Black spruce (Picea mariana (Mill.) BSP)-forested peatlands are widespread ecosystems in boreal North America in which peat accumulation, known as the paludification process, has been shown to induce forest growth decline. The continuously evolving environmental conditions (e.g., water table rise, increasing peat thickness) in paludified forests may require tree growth mechanism adjustments over time. In this study, we investigate tree ecophysiological mechanisms along a paludification gradient in a boreal forested peatland of eastern Canada by combining peat-based and tree-ring analyses. Carbon and oxygen stable isotopes in tree rings are used to document changes in carbon assimilation rates, stomatal conductance, and water use efficiency. In addition, paleohydrological analyses are performed to evaluate the dynamical ecophysiological adjustments of black spruce trees to site-specific water table variations. Increasing peat accumulation considerably impacts forest growth, but no significant differences in tree water use efficiency (iWUE) are found between the study sites. Tree-ring isotopic analysis indicates no iWUE decrease over the last 100 years, but rather an important increase at each site up to the 1980s, before iWUE stabilized. Surprisingly, inferred basal area increments do not reflect such trends. Therefore, iWUE variations do not reflect tree ecophysiological adjustments required by changes in growing conditions. Local water table variations induce no changes in ecophysiological mechanisms, but a synchronous shift in iWUE is observed at all sites in the mid-1980s. Our study shows that paludification induces black spruce growth decline without altering tree water use efficiency in boreal forested peatlands. These findings highlight that failing to account for paludification-related carbon use and allocation could result in the overestimation of aboveground biomass production in paludified sites. Further research on carbon allocation strategies is of utmost importance to understand the carbon sink capacity of these widespread ecosystems in the context of climate change, and to make appropriate forest management decisions in the boreal biome.

中文翻译：

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加拿大北方森林泥炭地的沼泽化降低了黑云杉的生长速度，但不会改变树木的水分利用效率


黑云杉（Picea mariana (Mill.) BSP）森林泥炭地是北美洲北部广泛分布的生态系统，其中泥炭的积累（称为沼泽化过程）已被证明会导致森林生长衰退。荒芜森林中不断变化的环境条件（例如，地下水位上升、泥炭厚度增加）可能需要随着时间的推移调整树木生长机制。在这项研究中，我们通过结合泥炭分析和树木年轮分析，研究了加拿大东部北方森林泥炭地沿荒漠化梯度的树木生态生理机制。年轮中的碳和氧稳定同位素用于记录碳同化率、气孔导度和水分利用效率的变化。此外，还进行了古水文学分析，以评估黑云杉树对特定地点地下水位变化的动态生态生理调整。泥炭积累的增加极大地影响了森林生长，但研究地点之间的树木水分利用效率（iWUE）没有显着差异。树木年轮同位素分析表明，在过去 100 年里，iWUE 没有减少，但直到 1980 年代，iWUE 稳定之前，每个地点都有显着增加。令人惊讶的是，推断的断面积增量并未反映这种趋势。因此，iWUE 的变化并不能反映生长条件变化所需的树木生态生理调整。当地地下水位的变化不会引起生态生理机制的变化，但 20 世纪 80 年代中期在所有地点都观察到 iWUE 的同步变化。我们的研究表明，沼泽化会导致黑云杉生长下降，但不会改变北方森林泥炭地树木的水分利用效率。 这些发现强调，未能考虑与荒漠化相关的碳使用和分配可能会导致高估荒漠化地点的地上生物量生产。对碳分配策略的进一步研究对于了解气候变化背景下这些广泛的生态系统的碳汇能力以及在北方生物群落中做出适当的森林管理决策至关重要。