P2C2: Amazonian Tree-Ring Chronologies for Climate and Streamflow Reconstruction

该项目旨在开发亚马逊盆地 200-400 年的树木年表，以扩展降水和水流的简短仪器记录。亚马逊河流量是地球上最大的流量，部分受到热带大西洋和太平洋的海洋大气强迫的调节。自 1902 年以来，亚马逊流域的流量一直受到监测，并且似乎已经进入了水文变异性放大的新状态。最近最小和最大流量的分歧在短期工具记录中是没有先例的。亚马逊流域水文循环加剧的原因尚不清楚，但可能是由于森林砍伐或与观测到的热带北大西洋变暖相关的水汽平流变化引起的。根据 CMIP5 气候模型集合模拟，在温室气体排放和土地覆盖变化照常的情况下，预计 21 世纪中叶亚马逊流域大部分地区的年降水量将严重下降。 在全新世晚期自然气候变率的背景下，最近和预测的水文气候变化可能是多么前所未有，无法从亚马逊盆地的少数短暂且空间不连续的仪器观测中凭经验确定。树木年轮重建提供了对过去特大干旱和雨雪的过时的空间分布估计，需要验证北美和亚洲十年间水分状况动态的模型模拟。 然而，世界上生物多样性最丰富的森林生态系统——亚马逊森林，并没有免费产生可用于精确年代测定或气候重建的多世纪树木年表。在没有寒冷的冬季休眠季节的情况下，大多数热带树种不会产生解剖学上独特的年轮，因此不能用于常规树木年代学。 然而，一些热带森林中确实存在强烈的环境节律，并且可能导致一些本地物种形成年度树木年轮。 这些年度变化包括亚马逊中部、东部和南部低地森林深刻的降水季节性和长期的洪水脉冲。研究小组的同事已经证明，这些淹没森林中的Macrolobium acaciifolium存在年轮，并证明了它们在水文气候应用中的潜在价值。巴西和玻利维亚东部季节性干燥高地森林中的Bertholletia excelsa、Cedrella fissilis、C. odorata 和 Centrolobium microchaete 也已证实有年轮，而年轮的宽度和氧同位素年代学与大规模的气候变化有关。该项目将使用新的树木年轮年表来开发网格区域湿度重建，利用过去千年和21世纪的CMIP5集合模拟来构建气候变化背景下最极端的前仪器和现代干旱，并探索亚马逊流域海洋-大气强迫气候的稳定性。该项目还将利用研究资源，联合来自美国、巴西、玻利维亚和阿根廷的学生和研究人员，应用选择亚马逊流域热带硬木的树木气候学。气候学家、树木年代学家和森林科学家组成的国际合作组织，拥有解决热带硬木树木年轮精确测定所涉及的复杂问题所需的专业知识。

This project aims to develop 200-400 year-long tree-ring chronologies from the Amazon Basin to extend the short instrumental record of precipitation and streamflow.Amazon River discharge, the largest on Earth, is modulated in part by ocean-atmospheric forcing from the tropical Atlantic and Pacific. Amazon discharge has been monitored since 1902 and appears to have entered a new regime of amplified hydrological variability. The recent divergence in minimum and maximum flows has no precedent in the short instrumental record. The cause of this intensified hydrological cycle over Amazonia is not known, but may arise from deforestation or moisture advection changes associated with the observed warming of the tropical North Atlantic.A severe decline in annual precipitation is predicted for the mid-21st century over most of Amazonia based on CMIP5 climate model ensemble simulations under business-as-usual greenhouse gas emissions and land cover changes. How unprecedented the recent and predicted hydroclimatic changes might be in the context of natural climate variability during the late Holocene cannot be determined empirically from the few short and spatially discontinuous instrumental observations from the Amazon Basin.Tree-ring reconstructions have provided well-dated spatially distributed estimates of past megadroughts and pluvials needed for verifying model simulations of the dynamics responsible for decadal moisture regimes over North America and Asia. However, the most biodiverse forest ecosystems in the world, the forests of Amazonia, have not produced freely available multi-century tree-ring chronologies useful for exact dating or climate reconstruction. In the absence of a cold winter dormant season, most tropical tree species do not produce anatomically distinctive annual growth rings and therefore cannot be used for routine dendrochronology. However, strong environmental rhythms do exist in some tropical forests and can induce annual tree-ring formation in a few native species. These annual changes include the profound precipitation seasonality and prolonged flood pulse of lowland forests in the central, eastern, and southern Amazon. Colleagues on the research team have proven that annual rings exist in Macrolobium acaciifolium in these inundation forests and have demonstrated their potential value for hydroclimatic applications. Annual rings have also been proven in Bertholletia excelsa, Cedrella fissilis, C. odorata, and Centrolobium microchaete from seasonally dry upland forests of Brazil and eastern Bolivia, and ring width and oxygen isotope chronologies derived from them are related to large-scale climate variability. This project will use new tree-ring chronologies to develop gridded regional moisture reconstructions, to frame the most extreme pre-instrumental and modern droughts in the context of climate change using CMIP5 ensemble simulations of the past millennium and the 21st century, and to explore the stability of ocean-atmospheric forcing of climate over Amazonia.The project will also leverage research resources and unite students and investigators from the United States, Brazil, Bolivia, and Argentina in the application of dendroclimatology to select tropical hardwoods in Amazonia. An international collaboration of climatologists, dendrochronologists, and forest scientists has been organized with the expertise required to solve the complexities involved in the exact tree-ring dating of tropical hardwoods.