Many countries, and formerly even states within the United States, have laws prohibiting or strictly regulating the transfer across borders of plants. The reason for this has a direct bearing on the issue of protection of existing plant species and ecosystems.
Many plants are native to certain locations, as many species of animals are native to certain locations. Because plants are an integral part of an ecosystem, any non-native plants introduced into their environment can adversely affect the native species. Cross-pollination, for example, can decimate a type of plant that is essential for the survival of animals that are dependent upon it for nourishment. Additionally, non-native plants compete with native plants for nutrients in the soil that they all need to live. Entire agricultural sectors can be badly damaged by foreign plants introduced into a new environment.
It is not just, or, in many instances, primarily the plant itself that is of concern. More often, the threat to native plants are the insects that are transported across borders in plants. Firewood or Christmas trees are highly likely to be carriers of insects that can adversely affect ecosystems to which they are not native.
Recognizing the threat cross-border transfers of plants -- and animals -- can have upon native ecosystems, many countries began to look for ways to better regulate such transfers. The result was the United Nations Convention on International Trade in Endangered Species of Wild Fauna and Flora, which entered into force in 1975 and currently includes 175 countries. This Convention is intended to both protect endangered species that are subject to poaching and illicit trafficking and to protect native ecosystems.