PLSS Biomass by Species and PFT, Upper Midwest: Level 1

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. Gridded (8x8km) estimates of pre-settlement (1800s) forests from the upper Midwestern US (Minnesota, Wisconsin, and most of Michigan) using 19th Century Public Land Survey System (PLSS) records provide relative composition, biomass, stem density, and basal area for 26 tree genera. This mapping is more robust than past efforts, using spatially varying correction factors to accommodate sampling design, azimuthal censoring, and biases in tree selection. 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 analogue, and novel forests, modern forests with no past analogs. Differences between PLSS and FIA forests are spatially structured as a result of differences in the underlying ecology and land use impacts in the Upper Midwestern United States. Modern biomass is higher than pre-settlement biomass in northern Minnesota, northwestern and south central Wisconsin along the former prairie-forest border through Minnesota that was largely open savanna and the Big Woods of Minnesota. PLSS biomass was higher than today in northern Wisconsin and upper and lower Michigan due to shifts in species composition and, presumably, average stand age. Modern forests are more homogeneous, and ecotonal gradients are more diffuse today than in the past. Novel forest assemblages represent 29% of all FIA cells, while 25% of pre-settlement forests no longer exist in a modern context. Lost forests are centered around the forests of the Tension Zone, particularly in hemlock dominated forests of north-central Wisconsin, and in oak-elm-basswood 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 to land managers if landscape restoration is a priority in the region. 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. This material is based upon work supported by the National Science Foundation under grants #DEB-1241874, 1241868, 1241870, 1241851, 1241891, 1241846, 1241856, 1241930.欧美土地利用及其遗留效应重塑了美利坚合众国（United States, US）全境的森林结构与组成。精准重建历史森林状态，对于解析驱动过去、当前及未来森林动态的核心过程至关重要。本研究依托19世纪公共土地测量系统（Public Land Survey System, PLSS）的记录，对上中西部美国（明尼苏达州、威斯康星州及密歇根州大部）的定居前（19世纪）森林开展网格化（8×8km）估算，获取了26个树木属的相对组成、生物量、茎密度与断面积数据。相较于既往研究，本次制图更为稳健——通过引入空间变化的校正因子，适配了采样设计、方位截尾与树木选择偏差带来的影响。 我们借助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，以此揭示三类森林的分布特征：消失森林、无现代对应类型的定居前森林，以及无历史对应类型的新型森林。受美国中北部地区基础生态学特征与土地利用影响的空间异质性影响，PLSS与FIA森林数据集间的差异呈现显著的空间结构化特征。在明尼苏达州北部、威斯康星州西北部及中南部沿昔日稀树草原-森林交界带（该区域曾为开阔稀树草原与明尼苏达大森林（Big Woods）），现代森林生物量高于定居前水平。而在威斯康星州北部以及密歇根州上下部，PLSS记录的生物量则高于当代，这一差异源于物种组成的转变，推测同时与林分平均年龄变化相关。现代森林同质化程度更高，生态过渡带的梯度相较历史时期更为弥散。新型森林组合占全部FIA网格单元的29%，另有25%的定居前森林已在现代生态系统中不复存在。 消失的森林集中分布于张力带（Tension Zone）周边区域，尤以威斯康星州中北部以铁杉为主导的森林，以及明尼苏达州中南部、威斯康星州东部沿森林-草原交界带的栎-榆-椴森林最为显著。新型FIA森林组合在研究区域内分布相对均匀，但森林新颖性与距离中北部残存森林的空间距离存在强相关关系：根据历史森林类型的差异，距离残存林地20至60km处的森林新颖性水平最高。 残存森林与新型森林间的空间关联、生态过渡带结构的变迁，以及历史森林类型的消亡，均为该区域以景观修复为核心目标的土地管理者带来了严峻挑战。森林新颖性与生态变化的空间信号同时表明，若要理解物种与群落对气候变化的潜在响应，直接沿用现代物种与群落的空间分布及其与地球物理、气候属性的关联开展研究，可能存在潜在局限。人类定居活动对现代森林的影响广泛且存在空间异质性，相较于历史森林，该影响推动了现代森林的同质化进程，这一结论对未来森林管理具有重要指导意义。本研究成果依托美国国家科学基金会（National Science Foundation）的资助项目完成，资助编号为DEB-1241874、1241868、1241870、1241851、1241891、1241846、1241856、1241930。