Stem succulence controls flower and fruit production but not stem growth in the desert shrub ocotillo (Fouquieria splendens).

PREMISE OF THE STUDY

The C desert shrub ocotillo (Fouquieria splendens) completely lacks xeromorphic leaves but is uncommonly both stem succulent and repetitively drought deciduous (documented to have produced many foliation-defoliation cycles during a growing season). Both adaptations conserve water in this xerophyte, but are the roles of succulence and deciduousness merely redundant? The observation that year-to-year reproductive effort was relatively consistent while vegetative growth was not offered a critical clue that, coupled with long-term precipitation data, helped answer this question.

METHODS

At two sites in the Chihuahuan Desert in southern New Mexico, United States, 22 ocotillos were studied annually for more than two decades to explore the relationships among reproductive effort, vegetative stem growth, and patterns of precipitation.

KEY RESULTS

Vegetative stem growth occurred in mid- to late summer (July-September), the season of maximum precipitation in the Chihuahuan Desert, and was significantly related to summer precipitation received in the year of growth. Reproductive effort occurred in early to late spring (April-June), which with winter account for minimum precipitation during the year, but was significantly related to summer precipitation received in the previous year, suggesting the importance of stem succulence and stored water.

CONCLUSIONS

While highly variable summer precipitation was responsible for enormous fluctuations in annual ocotillo stem growth, stem succulence insulated reproductive effort from such immense variability. Stem-stored water allowed the production of flowers and fruits to proceed relatively consistently during the driest years and during the driest time of year in the Chihuahuan Desert.