Many aspects of forest succession are well known and have been successfully incorporated into predictive models (Huston & Smith 1987; Botkin 1993; Pacala et al. 1996). The establishment phase, however, remains one of the least understood and most often ignored components of forest dynamics (Canham 1989; Connell 1989; Lieberman et al. 1989). Botkin (1993) considers the modelling of seedling germination to be ‘computationally unwieldy’ and the prediction of seedling recruitment to be difficult due to the lack of general patterns among species. Models of forest dynamics often ignore small seedlings (Shugart 1984; Botkin 1993; Pacala et al. 1996), make very simplified assumptions about their spatial distribution and recruitment (Pacala et al. 1996), or start from known seedling populations without modelling new recruitment (Mou et al. 1993).
In addition to computational difficulties, a scarcity of long-term demographic data limits the incorporation of small seedlings into forest dynamics models. Most published studies focus on survival of planted seedlings (Ashton 1995; Ashton et al. 1995; McKee 1995), establishment in gaps (Denslow 1980; Augspurger 1984; Schupp et al. 1989; Tappeiner & Zasada 1993) or relatively short-term (<3 years) survival (Augspurger 1984; Turner 1990a,b; Houle 1992). Few studies have examined natural establishment and long-term survival in forest understories away from gaps (Hett & Loucks 1971, 1976; Woods 1984; Clebsch & Busing 1989; Busing 1994; De Steven 1994; Hart 1995; Boerner & Brinkman 1996). Such non-gap seedlings, via a process known as advance regeneration (Smith 1962), can however contribute to, or even dominate, the canopy replacement process (Smith 1962; Woods 1984; Connell 1989; Silvertown & Lovett Doust 1993).
Advance regeneration seedling pools are dynamic. Rates of seed input and seedling mortality vary among species, and also vary within species across space and time (Augspurger 1984; Canham 1989; Streng et al. 1989; Jones et al. 1994). Potential causes of mortality include abiotic stresses such as shade, drought and flooding (Walker et al. 1986; Streng et al. 1989; Walters et al. 1993) and biotic influences such as herbivory, disease or root competition (Janzen 1970; Woods 1979; Hubbell 1980; Maguire & Forman 1983; Augspurger 1984; Walker et al. 1986; Janzen & Hodges 1987; Streng et al. 1989; Jones et al. 1994). Microtopography, which often influences both seed dispersal and seedling stress, can also be important (Harcombe et al. 1982; Augspurger 1984; Beatty 1984), especially in floodplain forests (Huenneke & Sharitz 1986).
If responses of understorey seedlings to these various factors can be reliably predicted, forest succession models may be better able to incorporate seedling dynamics. However, most of our current knowledge of advance regeneration ecology is derived from anecdotal observation of seedlings in natural systems or laboratory experiments (Hook 1984; Burns & Honkala 1990a,b; Walters et al. 1993; Walters & Reich 1996). Few studies have used detailed, long-term (i.e. >5 years) observations of natural populations. We are aware of only two published studies on long-term demography of advance regeneration in floodplain forests (Streng et al. 1989; Jones et al. 1994).
In addition to their utility for modelling community dynamics, long-term demographic studies can test hypotheses concerning corollaries (and mechanisms) of survival. For example, several studies have suggested that tree seedling survival (either before or after disturbance) is more closely linked to size than to age (Johnson 1977; Marquis 1982; Streng et al. 1989; Loftis 1990). This makes sense for floodplain forests where taller seedlings may be able to avoid the often lethal effects of leaf inundation (Hosner 1958; Hook 1984; Jones et al. 1989). However, some understorey seedlings in floodplains grow very slowly (Streng et al. 1989; Jones et al. 1994) and it is therefore possible that growth rates may be a key to survival, as has been found for other tree seedlings (Walters & Reich 1996).
In this paper we examine long-term demographic patterns for understorey tree seedlings in four floodplain forests to answer two questions. First, is there a common pattern of seedling survival in forest understories that can be exploited to model density and species composition (assuming that seed input and germination are known)? Secondly, are there key aspects of the seedlings and of the understorey environment that may cause variations in the general pattern? Based on past studies (Streng et al. 1989; Jones et al. 1994), we hypothesize that survival differences among species will be large, that soil moisture (including flooding and drought) will affect survival in most species, and that taller seedlings will survive better.