BOTANICAL AND ECOLOGICAL CHARACTERISTICS
SPECIES: Betula nigra | River Birch 



GENERAL BOTANICAL CHARACTERISTICS : 
River birch is a medium-sized, native, deciduous tree.  Isolated
specimens have reached 100 feet (30.5 m), but the usual height range is
50 to 80 feet (15.2-24.4 m) and 24 to 36 inches (61-91 cm) d.b.h. [13].
In Wisconsin, it is usually a small, multistemmed tree [2].  The bark is
separated into thin papery scales, with coarse scales on lower trunks
[7].  It is fairly short-lived [5,34].


RAUNKIAER LIFE FORM : 
   Phanerophyte


REGENERATION PROCESSES : 
Good seed crops are usually produced annually [13].  The winged seeds
are wind or water disseminated.  Water dissemination is probably more
important because water deposits the seeds on moist shores favorable to
germination and establishment [39].  The seeds germinate rapidly in
moist alluvial soil, often in large numbers, forming thickets on
sandbars [13].  The seeds are apparently viable only a few days [24].
However, Koevenig [42] reported that seeds with the fruit wall and seed
coat removed will germinate even after 5 months in storage.  He
concluded that a germination inhibitor builds up in either the fruit
wall or seed coat.

River birch does not spread vegetatively, but multiple stems arising
from stump sprouts are common [13].  Because of this, river birch is
resilient to flood damage.  On a frequently flooded site in Wisconsin,
77 percent of river birch stems were of sprout origin, and the remainder
were from seedlings [2].


SITE CHARACTERISTICS : 
River birch occurs in wide range of climatic conditions.  It is
primarily a tree of alluvial soils (Entisols) [13].  River birch occurs
largely on lowlands, floodplains, streambanks, and lake margins.
Typical sites are sandbars and new land near streams, inside the natural
levee or front [15].  It is occasionally found on scattered upland sites
[7].  It is positively associated with clay soils [40].  Soils can be
either well- or poorly drained, as long as they are at or near field
capacity year-round [13,24].  River birch often occurs on soils that are
too acid for most other hardwoods (pH range 2 to 4) [13], but also
occurs on soils of higher pH [37].

River birch is moderately tolerant to flooding [13].  In a laboratory
experiment, river birch seedlings survived up to 30 days of flooding,
forming adventitious roots and prominent lenticel intumescences [32].
Of five floodplain species tested, river birch seedlings were moderately
tolerant of inundation (complete coverage), and tolerant of waterlogging
(soil saturated only) [19].  Mature trees exhibited 77 percent survival
after 240 days of flooding, but none survived 730 days [16].  Seedlings
are stunted by extended periods of flooding, but remain healthy if
flooded for less than 24 percent of the growing season.  River birch can
occur in soils that are waterlogged about 50 percent of the time [16,24].


SUCCESSIONAL STATUS : 
Facultative Seral Species

River birch is intolerant of shade.  It is an early pioneer on stream
bank alluvium, and requires high soil moisture coupled with no shade for
germination and establishment [13].  River birch may be the initial
colonizer of sandbars, or may establish after sandbars are stabilized by
more flood-tolerant alders (Alnus spp.) or willows (Salix spp.) [25].
In Maryland, small stream valleys with shallow surface water are
colonized by hazel alder (Alnus serrulata), and then invaded by river
birch [17].  River birch is the most common species on disturbed
streambanks in Tennessee.  It readily establishes on the soils exposed
by stream channelization projects and remains important for a number of
years, even after canopy closure [18].  Myers and Buchman [24] classify
river birch stands as subclimax; river birch usually follows willows and
is replaced by other hardwoods, generally oaks.


SEASONAL DEVELOPMENT : 
Male catkins are formed on twig tips in the fall and mature the
following April or May.  Female catkins appear with the leaves and open
in early spring.  The fruit matures in late spring or early summer, and
is dispersed upon maturity [13].

Related categories for Species: Betula nigra | River Birch