Don't let the name trick you - there are no thorns on this one! Also, it's not related to the Alder family just to add to the confusion! This is a native tree, although in most cases it is more of a large shrub. It's not very common so should be encouraged to grow where possible. I've only come across them at the RSPB Aylesbeare Common reserve whilst volunteering. The buds are very distinct making it a little easier to identify in the winter. The bark and berries are poisonous.
The name Alder Buckthorn is thought to be derived from its similar appearance to Alder and the fact that the two trees are often found growing together. Alder buckthorn is in the Rhamnaceae family and alder is in the Betulaceae (birch family) so botanically the two species are unrelated. Below are some useful links:​
One of many Alder Buckthorn shrubs on the RSPB Aylesbeare Common reserve
Close up up Alder Buckthorn trunk and bark
The leaves are matt green, oval, 3-7cm long and slightly hairy. They have a rounded tip but are tapered towards the stalk. Mature trees can grow to a height of 6m so not very tall. The outer bark is dark brown but the inner bark is bright yellow when exposed. The twigs are smooth and straight, purple-brown in colour and have fine white streaks. In winter the buds do not have scales and are hairy. Peeling back the bark surface reveals yellow bark underneath.
General view of the Alder Buckthorn leaves
Close up of the leaves of Alder Buckthorn
The leaves are placed alternately along the twig and have 6-10 pairs of lateral veins that do not curve towards the tip.
General view of Alder Buckthorn flowers
Close up of the Alder Buckthorn flowers
Alder Buckthorn is hermaphrodite, meaning that male and female reproductive parts are contained within the same flower. The star-shaped flowers are small, 3–5 mm diameter, with five greenish-white triangular petals. They appear in May to June in clusters in the leaf axils. Once pollinated by insects, the flowers develop into a small berry, which ripens from green to red in late summer, eventually to a dark purple or black in early autumn. 
PHOTOS
QUICK FACTS
  • ​Alder Buckthorn is unrelated to Alder
  • The outer bark is dark brown but the inner bark is bright yellow when exposed
  • It is hermaphrodite, meaning that male and female reproductive parts are contained within the same flower
  •  It grows best in wet soils and open woods, thriving in scrubs, hedgerows, wet heathland, river banks and bogs
  • Its flowers provide a source of pollen and nectar for bees and other insects, and its berries are eaten by birds
  • It was once used as a purgative, which was thought to help rid the body of illness and disease
  • A yellow dye is obtained from the leaves and bark. It is used in Russia and turns black when mixed with salts of iron
  • A green dye is obtained from the unripe fruit, and a blue or grey dye is obtained from the ripe berries
  • The wood has been used to make wooden nails and shoe lasts
  • Alder Buckthorn charcoal is prized in the manufacture of gunpowder
  • It is particularly valued for time fuses because it has a very even burn rate
  • It is widespread, but scarce, across England and Wales
  • Both Alder and Purging Buckthorn are the main foodplants of the Brimstone Butterfly
  • The bark and berries are poisonous
OTHER USEFUL DATA
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Zelimir Borzan, University of Zagreb, Bugwood.org
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Gallaher's cigarettes tree card - credit to cigcardpix via Flickr
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Will's cigarettes tree card - credit to cigcardpix via Flickr
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Rington's tea (scanned from personal collection)
OTHER USEFUL RESOURCES
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www.exmoortrees.co.uk
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www.rhs.org.uk
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www.naturespot.org.uk
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https://en.wikipedia.org
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www.tree-guide.com
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www.woodlandtrust.org.uk
PLEASE LEAF ME ANY FEEDBACK / COMMENTS
If there is anything out of place or wrong please contact me. Equally if there is anything you wish to add please let me know. The more information we have about Alder Buckthorn the better. Many thanks!