Competition can vary in intensity depending on resource availability and type

Plants can compete for aboveground resources, such as light, as well as for belowground resources, such as soil nutrients. Researchers have suggested that the relative importance of aboveground and belowground competition in plants might change depending on whether aboveground or belowground resources are more scarce: belowground competition, for example, might be expected to increase in intensity when the competing plants are growing in nutrient-poor soils.

Wilson and Tilman selected a series of 5 ? 5 m plots of natural vegetation growing in sandy, nitrogen-poor soils. For 3 years, they treated half of the plots with high-nitrogen fertilizer each year. This 3-year period gave the plant communities in the fertilized plots time to adjust to the experimentally imposed change in soil nitrogen levels. At the end of the 3-year period, they planted Schizachyrium individuals in all the plots.

Once they were added to the high-nitrogen (fertilized) and low-nitrogen (unfertilized) plots, Schizachyrium individuals were grown under three treatments: (1) with neighbors present (competition), (2) with neighbor roots present but neighbor stems tied back (which prevented light competition with Schizachyrium), or (3) with neighbor roots and stems both removed (no competition). Wilson and Tilman found that while total competition (the sum of belowground and aboveground competition) did not differ between the low- and high-nitrogen plots, belowground competition was most intense in the low- nitrogen plots (FIGURE 14.5A). They also found that aboveground competition increased when light levels were low (FIGURE 14.5B). Thus, their work demonstrates that the intensity of competition can increase when the particular resource being competed for is scarce.

FIGURE 14.5 Resource Availability Affects the Intensity of Competition (A)Intransplant experiments with the grass Schizachyrium scoparium, belowground competition between plant species for nutrients increased in intensity when soil nutrients were scarce. (B) Similarly, aboveground competition for light increased as light levels decreased. (After S. D. Wilson and D. Tilman. 1993. Ecology 74: 599-611.) View larger image