Stem Branching of Cycad Plants Informs Horticulture and Conservation Decisions

Abstract

The number of branches in male and female plants of Cycas micronesica K.D. Hill, Cycas edentata de Laub., Cycas wadei Merr., and Zamia encephalartoides D.W. Stev. were counted in Guam, Philippines, and Colombia, to confirm earlier reports that female plants develop fewer branches than males. Cycas plants produce determinate male strobili and indeterminate female strobili, but Zamia plants produce determinate strobili for both sexes. More than 80% of the female trees for each of the Cycas species were unbranched with a single stem, but more than 80% of the male trees exhibited two or more branches. The mean number of branches on male plants was more than double that of female plants. The number of branches of the Zamia male plants was almost triple that of female plants. Moreover, the Zamia plants produced 2.8-fold greater numbers of branches than the mean of the Cycas plants. Most of Guam’s unsexed C. micronesica trees in 2004 were unbranched, but after 15 years of damage from non-native insect herbivores, most of the remaining live trees in 2020 contained three or more branches. The results confirm that male Cycas and Zamia plants produce more branches than female plants and suggest cycad species with determinate female strobili produce more branches on female plants than species with indeterminate female strobili. Our results indicate that the years of plant mortality on Guam due to non-native insect herbivores have selectively killed more female C. micronesica trees. Horticulture and conservation decisions may be improved with this sexual dimorphism knowledge.

Biotic Threats to Cycas micronesica Continue to Expand to Complicate Conservation Decisions

“…The pachycaulous cycad stem is uniquely designed with a large primary thickening meristem and very few branches [ 64 ]. In fact, more than half of Guam’s female C. micronesica trees are unbranched [ 65 ]. Stem height growth is relatively slow, averaging about 3 cm per year for Guam’s population [ 66 ].…”

“…The consequences of non-native insect herbivores on a native host tree are not restricted to mortality. We have reported that chronic herbivory of C. micronesica by these non-native insects has caused reductions in stem carbon dioxide efflux [ 90 ], has selectively killed more unbranched trees than branched trees to indicate more female trees have died than male trees [ 65 ], has compromised the success rate of adventitious root formation on stem cuttings [ 91 ], and has reduced the rate of tree height growth [ 66 ]. Additionally, we have reported substantial changes in leaf litter chemistry following herbivory in a manner that predicts increased decomposition rates [ 92 , 93 ].…”

Special Issue: Feature Papers 2020

“…Conservation of unique plant species has significant ecological and horticultural implications. Marler and Calonje [14] observed that male Cycas and Zamia plants produced more branches than female plants, and cycad species with determinate female strobili produced more branches on female plants than species with indeterminate female strobili. Horticultural and conservation decisions may be improved with this sexual dimorphism knowledge.…”

Aulacaspis yasumatsui Delivers a Blow to International Cycad Horticulture

“…Branching behaviors of cycad trees are unique, and many female trees remain unbranched for life. In contrast, most male trees exhibit several points of dichotomous branching [63]. The pre-invasion Guam populations of C. micronesica were comprised mostly of unbranched trees, but after 15 years of A. yasumatsui damage most of the remaining live trees contained three or more branches.…”