According to the ‘fitness-suitability' hypothesis, ongoing changes in climate are expected to affect habitat suitability and hence species' fitness. In trees, differences in fitness may manifest as changes in growth rates, which will alter carbon uptake. Using tree-ring data, we calculated > 1.5 million annual stem growth rate estimates (standardized for tree size) for 15 677 trees representing 37 species from 558 populations throughout eastern North America. We used collections data and species distribution models to estimate each population's climatic suitability from 1900 to 2010. We then assessed the relationships between growth, suitability and time using linear mixed-effects models. We found that stem growth rates decreased significantly through time independent of changes in climate suitability and that relationships between growth rates and climate suitability were highly variable across species. Contrary to expectations, we found that growth rates were negatively correlated with species' climate suitability, a relationship that was consistent over time for gymnosperms and became more negative through time for angiosperms. These results may suggest that stem growth rates are not a good proxy for fitness and/or that unidentified factors may be slowing tree growth and outweighing any potential benefits of climate change and increasing atmospheric CO₂ concentrations. Regardless of the cause, this finding indicates that we should not count on the increased growth of eastern North American trees to help offset anthropogenic carbon emissions.

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北美东部树木气候适宜性和生长率的变化

根据“适应性-适宜性”假设，气候的持续变化预计会影响栖息地适宜性，从而影响物种的适应性。在树木中，适应性差异可能表现为生长速度的变化，这将改变碳吸收。使用树木年轮数据，我们计算了超过 150 万棵年茎生长速率估计值（根据树木大小标准化），代表北美东部 558 个种群的 37 个物种的 15677 棵树木。我们使用收集数据和物种分布模型来估计每个种群从 1900 年到 2010 年的气候适宜性。然后，我们使用线性混合效应模型评估了生长、适宜性和时间之间的关系。我们发现，随着时间的推移，茎的生长速率显着下降，与气候适宜性的变化无关，而且生长速率和气候适宜性之间的关系在不同物种之间存在很大差异。与预期相反，我们发现生长速率与物种的气候适宜性呈负相关，这种关系对于裸子植物来说随着时间的推移是一致的，而对于被子植物来说随着时间的推移变得更加消极。这些结果可能表明，茎的生长速度不能很好地代表健康和/或不明因素可能会减缓树木的生长，并超过气候变化和增加大气二氧化碳的任何潜在好处 气候适宜性，随着时间的推移，裸子植物的关系是一致的，而被子植物的关系随着时间的推移变得更加消极。这些结果可能表明，茎的生长速度不能很好地代表健康和/或不明因素可能会减缓树木的生长，并超过气候变化和增加大气二氧化碳的任何潜在好处 气候适宜性，随着时间的推移，裸子植物的关系是一致的，而被子植物的关系随着时间的推移变得更加消极。这些结果可能表明，茎的生长速度不能很好地代表健康和/或不明因素可能会减缓树木的生长，并超过气候变化和增加大气二氧化碳的任何潜在好处_2个浓度。无论原因如何，这一发现表明我们不应指望北美东部树木的生长增加来帮助抵消人为碳排放。