The literature on the importance of competition across productivity gradients has yielded conflicting results. This inconsistency may be due to the tendency of these studies to define productivity using variation in mean resource levels, whereas in nature, resources are usually heterogeneous over time. Earlier studies have demonstrated that the timing of resource availability can have as large an effect on plant survival and growth as total amount for both arid and Mediterranean species. Given this, community productivity and the relative importance of competition should be affected by the timing of resource renewal. We investigated the effect of timing and total amount of water application for growth and survivorship of plants from two congeneric pairs of grasses, each with a species from a low (desert) and a high (Mediterranean) productivity community, and each growing alone and with diffuse competition. We tested the hypothesis that the length of the intervals between resource-renewal events (interpulse periods) can have as large an effect on productivity and the intensity of competition as changing total amount of resource available. The highest target survival and growth was generally found at high water levels and intermediate interpulse lengths as predicted, but the effects of water treatment on competitive dynamics differed between species. Although the intensity of competition increased with increased productivity in some cases, the variation in response of competition intensity to pulsing was considerable between genera and source sites. Such differences in competitive response illustrate a possible source of inconsistency in the literature, because species differ in their response to the temporal aspect of resource renewal.

Key words: Cutandia, Negev desert, plant competition, productivity gradient, resource pulse, Vulpia