Abstract
(1) Plants of Mimulus primuloides in populations growing at different altitudes in California have end-of-season values of both total biomass and proportion of biomass in vegetative reproductive organs (vegetative reproductive allocation) which show a maximum value in the middle-altitude population (2500 m). (2) It is suggested that intraspecific competition reduces plant size at low altitudes, resulting in lowered vegetative reproductive allocation; but that a reduction in plant density at the middle altitude allows increased plant size, and therefore increased allocation. At the highest altitude (3400 m), plant size is reduced by the severe environment, again lowering this allocation. (3) Results of controlled environment chamber experiments with plants from five sites grown at three different densities supported this explanation. (4) The number of successful inflorescences per plant decreased with increasing altitude in natural populations. A controlled environment chamber experiment with plants from five sites revealed a positive correlation between total weight per plant and flower weight per plant; a high, genetically-determined, flower production at the highest altitude; and no change in the proportion of biomass allocated to inflorescences with changes in total plant biomass. (5) Although seeds were produced to some extent at all altitudes, seedling establishment was rare. A comparison of vegetative and sexual reproductive allocation in terms of propagules produced showed that the least emphasis on sexual reproduction was at the highest-altitude site. A similar comparison in terms of biomass allocation showed higher allocation to vegetative than to sexual reproduction at all altitudes, but the difference was least at the highest altitude. This increased allocation to sexual reproduction at the highest altitude may be an adaptation for greater potential dispersal or seedling establishment.
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