The climate envelope of Alaska’s northern treelines: Implications for controlling factors and future treeline advance

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Colin T. Maher, Environment and Natural Resources Institute, University of Alaska, Anchorage, AK, Roman Dial, Environmental Science, Alaska Pacific University, Anchorage, AK, Neal J. Pastick, KBR Inc., contractor to the U.S. Geological Survey, Earth Resources and Observation Science Center, Sioux Falls, SD, Rebecca E. Hewitt, Center for Ecosystem Science and Society, Northern Arizona University, Flagstaff, AZ, M. Torre Jorgenson, Alaska Ecoscience and Patrick F. Sullivan, Environment and Natural Resources Institute, University of Alaska Anchorage, Anchorage, AK

Background/Question/Methods
Understanding the key mechanisms that control northern treelines is important to accurately predict biome shifts and terrestrial feedbacks to climate. At a global scale, it has long been observed that elevational and latitudinal treelines occur at similar mean growing season air temperature (GSAT) isotherms, inspiring the growth limitation hypothesis (GLH) that cold GSAT limits aboveground growth of treeline trees, with mean treeline GSAT ~6-7 °C. Treelines with mean GSAT warmer than 6-7 °C may indicate other limiting factors. Many treelines globally are not advancing despite warming, and other climate variables are rarely considered at broad scales. Our goals were to test whether current boreal treelines in northern Alaska correspond with the GLH isotherm, determine which environmental factors are most predictive of treeline presence, and to identify areas beyond the current treeline where advance is most likely. We digitized ~12,400 km of treelines (> 26K points) and computed seasonal climate variables across northern Alaska. We then built a generalized additive model predicting treeline presence to identify key factors determining treeline position.
Results/Conclusions
Two metrics of mean GSAT at Alaska’s northern treelines were consistently warmer than the 6-7 °C isotherm (means of 8.5 °C and 9.3 °C), suggesting that direct physiological limitation from low GSAT is unlikely to explain the position of treelines in northern Alaska. Our final model included cumulative growing degree-days (GDD), near-surface (≤ 1 m) permafrost, and growing season total precipitation (GS_Ppt), which together may represent the importance of soil temperature. Our results indicate that mean GSAT may not be the primary driver of treeline in northern Alaska or that its effect is mediated by other more proximate, and possibly non-climatic, controls. Our model predicts treeline potential in several areas beyond current treelines, pointing to possible routes of treeline advance if unconstrained by non-climatic factors.