Effects of water pulsing on individual performance and competitive hierarchies in plants

In a glass house experiment, we investigated the effect of both the frequency of water pulses and the total amount of water supplied on individual performance in the absence and presence of neighbors. We used monocultures and all combinations of pairs of seedlings of three species of perennial grasses, characteristic of different points along a soil moisture gradient within a semi-arid grassland in New Mexico, USA. In the absence of neighbors, higher total water or more frequent (but smaller) pulses significantly increased growth of all three species. The species with the fastest intrinsic growth rate, and from the most productive habitat, exhibited the largest increase in absolute and relative growth in response to higher total water quantity. Competitive effects were highly significant overall and there were significant hierarchies of competitive ability. Under frequent pulses, the fast-growing species from the most productive environment was the best competitor in terms of both ability to suppress other plants and ability to tolerate the presence of neighbors. However, under infrequent pulses, the slowest growing species from the least productive environment became a much stranger competitor, again in terms of both suppression and tolerance of neighbors. While differences in total water availability had greater effects than differences in pulsing regime on individual plant performance in the absence of competition, pulsing regime had much stronger effects on relative competitive abilities and thus may be more likely to influence field distribution patterns.