While mature forests are declining worldwide, tree plantations could provide habitats of conservation value for forest-adapted species. However, to what degree the fauna in tree plantations matches the diversity and composition of mature forest communities is still debated. In this meta-analysis, we used beetle species (Coleoptera) as biodiversity indicators to assess the conservation value (i.e. the log response ratio in species richness and abundance and the Soerensen similarity) of tree plantations versus reference old-growth forests. We tested the effects of biome, plantation age, tree species richness and origin, surrounding habitat and the trophic position of the focal guild on the diversity and composition of beetle communities. Our results showed that tree plantations generally harboured communities with 47% less individuals, 33% less species and a significantly distinct composition than old-growth forests. In the case that tree plantations had a similar number of species or individuals as old-growth forests, those plantations still displayed a 40% difference in species composition. Beetle abundance was significantly lower in tropical/subtropical plantations and for detrivorous species. We found no further significant drivers of beetle diversity and community composition in tree plantations. We conclude that, although tree plantations can provide crucial habitats for forest-adapted species, they generally harbour less diverse and significantly different communities, highlighting the conservation value of old-growth forests as biodiversity refuges. To properly assess the conservation value of plantations in different taxa, we propose the use of species composition instead of single diversity indices.