Award Date

May 2024

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Life Sciences

First Committee Member

Elizabeth Stacy

Second Committee Member

Daniel Thompson

Third Committee Member

Jeffery Shen

Fourth Committee Member

Mira Han

Fifth Committee Member

Amei Amei

Number of Pages

138

Abstract

Plant-plant interactions play an important role in assembling plant communities. Interactions between neighboring plants can vary as a result of the genetic relatedness of neighbors, impacting rates of growth and patterns of resource allocation. When growing alongside close relatives, some species decrease their growth in a form of cooperation, while others grow faster through facilitation. A complication of plant interaction studies arises because decreased growth in the presence of close relatives can also be due to competition for resources, which increases with phenotypic similarity. Further complicating matters, mycorrhizal fungi, through their connections with plant roots, may strongly influence plant interactions.

My research compares the nature of plant-plant interactions within and between early- and late-successional varieties of Hawaiian trees, Metrosideros polymorpha var. incana (hereafter incana) and var. glaberrima (hereafter glaberrima), that differ in population density and the prevalence of mycorrhizal fungi in their native environments. I predicted that seedlings would respond (grow) differentially in the presence of genetically different neighbors and that the pattern of response would differ between the varieties in the presence/absence of mycorrhizae. Germinants of the two varieties were planted in pairs in experimental pots such that each pot contained a target seedling and a single neighbor, with the genetic relatedness of neighboring seedlings varying among treatments. One-half of the pots were supplemented with mycorrhizal fungi, and all were kept under ambient greenhouse conditions. After ~15 months, growth rates of the target seedlings were measured under the 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, hyphal growth was quantified in each pot with mycorrhizae to assess the relative roles of cooperation versus competition; cooperating seedlings were expected to increase the flow of photosynthates (i.e., carbohydrates) to their mycorrhizal symbionts, thus increasing hyphal growth.

Based on the results, an increase in nutrient uptake with mycorrhizal fungi seems to be common in both the early- and late-successional varieties, but the seedling behaviors especially towards siblings seem to be different between incana and glaberrima. The overall growth of both varieties of M. polymorpha was increased, and allocation to root length relative to shoot length was reduced in the presence of mycorrhizae compared to the absence of mycorrhizae, suggesting an increase in nutrient uptake with mycorrhizal fungi. Differences among treatments were completely restricted to the sibling treatment versus the pop and var treatments. In the presence of sibling neighbors, target seedlings of incana and glaberrima had lower and higher root:shoot length ratios, respectively, than those grown with more genetically distant neighbors. Interestingly, root:shoot mass ratios did not vary among treatments for either variety. These results imply that seedlings of glaberrima grown with sibling neighbors prioritized vertical root growth over horizontal root growth to obtain nutrients. Also in the sibling treatment, but in the absence of mycorrhizal fungi, higher specific leaf area (SLA) of target seedlings was observed in both incana and glaberrima, but statistically supported only in glaberrima. In glaberrima, both the greater SLA of target seedlings grown with sibling neighbors and the constant shoot length among treatments suggest the ability of seedlings of this variety to increase light capture without shading sibling neighbors. Further, with a single exception, the sizes of target and neighboring seedlings were negatively correlated as expected, due to competition; the exception was the sibling treatment for which there was a significant positive relationship for glaberrima alone, not incana. Finally, the greatest mycorrhizal hyphal length was observed in both varieties in the sibling treatment, suggesting cooperative behavior, yet this increase was statistically significant only in glaberrima. 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.

Beyond their responses to neighbor genetic relatedness, incana and glaberrima seemingly have different strategies for resource allocation affecting growth of both seedlings and their mycorrhizal symbionts that is consistent with their differential adaptation to early- and late-successional environments. Higher overall growth rates (i.e., final sizes) were observed in seedlings of incana relative to glaberrima regardless of neighbor treatment or the presence or absence of mycorrhizal symbionts. Rapid seedling growth in incana may reflect adaptation of this variety to the harsh conditions of early-successional environments where seedling establishment is likely restricted to occasional, brief periods of favorable conditions. In addition, mycorrhizal hyphal density was higher in incana-target pots relative to glaberrima-target pots. Despite a lack of correlation between total dry mass and hyphal density for incana or glaberrima separately or combined, the relatively higher growth rates of both target seedlings and mycorrhizal fungi in the incana pots may suggest a mutually positive relationship between incana and mycorrhizal fungi. Such a relationship may be expected in incana given the mycorrhizae-limiting nature and low population density characteristic of early-successional environments in Hawaii, where rapid seedling growth may be favored and can occur without intensifying competition among well-spaced neighbors. In contrast, growth of seedlings of glaberrima and their mycorrhizal symbionts may be restricted to minimize competition for resources in the high population densities of late-successional forests. 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.

Keywords

kin recognition; kin selection; Metrosideros; mycorrhizal symbiosis; plant succession; trees

Disciplines

Communication | Environmental Sciences | Plant Sciences | Terrestrial and Aquatic Ecology

Degree Grantor

University of Nevada, Las Vegas

Language

English

Rights

IN COPYRIGHT. For more information about this rights statement, please visit http://rightsstatements.org/vocab/InC/1.0/

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