In the present treatment Taxodiaceae is included in Cupressaceae (see for instance Watson & Eckenwalder 1993, Mabberley 2008). Both families are surely closely related and perhaps merely distinguished by leaves alternate and opposite or whorled respectively. The present circumscription is supported by recent phylogenetic research.
In the study area the non-native representatives of Cupressaceae are usually found in the juvenile, non-reproductive phase. Morphology and dimensions of scale leaves differ much in juvenile and adult plants and young plants often key out in a different way than adult plants! As a consequence, accurate identification of juvenile plants without presumed parental plants seems to be almost impossible without experience.
Juniperus communis L. is a rare and local native in Belgium (especially eastern Kempen in Flanders and parts of Wallonia). However, it is also cultivated for ornament and could occur in places where it is not native. Numerous additional taxa of Juniperus are also introduced and might escape (for an extensive account of Juniperus in cultivation in Europe, see Welch 1986).
1. Leaves needlelike (usually at least 10 mm long), in whorls of 3 or alternate, acicular. Branchlets not in flattened sprays. Mature female cones berrylike or dry and woody === 2
1. Leaves scale-like, minute (usually less than 5 mm long), opposite, and not acicular. Branchlets in flattened sprays. Mature female cones dry and woody === 4
2. Leaves in whorls of 3. Mature female cones berrylike (native) === Juniperus
2. Leaves not whorled. Mature female cones dry and woody, with peltate scales === 3
3. Leaves of lateral twigs deciduous (soft and straight), distichously arranged === Taxodium
3. All leaves persistent (tough and incurved), spirally arranged === Cryptomeria
4. Lateral scale leaves separated or touching only at base. Branchlets wider, up to 3 mm wide. Foliage usually glossy. Mature female cones ellipsoid with flattened scales. Leaves with sweet, fruity smell when crushed === Thuja
4. Lateral scale leaves confluent in the lower half. Branchlets narrow, up to 2 mm wide. Foliage usually dull. Mature female cones globose with peltate scales. Leaves with resinous or oily (sometimes unpleasant) smell when crushed === 5
5. Scale leaves with distinct white markings beneath (best seen in young, ultimate branchlets). Gland on median scale leaf usually present, linear. Leaves with resinous or oily smell when crushed === Chamaecyparis
5. Scale leaves without white markings beneath. Gland on median scale leaf usually absent (if present, then circular). Leaves with unpleasant smell when crushed === Xanthocyparis
Christensen K.I. (2000) Coniferopsida. In: Jonsell B. (ed.), Flora Nordica, vol. 1. The Bergius Foundation, Stockholm: 91-115.
De Koning J., Van den Broek J.W., Van de Laar H.J. & Fortgens G. (2000) Nederlandse dendrologie (13e druk). H. Veenman & zonen, Ede: 585 p.
Farjon A. (2005b) A monograph of Cupressaceae and Sciadopitys. Kew Publishing: 648 p.
Mabberley D.J. (2008) Mabberley’s plant-book (3th ed.). Cambridge University Press, Cambridge: XVIII + 1021 p.
Roloff A. & Bärtels A. (2006) Flora der Gehölze (2e Auflage). Ulmer, Stuttgart: 844 p.
Schulz C., Knopf P. & Stützel Th. (2005) Identification key to the Cypress family (Cupressaceae). Feddes Repert. 116(1-2): 96-146.
Sell P.D. (1998) Conifers. In: Rich T.C.G. & Jermy A.C. (eds.), Plant crib: 35-40. BSBI, London.
Watson F.D. & Eckenwalder J.E. (1993) Cupressaceae. In: Flora of North America Editorial Committee (ed.), Flora of North America north of Mexico, vol. 2. Oxford University Press, New York-Oxford: 399-422.
Welch H.J. (1986) Juniperus. In: Walters S.M. & al. (eds.), The European Garden Flora, vol. 1: 84-87. Cambridge University Press, Cambridge.