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

0212917Hu拟议研究的主要目标是确定太平洋西北部内陆地区西部铁杉和西部红柏的全新世迁徙滞后时间，并检查气候变化、扩散限制和林分水平入侵作为迁徙控制因素的相对重要性。这两个物种目前在沿海分布区和内部分布区共同支配着演替后期的森林，中间隔着160公里宽的半干旱草原和松林的扩散屏障。初步数据表明，这些物种进入内陆地区的入侵比建立合适的气候条件晚了3000年，比之前研究假设的任何迁徙延迟都要长。这种可能性与几十年来对沿海地区进行的古生态研究得出的结论形成鲜明对比，即在太平洋西北部的异质地貌中，迁徙滞后通常不重要，大多数物种在冰川消融后不久到达那里。然而，由于缺乏内陆地区的古生态和古气候数据，铁杉和雪松进入内陆的范围扩展在多大程度上滞后于气候变化以适应条件，以及气候限制、扩散限制和林分水平入侵对它们的扩展是如何影响的，目前尚不清楚。为了达到研究目标，本研究将使用一套古生态工具和最先进的古气候方法来重建内陆范围内纬度梯度上的三对地点的植被、火灾干扰和气候。具体地说，将对这些地点的沉积物岩心进行花粉、木质化气孔、木材纤维和植物大型化石的分析，以检测铁杉和雪松在三个空间尺度上的范围扩展(沿海和内陆范围之间，内部范围内，以及到达湖泊附近)。这些沉积物岩心的宏观木炭分析将用于识别火灾事件，并评估火灾对林分侵入性的影响。有效水分和生长季温度的重建将通过氧同位素和微量元素分析以及硅藻和侏儒Ransfer函数进行。时间控制将基于植物宏观化石的AMS 14C年代，Tephra层的年龄，这些结果将被综合起来，以量化铁杉和雪松的迁移滞后时间，并评估各种依赖于尺度的迁移控制。据我们所知，这是第一次利用植被、干扰和气候的独立测量来研究扩散限制、林分尺度入侵和气候限制在范围扩展中的相对作用的综合古生态研究。这个项目将产生一个大型的多代理数据集，与理解北方温带地区破碎的地形上未来的范围变化有关。在范围间尺度上的大迁移滞后的证据表明，远距离扩散事件发生的频率很低。相反，气候限制(即没有迁徙滞后)的证据表明，在过去4000-2000年间，向更凉爽和更潮湿的气候条件发生的相对较小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。对比沿海和内陆范围以及内陆范围内的迁徙滞后，可以深入了解在高度破碎化的景观中与在没有扩散屏障的情况下植物迁徙的模式。在局部尺度上，火介导入侵的证据将表明，立地尺度入侵是控制这些物种范围扩大的另一种重要机制。这种多代理/多尺度方法代表了在解释全新世植物迁徙记录方面的重大进步。

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.