Effects of habitat, burial, age and passage through birds on germination and establishment of chinese tallow tree in coastal South Carolina

Journal of the Torrey Botanical Society, Apr-Jun 2001 by Renne, Ian J, Spira, Timothy P, Bridges, W C

Discussion. GREENHOUSE EXPERIMENTS. Although an avian gastrointestinal treatment is not a germination requirement, defecated and acid-- treated seeds had higher percent germination and reduced germination time relative to unhandled seeds (also see Glyphis et al. 1981; Barnea et al. 1991). If early emergence leads to greater biomass accumulation during the growing season relative to later-emerging seedlings (Ross and Harper 1972; Fowler 1988), then the seedling survival advantage conferred by seed passage through birds may contribute to the northward spread of tallow tree by reducing germination time. Indeed, Draper (1982) and Jubinsky and Anderson (1996) report frost damage is a limitation to its distribution, and Renne (unpubl. data) found plant size is a strong determinant affecting tallow tree overwintering survival. Early emergence can also confer a competitive advantage over late-emerging conspecifics or other plants (Ross and Harper 1972; Weaver and Cavers 1979), but this advantage may vary in space and time (Fowler 1984; Kalisz 1986; Jones and Sharitz 1989).

Potential advantages of rapid aril removal by acid treatment or digestion by birds include increased imbibition and gas exchange of seeds, two requirements for germination (Baskin and Baskin 1998). Gas exchange of surface seeds was at least as great as those that were buried, whereas imbibition was likely greater for buried seeds because more seed surface was in direct contact with soil. The higher germination of buried seeds suggests increased imbibition efficiency was the primary mechanism influencing their germination (also see Bruce 1993).

Defecated and freshly collected seeds had higher seed viability than unhandled and one-- year old seeds, respectively, and one-year old unhandled seeds had the lowest viability of any treatment. Loss of viability occurred under our outdoor storage conditions, but was slower for seeds that passed through the avian gut. Reduction in seed moisture (Harrington 1973) due to removal of the waxy aril may have prolonged longevity, but it is not clear if bird-dispersed seeds have greater longevity than unhandled seeds under field conditions.

FIELD SEEDLING ESTABLISHMENT. Tallow tree is seed-limited in many habitats in coastal South Carolina because sowing seed along transects and in plots resulted in higher seedling establishment rates relative to control areas with no sown seeds (Louda 1982; Shaw and Antonovics 1986). Disturbance was not simulated along transects (i.e., no vegetation was removed), thus the possibility of microsite limitation within habitats cannot be excluded (Eriksson and Ehrlen 1992). In 1998, microsite differences for germination and initial establishment existed among habitats, but because seedling survival did not differ, the transition probability from seed to established seedling was not different among habitats.

In 1999, however, final seedling number differed among habitats (Fig. 4b). If conditions affecting adult growth and survival are similar to those affecting seedling establishment, then many of the coastal forests we sampled will continue to be invaded, albeit at different rates, by tallow tree. Renne (unpubl. data) found that tallow tree population growth rates differed among the five forest types, but all were positive from 1997-1999. Moreover, population growth corresponded well with our seedling emergence and survival data, with the slowest (BLH) and most rapidly increasing (CC) populations having the lowest and highest establishment rates, respectively. As these populations continue to grow and age, they will likely become microsite-limited as available sites become colonized.