Drought‐induced stomatal closure probably cannot explain divergent white spruce growth in the Brooks Range, Alaska, USA

• Published in: Ecology (Durham) Vol. 97; no. 1; pp. 145 - 159
• Main Authors: Brownlee, Annalis H, Sullivan, Patrick F, Csank, Adam Z, Sveinbjörnsson, Bjartmar, Ellison, Sarah B. Z
• Format: Journal Article
• Language: English
• Published: United States Brooklyn Botanical Garden 2016; ECOLOGICAL SOCIETY OF AMERICA; Ecological Society of America
• Subjects: 20th century; Alaska; Arctic treeline; boreal forests; Brooks Range, Alaska, USA; Carbon; Carbon dioxide; Carbon Isotopes; Cellulose; Climate; climate change; Computing time; Cores; Correlation analysis; Density; Divergence; Drought; Droughts; Estimates; Evergreen trees; Exchanging; Fluctuations; Flux density; Gas exchange; growing season; growth rings; isotopes; Mathematical analysis; Moisture; Photosynthesis; Picea - growth & development; Picea - physiology; Picea glauca; Plant growth; Plant Stomata - physiology; Plant Transpiration; Polar environments; precipitation; sap flow; SAP protein; Stomata; stomatal conductance; stomatal movement; Strength; Stress; Stress concentration; Taiga; Temperature; Temperature effects; Time Factors; tree growth; Treeline; Trees; tree‐ring; Trends; water stress; water use efficiency; Watersheds; white spruce (Picea glauca); Xylem; Alaska; Arctic region; PN, Arctic; INE, USA, Alaska
• ISSN: 0012-9658; 1939-9170
• DOI: 10.1890/15-0338.1

Increment cores from the boreal forest have long been used to reconstruct past climates. However, in recent years, numerous studies have revealed a deterioration of the correlation between temperature and tree growth that is commonly referred to as divergence. In the Brooks Range of northern Alaska, USA, studies of white spruce (Picea glauca) revealed that trees in the west generally showed positive growth trends, while trees in the central and eastern Brooks Range showed mixed and negative trends during late 20th century warming. The growing season climate of the eastern Brooks Range is thought to be drier than the west. On this basis, divergent tree growth in the eastern Brooks Range has been attributed to drought stress. To investigate the hypothesis that drought‐induced stomatal closure can explain divergence in the Brooks Range, we synthesized all of the Brooks Range white spruce data available in the International Tree Ring Data Bank (ITRDB) and collected increment cores from our primary sites in each of four watersheds along a west‐to‐east gradient near the Arctic treeline. For cores from our sites, we measured ring widths and calculated carbon isotope discrimination (δ¹³C), intrinsic water‐use efficiency (iWUE), and needle intercellular CO₂ concentration (Cᵢ) from δ¹³C in tree‐ring alpha‐cellulose. We hypothesized that trees exhibiting divergence would show a corresponding decline in δ¹³C, a decline in Cᵢ, and a strong increase in iWUE. Consistent with the ITRDB data, trees at our western and central sites generally showed an increase in the strength of the temperature–growth correlation during late 20th century warming, while trees at our eastern site showed strong divergence. Divergent tree growth was not, however, associated with declining δ¹³C. Meanwhile, estimates of Cᵢ showed a strong increase at all of our study sites, indicating that more substrate was available for photosynthesis in the early 21st than in the early 20th century. Our results, which are corroborated by measurements of xylem sap flux density, needle gas exchange, and measurements of growth and δ¹³C along moisture gradients within each watershed, suggest that drought‐induced stomatal closure is probably not the cause of 20th century divergence in the Brooks Range.