In a natural ecosystem all the species of organisms have predators or there is a limited supply of food and other essential resources that keeps their population under control.
Human technology and the process of exchange of goods between different countries has created a means by which species of organisms can easily reach ecosystems they are not native to. In many cases, these species do not have predators and the resources in the new ecosystem allows an uncontrolled expansion in population.
This can be very harmful for the native species and could even lead to their extinction. A species that is harmless in its native ecosystem could prove to be extremely harmful in a new ecosystem. There are thousands of such animals and plants that have invaded new lands and cause a lot of damage, environment and economic.
The law in Canada stopping the import of firewood and Christmas trees that have been grown in other countries is meant to prevent the arrival of species that could be disruptive to the ecosystem. Trees by themselves can cause a lot of damage; in addition they could become a means of transport for many other invasive species like insects, small animals, etc.