Tree demography dominates long-term growth trends inferred from tree rings.
Tree demography dominates long-term growth trends inferred from tree rings.
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DOI：
10.1111/gcb.13410
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发表时间：
2017-03
影响因子：
11.6
通讯作者：
Ziv G
中科院分区：
文献类型：
--
作者：
Brienen RJ;Gloor M;Ziv G
了解森林对二氧化碳和温度升高的反应是一项重要的挑战，但并非易事。树木年轮越来越多地用于研究这种反应。在最近的一项研究中，van der Sleen et al. (2014) Nature Geoscience, 8,4使用了12种热带树种的树木年轮，发现尽管内在水分利用效率有所提高，但没有观察到生长刺激。这挑战了增加二氧化碳会刺激经济增长的观点。不幸的是，树木年轮分析可能会受到偏见的困扰，导致虚假的生长趋势。虽然他们的研究评估了几种偏见，但并不能解释所有的偏见。特别是，一个偏差可能严重影响了他们的结果。有些物种的繁殖模式并不统一，而是集中在某一年。如果按固定大小类别计算增长率，这将导致虚假的负增长趋势，因为“快速生长”的树木比缓慢生长的树木更早达到采样直径，因此快速生长的树木往往具有更早的日历日期。我们评估了这种“非均匀年龄偏差”对观察到的生长趋势的影响，发现van der Sleen关于缺乏生长刺激的结论并不成立。增长趋势（至少部分）是由潜在的招聘或年龄分布驱动的。年龄分布越聚集的物种表现出更多的负增长趋势，对物种年龄分布影响的模拟结果表明，物种的生长趋势与观测结果接近。对增长数据的重新评估和对偏差的修正导致整个时期每十年1-2%的显著正增长趋势，自1950年以来为3-7%。然而，这些观察结果应该谨慎对待，因为多重偏差会影响这些趋势估计。总之，我们的研究结果强调，如果不仔细考虑人口统计过程，长期生长趋势的树木年轮研究可能会受到偏差的强烈影响。
Understanding responses of forests to increasing CO 2 and temperature is an important challenge, but no easy task. Tree rings are increasingly used to study such responses. In a recent study, van der Sleen et al. (2014) Nature Geoscience, 8, 4 used tree rings from 12 tropical tree species and find that despite increases in intrinsic water use efficiency, no growth stimulation is observed. This challenges the idea that increasing CO 2 would stimulate growth. Unfortunately, tree ring analysis can be plagued by biases, resulting in spurious growth trends. While their study evaluated several biases, it does not account for all. In particular, one bias may have seriously affected their results. Several of the species have recruitment patterns, which are not uniform, but clustered around one specific year. This results in spurious negative growth trends if growth rates are calculated in fixed size classes, as ‘fast‐growing’ trees reach the sampling diameter earlier compared to slow growers and thus fast growth rates tend to have earlier calendar dates. We assessed the effect of this ‘nonuniform age bias’ on observed growth trends and find that van der Sleen's conclusions of a lack of growth stimulation do not hold. Growth trends are – at least partially – driven by underlying recruitment or age distributions. Species with more clustered age distributions show more negative growth trends, and simulations to estimate the effect of species’ age distributions show growth trends close to those observed. Re‐evaluation of the growth data and correction for the bias result in significant positive growth trends of 1–2% per decade for the full period, and 3–7% since 1950. These observations, however, should be taken cautiously as multiple biases affect these trend estimates. In all, our results highlight that tree ring studies of long‐term growth trends can be strongly influenced by biases if demographic processes are not carefully accounted for.
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