In sustainable forestry, foresters often plant red alder (Alnus rubra) alongside Douglas fir (Pseudotsuga menziesii) in soils that are highly depleted of nitrogen. Red alder has a mutualistic relationship with nitrogen-fixing Frankia bacteria located in its root nodules.
Explain how interplanting red alder with Douglas fir increases the growth and timber yield of the Douglas fir trees without the application of commercial nitrogen fertilisers.
