Winner and losers: examining biotic interactions in forbs and grasses in response to changes in water and temperature in a semi-arid grassland

Abstract

Warming and changing water amount can alter the outcome of biotic interactions in native and exotic plants between facilitation and competition. Exotic plants may adapt better to changing environmental conditions, such that they may compete better than native plants. We conducted competition trials for four plant species, two exotic forbs (Centaurea stoebe and Linaria vulgaris) and two grasses (exotic Poa compressa and native Pseudoroegneria spicata), commonly found in Southern interior British Columbia. We compared the effects of warming and changing water on target plant shoot and root biomass, and on pair-wise competitive interactions among all four species. We quantified interactions using the Relative Interaction Intensity index, which has values from -1 (complete competition) to +1 (complete facilitation). C. stoebe biomass was highest under low water and no competition. Facilitation of C. stoebe was found under high water and low temperatures but shifted to competition under low water and/or warming. Competition in L. vulgaris decreased due to reduced water and increased due to warming. Grasses were less competitively suppressed by warming but more competitively suppressed by reduced water input. The response of exotic plants to climate change can differ by plant species, moving in opposite directions for both forbs, but grasses appear to respond similarly. This has consequences for grasses and exotic plants in semi-arid grasslands.

Keywords: Climate change; exotic; forbs; grasses; native; warming; water.

Figure 1.

Mean ± 1 SE of (A) shoot biomass and (B) root biomass of the four plants in this experiment in response to water input and competition. Bars sharing the same letter are not significantly different at P < 0.05. Bars without letters show no significant interaction effects between water and competition.

Figure 2.

Mean ± 1 SE of relative interaction indices (RII) calculated using (A) shoot biomass and (B) root biomass of spotted knapweed (C. stoebe) and Canada bluegrass (P. compressa) in response to changes in water input and temperature. Bars sharing the same letter are not significantly different at P < 0.05. Bars without letters show no significant interaction effects between water and temperature.

Figure 3.

Mean ± 1 SE of relative interaction indices (RII) calculated using (A) shoot biomass and (B) root biomass of yellow toadflax (L. vulgaris) and bluebunch wheatgrass (P. spicata) in response to changes in water input. Bars sharing the same letter are not significantly different at P < 0.05. Bars without letters show no significant effects of water input.

Figure 4.

Mean ± 1 SE of relative interaction indices (RII) calculated using (A) shoot biomass and (B) root biomass of yellow toadflax (L. vulgaris) and bluebunch wheatgrass (P. spicata) in response to changes in temperature. Bars sharing the same letter are not significantly different at P < 0.05. Bars without letters show no significant effects of temperature.