Abstract
Arctic Treeline is the transition from the boreal forest to the treeless tundra and may be determined by growing season temperatures. The physiological mechanisms involved in determining the relationship between the physical and biological environment and the location of treeline are not fully understood. In Northern Alaska, we studied the relationship between temperature and leaf respiration in 36 white spruce (Picea glauca) trees, sampling both the upper and lower canopy, to test two research hypotheses. The first hypothesis is that upper canopy leaves, which are more directly coupled to the atmosphere, will experience more challenging environmental conditions and thus have higher respiration rates to facilitate metabolic function. The second hypothesis is that saplings [stems that are 5–10cm DBH (diameter at breast height)] will have higher respiration rates than trees (stems ≥10cm DBH) since saplings represent the transition from seedlings growing in the more favorable aerodynamic boundary layer, to trees which are fully coupled to the atmosphere but of sufficient size to persist. Respiration did not change with canopy position, however respiration at 25°C was 42% higher in saplings compared to trees (3.43±0.19 vs. 2.41±0.14μmolm−2 s−1). Furthermore, there were significant differences in the temperature response of respiration, and seedlings reached their maximum respiration rates at 59°C, more than two degrees higher than trees. Our results demonstrate that the respiratory characteristics of white spruce saplings at treeline impose a significant carbon cost that may contribute to their lack of perseverance beyond treeline. In the absence of thermal acclimation, the rate of leaf respiration could increase by 57% by the end of the century, posing further challenges to the ecology of this massive ecotone.
Highlights
At the global scale, plant respiration represents a massive flux of carbon from the biosphere to the atmosphere (Li et al, 2018) and plays a critical role in the global carbon cycle
We show that saplings have significantly higher rates of leaf respiration than trees
The forest tundra ecotone (FTE), is defined by the lack of trees, the implication is that germlings, seedlings, and saplings growing at this transitional boundary between tundra and forest, do not represent the permanent establishment of the boreal forest biome, as they are unlikely to survive
Summary
Plant respiration represents a massive flux of carbon from the biosphere to the atmosphere (Li et al, 2018) and plays a critical role in the global carbon cycle. While our understanding of biological and environmental controls of respiration rates remains incomplete, it is clear that temperature has a first order effect, setting up potential feedbacks between environmental change and respiratory CO2 release (O’Sullivan et al, 2013, 2017; Heskel et al, 2016). Respiration represents a significant carbon loss and offsets photosynthetic carbon gain, which may have a controlling effect on ecosystem productivity. The accelerated rate of warming near the poles (Cohen et al, 2014), the massive carbon stores across northern biomes (Fisher et al, 2014), and the uncertainty in the future strength of northern latitudes as a strong carbon sink (Schuur et al, 2015) all point to an acute need for understanding the response of respiration to temperature in these systems
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