How does Frankia help alders and she-oaks live in nutrient-poor habitats?

Frankia bacteria assist plants like alders and she-oaks in nutrient-poor habitats through a mutualistic symbiotic relationship known as nitrogen fixation.12

These bacteria invade the root hairs of the host plant, stimulating the formation of root nodules—specialized structures where the plant and bacteria exist in an intimate association. Within these nodules, Frankia converts atmospheric nitrogen (which is otherwise unavailable to plants) into ammonia. The host plant then utilizes this ammonia for its development, allowing it to thrive in environments where nitrogen is otherwise scarce.13

While this process is similar to the symbiosis between legumes and Rhizobium, the relationship between Frankia and its host plants (often referred to as actinorhizal plants) represents an independent evolutionary development of nitrogen-fixing capabilities.12