Light availability drives vertical canopy gradients in photosynthetic functioning and carbon (C) balance, yet patterns of variability in these gradients remain unclear. We measured light availability, photosynthetic CO 2 and light response curves, foliar C, nitrogen (N) and pigment concentrations, and the photochemical reflectance index (PRI) on upper and lower canopy needles of white spruce trees ( Picea glauca ) at the northern and southern extremes of the species’ range. We combined our photosynthetic data with previously published respiratory data to compare and contrast canopy C balance between latitudinal extremes. We found steep canopy gradients in irradiance, photosynthesis, and leaf traits at the southern range limit, but clear convergence across canopy positions at the northern range limit. Thus, unlike many tree species from tropical to mid-latitude forests, high latitude trees do not require vertical gradients of metabolic activity to optimize photosynthetic C gain. Consequently, accounting for self-shading is less critical for predicting gross primary productivity at northern relative to southern latitudes. Northern trees also had a significantly smaller net positive C balance than southern trees suggesting that, regardless of canopy position, low photosynthetic rates coupled with high respiratory costs may ultimately constrain the northern range limit of this widely distributed boreal species. the

Vertical gradients in physiological function and resource allocation of white spruce diverge at the northern- and southern-most range extremes

As a temperature-delineated boundary, the Arctic treeline is predicted to shift northward in response to warming. However, the evidence for northward movement is mixed, with some sections of the treeline advancing while others remain stationary or even retreat. To identify the drivers of this variation, we need a landscape-level understanding of the interactions occurring between climate, tree growth, and population regeneration. In this study, we assessed regeneration alongside annual tree growth and climate during the 20th century. We used an ageheight model combined with tree height from aerial lidar to predict the age structure of 38,652 white spruce trees across 250 ha of Arctic treeline in the central Brooks Range, Alaska, USA. We then used age structure analysis to interpret the trends in regeneration and tree-ring analysis to interpret changes in annual tree growth. The climate became significantly warmer and drier circa 1975, coinciding with divergent responses of regeneration and tree growth. After 1975, regeneration of saplings (trees ≤ 2m tall) decreased compared to previous decades whereas annual growth in mature trees (trees >2m tall) increased by 54% (p<0.0001, Wilcoxon test). Tree-ring width was positively correlated with May-August temperature (p<0.01, Pearson coefficient) during the 20th century. However, after circa 1950, the positive correlation between temperature and growth weakened (i.e., temperature divergence) while the positive correlation with July precipitation strengthened (p<0.01, Pearson coefficient), suggesting that continued drying may limit future growth at this section of Arctic treeline. We conclude that while warmer temperatures appear to benefit annual growth in mature trees, the warmer and drier environmental conditions in spring and summer inhibit regeneration and therefore may be inhibiting the northward advance at this Arctic treeline site. Researchers should consider the interactions between temperature, water availability, and tree age when examining the future of treeline and boreal forest in a changing climate.

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Growth increases but regeneration declines in response to warming and drying at Arctic treeline in white spruce (Picea glauca)

ABSTRACT Light availability drives vertical canopy gradients in photosynthetic functioning and carbon (C) balance, yet patterns of variability in these gradients remain unclear. We measured light availability, photosynthetic CO 2 and light response curves, foliar C, nitrogen (N) and pigment concentrations, and the photochemical reflectance index (PRI) on upper and lower canopy needles of white spruce trees ( Picea glauca ) at the species’ northern and southern range extremes. We combined our photosynthetic data with previously published respiratory data to compare and contrast canopy C balance between latitudinal extremes. We found steep canopy gradients in irradiance, photosynthesis, and leaf traits at the southern range limit, but a lack of variation across canopy positions at the northern range limit. Thus, unlike many tree species from tropical to mid-latitude forests, high latitude trees may not require vertical gradients of metabolic activity to optimize photosynthetic C gain. Consequently, accounting for self-shading is less critical for predicting gross primary productivity at northern relative to southern latitudes. Northern trees also had a significantly smaller net positive leaf C balance than southern trees suggesting that, regardless of canopy position, low photosynthetic rates coupled with high respiratory costs may ultimately constrain the northern range limit of this widely distributed boreal species. SUMMARY STATEMENT Canopy gradients in photosynthetic capacity of white spruce diminish at high compared to low latitudes. Low carbon balance in high latitude trees may determine the extent of northern treeline. 

/pdf/vertical-gradients-in-photosynthetic-physiology-diverge-at-1b19w8rk.pdf

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

White spruce (Picea glauca) spans a massive range from arctic treeline to temperate forests. Yet the variability in respiratory physiology and the implications for tree carbon balance at the extremes of this distribution remain enigmasWorking at Arctic and Temperate sites more than 5000 km apart, we measured the short-term temperature response of dark respiration (R/T) at upper and lower canopy positions. R/T curves were fit to a polynomial model and model parameters (a, b, and c) were compared between locations, canopy positions, or with published data. Respiration measured at 25{degrees}C (R25) was 68% lower at the southern location than the northern location, resulting in a significantly lower a parameter of the R/T response in temperate trees Only at the southern location did upper canopy leaves have a steeper temperature response than lower canopy leaves, likely reflecting steeper canopy gradients in light. No differences were manifest in the maximum temperature of respiration. At the northern range limit, respiration appears extreme. This high carbon cost likely contributes to the current location of northern treeline. We find that respiration will increase with end-of-the-century warming and will likely continue to constrain the future range limits of this important boreal species. Summary StatementWhite spruce (Picea glauca) needle respiration at the northern limit of the species range is three times higher than at the southern range limit. This extreme carbon cost likely challenges tree survival and contributes to the location of the northern treeline.

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Zoe M. Griffin

Papers

Variation in white spruce needle respiration across the species range