Climatic and Ecological Controls on the Holocene Range Expansion of Western Hemlock and Western Redcedar in the Interior Pacific Northwest

[0212917]本研究的主要目的是确定西铁杉和西红杉在太平洋西北部内陆地区的全新世迁移滞后，并研究气候变化、扩散限制和林分水平入侵对迁移的相对重要性。这两个物种目前在沿海分布范围和内陆分布范围内共同主导晚演替森林，这些分布范围被160公里宽的半干旱草地和松林分散屏障隔开。初步数据表明，这些物种入侵内陆地区的时间比适宜气候条件的建立晚了3000年，比以往研究假设的任何迁移延迟都要长。这种可能性与沿海地区几十年的古生态学研究得出的结论形成鲜明对比，后者认为，在太平洋西北部的异质景观中，迁移滞后通常不重要，因为大多数物种是在冰川消退后不久到达的。然而，由于缺乏来自内陆地区的古生态和古气候数据，尚不清楚铁杉和雪松向内陆扩张的范围在多大程度上落后于气候变化，以及气候约束、扩散限制和林分入侵如何影响它们的扩张。为了实现研究目标，本研究将采用一套古生态工具和最先进的古气候方法，在内部范围内跨越纬度梯度的三对地点重建植被、火灾干扰和气候。具体而言，将分析这些地点的沉积物岩心中的花粉、木质化气孔、木纤维和植物大化石，以检测铁杉和雪松在三个空间尺度上（沿海和内陆之间、内陆范围内和靠近湖泊）的范围扩张。这些沉积物岩心的宏观木炭分析将用于识别火灾事件和评估火灾对林分入侵的影响。利用氧同位素和微量元素分析以及硅藻和小转移函数重建有效水分和生长季节温度。年代学控制将基于植物宏观化石的AMS 14c测年、薄片层年龄和大块沉积物的210pb测年。这些结果将被整合，以量化铁杉和雪松的迁移滞后，并评估各种规模相关的迁移控制。据我们所知，这是第一次使用植被、干扰和气候的独立测量来解决分布限制、林分尺度入侵和气候限制在范围扩展过程中的相对作用的综合古生态研究。该项目将产生一个大型的多代理数据集，与了解北温带地区破碎景观的未来范围变化有关。在范围内尺度上存在较大迁移滞后的证据表明，远距离迁移事件很少发生，因此，在预期的气候变化期间，类似尺度上的范围迁移可能受到有限的远距离迁移的限制。相比之下，气候限制（即没有迁移滞后）的证据表明，在过去4000-2000年间，向更冷、更湿的气候条件发生的相对较小幅度的变化是铁杉和雪松入侵内陆范围所必需的，因此这些森林将容易受到未来变暖的影响。对比沿海地区和内陆地区以及内陆地区的迁移滞后，应该能让我们深入了解在高度碎片化的景观中与在没有扩散屏障的情况下植物迁移的模式。在局部尺度上，火介导入侵的证据表明，林分尺度的入侵是控制这些物种范围扩张的另一个重要机制。这种多代理/多尺度方法在解释全新世植物迁移记录方面取得了重大进展。

0212917HuThe main objectives of the proposed research are to determine Holocene migration lags of westernhemlock and western redcedar in the interior Pacific Northwest and to examine the relative importance of climatic change,dispersal limitation,and stand-level invasibility as controls of migration. These two species currently co-dominate late-successional forests in a coastal distributional range and an interior distributional range separated by a 160-km wide dispersal barrier of semi-arid grassland and pine forest. Preliminary data suggests that invasion of these species into the interior range lagged behind the establishment of suitable climatic conditions by 3000 years, longer than any migration lag hypothesized by previous studies. This possibility stands in sharp contrast to the inference from several decades of paleoecological studies in the coastal range that migration lags were generally unimportant in the heterogeneous landscapes of the Pacific Northwest where most species arrived shortly after deglaciation. Because of the dearth of paleoecologic and paleoclimatic data from the interior range, however, it remains uncertain to what extent the range expansion of hemlock and cedar into the interior lagged behind climatic change to suitable conditions and how climatic constraints, dispersal limitation, and stand-level invasibility affected their expansion. To address the research objectives, this study will employ a suite of paleoecologic tools and state-of-the-art paleoclimatic methods to reconstruct vegetation, fire disturbance, and climate at three pairs of sites across a latitudinal gradient in the interior range. Specifically, sediment cores from these sites will be analyzed for pollen,lignified stomata,wood fibers,and plant macrofossils to detect range expansion of hemlock and cedar at three spatial scales (between the coastal and interior ranges, within the interior range, and arrival adjacent to lakes).Macroscopic-charcoal analysis of these sediment cores will be used to identify fire events and assess the effect of fire on stand invasibility.Reconstructions of effective moisture and growing-season temperature will be made from oxygen-isotope and trace-element analyses as well as diatom and midget ransfer-functions.Chronological controls will be based on AMS 14 C dates of plant macrofossils,ages of tephra layers,and 210 Pb dates of bulk sediments.These results will beintegrated to quantify migration lags of hemlock and cedar and assess various scale-dependent migration controls.To our knowledge,this is the first comprehensive paleoecological study using independent measures of vegetation,disturbance,and climate to address the relative roles of dispersal limitation,stand-scale invasibility,and climatic constraints during range expansion.This project will result in a large multi-proxy data set relevant for understanding future range shifts on the fragmented landscapes of the northern temperate regions.Evidence of a large migration lag at the inter-range scale would suggest that long-distance dispersal events occur infrequently,and thus range shifts at similar scales during anticipated climatic change may be constrained by limited long-distance dispersal.In contrast,evidence of climatic limitation (i.e.,no migration lag)would imply that the relatively small-magnitude changes toward cooler and wetter climatic conditions in the last 4000-2000 years were necessary for the invasion of hemlock and cedar into the interior range,and thus these forests will be vulnerable to future warming. Contrasting migration lags across the coastal and interior ranges and within the interior range should offer insights into the patterns of plant migration in highly fragmented landscapes versus in the absence of a dispersal barrier.At the local scale,evidence for fire-mediated invasion would show that stand-scale invasibility is an additional important mechanism controlling the range expansion of these species.This multiple-proxy/multiple-scale approach represents a significant advance in interpreting Holocene records of plant migration.