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North America’s oldest boreal trees are more efficient water users due to increased [CO2], but do not grow faster

Claudie Giguère-Croteau,É. Boucher,Y. Bergeron,M. Girardin,I. Drobyshev,Lucas C. R. Silva,J. Hélie,M. Garneau

Published 2019 in Proceedings of the National Academy of Sciences of the United States of America

ABSTRACT

Significance The metabolism of North America’s oldest boreal trees (Thuja occidentalis L.) is strongly affected by rising anthropogenic CO2 emissions. Intrinsic water use efficiency (iWUE) increased dramatically, although nonlinearly, since the beginning of the industrial Era. Our study shows that while T. occidentalis L. acclimated to rising [CO2], no stem growth was observed, suggesting that trees could not benefit from the increased iWUE. Due to anthropogenic emissions and changes in land use, trees are now exposed to atmospheric levels of [CO2] that are unprecedented for 650,000 y [Lüthi et al. (2008) Nature 453:379–382] (thousands of tree generations). Trees are expected to acclimate by modulating leaf–gas exchanges and alter water use efficiency which may result in forest productivity changes. Here, we present evidence of one of the strongest, nonlinear, and unequivocal postindustrial increases in intrinsic water use efficiency (iWUE) ever documented (+59%). A dual-isotope tree-ring analysis (δ13C and δ18O) covering 715 y of growth of North America’s oldest boreal trees (Thuja occidentalis L.) revealed an unprecedented increase in iWUE that was directly linked to elevated assimilation rates of CO2 (A). However, limited nutrient availability, changes in carbon allocation strategies, and changes in stomatal density may have offset stem growth benefits awarded by the increased iWUE. Our results demonstrate that even in scenarios where a positive CO2 fertilization effect is observed, other mechanisms may prevent trees from assimilating and storing supplementary anthropogenic emissions as above-ground biomass. In such cases, the sink capacity of forests in response to changing atmospheric conditions might be overestimated.

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