Accuracy of node and bud-scar counts for aging two dominant conifers in western North America

Accurately aging trees is critical for understanding tree demography and tree responses to environmental change. Given the proliferation of studies aimed at understanding the effects of climate and disturbance on forest ecosystems, it is important to understand the tradeoffs between field-based age estimates and precise dendrochronological techniques. We assessed the accuracy of age estimates from node counts in the field against precise tree-ring counts at the root-shoot boundary, in 1279 ponderosa pine and 1268 Douglas-fir seedlings sampled from across three study regions in the western U.S. We also assessed the accuracy of age estimates from bud-scar counts in the field against node counts and precise tree-ring counts in a subset of 757 seedlings from the Northern Rockies. Node counts systematically underestimated ring counts by an average of 4.1 years, with bias increasing with tree age. At annual, ±1-, ±2-, and ±5-yr precision, the accuracy of node counts was 5%, 15%, 29%, and 74% across all regions and species, respectively. Similar results were found for bud scars. Given the magnitude of the bias between field-based methods and ring counts, it is critical to select appropriate aging methods, based on the precision required to answer specific ecological questions. To improve the accuracy of field-based age estimates in these species, we provide a tool for correcting for the bias when precise dendrochronological aging is not feasible.

精准测定树木年龄，对于理解树木种群动态及其对环境变化的响应至关重要。鉴于当前旨在阐明气候与干扰对森林生态系统影响的研究日益增多，明晰野外年龄估算方法与精准树轮年代学（dendrochronological）技术之间的权衡关系显得尤为关键。我们以美国西部三个研究区域内采集的1279株黄松（ponderosa pine）幼苗与1268株花旗松（Douglas-fir）幼苗为研究对象，以根-茎交界区（root-shoot boundary）的精准树轮计数为基准，评估了野外节点计数法（node counts）所得年龄估算结果的准确性。此外，我们针对北落基山脉区域的757株幼苗子集，以节点计数与精准树轮计数为参照，评估了野外芽痕计数法（bud-scar counts）所得年龄估算结果的准确性。节点计数法会系统性低估树轮计数结果，平均低估4.1年，且低估偏差随树木年龄增长而增大。在年际、±1年、±2年以及±5年的精度要求下，节点计数法在所有区域与物种中的准确率分别为5%、15%、29%与74%。芽痕计数法也得到了相似的结果。鉴于野外估算方法与树轮计数法之间的偏差幅度，根据特定生态学问题所需的精度来选择合适的年龄测定方法至关重要。为提升这类物种野外年龄估算的准确性，我们提供了一种校正工具，可在无法开展精准树轮年代学年龄测定时使用。