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- Author or Editor: Gil N. Cruz x
Knowledge about the position of tree roots is needed to understand tree biology and inform management decisions. Cycad species produce pachycaul stems and are not represented in the published studies on the relationships between aboveground tree traits and root system spread. We measured the radius of root egress from 6-year-old Cycas edentata de Laub., Cycas micronesica K.D. Hill, and Cycas nitida K.D. Hill & A. Lindstrom plants in an ex situ cycad germplasm collection into an 8-m-wide buffer zone in which no trees were growing. We then determined the relationships of root system radius to aboveground plant traits. The root system radius extended 5.2 to 7.5 m from the base of the stems, and was 3.5 times the plant height, 32 to 36 times the stem diameter, and 3.6 to 4.2 times the radius of the canopy dripline. Values for root radius in relation to plant height and canopy radius exceeded published data for leptocaul tree species, but values for stem diameter were less than these data. These findings on the location of cycad roots provide adaptive management cycad knowledge to inform horticulture decisions such as avoidance of roots during soil disturbance and placement of fertilizers and systemic pesticides.
Ubiquitous Aulacaspsis yasumatsui Takagi infestations of Guam’s Cycas micronesica K.D. Hill trees cause direct herbivory of most exposed organ surfaces, including developing naked ovules and seeds. The nonstructural carbohydrates of infested vs. noninfested seeds were quantified to understand more fully the influences on seed quality for propagation purposes. Two studies compared seeds from healthy trees with those of unhealthy trees suffering from whole-tree A. yasumatsui infestations. The sugars fructose, glucose, maltose, and sucrose were in greater concentrations and herbivory reduced these free sugars by a greater percentage in sarcotesta tissue than in sclerotesta and gametophyte tissues. Starch concentration was greatest in gametophyte tissue, but herbivory reduced starch by a greater percentage in sarcotesta tissue. A third study was used to manipulate seeds of unhealthy infested trees such that some seeds were uninfested and some seeds were infested and revealed the nonstructural carbohydrates of the uninfested seeds were greater than those of the infested seeds in patterns that were similar to those when the entire tree was protected from herbivory. The combined results indicated that both source and sink relations were involved in the reductions of seed carbohydrates by A. yasumatsui herbivory. The reduction in seed resource pool by the herbivore feeding may be one of the mechanisms that results in reduced germination percentage and increased seedling mortality.