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 (H0). The first H01 is that canopy position will not influence leaf respiration. The associated alternative hypothesis (HA) is that the 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 H02 is that tree size will not influence leaf respiration. The associated HA is that saplings (stems that are 5-10 cm DBH (diameter at breast height)) will have higher respiration rates than trees (stems ≥ 10 cm 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 ± 0.14 μmol m-2s-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 are extreme, imposing 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

“…We extend this to examine the overall temperature response of respiration. Second, while our previous study did not find canopy position differences in respiration at the FTE (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. Light acclimation has been shown to affect respiration rates in white spruce dramatically (Awada & Redmann, 2000) and there are intra-canopy gradients in light absorption in the dense canopy at Black Rock Forest (Schmiege et al, pers com).…”

“…Leaves used for the respiration measurements from the southern site were from south facing branches collected in late June and early July of 2018 from six study trees and came from the upper (1 m below the apical meristem) and lower (1.37 m from the ground) canopy positions. As in our FTE study (Griffin et al, 2021), the terminal portions of several branches were cut with sharp pruners and the removed portion of the stem was immediately wrapped with wet paper towels, sealed in a plastic bag with ample air and placed in cooler where they could be kept dark and transported to the lab. The top of the canopy was accessed with an articulating boom lift.…”

“…No saplings were sampled at the southern location and the effect of tree size/development is not specifically of interest here. Thus, the Griffin et al (2021) dataset was trimmed to exclude all saplings (stems < 10cm DBH), leaving 18 individual trees (≥ 10 cm DBH) and re-analyzed for the present study. No part of the analysis presented here was included in the earlier publication.…”

“…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 exceptionally high respiratory costs (Griffin et al, 2021). This species has a transcontinental range in North America and while it is one of the most common tree species defining the FTE (Sutton, 1969), it also has a remarkably large species distribution stretching from the west coast of Alaska to the east coast of Canada and New England (US Geological Survey, 1999) (Figure 1).…”

“…The goal of this research 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.…”