PLSS Forests Lost Compared to Modern Forests and Novel Modern Forsets Gained Compared to PLSS Forests, 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（EuroAmerican）土地利用及其遗留效应重塑了美国全境的森林结构与组成。精准还原历史森林状态，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。本研究借助19世纪公共土地测量系统（Public Land Survey System, PLSS）记录，生成了美国中西部北部（明尼苏达州、威斯康星州及密歇根州大部）殖民前（19世纪）森林的网格化（8×8km）估算数据，涵盖26个树木属的相对组成、生物量、茎密度与断面积（basal area）。相较于既往研究，本制图方法更为稳健：通过引入空间变化的校正因子，适配采样设计、方位截尾以及树木选择偏差。

本研究依托美国林务局森林清查与分析（Forest Inventory and Analysis, FIA）数据对比殖民前与现代森林，以揭示消失森林（无现存对应类型的殖民前森林）与新型森林（无历史对应类型的现代森林）的分布特征。PLSS与FIA森林数据间的差异呈现空间分布格局，这源于美国中西部北部区域的底层生态差异与土地利用影响的空间异质性。在明尼苏达州北部、威斯康星州西北部及中南部的原草原-森林过渡带（该区域曾以开阔稀树草原与明尼苏达州大森林（Big Woods）为主），现代森林生物量高于殖民前水平；而在威斯康星州北部以及密歇根州上下半岛，PLSS估算的生物量则高于当前水平，这源于物种组成变化及推测的林分平均年龄差异。相较于历史状态，现代森林同质化程度更高，生态交错带（ecotone）梯度也更为弥散。新型森林群丛占FIA网格单元总数的29%，而25%的殖民前森林已在现代背景下不复存在。

消失森林主要集中于张力带（Tension Zone）森林，尤其是威斯康星州中北部以铁杉（hemlock）为主的森林，以及明尼苏达州中南部与威斯康星州东部沿森林-草原边界的栎-榆-椴森林。FIA的新型森林群丛在研究区域内分布相对均匀，但新型性与该区域残存森林（remnant forests）的空间距离存在显著关联：根据历史森林类型的不同，距离残存森林20~60km处的新型性水平最高。

若该区域将景观恢复作为优先目标，则残存森林与新型森林的空间关联、生态交错带结构变化以及历史森林类型的丧失，将为土地管理者带来严峻挑战。新型性与生态变化的空间信号也表明，若要解析物种与群落的现代空间分布及其与底层地球物理、气候属性的关联，进而理解其对气候变化的潜在响应，将面临潜在挑战。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，该影响促使现代森林同质化，这对未来森林管理具有重要启示意义。

本研究成果基于美国国家科学基金会（National Science Foundation）资助的项目，项目编号分别为DEB-1241874、1241868、1241870、1241851、1241891、1241846、1241856及1241930。