Ecologists restoring a former industrial wasteland plant alder trees (Alnus glutinosa) to colonise the highly nitrogen-deficient soil. After several years of growth, the alder trees are cleared, and saplings of climax oak species (Quercus robur) are planted in their place.
Explain how the cultivation and subsequent clearing of alder trees can enrich the soil with nitrogen-containing compounds, reducing the need for artificial nitrogen fertilizers to support the growth of the oak saplings.
