Abstract
Boreal forests play critical roles in global carbon, water and energy cycles. Recent studies suggest drought is causing a decline in boreal spruce growth, leading to predictions of widespread mortality and a shift in dominant vegetation type in interior Alaska. We took advantage of a large set of tree cores collected from random locations across a vast area of interior Alaska to examine long-term trends in carbon isotope discrimination and growth of black and white spruce. Our results confirm that growth of both species is sensitive to moisture availability, yet show limited evidence of declining growth in recent decades. These findings contrast with many earlier tree-ring studies, but agree with dynamic global vegetation model projections. We hypothesize that rising atmospheric [CO2] and/or changes in biomass allocation may have compensated for increasing evaporative demand, leaving recent radial growth near the long-term mean. Our results highlight the need for more detailed studies of tree physiological and growth responses to changing climate and atmospheric [CO2] in the boreal forest.
Highlights
The boreal forest is a critical component of global carbon, water and energy cycles[1]
Spruce are relatively isohydric in the sense that complete stomatal closure and loss of hydraulic conductance both occur at high xylem water potential
Near complete stomatal closure was observed at a xylem water potential of ~−2.0 MPa19, while 50% loss of hydraulic conductance was detected at a xylem water potential of ~−4.0 MPa20
Summary
The boreal forest is a critical component of global carbon, water and energy cycles[1]. Over the last two decades, numerous tree-ring studies have been completed in interior Alaska, with most concluding that declining growth of black (Picea mariana) and white spruce (Picea glauca) is a result of drought stress, induced by rising air temperature and increasing evaporative demand[6,7,8,9,10,11,12,13,14,15]. We carried out a more thorough evaluation of the relationships between climate and growth, used stable isotopes in tree-ring alpha-cellulose to examine potential changes in gas exchange physiology over time and analyzed a wide range of habitat variables with the aim of identifying important controls on spatial variation in recent growth of black and white spruce. We anticipated an overall positive correlation between precipitation and growth
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