In the past, the family was often divided into two families, Betulaceae (Alnus,Betula) and Corylaceae (the rest). Recent treatments, including theAngiosperm Phylogeny Group, have described these two groups as subfamilies within an expanded Betulaceae:Betuloideae andCoryloideae.
Betulaceae flowers are monoecious, meaning that they have both male and female flowers on the same tree. Their flowers present as catkins and are small and inconspicuous, often with reduced perianth parts. These flowers have large feathery stamen and produce a high volume of pollen, as they rely on wind pollination. Their leaves are simple, with alternate arrangement and doubly serrate margins.
The Betulaceae are believed to have originated at the end of theCretaceous period (about 70 million years ago) incentral China. This region at the time would have had aMediterranean climate due to the proximity of theTethys Sea, which covered parts of present-dayTibet andXinjiang into the earlyTertiary period. This point of origin is supported by the fact that all six genera and 52 species are native to this region, many of those beingendemic. All six modern genera are believed to have diverged fully by theOligocene, with all genera in the family (with the exception ofOstryopsis) having afossil record stretching back at least 20 million years from the present.
The other genera include a number of popularornamental trees, widely planted in parks and large gardens; several of the birches are particularly valued for their smooth, brightly colouredbark.
Thewood is generally hard, tough and heavy,hornbeams particularly so; several species were of significant importance in the past where very hard wood capable of withstanding heavy wear was required, such as forcartwheels,water wheels,cog wheels,tool handles, chopping boards, and wooden pegs. In most of these uses, wood has now been replaced bymetal or other man-made materials.
^Angiosperm Phylogeny Group (2009), "An update of the Angiosperm Phylogeny Group classification for the orders and families of flowering plants: APG III",Botanical Journal of the Linnean Society,161 (2):105–121,doi:10.1111/j.1095-8339.2009.00996.x,hdl:10654/18083
^abSoltis DE, Smith SA, Cellinese N, Wurdack KJ, Tank DC, Brockington SF, Refulio-Rodriguez NF, Walker JB, Moore MJ, Carlsward BS, Bell CD, Latvis M, Crawley S, Black C, Diouf D, Xi Z, Rushworth CA, Gitzendanner MA, Sytsma KJ, Qiu YL, Hilu KW, Davis CC, Sanderson MJ, Beaman RS, Olmstead RG, Judd WS, Donoghue MJ, Soltis PS (2011). "Angiosperm phylogeny: 17 genes, 640 taxa".Am J Bot.98 (4):704–730.Bibcode:2011AmJB...98..704S.doi:10.3732/ajb.1000404.hdl:2027.42/142064.PMID21613169.S2CID8204080.
^Xiang XG, Wang W, Li RQ, Lin L, Liu Y, Zhou ZK, Li ZY, Chen ZD (2014). "Large-scale phylogenetic analyses reveal fagalean diversification promoted by the interplay of diaspores and environments in the Paleogene".Perspectives in Plant Ecology, Evolution and Systematics.16 (3):101–110.Bibcode:2014PPEES..16..101X.doi:10.1016/j.ppees.2014.03.001.
