Stem tilting and pseudocephalium orientation in Cephalocereus columna-trajani (Cactaceae): a functional interpretation

This paper analyzes the functional implications of stem tilting and pseudocephalium orientation in the giant columnar cactus Cephalocereus columna-trajani. This species shows a consistent northern orientation of its pseudocephalium (a nonphotosynthetic hairy structure where flowers are produced) and stem tilting in the same direction. Analysis of pseudocephalium orientation was made on field data gathered from subpopulations of C. columna-trajani from slopes with different exposures. Additionally, from morphometric characteristics measured in the field, a model cactus was constructed with the purpose of simulating radiation interception by different morphologies. Variations of this model cactus allowed the simulation of irradiance on erect and tilted cacti, as well as on plants with varying pseudocephalium orientation. Results of irradiance interception by different morphologies were related to actual data of growth rates, flowering period, and rainfall and temperature patterns on the study zone. Sampled individuals of C. columna-trajani showed a significant north-northwest pseudocephalium orientation (angular mean = 339⚬± 22⚬). Simulations showed that tilted cacti with pseudocephalia facing northwards increase yearly interception of direct solar radiation by the whole plant compared to erect cacti with or without a pseudocephalium (2 and 7% increase, respectively), and with tilted cacti with the pseudocephalium facing away from the north (9-10% increase). Additionally, the observed morphology decreases radiation interception during the hottest and driest period of the year. From our results, pseudocephalium orientation and stem tilting in C. columna-trajani appear to be morphological adaptations that allow the fine-tuning of a columnar morphology to its thermal and radiation environment. However, the cost of tilting in this giant columnar cactus is that branching (which increases photosynthetic area and reproductive output) appears to be almost impossible without serious risk of stem breakage.