Factors affecting seed production by the endangered chaparral shrub Fremontodendron californicum subsp. decumbens (Sterculiaceae)

Insect herbivory can greatly decrease plant reproductive success. This experiment reduced insect herbivore damage to branches of the federally endangered Pine Hill flannel bush (Fremontodendron californicum subsp. decumbens) during two flowering seasons to explore interactions among factors that can limit reproductive output (e.g., resource limitation, pollen limitation, seed predation). Experimental treatments included insecticide and control (water spray) treatments in 1983, with these and an insecticide plus exclosure treatment in 1984. Exclosures were designed to decrease herbivory of fruits by rodents. Reproductive stages (flower buds, flowers, and fruits) were counted and examined periodically for damage, and mature fruits were collected and seeds counted. In both years, insecticide treatment greatly decreased insect herbivory, increasing survival of flower buds, flowers, and fruits so that seed production increased 14-to 27-fold. Pollination success was unaffected by treatments, but fruit abortion increased significantly on insecticide-treated branches. Rodent herbivory of fruits greatly impacted fruit production on insecticide-treated branches in 1983, an El Niño-Southern Oscillation (ENSO) year, but was relatively unimportant in 1984 (a non-ENSO year). I concluded that: 1) seed output is limited by insect herbivores and can be greatly increased by insecticide treatment; 2) other limiting factors (resource limitation, rodent herbivory) increase in importance and partially compensate for increased fruit survival when insect herbivory is lessened by insecticide treatment; and 3) rodent fruit herbivory may be influenced by climate, being greater during years characterized by an ENSO event.