Contrasting life-history traits of black spruce and jack pine influence their physiological response to drought and growth recovery in northeastern boreal Canada

An increase in frequency, intensity and duration of drought events affects forested ecosystems. Trees react to these changes by adjusting stomatal conductance to maximize the trade-off between carbon gains and water losses. A better understanding of the consequences of these drought-induced physiological adjustments for tree growth could help inferring future productivity potentials of boreal forests. Here, we used samples from a forest inventory network in Canada where a decline in growth rates of black spruce (Picea mariana (Mill.) B.S.P.) and jack pine (Pinus banksiana Lamb.) occurred in 1988-1992, an exceptionally dry period, to verify if this growth decline resulted from physiological adjustments of trees to drought. We measured carbon and oxygen isotope ratios in growth rings of 95 spruces and 49 pines spanning 1985-1993. We used C-13 discrimination (Delta C-13) and O-18 enrichment (Delta O-18) as proxies for intrinsic water use efficiency and stomatal conductance, respectively. We studied how inter-annual variability in isotopic signals was linked to climate moisture index, vapor pressure deficit and annual snowfall amount. We found significantly lower Delta C-13 values over 1988-1990, and significantly higher Delta O-18 values in 1988-1989 and 1991 compared to the 1985-1993 averages. We also observed that a low climatic water balance and a high vapor pressure deficit were linked with low Delta C-13 and high Delta O-18 in the two study species, in parallel with low growth rates. The latter effect persisted into the year following drought for black spruce, but not for jack pine. These findings highlight that small differences in physiological parameters between species could translate into large differences in post-drought recovery. The stronger and longer lasting impact on black spruce compared to jack pine suggests a less efficient carbon use and a lower acclimation potential to future warmer and drier climate conditions. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

The increase in frequency, intensity, and duration of drought events affects forested ecosystems by causing trees to adjust stomatal conductance to maximize the trade-off between carbon gains and water losses. This study found that a decline in growth rates of black spruce and jack pine in an exceptionally dry period from 1988 to 1992 was related to physiological adjustments of trees to drought. Isotopic signals were linked to climate moisture index, vapor pressure deficit, and annual snowfall amount, impacting tree growth and physiological responses.