Recruitment is often the critical stage for the maintenance of plant populations, and can thus influence their distribution and abundance (Harper 1977; Fenner 1985). While recruitment is frequently limited by interference from neighbouring adults (Goldberg 1987b; Bertness & Yeh 1994), there are situations where adults may facilitate recruitment (Callaway & Walker 1997). It is therefore important to understand what influences the balance between the negative and positive interactions with neighbouring plants.
Most hypotheses concerning patterns in the net magnitude of these interactions have specifically concerned the influence of productivity and/or the suitability of environmental conditions. For example, although competition may predominate at all productivity levels, its intensity may be low in unproductive environments due to harsh abiotic conditions, and increase with productivity due to greater amounts of neighbour biomass and frequency of size–asymmetric interactions (Grime 1979; Huston 1979; Thompson & Grime 1988; Keddy 1989; Wisheu & Keddy 1992; Fig. 1a, number 1). Alternatively, competitive intensity may be constant with respect to productivity, although the limiting resources may shift from below-ground resources (e.g. nutrients and water) to light as productivity increases (Newman 1973; Grubb 1985; Tilman 1988; Fig. 1a, number 2). Bertness & Callaway (1994) suggest another alternative: interactions are not necessarily competitive along the entire productivity gradient. They predict that in areas with harsh edaphic conditions, vegetation will facilitate recruitment, but in physically benign areas competitive interactions will prevail (Fig. 1a, number 3).
The available empirical data that can be used to test these predictions have produced conflicting results to date; different studies support each of the three predictions (Gurevitch 1986; Wilson & Tilman 1991; Wilson 1993; Kadmon 1995; Twolan-Strutt & Keddy 1996; Goldberg et al. 1999). Although some of this conflict may be due to the environmental gradient, community type, or life-stage studied, we suggest that another crucial factor has been largely overlooked. Conflicting results concerning the changes in interaction intensity across gradients may be due to how researchers incorporate effects of plant litter in their studies. Litter is another very important factor in determining recruitment patterns across environments and may thus influence interaction intensity.
Removal studies addressing vegetation effects do not treat litter effects uniformly; in fact, many do not specify whether or not litter was removed along with live vegetation (Goldberg 1987a; Tilman 1989; Wilson & Tilman 1991; Bertness & Shumway 1993; Wilson 1994). If litter is removed along with vegetation, comparison of this removal with control conditions quantifies the total effects of both vegetation and litter. These total effects are the net balance of the direct effects of each, the indirect effect of one on the other, and any interaction modifications (sensuWootton 1993; Fig. 2a). Studies that remove only vegetation and leave litter intact assess the net balance of the direct effect of vegetation, its indirect effect on litter and the modification of the litter effect, as well as the indirect effect and interaction modification of litter on vegetation (Fig. 2b). The direct effect of live vegetation in the absence of any influence of litter is seldom addressed. Although these comparisons differ in the interactions with litter that they encompass, they have not been distinguished as measures of interaction intensity. This lack of differentiation may be problematic because the mechanisms through which litter and vegetation affect resources are often very distinct (Facelli & Pickett 1991). Hence, patterns of interaction intensity across environmental gradients may be very different depending on if, and how, litter effects are incorporated.
It has become clear that litter can strongly influence community structure, although little work has tested empirically how these effects vary across environments. Litter is often thought to interfere with the emergence and growth of seedlings (Knapp & Seastedt 1986; Bergelson 1990; Carson & Peterson 1990) because they may face altered germination cues (Grime & Jarvis 1975), endure phytotoxic effects of the litter leachate (Van der Valk 1986), grow under low light levels, and/or devote energy and time to penetrate the mat (Facelli & Pickett 1991). Thus, increased competitive intensity in productive areas may not be solely due to the direct result of resource competition, but also the result of litter accumulation due to past productivity (Tilman 1987; Carson & Peterson 1990; Foster & Gross 1997). However, in unproductive environments these costs may be outweighed by increased shading, and thus increased water availability (Fowler 1986, 1988; Knapp & Seastedt 1986). These arguments predict that effects of litter would be facilitative in less productive areas but would become negative as productivity increases (Carson & Peterson 1990; Barton 1993; Foster & Gross 1997; Fig. 1b).
Because litter does have strong effects under some conditions, how litter is incorporated into studies of interactions across productivity gradients could strongly influence tests of the predictions described previously. In this paper, we present results of a field experiment designed to examine the direct and indirect effects of vegetation and litter in replicate sites of several community types that differ in a range of environmental characteristics related to productivity. Focusing on the recruitment phase, we asked three questions. (i) Do recruitment levels and the effects of vegetation and litter on recruitment levels vary along this productivity gradient? (ii) If so, how does the incorporation of different types of interactions with litter affect variation in interaction intensity across the gradient? (iii) Is this variation consistent with any of the predictions above?
Most studies have used short-term growth of individuals of one or a few long-lived perennial species to test these hypotheses concerning patterns along productivity gradients (Goldberg et al. 1999). In this study, we tested whether any of these hypothesis are consistent with patterns of early emergence and survival of the entire seedling community. It is important to test these predictions in a range of life-history stages to be able to interpret the population consequences of local processes (Peckarsky et al. 1997; Goldberg et al. 1999). For instance, Goldberg et al. (1999) found that plant survival may show different patterns of competitive intensity along productivity gradients from those exhibited by plant growth. Moreover, examining community-wide patterns, although it misses species-specific information, is more representative of the response of the whole community than a study of one or two species from that community.