Croton californicus is a drought-deciduous, subshrub inhabiting sandy soils of southwestern North America. Previously described as a dioecious species, monoecious individuals have been reported and were found in all populations. Existence of monoecious morphs indicates an unusual breeding system and the possibility of labile sex expression in C. californicus. This study demonstrates sex lability in response to various environmental influences. Three experimental populations in southern California were studied monthly during the 1996-97 season. Two inland populations were located in costal sage scrub communities and one desert population in creosote bush scrub. Ten plants (five males and five females) were assigned to each of three treatments and a control group for a total of 40 plants per population. Plants chosen for the experiment were flowering and similar in size. The three experimental treatments were as follows: 1) crown pruning, 2) monthly watering, and 3) applications of gibberellic acid. Combined treatment groups contained a significantly greater (c2 = 4.44, df = 1, P < 0.05) number of sexually labile plants when compared to the control. Plants in the control group did not exhibit sex lability. The desert population contained significantly more (c2 = 6.74 , df = 2, P < 0.05) sexually labile individuals when compared to the inland populations. Results indicate that sex expression is environmentally influenced in C. californicus and that pruning and gibberellic acid treatments appear to have a greater effect than watering. These data are consistent with the idea that increased sex lability may be an adaptive response in extreme environments.

Key words: Croton californicus, environmental effects, Euphorbiaceae, sex expression, sex lability