The simplest and most straightforward answer being, the Alder is the fastest growing native Irish tree species. This is the often proffered and rarely questioned answer but there is a very close challenger in the Willow. In my opinion, the Alder gets there on a technicality.
Technically speaking, Willow outgrows Alder, hands down in my experience. If you look at the below photos you’ll see an Alder and a Willow, both planted in the back field here on the Iveragh Peninsula, Ireland.
From the above images, it would be fair to say the Alder has it clear and simple, in five years it’s the far taller tree, but that doesn’t immediately equate to it having grown more.
What the picture doesn’t show is the Willow pictured has also given birth to three siblings that are happily growing throughout the garden, it’s also been the source material for a willow dome structure. So taking this into account the Willow has grown more, as in, produced more wood.
Why isn’t Willow Ireland’s fastest growing native tree?
Well, it’s a native species based issue. The varieties of Willow, notably Weeping Willow (Salix babylonica) or the hybrid Crack Willow (Salix × fragilis) can grow 6-10 feet in a year and would easily outpace te Alder, are not native to Ireland. The native willows of Ireland include species like the Goat Willow (Salix caprea) and the Bay Willow (Salix pentandra), which, while still quite fast-growing, do not typically reach the extreme growth rates of their non-native relatives. What we have here, pictured in the back field is Purple or Basket Willow (Salix purpurea), which although Biodiversity Ireland has records of it’s presence dating back to the 1890’s is not deemed a native species. So whilst it may have outproduced it’s native competitor, it’s not native, so not a competitor.