Have you noticed that there are catkins on trees that look similar to hazel catkins, but which also have brown spheres hanging from them too, and when you look closer, the spheres look just like tiny pine cones? If so, you are looking at a alder tree. In the picture above you can see the darker, unopened, male catkins; you can see lots of the yellow open catkins, and you can see the dark brown "cones". These cones are actually last year's fruits, which developed from the female parts of the tree, the flowers, once they have been fertilised from the pollen released from the catkins of another tree (like hazel, alders don't self fertilise). The fruits open up in the winter and release their seeds, just like pine cones.
But can you spot the female part of the tree, the flowers that form these cones? Like the hazel, they are situated above the male catkins, and in the top picture they look a bit like the bulbous end of a matchstick. In the picture below you can see that they have reddish colour to them and they have opened up to receive pollen. Check out alder trees later in the year and you will spot the green fruits in little clusters, which will dry out and form the brown cone-like structures next winter.