PARENT SESSION
Oral Session #28: Invasions - Resistance and facilitation.
Presiding: D. Holway
Tuesday, August 6. 8:00 AM to 11:30 AM. Gila Meeting Room, TCC.
Abiotic and biotic resistance to plant invasions in vernal pools.
GERHARDT, FRITZ*,1, COLLINGE, SHARON1, 1 University of Colorado, Boulder, Colorado
ABSTRACT- To understand why some communities are more vulnerable to invasions than others, we evaluated the relative importance of abiotic constraints and biotic resistance in determining community invasibility. We tested three hypotheses: 1) abiotic constraints alone determine the invasibility of different sites, 2) abiotic constraints and biotic interactions determine invasibility at different sites along abiotic gradients, and 3) the strength and direction of biotic interactions determining invasibility differ along abiotic gradients. To test these hypotheses, we conducted field experiments with six exotic plant species in 11 natural and 11 restored vernal pools at Travis AFB in the Central Valley of California. Early in the growing season, exotic plants were negatively affected by abiotic factors associated with increasing water depth. Because most species germinated prior to flooding, germination was generally not affected by water depth. However, germination of Rumex crispus, which occurred more slowly, was greater at shallower depths. Survival of three species was greater at shallower depths, and three species also produced more leaves and grew taller at shallower depths. Later in the growing season, interactions with the surrounding plant community played a more important role in determining exotic plant performance. Although germination was not affected by the presence of neighbors, survival of Rumex crispus was greater without neighbors, and Lolium multiflorum produced more leaves without neighbors. Collectively, these experiments suggest that the invasibility of vernal pools is determined by a complex interaction of abiotic constraints and biotic interactions with the existing plant community. Early in the growing season, exotic plants are unable to tolerate the abiotic conditions in the pools, but interactions with other plants plays a more important role in determining the success of invaders later in the growing season.
KEY WORDS: abiotic constraints, biotic resistance, community invasibility, vernal pools