Tomoko Sakishima, Ph.D. Candidate

Department of Biological Sciences

Plant-plant Interaction in Early vs Late-Succesional Varieties of the Hawaiian Landscape-Dominant Tree Metrosideros Polymorpha

Committee Members:

• Elizabeth Stacy, Ph.D., Advisory Committee Chair
• Jeffery Shen, Ph.D., Advisory Committee Member
• Daniel Thompson, Ph.D., Advisory Committee Member
• Mira Han, Ph.D., Advisory Committee Member
• Amei Amei, Ph.D., Graduate College Representative 

Abstract:

• Plant-plant interactions play an important role in assembling plant communities. Interactions between neighboring plants can vary as a result of the degree of genetic relatedness between them and the influence of mycorrhizal networks, and these interactions can then impact rates of growth and patterns of resource allocation in plants. My research compares the nature of plant-plant interactions, with and without mycorrhizae, within and between early-successional (incana) and late-successional (glaberrima) varieties of Hawaii’s landscape-dominant tree species, *Metrosideros polymorpha*. Incana and glaberrima differ in both population density and the prevalence of mycorrhizal fungi in their native environments and thus in the frequency of plant-plant and plant-mycorrhizal interactions. To examine the nature of these interactions in this system, I grew pairs of seedlings in experimental pots for ~15 months, such that each pot contained a target seedling of either incana or glaberrima and a single neighbor seedling. Growth rates of the target seedlings were measured under three treatments (sib = sibling neighbor, pop = neighbor derives from a different population of the same variety, and var = neighbor is from the opposite variety), as well as from control seedlings grown alone, both with and without mycorrhizal fungi. Additionally, I quantified hyphal growth to assess the relative roles of cooperation versus competition between neighboring seedlings; cooperating seedlings were expected to increase the flow of photosynthesis-derived carbohydrates to their mycorrhizal< symbionts, thus increasing hyphal growth. Growth rates of both varieties< were higher in the presence of mycorrhizae, and overall growth rate and mycorrhizal density were higher for incana than for glaberrima, consistent with their differential adaptation to early- and late-successional environments. Interestingly, cooperation between siblings was observed in glaberrima through increases in root:shoot length ratio, specific leaf area, and mycorrhizal density in the sib treatment relative to the pop and var treatments, as well as through a uniquely positive relationship between target seedling size and neighbor seedling size in the sib treatment. These four lines of evidence suggest that seedlings of late-successional glaberrima may be adapted to recognize and/or respond to genetically different neighbors, while such evidence was minimal or absent in seedlings of early-successional incana. Given that the differential response of seedlings to genetically different neighbors occurred even in the absence of mycorrhizal fungi, mycorrhizal symbionts do not appear to facilitate kin recognition or response in *Metrosideros*. Results of this research provide novel insights into the poorly known world of neighboring< interactions in trees affected by genetic relatedness and successional stage, with implications for forest restoration.