In situ soil N mineralization in gopher mounds and soil under gopher mounds:Experiment 014 - Successional Dynamics on a Resampled Chronosequence

The purpose of this observational study is to describe the dynamics of ecosystem succession. The change in the number, type, and amount of plant and grazing animal species is monitored in more than 20 fields. 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. Permanent transects have been established in these abandoned fields for purposes of sampling in a consistent location from year to year. Permanent plots along these transects have been used to sample soil nutrients, (in particular, nitrogen) abundance of vegetation, species composition and herbivore populations. The sampling occurs approximately every 6 years. In the initial survey, 100 quadrats of size 1 by 0.5 m were sampled per field in 23 different fields. Abandoned fields included in E014 are 4, 5, 10, 21, 24, 26, 27, 28, 32, 35, 39, 40, 41, 44, 45, 47, 53, 70, 72, 76, 77. Fields 22(B), 29(A), and 69(C) were originally included in E014 but used for other purposes shortly after the start of the study. This experiment was established in 1983 and 1989 by principal investigators Johannes Knops and David Tilman. 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.本观测研究旨在阐明生态系统演替（ecosystem succession）的动态过程。研究团队对20余块农田中的植物与放牧动物物种的数量、类群及丰度变化进行长期监测。这些农田此前均有耕作历史，后于不同时期被弃耕。弃耕后农田不再受人为干扰，植物可依托土壤原位种子或经风力、动物携带进入农田的外来种子完成定植。为实现年度间固定点位的标准化采样，研究人员在各弃耕农田中布设了永久样带（permanent transects），沿样带设置永久样方以采集土壤养分、植被多度、物种组成以及草食动物种群数据。采样周期约为每6年一次。初始调查阶段，研究人员在23块农田中每块采集100个尺寸为1×0.5米的样方（quadrats）。纳入E014数据集的弃耕农田编号为4、5、10、21、24、26、27、28、32、35、39、40、41、44、45、47、53、70、72、76、77。农田22(B)、29(A)与69(C)最初也纳入E014数据集，但在研究启动后不久便被挪作他用。本实验于1983年与1989年由首席研究员约翰内斯·克诺普斯（Johannes Knops）与戴维·蒂尔曼（David Tilman）主持建立。此前在CDR及其他研究地点开展的前期研究已证实，火烧频率对维持草原-森林交错带的草原开阔生境与栎树稀树草原（oak savanna）具有决定性调控作用。火烧制度会对部分物种种群产生抑制作用，同时促进另一部分物种的繁衍，并驱动光照与养分循环动态，进而影响群落功能属性与多样性水平。最终，火烧频率会与气候、氮沉降（N deposition）、土地利用及生物入侵（biotic invasion）等因子相互作用，共同决定树木-草本植物互作的结局，以及明尼苏达州草原-森林交错带等生态交错带（ecotone）的植被动态。2006年，每块农田均被均分为两个区域，随机选取其中一半开展周期性控制性火烧（prescribed burning，即每两年实施一次火烧）。研究团队预计，经火烧处理的区域将持续向草原草地（prairie grassland）方向演替；未受火烧处理的区域若周边存在白皮松种源，则将逐渐发展为白皮松林（white pine stands），否则将以极慢的演替速率逐步形成栎树林。