In botanical terms, this is not a fruit: it's an accessory fruit. It is formed when several flowers of an inflorescence form fruits, fuse together and develop overlapping scales similar to those of the cone of a conifer.

The alder belongs to the group of plants known as Gymnosperms, which produce "naked" seeds. In botanical terms, it does not bear fruits, but rather modified cones.

The same tree can bear fruit of three different generations simultaneously: miniature fruits that will mature in a year; the fruits of the current year, which are mature and will dry out in preparation for releasing their seeds; and those of the previous year, which are open, empty and seedless.

This is a good example of anemochory: when the fruit opens, the seeds are dispersed by the wind.