NOAA/WDS Paleoclimatology - Voelker - MOFEP Site 5 - QUCO - ITRDB MO065

Despite experimental evidence showing that elevated CO2 levels increase growth in most plants, the isolation of a signal consistent with anthropogenically caused increases in atmospheric CO2 from the dendrochronological record has shown mixed results. Our extensive sets of tree ring data from the Ozark Mountains in Missouri showed that since 1850, Quercus velutina Lam., Quercus coccinea Muench., and Pinus echinata Mill. trees increased in stem growth coincidently with increases in atmospheric CO2. Those long-term increases in radial growth appear unrelated to historical disturbance levels for the region, to long-term changes in relevant climatic variables, or to productivity of sites sampled for the purpose of creating a time sequence of tree ring growth. It is still unclear what the potential role of nitrogen deposition might have been for tree growth. We cross-dated a large number of increment cores and aligned the ring width data by pith date for accurate age constant assessments of growth over the past 150 years. Thus, we circumvented changes in growth trend associated with differences in physiological functioning during development, as well as the need for statistical detrending that removes an unknown degree of long-term environmental signal, the so called segment length curse that applies to standard dendrochronological investigations. When the positive relationship between CO2 and ring width was examined at different ages, an ontogenetic decline in the rate of growth stimulation was found. Specifically, both the pooled Quercus spp. and P. echinata were characterized by a negative exponential pattern of response over a developmental sequence through age 50. Further knowledge of an intrinsic decline in CO2 sensitivity with tree age or size such as this may be important for increased accuracy in estimating terrestrial carbon stocks across successional landscapes.

尽管已有实验证据表明，大气CO₂浓度升高可促进多数植物的生长，但从树木年代学（dendrochronology）记录中提取与人为活动引发的大气CO₂浓度升高相关的信号时，却得到了不尽一致的结果。我们从密苏里州欧扎克山脉获取的大量树木年轮数据集显示，自1850年以来，黑栎（*Quercus velutina* Lam.）、猩红栎（*Quercus coccinea* Muench.）与短叶松（*Pinus echinata* Mill.）的茎干生长量随大气CO₂浓度升高同步增加。这些径向生长的长期增长似乎与该区域历史干扰水平、相关气候变量的长期变化，以及为构建树木年轮生长时间序列所采样样地的生产力均无关联。目前仍不清楚氮沉降对树木生长可能发挥的潜在作用。我们对大量树芯样本进行了交叉定年，并以髓心年代为基准对齐年轮宽度数据，以此实现对过去150年树木生长的精准恒定年龄评估。借此，我们规避了与树木发育阶段生理功能差异相关的生长趋势变化，同时也无需进行统计去趋势处理——这类去趋势操作会移除一定程度未知的长期环境信号，即适用于标准树木年代学研究的所谓“样段长度诅咒”。当在不同树龄下考察CO₂浓度与年轮宽度之间的正相关关系时，我们发现生长促进速率随个体发育呈现下降趋势。具体而言，合并后的栎属（*Quercus* spp.）与短叶松（*Pinus echinata*）在50年的发育序列中均表现出负指数型的响应模式。针对此类“树木年龄或体型越大，对CO₂的敏感性内在下降”这一规律的进一步研究，对于提升演替景观中陆地碳储量估算的精准度具有重要意义。