Reading between the rings: climatic and biotic controls of shrub growth and expansion in the tundra biome

Abstract

The tundra biome has undergone dramatic vegetation shifts in recent decades, which have been partly attributed to climate warming. Shrub species in particular are expanding widely throughout the Pan-Arctic region, and are involved in complex vegetation-atmosphere interactions that have important implications for the global energy balance and carbon budget. However, projections of vegetation change and associated feedbacks are complicated by the high variability in the sensitivity of shrub growth to temperature among sites and species. A mechanistic understanding of the individual-to-regional controls of climate sensitivity is therefore needed to accurately predict future vegetation change at the biome scale. This thesis quantifies the influence of environmental and ecological factors, and especially of plant-plant interactions, on the growth response of Arctic shrub communities to climate change. Climate change in the Arctic has resulted in warmer, but also longer growing seasons in many locations due to earlier snowmelt. These two factors are often treated as one single control of plant growth, but with scarce records of green-up and senescence dates for the Arctic, few studies have measured the sensitivity of shrub growth to changes in growing season length. Using radial growth time series from over 300 shrubs collected at four sites of contrasting climatic regimes and greening trajectories in Northern Canada, I measured the sensitivity of shrub growth to summer temperature and satellite-derived growing season length. I found that growing season length and summer temperature were decoupled within sites and had inconsistent effects on growth across the four sites. My findings indicate that longer and warmer growing seasons do not necessarily act as combined drivers of vegetation change across the biome. My research also demonstrated that growth at the root collar of shrubs is more climate sensitive than stem growth, possibly indicating differential internal resource allocation strategies, and highlighting the importance of standardised protocols when comparing dendroecological data across multiple sites. Individual and species traits are thought to play an important role in the response of tundra vegetation to climate change. Taller shrub species have been shown to be more climate-sensitive than dwarf shrubs, but whether this relationship holds at the individual level is unknown. I tested whether plant size, as a proxy for competitive ability, explained variation in the climate sensitivity of shrub growth using 1085 individual size and growth-ring records from 16 species at 18 sites across the tundra biome. I did not find evidence that taller shrubs were more climate sensitive, and found that height became a progressively poorer predictor of other growth dimensions at higher latitudes. This suggests that predictions of functional and structural change based on allometric equations from boreal or sub-Arctic populations may not be valid for the tundra biome as a whole. Plant-plant interactions are a strong driver of community dynamics. With increasing shrub densities in the circumpolar region, competition could have an increasingly important influence on shrub growth, potentially limiting climate-driven expansion. I found that competition with trees might slow down shrub expansion in the boreal forest biome, as the climate sensitivity of shrub growth was much lower in a boreal forest in southwest Yukon compared to shrubs growing in the alpine tundra in the same region. However, my findings did not indicate a strong control of shrub-shrub competition on growth. A canopy removal experiment did not reveal any difference in the growth rate of shrubs having experienced a decrease in aboveground competition compared to shrubs growing in intact shrub patches. Additionally, shrubs experiencing more competition were generally as climate sensitive as those with fewer or more distant neighbours, as I demonstrated through spatial analysis at four sites across the Canadian Arctic. However, their spatial arrangement, with positive size-distance relationships between pairs of neighbours, suggested that competition does play a role in the life history of these shrubs, especially at more productive sites. Finally, I found evidence of physical and chemical interference of ground vegetation on the germination of deciduous shrub seeds, indicating that interactions with other plant functional groups may control rates of shrub expansion. Shrub expansion at the plot to landscape scale has been heavily documented over multiple decades through several lines of evidence including long-term monitoring, remote sensing, and experimental studies. The increase in shrub biomass in the tundra has high certainty both in detection and in attribution to climate warming. However, my thesis highlights the complexity and variability of growth responses when using radial growth as an indicator of climate sensitivity. I detected this variability at multiple scales, from plant parts within an individual showing inconsistent climatic signals, to site-scale sensitivity responding to different facets of global change. I did not find strong or consistent influences of biotic and abiotic controls on the growth responses of tundra shrubs; however, these relationships may change over time as shrub densities continue to increase and exacerbate resource limitations. With 80% of tundra biomass potentially located below ground, understanding whole-plant and community-level responses to climate will be critical to improve projections of tundra plant community responses to global change. Understanding the different drivers of primary and secondary growth will be key to using estimates of climate sensitivity derived from growth-ring records to project biomass change and associated feedbacks across the tundra biome.