Alder trees (Alnus species) range in size from only 15 feet to a tall 50 feet and belong to the birch tree family. They grow either as single-trunked trees or small shrubs. Three common species are the European alder (Alnus glutinosa), the speckled alder (Alnus rugosa) and the white alder (Alnus incana). Alder trees grow native across the United States, Canada, parts of north Africa, Russia, Iran, Turkey and Europe.
The alder tree is monoecious, which means it is both male and female, enabling it to self-pollinate. The male flowers appear yellowish-brown and female flowers appear red. Flowering occurs the latter part of March and into April. Pollination is achieved through the use of the wind.
All varieties of alder trees produce flowers that form into strobili, or fruit, when pollinated. The strobili appear as tiny nutlike cones that measure 1 inch in length. The seed cones mature in October. Each cone releases several flat reddish seeds. They are widely favored as a food source for wildlife. The cones remain on the tree into the springtime and add wintertime interest.
Each tiny seed has air-filled membranes that appear like tiny wings. These allow the seeds to easily float down streams and rivers. Many seeds that float on the surface of the water will begin to germinate. Once the seeds reach land they quickly begin to take root into the soil. Due to this unique ability to germinate in water, the tree grows abundantly along streams, rivers, lakes and other water bodies. It enjoys moist soil conditions and thrives in a riparian setting.
The alder tree's deep and abundant roots offer soil stability along water bodies. The roots often penetrate the stream bed or lake's edge, which gives fish a hiding place. Once the foliage of the tree falls into water it begins to decompose. The decomposing leaves provide food for fish and invertebrates. They also add nutrients to the water.
Frankia Alni Bacteria
The Frankia alni bacteria forms in the nodes of the alder tree's root system. The bacteria has the ability to fix nitrogen from the air and then supply it to the tree in a form that the tree can benefit from. This process helps to make the surrounding soil around the tree's root system nutrient rich. The Frankia alni bacteria enjoys life with the alder tree because the tree provides the bacteria with carbon.