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

Effects of habitat, burial, age and passage through birds on germination and establishment of Chinese tallow tree in coastal South Carolina1

RENNE, I. J., T. P SPIRA (Department of Biological Sciences, Clemson University, Clemson, South Carolina 29634) AND W. C. BRIDGES, JR. (Department of Experimental Statistics, Clemson University, Clemson, South Carolina 29634). Effects of habitat, burial, age and passage through birds on germination and establishment of Chinese tallow tree in coastal South Carolina. J. Torrey Bot. Soc. 128:000-000. 2001.-Factors affecting germination and seedling establishment of the nonindigenous, invasive Chinese tallow tree (Sapium sebiferum (L.) Roxb.) were investigated under greenhouse and field conditions. In greenhouse experiments, buried seeds had higher germination rates and percentages than surface seeds, and simulated and actual seed passage through the avian gut enhanced germination. One year of aging reduced germination and seed viability, particularly for seeds unhandled by birds. In the field, seedling emergence and survival were greater in mixed pine-hardwood forest than in pine-turkey oak forest or spoil areas. Planting date did not affect overall seedling emergence or survival. Seeds sown in March 1998 and 1999 in five coastal forests that contained established tallow trees emerged throughout the growing season. While seedling emergence rates varied, final seedling numbers were similar across habitats in 1998, but were lower and differed across habitats in 1999. Recruitment was thus spatially and temporally variable. Viability of seeds buried for one and two years ranged from 16 to 69% in the five habitats, but did not differ among habitats or length of burial time. Because habitat type did not affect seed dormancy (both years) and final seedling number (1998 only), we conclude that other factors influence the differential success of tallow trees among coastal forests in South Carolina. Management strategies (e.g., fire) should consider the seedling phenology and seed bank capability of tallow tree.

Key words: Sapium sebiferum, exotic plant, invasion, avian seed dispersal, germination, dormancy, seedling establishment.

Chinese tallow tree (Sapium sebiferum (L.) Roxb.) is a nonnative, aggressive plant that has colonized coastal prairie and other habitats along the Gulf Coast (Bruce et al. 1995; Neyland and Meyer 1997; Wall and Darwin 1999) as well as several forest types along the southeastern coastal plain (Helm et al. 1991). In South Carolina, not all habitats are equally susceptible to tallow tree invasion (Renne, unpubl. data). Spoil dredge areas and other highly disturbed sites generally harbor the highest density of large reproductive trees, whereas many less disturbed forest types have smaller, more widely spaced individuals. In particular, longleaf pine-turkey oak and pine flatwood communities contain almost no tallow trees. These habitats may not receive sufficient numbers of seeds to successfully establish populations (i.e., seed-limited) and/or conditions may be unsuitable for seed germination and seedling establishment (i.e., microsite-- limited; Klinkhamer and de Jong 1989; Eriksson and Ehrlen 1992).

Determining the factors) limiting tallow tree population growth among habitats can be useful for predicting its future local and regional success. If seed input is high, for example, seed-- limited areas would likely become invaded, whereas microsite-limited areas may be more resistant, depending on the spatial and temporal availability of `safe sites' (Harper 1977) and other environmental factors affecting adult growth, survival and fecundity.

Renne et al. (2000) found that tallow tree seeds are consumed in large numbers by many bird species, but the fate of these propagules is unknown (see Renne 1996). Birds may digest the seeds or they may defecate viable seeds in inappropriate sites for establishment. Viable seeds could also be deposited in suitable areas where they do not immediately germinate. Seed banks can be important to the growth and persistence of plant populations by spreading the risk of propagule mortality in a spatially and temporally variable environment (Cohen 1966; Brown and Venable 1986; Kalisz and McPeek 1993).

Seed dispersal and seedling establishment represent an important demographic link in the life history of plants (Harper 1977; Herrera et al. 1994; Houle 1995). Environmental factors affecting germination, seedling establishment and other life history phases such as adult survival, growth and fecundity may be positively correlated. However, spatial and temporal fluctuations in microsite differences (e.g., local variation in predation, competition, edaphic conditions, etc.) can cause considerable discordance between seed bank longevity, germination and subsequent seedling recruitment (Herrera et al. 1994; Houle 1995, 1998; Schupp 1995). This in turn can lead to weak correlations between seed rain pattern and adult plant distribution.

In this study, we investigated factors affecting germination and seedling establishment of Chinese tallow tree. The effects of seed passage through a bird's gut, seed burial and seed age on germination were assessed in a greenhouse. In a series of field experiments, we investigated whether seedling emergence and survival differed among planting dates, years and forest types in coastal South Carolina. The potential for a two-year soil seed bank was also assessed. By studying the germination behavior and establishment of this invasive plant, a better understanding of its demography and management may be achieved.