Stem tilting, pseudocephalium orientation, and stem allometry in Cephalocereus columna-trajani along a short latitudinal gradient

Cephalocereus columna-trajani is a giant columnar cactus endemic of the Tehuacán-Cuicatlán Valley in Central Mexico. Stem tilting and northward pseudocephalium azimuth in C.␣columna-trajani have functional advantages in terms of interception of direct solar radiation at the northernmost portions of its range. Since the success of both characters strongly depends on the apparent position of the sun during the growing season, in this paper we test the hypothesis that the occurrence of such columnar morphology is restricted geographically and imposes mechanical restrictions that limit column height. Following a latitudinal gradient along the Tehuacán-Cuicatlán Valley, we selected five populations, recorded tilting angle and pseudocephalium azimuth, and carried out allometric and biomechanical analyses of height–diameter relationships. Northern populations showed higher tilting angles. Pseudocephalium azimuth significantly differed among populations, and pseudocephalium orientation was consistently North-Northwestern. Stem allometry showed that the stems of the southern populations increased in height at a far greater rate with respect to diameter than the northern populations. The southernmost population showed the lowest safety factor. These results support the hypothesis that stem tilting in C.␣columna-trajani is functionally advantageous in a restricted geographical range, and imposes mechanical restrictions to column height.