Intraspecific Competition in Chenopodium album Varies with Resource Availability

-T~~ intensity of intraspecific competition under different levels of light and nitrogen supply was assessed for the temperate zone herb, Chenopodium album. Plants were grown at two densities (one or six plants/pot) in a modified factorial design with three levels each of light and nitrogen availability. This yielded five light/nitrogen treatments which comprised two gradients, one extending from low light and nitrogen to conditions of high light and nitrogen, the other from conditions of low light/high nitrogen to conditions of low nitrogen/high light. Competition among plants grown six per pot significantly affected growth in all treatments and was most intense where resource supply was greatest. When one or both resources were limiting, competition was less intense. This pattern held whether assessed for total biomass, aboveground biomass, or belowground biomass only. Thus, it seems that the relative amount of resource limitation, not the source (light or nitrogen limitation), determined the intensity of intraspecific competition for this species. Those resource conditions most conducive to intense competition were also the most conducive to high productivity (dry biomass of plants grown alone). The pattern of competitive intensity observed in Chenopodium album suggests that the intensity of intraspecific competition is greatest under optimal resource conditions. M7e suggest that patterns of intraspecific competition may be important in community dynamics, and that the interpretation of future interspecific competition studies would be more complete if intraspecific competition were also considered. The importance of intraspecific competition to the ecology and evolution of species is widely recognized. However, most investigations of intraspecific plant competition have focused on tests of the -3/2 thinning rule (e.g., Kikuzawa, 1988; Firbank and Watkinson, 1990), on the development of size hierarchies (e.g., Weiner, 1985; Weiner and Thomas, 1986; Jurik, 1991), on the characteristics of neighborhood competitive interactions (e.g., Pacala and Silander, 1985; Silander and Pacala, 1985), or on variation in competitive ability among ecotypes under different environmental conditions (e.g., Clay and Levin, 1986; Linhart, 1988). Mechanistic studies of the effect of resource availability on the intensity or importance of intraspecific plant competition in nonagricultural species are particularly few (Clay and Levin, 1986). For many species, especially those that establish dense, primarily monospecific stands, intraspecific competition can be critical to the growth, survival and reproductive success of individuals (e.g., McAuliffe and Janzen, 1986; Ungar, 1992; Aguilera and Lauenroth, 1993). In contrast to intraspecific competition, the importance and intensity of interspecific plant competition under different resource availability regimes, and the effects of interspecific plant competition on community structure, have been investigated by numerous researchers. Some of these studies show that at low resource levels interspecific competition between individuals is less intense (Grime, 1979, 1987; Wilson and Keddy, 1986; Wilson, Present address: Department of Ecology and Evolutionary Biology, University of Connecticut, Storrs 06269 1988; Reader and Best, 1989; Goldberg, 1990; Aerts et al., 1991; Campbell et al., 1991), perhaps because under such conditions individuals are too sparse or slow-growing to affect neighbors strongly. Other studies (Tilman, 1988; Tilman and Cowan, 1989; Wilson and Tilman, 1991; Reader et al., 1994) show that interspecific competition is equally intense along a gradient of increasing plant productivity (defined as standing biomass). The latter studies suggest that resource limitation typically shifts from belowground to aboveground as light limitation increases and nitrogen limitation decreases. Studies of interspecific plant competition often either do not consider the importance of intraspecific competition (e.g., Reader et al., 1994), or use plants grown in monoculture as control treatments (e.g., Tilman and Cowan, 1989; Aerts et al., 1991; Campbell et al., 1991; Wilson and Tilman, 1991; M7ilson and Tilman, 1993). In these studies, different interspecific competition mixtures are compared to the intraspecific competition control. However, changes in the dynamics of intraspecific competition along density or resource supply gradients are rarely explicitly examined within the monoculture control groups. The intensity of intraspecific competition probably varies across density or resource supply gradients, and such changes might influence interpretation of interspecific interactions. The objective of this study was to test whether the intensity of intraspecific plant competition varies under different conditions of resource supply. Because limiting resources such as light and soil nutrients often interact in their effects on plant traits (Peace and Grubb, 1981; Swank and Oechel, 1991), and because it has been proposed that competition for aboveand belowground resources changes over resource supply gradients (Tilman, 1988; Wedin and Tilman, 1993), we considered the effects of both aboveand belowground resource limitation on the intensity of intraspecific competition. Resource effects and the intensity of intraspecific competition were assessed by growing Chenopodium album alone and in competition with conspecific neighbors in a modified factorial design with three levels each of light and nitrogen availability.