DOI: 10.5061/dryad.h45mr
title: Data from: Changes in spatial variance during a grassland to shrubland state transition

description: State transitions are changes in ecosystem structure and self-reinforcing
 feedbacks that are initiated when an exogenous driver variable crosses a
 threshold. Reversing state transitions is difficult and costly. While some
 state transitions are relatively rapid, many take years to decades.
 Outside of theoretical models, very little is known about slower state
 transitions and how they unfold in time and space. We quantified changes
 in spatial variance as a mesic grassland ecosystem shifts to a
 shrub-dominated state, using long-term experiments and simulations that
 maintain grasslands with annual fires or initiate a state transition to
 shrub dominance by decreasing fire frequency. In the experiments, the
 susceptibility to state transitions varied substantially in space. In the
 less frequent fire treatment, some plots became shrub-dominated around
 year 20 and grass extirpations began in year 25, but a third of the plots
 were still grass-dominated in year 37. Variable rates of state transition
 resulted in increasing spatial variance of grass cover over time, whereas
 shrub cover variance decreased. In the annually burned treatment, grasses
 remained dominant and the spatial variance of grass cover declined. In a
 separate experiment, less frequent fires were maintained for 23 years and
 then switched to annual fires. The switch to annual fires occurred shortly
 after grass variance started to increase and a majority of these plots
 quickly returned to a grass dominated state. In simulations, spatial
 variance remained low and average grass cover was high under frequent
 fires. If fire frequency decreased below a threshold, the ecosystem
 transitioned to shrubland, with a transient increase in the spatial
 variance of grass cover during the transition between states. Synthesis.
 Spatial variability in the rate and susceptibility to state transitions is
 indicative of a system with a patchy spatial structure, high spatial
 heterogeneity and low connectivity between patches. Increases in spatial
 variance can serve as an indication that some patches have begun a state
 transition and that management interventions are needed to avoid
 widespread transitions. This is one of the first empirical examples where
 altering management after an increase in spatial variance prevented state
 transitions.
description: Nongrazed_konza_lter_lowlands_dominant_veg_coverThis data-set reports results from long-term experimental treatments manipulating fire frequency in a native tallgrass prairie in the Central Great Plains, U.S.A. (39’05” N, 96’35” W). All plots are from lowland locations and were not grazed by large grazers during the experiments. Methods for calculating cover are described in the “Methods” section of the main text and in Appendix S1. The column “Watershed_designation” uses the same watershed designations as Table 1 in the main text. “Fire_treatment” refers to the fire frequency treatment a plot was grouped into for all calculations (see Table 1). For each watershed there were twenty plots (see “Plot_number”) arrayed over four transects per watershed (see “Transect_number”). Note that from year 2000 onwards, plots in 4B were sometimes visited in a different order, leading to some mislabeling of “Plot_number”. Therefore, the plot labels might not be consistent between locations in 4b after year 2000, but the same physical locations were measured each year. “Grass_cover” is the summed cover of the four dominant C4 grasses species. “Shrub_cover” is the summed cover of shrub and tree species that consistently maintain foliage above the grass layer. Cover within both functional groups is in units of “the sum of plant canopy cover per species per unit ground area” and can therefore exceed 100%, due to the presence of multiple overlapping canopies of different species. “NA” in the “Grass_cover” or “Shrub_cover” indicates that data were not available due to a disruption of the fire frequency treatment (see Table 1). For more information, including updates to the data-set, see description of PVC02 on the Konza Prairie LTER website: http://www.konza.ksu.edu/knz/pages/home/home.aspxKonza_SlowShifts_Experimental_Data.csv