This months spotlight has been written by our incredibly knowledgeable Park Ranger, Sean Garvey, revealing revelations about native shrub species, Alder Buckthorn.
Let me introduce you to one of St Nicks’ scarcest shrubs: Alder Buckthorn , Frangula alnus . It’s an unassuming plant that is easily overlooked; but it has a fascinating, and literally explosive, back-story.
For a start, its name is totally misleading. It isn’t closely related to Alder, Alnus glutinosa (plenty of these near the becks, and around the Bund, at St Nicks) but is most often found close to these trees in damp woodland, as at Askham Bog and Skipwith Common. Nor is it thorny, unlike its only other British cousin in the Rhamnaceae family, Buckthorn, Rhamnus cathartica .
Alder Buckthorn is a native shrub or small tree that grows up to about 6 meters. It is found throughout Britain, but isn’t very abundant anywhere. During autumn its berries slowly change from green to red (see my photo, left) to purplish-black: this is perhaps the most distinguishing feature of this species, which can easily be confused with other shrubs and trees.
Typically for a St Nicks plant (and other organisms found here, including some of the humans), Alder Buckthorn is a bit of an urban rebel that ignores the rules. So instead of growing in wet woodland on acidic soils — as it should — the few Nickers specimens seem to be flourishing in drier, non-acidic conditions. There’s a youngish tree about half-way along the southern edge of the Butterfly Walk. And two smallish ones in the stretch of young hedge between the meadow and the Rawdon Avenue boundary that our wonderful Student Action Volunteers have recently been gapping-up (excuse the hedge-maintenance jargon).
As for the fire and brimstone… Brimstone (an old name for sulphur) is buttery-yellow. Like the dye that used to be obtained from the bark and leaves of Alder Buckthorn.
Brimstone butterflies are usually one of the first of their kind to grace us with their presence in spring at St Nicks, and one of the last (second-brooders these ones) to show up in autumn. And guess what: their caterpillars only feed on Alder Buckthorn, and its Buckthorn relative. See:
Alder Buckthorn wood is yellowish (naturally) and far too brittle to be of much use. Remember its Latin name, Frangula: from the Latin frangere= English “to shatter”, hence “frangible”.
Except that when turned into charcoal, Alder Buckthorn has long had the reputation as being the best ingredient (together with saltpetre – and, of course, sulphur/brimstone) for making gunpowder. Alder Buckthorn charcoal was still in great demand for specialist explosives, such as time-fuzes, during both of Britain’s twentieth-century world wars.
Please don’t try this stuff at home, kids! Not the charcoal-burning, and definitely not the explosives-making.
Instead, like me, get yourself awestruck by our amazing fellow creatures, the plants around us. Even (or particularly) the obscure ones, like Alder Buckthorn. They – and we – always have a story to tell.
Sean Garvey 19/11/2018
Photos taken by Sean Garvey and Cliff Wilton