This is quite evident in the case of mono-cultivated, intensive crops that use irrigation and increasing amounts of fertilizers and pesticides. Timber resources can be natural or cultivated and are important environmental resources in many countries. They provide inputs for construction and the production of furniture, cardboard, cellulose, paper and other products, and are also a fuel source. Timber resources are defined by the volume of trees, living and dead, which can still be used for timber or fuel.