Soil carbon and nitrogen: Old-Field Chronosequence: Plant Productivity

The goal of this research is to study the change in plant growth and species distribution during succession. Annual plant growth above ground is annually sampled in more than 20 fields from 4 permanently marked 3m x 4m plots in each field. These fields were previously cultivated, but then abandoned from agriculture at various times in the past. The fields were left undisturbed for plants to develop from seeds within the soil or brought into the fields by wind or animals. The fields included in this study are 4, 5, 10, 24, 26, 28, 35, 39, 41, 45, 53, 70, 72, 77 and the Lawrence strip that was abandoned in 1988. This experiment was started in 1987 by lead investigators David Tilman and Johannes Knops. In 2001 new sampling was started in positions similar to the E054 plots in these E014 fields: 21, 27, 32, 40, 44, 47, 76. Past work at CDR and elsewhere has demonstrated an overriding influence of fire frequency in maintaining prairie openings and oak savanna at the prairie-forest border. Fire regimes harm some types of species while favoring others and drive light and nutrient dynamics, which in turn drive community functional attributes and diversity levels. Ultimately, fire frequency interacts with climate, N deposition, land use, and biotic invasion to determine the outcomes of tree-grass interactions and the dynamics of vegetation at ecotones such as the prairie-forest border in Minnesota. In 2006 each field was divided in half, and one half randomly chosen for periodic prescribed burning (a fire every other year). We anticipate that the burned half will continue succession to prairie grassland while the unburned half will become white pine stands if seed sources are nearby, or will otherwise undergo extremely slow succession to oaks.

本研究旨在探究植物演替过程中的生长动态与物种分布变化。研究人员每年对20余块样地的地上一年生植物生长量开展采样，每块样地内设置4个永久标记的3m×4m样方。这些样地此前均为耕地，后于不同历史时期被弃耕，此后保持未受干扰状态，以使土壤内源种子或经风力、动物携带进入样地的植物种子完成萌发与定植。本研究涵盖的样地编号为4、5、10、24、26、28、35、39、41、45、53、70、72、77，以及1988年弃耕的Lawrence样带。本实验由首席研究员David Tilman与Johannes Knops于1987年启动。2001年，研究人员在E014系列样地（编号21、27、32、40、44、47、76）中设置了与E054样方位置匹配的新增采样点位。过往在CDR及其他区域开展的研究已证实，火频（fire frequency）在维持草原-森林交错带（ecotone）的草原开阔地与栎树稀树草原过程中发挥着决定性作用。火制度（fire regimes）会对部分物种产生抑制作用，同时促进另一些物种的存续，并调控光照与养分动态，进而影响群落功能属性与多样性水平。最终，火频（fire frequency）与气候、氮沉降、土地利用及生物入侵共同交互作用，决定了树木-草本植物互作的结果，以及明尼苏达州草原-森林交错带（ecotone）的植被动态。2006年，每块样地均被划分为两个半区，随机选取其中一半进行周期性计划火烧（prescribed burning，每两年实施一次火烧）。研究团队推测，火烧半区将持续向草原群落演替；未火烧半区若周边存在种源，则会发展为白松林分，否则将以极慢的演替速率向栎树林群落转变。