In the early 1990s, a young Canadian forest ecologist named Suzanne Simard, while studying logged temperate understoreys in north-western British Columbia, observed an interesting correlation. After a clear-cut had been replanted, she noticed that when the small paper birch shoots were weeded out shortly afterwards, their disappearance coincided with the stunting and subsequent premature death of the newly planted Douglas fir seedlings nearby.
For many years, foresters believed that such weeding was necessary so that the young birches ('weeds') would not deprive the young firs ('crops') of valuable soil resources. However, Simard began to doubt whether this simple model of competition reflected the truth. It seemed entirely possible to her that the paper birches were not hindering the firs, but in some way actually helping them: when the birches were cut down, the health of the firs suffered. But if such mutual assistance between species really does occur in tree relationships, what is its nature – and how are individual trees able to help one another across the expanses of the forest?