There are a few alders in the park; mostly saplings, and three old giants scattered along Melville Drive.
The old trees are Common Alder (Alnus glutinosa).
Alders keep their catkins and their fruits (which look like cones) well into spring, making them easy to identify and providing food for birds.
Two new types of alder have arrived recently in the park. Both can be distinguished from Common Alder because their leaves have a pointed tip; Common Alder's don't and are sometimes even indented.
One of the new arrivals is Grey Alder (Alnus incana)
As here on Melville Drive
The new catkins that form in early spring are a startling bright pink.
The other group of new alders is Italian Alder (Alnus cordata), again with the startling pink in its spring catkins
Here's one on Bruntsfield Links on a snowy day
and another in a tree guard in front of the University Library, late in summer while the catkins and fruits are green
Here's a map of where to see some of the park's alders, old and new.