Studies of cycad trees in Colombia, Guam, and the Philippines have revealed how knowledge of their bifurcation behavior can help make conservation decisions for endangered plants.In a study published in the December issue of the journal HorticulturaeScientists at the University of Guam and the Montgomery Botanical Center in Florida have shown that the number of times a cycad tree makes branches can be used to infer the sex of the tree.
The findings have a practical application for the use of the described sexual dimorphism. Cycads are unique seed-producing plants. Conservation activities are being carried out on many species around the world as cycads are threatened by human activity.
Tree-shaped cycad stems are constructed using an approach that botanists call pachycaulous. These stems are thick and have few branches compared to other tree species.
Several historical reports from the botanical garden revealed that more lifelong divergence events occurred in male cycad plants. We decided to examine some of the field data from four dendritic cycad species to determine if this form of sexual dimorphism also occurred in natural habitats. “
Michael Caronier, Research Co-author, Cycad Biologist, Montgomery Botanical Center
A robust analysis of branching behavior from 2,695 cycad trees in the wild confirmed that male trees tended to branch more frequently than female trees. As a result, it was confirmed that the male tree produces two to three times as many branches as the female tree. In fact, 80% of the female cycads of Guam and the Philippines were unbranched.
Understanding how often cycads experience reproductive events helps to interpret these results. This is because divergence often occurs immediately after the generation of reproductive structure.
“Cycad biologists call these structures strobili,” said Benjamin Deloso, a cycad expert at the University of Guam. “The branching behavior that occurs after strobili generation is called isotonicity, which means that one branch is split into two branches of the same size.”
According to Deloso, male cycads produce strobili much more frequently than female cycads, thus accumulating more times over a lifetime in which subsequent isotropic divergence events can occur.
Most natural populations of cycad trees contain approximately 50% male trees and 50% female plants. However, the chronic difficulty in conducting field surveys is that most trees do not show active strobili, making it difficult to clearly determine the sex of wild cycads. The authors argue that understanding branch dimorphism provides new tools that conservation activists can use to better assess population characteristics.
Important implications for Guam’s Cycas micronesica population
New knowledge reveals interesting results from the threat to the protection of cycad micronesica on Guam. Between 2003 and 2005, several non-native pests invaded the island, causing a series of events that killed 96% of the cycad population. Comparing the pre-intrusion branch data with the current branch data, you can see that most of the unbranched trees are killed and a few of the branched trees are killed.
According to Calonje and Deloso, these observations indicate that relentless herbivores by non-native insects preferentially killed more female trees. This information is very important to conservation decision makers, as the recovery of the island’s cycad population can be constrained by a shortage of female trees. The practical application of this new knowledge lies in an ongoing tree rescue project in which cycad trees are being rescued from a construction site. Conservation professionals can use branching data to create more accurate estimates of parity between male and female trees when choosing trees to rescue.
The results also provide examples of the benefits of research on healthy plant populations. For the findings to be most informative, conservationists need knowledge generated from plants and habitats that present no modern threat. That knowledge can inform conservation goals by providing benchmarks that serve as species recovery goals.
Marler, TE & Calonje, M. , (2020) Cycad plant stem divergence informs horticultural and conservation decisions. Horticulturae.. doi.org/10.3390/horticulturae6040065.
Understanding cycad divergence can help endangered plant conservation decisions
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