Experimental evidence suggests that competition among plant species is generally hierarchical and that relatively large species are at a competitive advantage when competition is predominantly above‐ground. However, regional species pools are dominated numerically by relatively small plant species, and small species generally have higher local densities of resident plants within natural communities. One explanation is that larger plant species suffer disproportionately more under effects of intra‐specific competition (i.e. greater density dependence). We tested this prediction using ten herbaceous plant species in a competition experiment.
Kingston, Ontario, Canada, glasshouse.
Using a glasshouse experiment, we tested whether relatively large species suffer disproportionately more in monoculture relative to mixtures of all ten herbaceous plant species. We measured the effects of competition on biomass production and survival by monitoring both in monocultures and mixtures of our species.
Larger plant species suffered more under intra‐specific relative to diffuse inter‐specific competition in terms of survival; however, the slope of this relationship was not significantly greater than one, indicating that larger species did not suffer disproportionate density‐dependent suppression.
Our results support a role for size in plant competition, but also indicate that this role is reduced because relatively larger species suffer greater density‐dependent mortality when competing with other, equally large plants. As such, size‐based competitive hierarchies may not function as clearly in natural systems because the increased negative density dependence for larger species contributes to balancing out competition across size hierarchies.
Ten study species were chosen — all herbaceous perennials that commonly co-exist in old-field vegetation in eastern Ontario — with a range of species sizes and abundances spanning about two orders of magnitude (Fig. 1), and a range of seed sizes (mass) spanning three orders of magnitude (Table 1). These included two perennial grasses, seven non-nitrogen fixing forbs, and one nitrogen-fixing forb. Sufficient seed for these species was purchased from commercial suppliers (Prairie Moon Nursury, Winona, MN, USA; Ontario Seed Company, Kitchener, ON, Canada). All seeds were stored in refrigerated conditions (three were stored wet as recommended) for a period of 6 wk prior to starting the experiment to simulate natural over-wintering conditions. Germination tests were conducted to determine the percentage of seeds expected to germinate for each of the ten species. These tests used five replicates of 50 seeds per species placed in pots and soil similar to those used in the experiment and monitored for 4 wk in a growth chamber. Growth chambers were maintained on a 16:8 h light:dark schedule at 25 °C, similar to greenhouse conditions for the experiment. For each species, the number of seeds sown experimentally wasadjusted to species-specific germination rates such that we could expect approximately the desired number of seedlings of each species for each treatment (1, 100, 1000 individuals).
Growth and survival of the ten species were compared based on relative performance in monocultures and polycultures (henceforth mixtures) with the same plant density. Five monoculture replicates with projected initial densities of 1, 100 and 1000 individuals per pot (4.87 L volume; 25.4 cm diameter; 125.3 cm2 area) were produced for each of the ten species (150 pots). Additionally, using the same pot size, 30 mixtures were prepared with projected total initial densities of 1000 individuals per 125.3 cm2 and projected per-species initial densities of 100 individuals. Densities of 1000 individuals per pot were chosen to exceed the maximum density of individual plants (rooted units) observed in the field (B.S. Schamp and L.W. Aarssen unpublished data). Field abundance was determined by counting the number of rooted units per species in 100 randomly located 25 9 25-cm plots, and maximum biomass per species was determined as the maximum dry mass accumulation (g) at flowering among 25 randomly selected samples per species. Rooted units were counted by enumerating the number of transitions between aboveground shoot and below-ground roots for each species. Densities of 100 individuals per pot were used to confirm that 1000 plant monoculture pots were indeed undergoing competition for space and/or light (Fig. 2) and to track germination success. A total of 180 pots were filled with a mixture of Premiere ProMix© consisting of 75–85% Sphagnum peat moss, perlite and vermiculite, mixed 3:1 with autoclaved sand.
All pots were seeded on 24 May 2005 in the greenhouse at Queen’s University. Pots were initially top watered carefully to minimize the movement of seeds, avoid pooling and prevent the formation of soil topography in pots. All 180 pots were placed on two greenhouse tables. Pots were aligned in six rows of 15 per table, with the long axis (15 per row) oriented approximately east– west, and were spread to four tables after germination and randomized alternately within, and then between tables, biweekly. Pots were top watered daily with RO water for the first 2 wk of growth and bottom watered thereafter (in an individual tray for each pot). Additionally, all pots were given approximately the same volume of 20-20-20 NPK solution weekly. The greenhouse was maintained at 16 h of daylight at 25 °C and at 20 °C for the remaining 8 h.