Gimondo, J. Austin (2022) Biotic and abiotic risks to Dirca spp. in managed landscapes. PhD Dissertation, Horticulture, Iowa State University.
As the sole genus of the Thymelaeaceae in the continental United States, Dirca L. represents an important component of the diversity of the region. And while Dirca spp. have noted ornamental value these shrubs have not been utilized by the horticulture industry. Incorporating new species of conservational importance, such as Dirca spp., has multifaceted benefits, but is not without risks. The research contained herein is aimed at addressing selected risks associated with the utilization of Dirca spp.—that of supply, potential distribution in relation to cold hardiness, and possible pests. The first objective of this work was identifying how environmental conditions influence phenology of a species of Dirca and how that phenology relates to fruit set of these shrubs. Through tracking phenology and fruit set through multiple years, I demonstrate that low autumnal precipitation is associated with delayed growth resumption and flowering, which corresponds with increased fruit set of this rare species. The next objective was evaluating how pollinator presence affects the fruit set of D. mexicana in central Iowa. I introduced pollinators into cages with plants of D. mexicana to measure the change in fruit set compared to plants where pollinators were excluded and to open-pollinated plants. Compared to a pollinator-excluded treatment, the inclusion of pollinators or open-pollination increased probability of fruit set by more than ten times, but fruit set did not differ among pollinator types. The third objective involved developing a means to analyze pollen viability and longevity of Dirca and quantifying those traits. By testing multiple types of artificial media and other pollen-germination strategies, my results indicate that pollen longevity may be a contributing factor to the low fruit set observed in D. mexicana planted in temperate regions. Measuring the extent of damage that the leaf miner L. dircella causes in populations of D. occidentalis was the fourth objective. Damage was observed at multiple sites containing D. occidentalis, but the extent of damage varied among locations. The final objective of the dissertation was assessing the cold hardiness of Dirca spp. in central Iowa, including selections of particular interest for horticultural use. All the study plants acclimated quickly and deeply enough to endure local average and record low temperatures throughout the winter. In this work I have identified that if the barriers to seed production of Dirca spp. can be overcome, the multifaceted benefits of using these taxa far outweigh the few risks associated with the use of this genus. [link to publication]