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Alder has darker leaves and bark.
I have lived in places where both Alder and Hazel grow for many years and have even got a profile picture that was crafted by wood peckers from an Alder stump.
The date of this observation is April, which is when Alder has newly flushed leaves, which do not have full cuticle....hence the lighter colouration.
Lat/Lng: 52.3759, -1.4348
OS grid ref: SP385754