Alnus viridis ssp. Fruticosa modulates local conditions to influence intra and interspecies growth

Abstract

The Arctic is warming rapidly due to anthropogenic greenhouse gas emissions. Concurrent with warming, some Arctic plant communities have transformed from short statured evergreen and graminoid shrub tundra to tall deciduous shrubs in recent decades. As warming continues, plant-plant interactions will likely change and influence future community composition. Alnus viridis ssp. fruticosa (Siberian alder) is rapidly expanding across Arctic regions and is particularly important because Siberian alder is the Arctic's only large N-fixing shrub, may alter N-cycling to further influence both C-cycling and community composition as it spreads. This dissertation addressed two main questions to better understand how Arctic deciduous shrub communities have changed, and may continue to change as Arctic warming proceeds. First, how did the climate sensitivity of Siberian alder's growth change over the past century (1920 - 2017), and how did climate sensitivity change as Siberian alder grew older? Second, how has Siberian alder affected the long-term growth of two nearby dominant deciduous shrub species: Betula nana ssp. exilis (dwarf birch) and Salix pulchra (diamondleaf willow)? I used dendrochronological techniques to assess how sensitive the growth of each of these three species was to climate over the last century. For dwarf birch and diamondleaf willow, I compared growth near and away from Siberian alder. I hypothesized that Siberian alder shrubs would become more sensitive to climate as they grew both older and larger. I expected that older alder would enhance soil N availability over time, due to the accumulated products of N-fixation. I also expected that the growth of larger, older alders would be more sensitive to climate than the growth of younger alder, because of having both a greater photosynthetic capacity (enabling more growth under good conditions), but also higher maintenance respiration (leading to less growth under poor conditions). I found that Siberian alder overall has become more sensitive to July air temperatures in the second half of the past century as climate has warmed. Also, older shrubs were more sensitive to June and July air temperatures than younger shrubs. Thus, these findings support my hypothesis. Siberian alder frequently grows in close association with dwarf birch and diamondleaf willow. However, these two deciduous neighbors differ in several functional traits. Dwarf birch is a low statured, many branched shrub that mainly grows laterally, and thus is often found on the margins of the Siberian alder canopy. In contrast, diamondleaf willow has fewer branches, grows more vertically, and often is found almost entirely within the Siberian alder canopy. Thus, I hypothesized Siberian alder would facilitate the growth of dwarf birch growth, by alleviating both resource and abiotic stressors, but would reduce diamondleaf willow growth, due to increased light competition. Siberian alder promoted dwarf birch growth and reproduction, likely by alleviating N-limitations and reducing frost damage, though growth was delayed by two weeks. In contrast, diamondleaf willows growing near alder had much smaller growth rings than diamondleaf willows growing away from alder, likely due to light competition. This negative effect on willow growth near alder occurred despite these willows likely receiving greater access to soil N and protection from herbivory damage from the neighboring alder. Overall, these results demonstrate that Siberian alder will likely grow better as they get both older and larger, and as the Arctic continues to warm. Siberian alder influence the growth of neighboring deciduous shrubs over the long term, and those effects are species-specific because they depend on the functional traits of their neighbors. Thus, the continued spread of Siberian alder will likely alter vegetation community composition, and thus influence C and N cycling.