SHER, ANNA A.1, DEBORAH E. GOLDBERG2*, and ARIEL NOVOPLANSKY3. 1Biology Dpt, U of New Mexico, Albuquerque, NM 87131; 2Biology Dpt, U of Michigan, Ann Arbor, MI; 3Mitrani Dept of Desert Ecology, Ben Gurion U, Sde Boqer, Israel 84990. - Pulsed-resource gradient effects on productivity and competitive response in desert and Mediterranean grass species in Israel.
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