Novel and Lost Forests in the Upper Midwestern United States, from New Estimates of Settlement-Era Composition, Stem Density, and Biomass

Background
EuroAmerican land-use and its legacies have transformed forest structure and composition across the United States (US). More accurate reconstructions of historical states are critical to understanding the processes governing past, current, and future forest dynamics. Here we present new gridded (8x8km) reconstructions of pre-settlement (1800s) forest composition and structure from the upper Midwestern US (Minnesota, Wisconsin, and most of Michigan), using 19th Century Public Land Survey System (PLSS), with estimates of relative composition, above-ground biomass, stem density, and basal area for 28 tree types. This mapping is more robust than past efforts, using spatially varying correction factors to accommodate sampling design, azimuthal censoring, and biases in tree selection.
Changes in Forest Structure
We compare pre-settlement to modern forests using US Forest Service Forest Inventory and Analysis (FIA) data to show the prevalence of lost forests (pre-settlement forests with no current analog), and novel forests (modern forests with no past analogs). Differences between pre-settlement and modern forests are spatially structured owing to differences in land-use impacts and accompanying ecological responses. Modern forests are more homogeneous, and ecotonal gradients are more diffuse today than in the past. Novel forest assemblages represent 28% of all FIA cells, and 28% of pre-settlement forests no longer exist in a modern context. Lost forests include tamarack forests in northeastern Minnesota, hemlock and cedar dominated forests in north-central Wisconsin and along the Upper Peninsula of Michigan, and elm, oak, basswood and ironwood forests along the forest-prairie boundary in south central Minnesota and eastern Wisconsin. Novel FIA forest assemblages are distributed evenly across the region, but novelty shows a strong relationship to spatial distance from remnant forests in the upper Midwest, with novelty predicted at between 20 to 60km from remnants, depending on historical forest type. The spatial relationships between remnant and novel forests, shifts in ecotone structure and the loss of historic forest types point to significant challenges for land managers if landscape restoration is a priority. The spatial signals of novelty and ecological change also point to potential challenges in using modern spatial distributions of species and communities and their relationship to underlying geophysical and climatic attributes in understanding potential responses to changing climate. The signal of human settlement on modern forests is broad, spatially varying and acts to homogenize modern forests relative to their historic counterparts, with significant implications for future management.

研究背景
欧洲裔美国人的土地开发活动及其遗留影响，重塑了全美范围内的森林结构与组成。对历史森林状态的精准重建，是解析驱动过去、当前及未来森林动态过程的关键基础。本研究基于19世纪公共土地测量系统（Public Land Survey System, PLSS），重建了美国中西部北部地区（明尼苏达州、威斯康星州及密歇根州大部）定居前（19世纪）的森林组成与结构，生成了分辨率为8×8千米的网格化数据集，包含28个树种的相对组成、地上生物量、茎密度及胸高断面积估算值。相较于既往研究，本研究通过引入空间异质性校正因子，适配采样设计、方位截尾效应及树木选择偏差，使得重建结果更为可靠。

森林结构变化
本研究依托美国林务局森林清查与分析（Forest Inventory and Analysis, FIA）数据，对比定居前与现代森林，以此阐明“消失森林”（无现存对应类型的定居前森林）与“新兴森林”（无历史对应类型的现代森林）的分布范围。
定居前与现代森林的差异呈现显著空间格局，这源于土地利用影响及伴随的生态响应存在空间异质性。相较于历史状态，现代森林群落更为均质，生态交错带的梯度也更为弥散。新兴森林群落在所有FIA样格中占比达28%，同时有28%的定居前森林在现代背景中不复存在。
消失森林包括明尼苏达州东北部的美洲落叶松（tamarack）林，威斯康星州中北部及密歇根州上半岛地区以铁杉（hemlock）、雪松（cedar）为主的森林，以及明尼苏达州中南部与威斯康星州东部森林-草原交错带内的榆（elm）、栎（oak）、椴树（basswood）、铁木（ironwood）混交林。FIA样格内的新兴森林群落在研究区域内分布相对均匀，但新兴性与该区域内现存森林斑块的空间距离存在显著关联：依据历史森林类型的不同，距离现存斑块20至60千米的区域内易出现新兴森林。
现存与新兴森林的空间关联、生态交错带结构的变化以及历史森林类型的丧失，意味着若将景观修复作为优先目标，土地管理者将面临诸多严峻挑战。新兴性与生态变化的空间特征同时表明，若要解析森林群落对气候变化的潜在响应，依托现代物种与群落的空间分布及其与下垫面地球物理、气候属性的关联开展研究，可能存在一定局限。人类定居活动对现代森林的影响范围广泛且存在空间异质性，相较于历史森林群落，现代森林被显著均质化，该结论对未来森林管理具有重要指导意义。