DOI: 10.5061/dryad.dbrv15f5t
title: Disturbance alters transience but nutrients determine equilibria during grassland succession with multiple global change drivers

description: Disturbance and environmental change may cause communities to converge on
 a steady state, diverge towards multiple alternative states, or remain in
 long-term transience. Yet, empirical investigations of successional
 trajectories are rare, especially in systems experiencing multiple
 concurrent anthropogenic drivers of change. We examined succession in old
 field grassland communities subjected to disturbance and nitrogen
 fertilization using data from a long-term (22-year) experiment. Regardless
 of initial disturbance, after a decade communities converged on steady
 states largely determined by resource availability, where species turnover
 declined as communities approached dynamic equilibria. Species favored by
 the disturbance were those that eventually came to dominate the highly
 fertilized plots. Furthermore, disturbance made successional pathways more
 direct under low nutrients, revealing an important interaction effect
 between nutrients and disturbance as drivers of community change. Our
 results underscore the dynamical nature of grassland and old field
 succession, demonstrating how community properties such as beta-diversity
 change through transient and equilibrium states.
description: Data were collected in successional grassland fields at the Cedar
 Creek Ecosystem Science Reserve in Minnesota, USA (CDR, Lat: 45.4 Long:
 93.2 W) from 1982 to 2004. In 1982, identical disturbance X nutrient
 addition experiments were established in three abandoned agricultural
 fields within 5 km of one another that were last tilled and farmed in 1968
 (Field A), 1957 (Field B), and 1934 (Field C). Within
 each of the three Fields, two grids (35 X 55 m) were established in 1982
 for nutrient application, one in an area that was thoroughly disked in the
 spring of 1982 (E002), and another in an adjacent area that remained
 intact with old field vegetation (E001). The disking treatment pulverized
 the existing vegetation, leaving bare soil which was then raked to remove
 clumps of vegetation. Each grid consisted of 54, 4 X 4 m vegetation plots,
 receiving one of nine nutrient treatments (applied annually in May or
 June) in a randomized block design, with 6 replicate plots per field. A
 nearby remnant grassland within a native oak savannah (Field D) that had
 never been clear-cut or plowed was also surveyed annually and provides a
 comparison for our study. The nutrient-addition
 treatment had nine levels representing different combinations of Nitrogen
 (0 – 27.2 g N yr-1 added as
 NH<sub>4</sub>NO<sub>3</sub>) and Other Nutrients
 (20 g m-2 yr-1 P205; 20 g m-2 yr-1 K20; 40 g m-2 yr-1
 CaCO<sub>3</sub>; 30.0 g m-2 yr-1
 MgSO<sub>4</sub>; 18 ug m-2 yr-1 CuSO<sub>4</sub>;
 37.7 ug m-2 yr-1 ZnSO<sub>4</sub>; 15.3 ug m-2 yr-1
 CoCO<sub>2</sub>; 322 ug m-2 yr-1
 MnCl<sub>2</sub>; and 15.1 ug m-2 yr-1
 NaMoO<sub>4</sub>). Beginning in 1982,
 vegetation was sampled by clipping a 10 X 300 cm strip each year within
 each plot at the ground level. After clipping, biomass was sorted into the
 previous year’s growth (litter), and the current year’s growth (live
 biomass). Live biomass was sorted by species, dried, and weighed to the
 nearest 0.01 g.  All plots in all fields were sampled annually with the
 exception of years 1995 (only E001 sampled), 2001 (only E001 sampled), and
 2003 (only E001 and Field C E002 sampled). Due to new treatments
 (experimental burning and fence removal) among the three experimental
 fields after 2004, we restricted our analyses to the time period 1982 -
 2004. Additionally, beginning in 1992, three randomly chosen replicate
 plots within each nutrient treatment in the E002 grid received nutrient
 cessation and experimental burning, which we excluded from our analyses
 from 1992 onwards. We conducted statistical analyses on plot-year
 combinations with original disturbance X nutrient treatments established
 in 1982. Prior to all multivariate analyses, we applied a ln(1 + x) data
 transformation where x = biomass (in g) of individual plant species within
 a plot in a given year. Code for analyses and figures
 in the manuscript can be found here: https://github.com/melissadesiervo1031/CedarCreekconvergence