Growth and Its Relationship to Individual Genetic Diversity of Mountain Hemlock (Tsuga mertensiana) at Alpine Treeline in Alaska: Combining Dendrochronology and Genomics

Abstract

Globally, alpine treelines are characterized as temperature-limited environments with strong controls on tree growth. However, at local scales spatially heterogeneous environments generally have more variable impacts on individual patterns of tree growth. In addition to the landscape spatial heterogeneity there is local variability in individual tree genetic diversity (level of individual heterozygosity). It has been hypothesized that higher individual heterozygosity will result in more consistent patterns of growth. In this article, we combine genomics and dendrochronology to explore the relationship between individual genetic diversity and tree growth at a mountain hemlock (Tsuga mertensiana Bong. Carr) alpine treeline on the Kenai Peninsula, Alaska, USA. We correlated average observed individual heterozygosity with average tree-ring width and variance in tree-ring width within individuals to test the hypothesis that trees with higher individual heterozygosity will also have more consistent growth patterns, suggesting that they may be more resilient to climate and environmental fluctuations at the alpine treeline. Our results showed that there was no significant relationship between tree growth and individual heterozygosity. However, there was a significant positive relationship between average tree-ring width and variance in tree-ring width implying that overall, fast growing trees in stressful environments, such as the alpine treeline, grow unstably regardless of the level of individual heterozygosity.

Tree-Ring Analysis and Genetic Associations Help to Understand Drought Sensitivity in the Chilean Endemic Forest of Nothofagus macrocarpa

“…Other studies have jointly analyzed genetic and dendroecological data (e.g., Babushkina et al, 2016;Johnson et al, 2017), and they have not found a significant correlation with growth traits. Despite the absence of a significant relationship, these authors hypothesized that genetic diversity could provide better fitness through increased growth when conditions are favorable.…”

“…Broadly speaking, it is assumed that trees populations with greater genetic diversity have higher growth rates due to growth consistency and resilience to climatic disturbances (Reed and Frankham, 2003). However, there is still no agreement on the relationship between individual genetic diversity and tree growth, with positive relationships (e.g., Rodríguez-Quilón et al, 2015;Neophytou et al, 2016;González-Díaz et al, 2020) or no associations (Babushkina et al, 2016;Johnson et al, 2017).…”

Long‐lived larch clones may conserve adaptations that could restrict treeline migration in northern Siberia

“…The taxa could carry historical adaptations to colder environments making them maladapted to the current strong warming and thereby restricting tundra colonization (Aitken, Yeaman, Holliday, Wang, & Curtis-McLane, 2008;Johnson, Chhetri, Krutovsky, & Cairns, 2017).…”

“…It is argued that this reproduction mode would allow populations to survive adverse times, but also it enables the transmission of genetic information through time (Arnaud‐Haond et al., 2006; Balloux, Lehmann, & de Meeûs, 2003), which comes with a threat: Individuals may find themselves maladapted to new situations. Survival in refugia may come with huge costs: The taxa could carry historical adaptations to colder environments making them maladapted to the current strong warming and thereby restricting tundra colonization (Aitken, Yeaman, Holliday, Wang, & Curtis‐McLane, 2008; Johnson, Chhetri, Krutovsky, & Cairns, 2017). Unfortunately, the roles of genetic adaptation and clonal reproduction of krummholz, which potentially enables millennia‐long survival of single genotypes in the landscape (Laberge, Payette, & Bousquet, 2000), in migration dynamics are not yet understood.…”

“…It is unknown whether the historical genotypes are responsible for the unexpectedly slow responses to global warming, as observed at the Siberian treeline, or how they will shape future migration dynamics. Furthermore, with a subsequent influx of other genotypes (by pollen or seed), hybrids may form (Aitken et al., 2008; effect called gene swamping, e.g., Johnson et al., 2017) bringing another unknown dynamic into play.…”