In Plant List Betula celtiberica Rothm. & Vasc. is treated as synonym of Betula pubescens Ehrh.
But in POWO (Plants of the World Online) Betula celtiberica Rothm. & Vasc. is treated as accepted species.
For detail see the book - Ashburner, K. & McAllister, H.A. (2013). The genus Betula: a taxonomic revision of birches: 1-431. Royal Botanic Gardens, Kew.