produce new leaves continuously throughout the year. Young growing leaves are especially vulnerable. Plants can protect them to some degree,however, either by producing leaves in the dry season when insects are few in number, or by producing synchronized flushes of leaves that meet the needs of the herbivore population. They may even do both, by producing a flush of leaves at the end of the dry season. As the leaves mature they become tougher and more resistant to insect attack.
Some plants shield their leaves with a thick, waxy cuticle.
Others have evolved forms of physical defence, such as thorns or spines, while the trunks of many forest trees, including palms and the ginseng family, have a thick armour of sharp spines to prevent larger animals from reaching their leaves, flowers and fruits. These chemical defences take a huge variety of forms, and are remarkably effective. The leaves of some plants, for instance, have stinging hairs which cause an immediate and severe burning sensation on the skin. Others, especially of the sumac family, contain chemicals that can cause an allergic reaction in the intestines of a herbivore which is strong enough to prevent it from trying the plant again. Yet other leaves contain substances which, while harmless themselves, are phototoxic , which means that they react with ultraviolet light to form poisonous compounds which disable or kill the unfortunate predator.