Rises in atmospheric carbon dioxide (atmCO2) levels are known to stimulate photosynthesis and increase intrinsic water‐use efficiency (iWUE) in trees. Stand‐level increases in iWUE depend on the physiological response of dominant species to increases in atmCO2, while tree‐level response to increasing atmCO2 depends on the balance between the direct effects of atmCO2 on photosynthetic rate and the indirect effects of atmCO2 on drought conditions. The aim of this study was to characterize the response of boreal jack pine (Pinus banksiana) stands in Northern Ontario to changes in atmCO2 and associated climatic change over the past 100 years. The impact of changes in growing season length, temperature, and precipitation, as well as atmCO2 on tree growth, was determined using stable carbon isotopes and dendrochronological analysis. Jack pine stands in this study were shown to be in progressive decline. As expected, iWUE was found to increase in association with rising atmCO2. However, increases in iWUE were not directly coupled with atmCO2, suggesting that the degree of iWUE improvement is limited by alternative factors. Water‐use efficiency was negatively associated with tree growth, suggesting that warming‐ and drought‐induced stomatal closure has likely led to deviations from expected atmCO2‐enhanced growth. This finding corroborates that boreal forest stands are likely to face continued stress under future climatic warming.

Abstract. With increasing awareness of the consequences of climate change for global ecosystems, the focus and application of tree ring research have shifted to reconstruction of long-term climate-related trends in tree growth. Contemporary methods for estimating and removing biological growth trends from tree ring series (standardization) are ill-adapted to shade-tolerant species, leading to biases in the resultant chronologies. Further, many methods, including regional curve standardization (RCS), encounter significant limitations for species in which accurate age estimation is difficult. In this study we present and test two tree ring standardization models that integrate tree size in the year of ring formation into the estimation of the biological growth trend. The first method, dubbed size-deterministic standardization (SDS), uses tree diameter as the sole predictor of the growth trend. The second method includes the combined (COMB) effects of age and diameter. We show that both the SDS and COMB methods reproduce long-term trends in simulated tree ring data better than conventional methods; this result is consistent across multiple species. Further, when applied to real tree ring data, the SDS and COMB models reproduce long-term, time-related trends as reliably as traditional RCS and more reliably than other common standardization methods (i.e. C-method, basal area increments, conservative detrending). We recommend the inclusion of tree size in the year of ring formation in future tree ring standardization models, particularly when dealing with shade-tolerant species, as it does not compromise model accuracy and allows for the inclusion of unaged trees.

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<p><strong>Abstract.</strong> With increasing awareness of the consequences of climate change for global ecosystems, the focus and application of tree-ring research has shifted to reconstruction of long-term climate-related trends in tree growth. Contemporary methods for removing the biological growth-trend from tree-ring series (standardization) are ill-adapted to shade-tolerant species, leading to biases in the resultant chronology. Further, many methods, including regional curve standardization (RCS), encounter significant limitations for species in which accurate age estimation is difficult. In this study we present and test two tree-ring standardization models that integrate tree size in the year of ring formation into the estimation of the biological growth-trend. The first method, dubbed size deterministic standardization (SDS), uses tree diameter as the sole predictor of the growth-trend. The second method includes the combined (COMB) effects of age and diameter. We show that both the SDS and COMB methods reproduce long-term trends in simulated tree-ring data better than conventional methods – this result is consistent across multiple species. Further, when applied to real tree-ring data, the COMB method is more parsimonious than its than RCS. We recommend the inclusion of tree size in the year of ring formation in future tree-ring standardization models, particularly when dealing with shade-tolerant species, as it does not compromise model parsimony and allows for the inclusion of unaged trees.</p>

Response to reviewers We thank both anonymous reviewers for their insightful and thoughtful comments on our manuscript. We have implemented a vast majority of the suggestions highlighted in their reviews and in doing so believe the results are more statistically robust and the reasoning is clearer. To assist comprehensibility of our response this document is structured as follows: 1) Reviewer comment 2) Author response 3) changes to C1