Lamont Doherty Earth Observatory, Columbia University, United States
Background/Question/Methods
As a temperature-delineated boundary, the Arctic treeline is expected to shift northward as the climate warms. 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 regeneration. Here, we (1) developed a novel method that combines a species-specific tree height-age model (R2 = 0.89; RMSE = 4.15 years) with aerial lidar to describe the current forest tree age structure and regeneration of 38,652 white spruce trees at an Alaskan section of the treeline. We then (2) identified trends in annual radial growth during the 20th century; and (3) interpreted these data alongside monthly temperature, vapor pressure deficit, and precipitation via correlation analyses to deduce the possible responses of growth and regeneration at treeline to climate change.
Results/Conclusions
We found striking and simultaneous shifts in climate, growth, and regeneration at the Alaskan treeline beginning circa 1975. While mean annual precipitation did not change significantly after 1975 (308 mm year-1; p = 0.51, Wilcox test), the monthly climate that was significantly correlated (Pearson correlation, p< 0.05) with tree ring width did change, making conditions relatively hotter and drier than in previous decades. In particular, June-July average air temperature increased 1°C (p < < 0.0001, Wilcox test), July VPD increased 0.3 kPa (p < 0.05, Wilcox test), and snow water equivalent decreased 15% (162 to 138 mm year-1; p < 0.01, Wilcox test). The changing climate likely alleviated temperature-limitation on growth, as tree ring width increased by 64% after 1975 (p < < 0.0001, +0.24 mm ring width). However, in contrast to growth, regeneration rates decreased relative to those before c. 1975, driving the unimodal age structure of white spruce at this treeline. Because increased recruitment is a vital prerequisite to northern treeline advance, we predict that this section of the Alaskan treeline will not advance as the climate warms unless there is an equivalent increase in water availability despite increases in radial growth.
