High Leaf Respiration Rates May Limit the Success of White Spruce Saplings Growing in the Kampfzone at the Arctic Treeline

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.

Variation in White spruce needle respiration at the species range limits: A potential impediment to Northern expansion

“…The much higher rates of respiration at the northern range limit of this species support our first research hypothesis and may help explain not only why white spruce is not found further to the north, but also why the boreal forest biome transitions to tundra at these high northern latitudes (Griffin et al, 2021). Plants acclimated to cold temperatures often have higher rates of respiration measured at a common temperature and a steeper respiratory temperature response (Atkin & Tjoelker, 2003; Körner, 1989; Reich & Oleksyn, 2004) as a means to maintain metabolic function in cold environments.…”

“…Second, while our previous study did not find canopy position differences in respiration at the northern FTE site (Griffin et al, 2021), we hypothesize that at our southern site, upper canopy leaves will have higher respiration rates and be less responsive to temperature than lower canopy leaves. Third, while our previous study found very high respiration rates at the FTE (Griffin et al, 2021), we hypothesize that southern white spruce respiration will be similar to the average for the Needle‐leaved Evergreen (NLEv) plant functional type (PFT) to which they belong. To quantify white spruce respiratory temperature response and make these comparisons, we use two models, the global polynomial model of Heskel et al (2016) and the thermodynamic model of Liang et al (2018).…”

“…Recently, we reported that white spruce [ Picea glauca (Moench) Voss] growing at the arctic treeline [which marks the transition between the boreal forest and treeless tundra occurring in the Forest Tundra Ecotone (FTE)] exhibit high respiratory costs (Griffin et al, 2021). As one of the hardiest coniferous species, white spruce has a suite of structural and functional traits that are adapted to cold temperatures and short growing seasons (Sutton, 1969).…”

“…The goal of this study was to gain a better understanding of how respiration varies between the two extremes of the white spruce species range and with canopy position at each of those extremes. Using our recent, detailed assessment of white spruce respiratory physiology at the Alaskan FTE (Griffin et al, 2021), we implement identical techniques to compare this to the physiological function of spruce growing more than 5000 km away at the opposite end of the species distribution. We test four research hypotheses.…”

Vertical gradients in photosynthetic physiology diverge at the latitudinal range extremes of white spruce

“…2021). Griffin et al (2021) posit that this high respiratory cost may constrain the northern limit of white spruce and the Arctic treeline in general. With the addition of photosynthetic traits measured on the same trees, we are now in a unique position to examine the degree to which this respiratory cost is matched by photosynthetic carbon gains and whether a lower carbon balance may contribute to the northern limits of white spruce’s latitudinal range.…”