Drought-induced stomatal closure probably cannot explain divergent white spruce growth in the Brooks Range, Alaska
Dates
Year
2015
Citation
Brownlee, Annalis H., Sullivan, Patrick F., Csank, Adam Z., Sveinbjörnsson, Bjartmar, and Ellison, Sarah B. Z., 2015, Drought-induced stomatal closure probably cannot explain divergent white spruce growth in the Brooks Range, Alaska: Ecology.
Summary
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, 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 [...]
Summary
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, 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 (?13C), 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 ?13C, 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 ?13C. 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 during the early 20th century. Our results, which are corroborated by measurements of xylem sap flux density, needle gas exchange and measurements of growth and ?13C 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.; 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, 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 (?13C), 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 ?13C, 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 ?13C. Meanwhile, estimate 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 during the early 20th century. Our results, which are corroborated by measurements of xylem sap flux density, needle gas exchange and measurements of growth and ?13C 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.
