Yellow River Watershed, Indiana Historical (1800s) and Modern Forests

Historically, closed eastern forests transitioned into open savannas and prairies in the US Midwest, but this transition is poorly understood. To investigate the eastern boundary of the prairie-forest ecotone, we conducted a case study of historic and modern vegetation patterns of the Yellow River watershed in northwest Indiana. Historic vegetation came from the Public Land Survey notes collected in the early 1800s, whereas modern vegetation came from the Forest Inventory Analysis and USGS National Land Cover Database. We mapped historical survey vegetation data using GIS to reconstruct the region’s past and current forest composition and structure. We also mapped climate, topography and soil composition across the watershed to investigate the relationship between historic vegetation and environmental gradients. We found a sharp transition in the pre-settlement forest structure and composition, with dense deciduous forests in the eastern portion of our study area and open oak savannas in the west. The savanna ecosystem dominated in sandy, well-drained soils and was at a slightly lower elevation than the adjacent closed forest. Modest environmental changes accompanied major vegetation changes in the past, which might suggest that fire and hydrological patterns helped maintain the sharp ecotone. By contrast, the modern forest shows no difference in tree density and composition across the watershed, which is consistent with major land use and hydrology changes in the watershed since settlement. On the modern landscape, land that was historically closed forest now has higher agricultural productivity compared to land that was historically savanna, whereas the historic savanna currently supports more mesic forest. These results suggest that the environmental gradient continues to subtly shape the landscape. Though land use change has largely removed the closed mixed hardwood and oak savannas from this area, a better understanding of the historic vegetation and the conditions that supported it can help inform land management and restoration, as well as reveal ecological processes that drive vegetation transitions.This material is based upon work supported by the National Science Foundation under grant #DEB-1241874

历史上，美国中西部地区的封闭东部森林曾逐步演化为开阔稀树草原与北美草原，但学界对这一演替过程的认知仍较为匮乏。为探究草原-森林生态交错带（ecotone）的东部边界，我们以印第安纳州西北部的黄河流域为研究对象，开展了针对该区域历史与现代植被格局的案例研究。历史植被数据取自19世纪初收集的公共土地测量记录，现代植被数据则来自森林资源清查分析（Forest Inventory Analysis）以及美国地质调查局（USGS）全国土地覆盖数据库。我们借助地理信息系统（GIS）对历史调查植被数据进行空间制图，以重建该区域过去与当前的森林组成及结构。同时，我们对流域内的气候、地形与土壤组成开展空间制图，以探究历史植被与环境梯度之间的关联。研究发现，定居前的森林结构与组成存在显著的空间分界：研究区域东部为茂密的落叶阔叶林，西部则为开阔的栎树稀树草原。稀树草原生态系统主要分布于沙质、排水良好的土壤中，且其海拔略低于相邻的封闭森林。过去的植被巨变伴随有小幅环境变化，这表明火与水文格局或许是维持该尖锐生态交错带的关键因素。与之形成对比的是，当前流域内的森林在树木密度与组成上已无显著差异，这与定居以来流域内发生的重大土地利用与水文变化相契合。在当前景观中，历史上的封闭森林用地如今的农业生产力高于历史稀树草原用地，而历史稀树草原区域则发育出了更为湿润的中生森林。上述结果表明，环境梯度仍在潜移默化地塑造该区域的景观格局。尽管土地利用变化已基本清除了该区域的封闭混交硬叶林与栎树稀树草原，但深入理解历史植被及其维持条件，可为土地管理与生态修复提供科学依据，同时也有助于揭示驱动植被演替的生态过程。本研究成果依托美国国家科学基金会（National Science Foundation）编号为DEB-1241874的资助项目完成。