Neotropical Betulaceae

William Milliken

Royal Botanic Gardens, Kew, UK. 

Description

Trees.  Leaves simple, alternate (sometimes almost distichous), with pinnate venation and serrate or dentate margins, sometimes with indumentum of simple hairs. Intra-petiolar stipules present. Flowers unisexual (plants monoecious), borne on catkins, subtended by bracts; petals absent; sepals 1-6, scaly. Male flowers with 2-12 stamens, anthers basifixed and dehiscing by longitudinal slits; staminodes absent. Female flowers syncarpous (2-3 carpels); styles not fused; ovary inferior with 2-3 locules. Fruits are one-seeded nuts: flattened, winged and borne in cone-like structure (Alnus Mill.), ovoid and subtended by winged bract giving the impression of a samara (Carpinus L.) or encased in inflated bracts on a loose catkin (Ostrya Scop.).

Notes on delimitation

Distribution in the Neotropics

  • Only one genus, Alnus, occurs in South America. 
  • Carpinus and Ostrya are found in Central America and Mexico, at the southern end of their ranges.

Distinguishing characters (always present)

Other important characters

  • Close, ascending secondary leaf veins.
  • Twice-toothed leaf margin (Alnus and Carpinus).

Key differences from similar families

Potentially confused with other members of the Fagales but distinguishable from them by the following features:

Number of genera

Three genera are represented by three species in the Neotropics:

  • Alnus acuminata Kunth (= A. jorullensis, which is sometimes regarded as a separate species), widely distributed through South and Central America between 1200 and 3200m altitude. It is generally associated with damp ground and occurs along the banks of rivers and marshes.
  • Carpinus caroliniana Walter is a small sub-canopy tree occurring in Mexico and Central America (Guatemala and western Honduras).
  • Ostrya virginiana (Mill.) K. Koch occurs in Mexico and Central America (to El Salvador and Honduras).

Useful tips for generic identification

The three species are most readily distinguished by their fruits:

  • Alnus - Small winged nuts borne in a cone-like structure (which could be confused for a single fruit dehiscing to release its seeds).
  • Carpinus - Ovoid nuts subtended by large wing-like bracts (giving the impression of a samara), borne on an elongated pendulous inflorescence.
  • Ostrya - ovoid nuts enclosed in inflated bracts, borne on an elongated pendulous inflorescence.

Status

  • Native but also sometimes cultivated.

General notes

  • All members of this family have associations with nitrogen-fixing bacteria in their roots (e.g. Frankia spp.).
  • The wood of Alnus acuminata is very hard and is used for many purposes including construction, furniture, matches, musical instruments, and tool handles. This species is planted as a timber tree and has also been used for agroforestry in Costa Rica (as a shade tree for coffee crops). 
  • Ostrya virginiana also produces very hard wood and has been used by native Americans for various medicinal purposes including the treatment of toothache. 
  • Both Alnus and Ostrya have been used for tanning leather.

Important literature

Burns, R.M. & Honkala, B.H. (1990). Silvics of Forest Trees of the United States.  Agriculture Handbook 654.  United States Department of Agriculture Forest Service, Washington.

Furlow, J.J. (1977). Betulaceae. In Burger, W. Flora Costaricensis. Fieldiana, Botany 40: 56-58.

Furlow, J.J. (1979). The systematics of the American species of Alnus (Betulaceae). Rhodora 81(825): 1-121.

Holdridge, L.R. (1951). The alder, Alnus acuminata, as a farm timber tree in Costa Rica. Caribbean Forester 12(2): 47-57.

Lentz, D.L.  (2004). Betulaceae.  pp 50-51 in: Smith, N.A. et al. (eds), Flowering plants of the Neotropics. Princeton University Press, New Jersey.

Manos, P. S. & Steele, K. P. (1997). Phylogenetic analyses of 'higher' Hamamelididae based on plastid sequence data. American Journal of Botany 84:1407-1419.

How to cite

Milliken, W. (2010). Neotropical Betulaceae. In: Milliken, W., Klitgård, B. & Baracat, A. (2009 onwards), Neotropikey - Interactive key and information resources for flowering plants of the Neotropics. http://www.kew.org/science/tropamerica/neotropikey/families/Betulaceae.htm.