Seed Production and Dissemination- Fruits ripen in September and October and are soon dispersed by such mechanisms as wind, gravity, and animals. Although the flower bracts are reported to aid in wind dispersal, fruits rarely are carried more than one or two tree lengths from the parent (24). In addition to their limited role in seed dispersal, bracts may act as "flags" to attract pollinators (especially nocturnal ones) to the inflorescences (2). Animals probably increase the seed dispersal significantly.
The seed-bearing age for basswood generally ranges from 15 to 100 years, but seed production at age 8 years (10 years from seed) has been noted (45). The number of ripened fruits averages 9,700 to 13,200/kg (4,400 to 6,000/lb); green fruit averages 5,070 to 5,950 seeds per kilogram (2,300 to 2,700/lb) of fruit (17,33,35). Based on a number of collections, seed weights varied from 12 to 38 mg (0.18 to 0.59 gr) and averaged 31 mg (0.48 gr) (4). In a study for 26 years of 19 species in northern Wisconsin, basswood was one of the most consistent fall-maturing seed producers (18). It produced good seed crops 62 percent of the time from 1949 to 1974. When crown-released, basswood that were about 50 years old did not increase their fruit production during the 5-year period following release. Moreover, the quality of fruit remained poor throughout this period. In the third year after release, for example, only 5 percent of the fruit collected from the ground contained sound seed (37).
The production of fruit without seed (parthenocarpy) and seed infestation by a lepidopterous larva are two common defects that affect seed viability. A pin hole in the pericarp indicates the presence of the larvae. The percentage of fruits with the pin hole was 3 percent in a September collection and 7 percent for an October collection in southeastern Ontario (35); 30 percent of fruits were insect infested in 45 collections from various parts of the natural range of basswood (4). In the same collections, the percentage of fruits with seed ranged from 0 to nearly 100, but the lack of sound seed on the forest floor seems to be the rule. Only 2 percent were sound out of more than 7,400 identifiable basswood seeds found in the litter in a northern Wisconsin stand. Seeds covered by leaves had rotted and most of the seeds lying on or in the upper litter layers had been destroyed by rodents (18).
Seedling Development- Basswood seeds show a pronounced dormancy and generally germinate poorly regardless of seedbed conditions. The primary cause for the lack of quick germination is an impermeable testa. Using organic acids to digest the pericarps of the fruits and to render the testas permeable improves germination (17). Correctly treated seeds commonly average from 20 to 30 percent germination following stratification at 2° to 5° C (36° to 41° F) for 110 to 130 days. Germination is epigeal. Early harvesting followed by immediate sowing has also been suggested for overcoming dormancy of basswood seeds. Collections should be made when seed coats turn brown but before they become dry and hard, or more specifically, when the moisture content is 20 to 40 percent of the green weight (7,29).
Shading aids the establishment and initial survival of basswood seedlings but heavy shade limits subsequent growth and development, and vigorous growth is unlikely under the forest canopy. Likewise, higher soil temperatures found in forest openings are suitable for greatest growth of basswood seedlings (3).
Basswood seedlings first develop a long taproot, which is soon supplemented by lateral roots. First-year seedlings had a root penetration of 20.3 cm (8 in) with a lateral spread of 7.6 cm (3 in), and second-year seedlings had a root penetration of 21.3 cm (8.4 in) and a lateral spread of 18.3 cm (7.2 in) (30). Stem height was 5.6 cm (2.2 in) the first year and 9.4 cm (3.7 in) the second year.
Cold storage of autumn-lifted basswood seedlings maintains root growth capacity and overall seedling vigor for spring planting. Autumn-lifted stock should be stored at a temperature of 5° C (41° F) and a relative humidity of 70-85 percent (46).
Basswood has been successfully planted in Ontario on cutover land and abandoned farmland. On cutover land, survival was best when a light overhead canopy (8.0 m/ha or 35 ft/acre of residual basal area) controlled competing vegetation (36). Release of the seedlings from the residual overstory and undergrowth was recommended after three growing seasons. Fall plantings failed to survive. Early failures of hardwoods planted on old-field sites in Ontario have been attributed to the absence of mycorrhizal fungi (30), insufficient site preparation, and insufficient postplanting weed control (42,44). Fertilization at the time of planting had little effect on seedling survival or growth (43).
