Experiment 014 - Successional Dynamics on a Resampled Chronosequence

Introduction

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.

Key Findings

In the old fields, soils lost up to ~80% of C due to agriculture and after abandonment have slowly regained C, at about 340 kg C ha-1 y-1 (Knops & Bradley 2009), with the rate of C accumulation seemingly controlled by atmospheric N deposition and legume N fixation (Knops and Tilman 2000). Because atmospheric N deposition has increased in the last 50 years, and other factors such as temperature, rainfall, growing season length, and legume abundance also have changed, past rates of C accumulation might not apply to the future. We are addressing this by resampling soils and vegetation every six years in a total of 2100 plots across 21 old fields, and are examining how these changes correspond with vegetation and climate.

We have examined the impact of fire, vegetation, and plant-insect interactions on biogeochemical cycling (Laungani and Knops 2009ab, Dijkstra et al. 2006ab, Kay et al. 2007, 2008, and Hobbie 2010, Knops and Bradley 2009) and plant, bird, and mutualist communities (Chapman and Reich 2007, Peterson et al. 2007, Peterson and Reich 2008, Dickie et al. 2005, 2007, 2009a), as well as the interactive role of plant-mutualist interactions (Dickie et al. 2007) and tree-shrub interactions (Pelc et al. 2011) on the dynamics of the plant community.

Methods for e014