Vegetative reproduction as related to the biomechanics and anatomy of stem junctions for Opuntia species in southern California

Vegetative reproduction via the rooting of detached stems is widely recognized for certain Opuntia (Cactaceae) species. Vegetative reproduction for opuntias is facilitated by the small connections (junctions) between their flattened or cylindrical stem segments (cladodes or joints, respectively). Because of their smaller cross-sectional area, junctions are considered to be especially susceptible to mechanical failure, which can lead to vegetative reproduction because Opuntia stems can root. Thus, taxa that utilize vegetative reproduction prolifically were hypothesized to have weaker stem junctions than other taxa. The ability to reproduce vegetatively was investigated for two platyopuntia species (opuntias with cladodes) and their hybrid in the chaparral of southern California and for four sympatric cholla species (opuntias with cylindrical stems) in the Sonoran Desert.;Hybridization between the introduced arborescent Opuntia ficus-indica and the shrubby native O. littoralis has resulted in populations, termed i that can form large thickets, which allow them to survive the periodic fires of the chaparral. Biomechanically, the cladode junctions of the hybrid deflected more under applied loads and failed mechanically under smaller bending moments than either of its parent species. The junctions of O. "occidentalis" also had smaller wood cross-sectional areas and the wood had fewer cells that function in support and reduce cellular shearing than did junctions of O. ficus-indica and O. littoralis. Thus, wood characteristics can explain why O. "occidentalis" has weaker junctions than O. ficus-indica and O. littoralis, which increases its ability to reproduce vegetatively.;Vegetative reproduction by the rooting of detached terminal joints of Opuntia acanthocarpa, O. bigelovii, O. echinocarpa, and O. ramosissima was related to the morphology of the terminal joints and their joint junctions, the biomechanics of their joint junctions, and the rooting ability of detached terminal joints. Of the four species, only Opuntia bigelovii (teddy-bear cholla) utilized vegetative reproduction in the field. Opuntia bigelovii was also the only species that had readily detachable terminal joints with a high rooting ability. Terminal joints of O. bigelovii also had the greatest number of spines per stem area, which would facilitate lodging into the coat or skin of a passing vertebrate for dispersal.