Population Differences    There are few genetic studies of river birch but parent trees from  Illinois, Indiana, and Kentucky varied randomly in leaf and seed  characteristics. Their progenies also varied in leaf traits, and  first-year seedling height was correlated with annual diameter  growth of the parent trees (12). Texas stands varied  significantly in wood specific gravity, and the variation was  correlated with diameter growth. Single-tree progenies also  varied significantly in height growth (6). Thus, it appears  possible to increase both growth and specific gravity of river  birch by selecting for fast diameter growth.

    Hybrids    Natural hybrids between river birch and paper birch (Betula  papyrifera) have been reported but have never been verified  and appear unlikely because interspecific hybridization involving  river birch is very difficult. River birch has been crossed with

    sweet birch (B. lenta), gray birch (B. populifolia),  paper birch (B. papyrifera), resin birch (B.  glandulosa), and low birch (B. pumila var. glandulifera)  and with the following introduced species: B. ermani Cham.,  B. raddeana Trautv., B. pendula Roth, B.  pubescens Ehrh., B. platyphylla Sukachev, and B.  maximowicziana Regel. Seed yield and viability have been low  and growth has been poor. In general, crossing attempts are more  likely to succeed if river birch is the female parent (2).