Willow
There are 400 species of willow, Salix, ranging from trees to sub shrubs, and hybrids are common. There could be many more as they are difficult to identify. You would require a good tree ID book in order to identify them.
The leaves are frequently elongated but some species have round or oval leaves.
Goat willow leaves
Goat willow catkins March
Catkins, later in March
Male goat willow catkins April
Female goat willow catkins April
Seeded catkins June
Further reading
Willows The Genus Salix - Christopher Newsholme.
Comments on this article
frank 26 January, 2012
This is most interesting