DOI: 10.5061/dryad.83bk3j9pc
title: Ecosystem recovery from disturbance depends on nutrient supply rate

description: Human disturbances alter the functioning and biodiversity of many
 ecosystems. These ecosystems may return to their pre-disturbance state
 after disturbance ceases, however humans have altered the environment in
 ways that may change the rate or direction of this recovery. For example,
 human activities have increased supplies of biologically-limiting
 nutrients, such as nitrogen (N) and phosphorus (P), which can reduce
 grassland diversity and increase productivity. We tracked the recovery of
 a grassland for two decades following an intensive agricultural
 disturbance under ambient and elevated nutrient conditions. Productivity
 returned to predisturbance levels quickly under ambient nutrient
 conditions, but nutrient addition slowed this recovery. In contrast, the
 effects of disturbance on diversity remained hidden for 15 years, at which
 point diversity began to increase in unfertilized plots. This work
 demonstrates that enrichment of terrestrial ecosystems by humans may alter
 the recovery of ecosystems and that disturbance effects may remain hidden
 for many years.
description: <strong>Experimental Design  </strong>
 The work described here was conducted at the Cedar Creek
 Ecosystem Science Reserve (CDR) a U.S. Long Term Ecological Research
 (USLTER) site located (Latitude 45.4 N, Longitude 93.2 W) in Minnesota,
 USA (Figure S1). CDR lies on a sandy, outwash plain formed during
 the Wisconsin Glaciation about 11,000 years ago. These sandy soils are
 very low in N relative to other grasslands, and as a result have a
 relatively low productivity compared to other grasslands (Fay et al.
 2015). The site has a mean annual precipitation of about 750 mm and a mean
 annual temperature of 6<sup>o</sup> C (Borer et al. 2014a).
 During the course of this experiment, precipitation ranged from 538 to
 1505 mm yr-1 (mean = 802 mm yr-1) and growing season (April – August)
 precipitation ranged from 281 to 970 mm yr-1 (mean = 483 mm yr-1). The
 experiment was replicated in three abandoned agricultural fields that were
 last tilled and farmed in 1968 (Field A), 1957 (Field B), and 1934 (Field
 C) (Figure S1). Detailed field descriptions are available in Tilman
 (1987). The field experiment was established in 1982
 and was composed of two treatments in a split-plot design: 1. Disturbance
 (Control or Disked, 35 x 55 m plots) and 2. Nutrient Addition (9 levels, 4
 x 4 m plots). The nutrient-addition treatment had nine levels representing
 different combinations of Nitrogen (0 – 27.2 g N
 yr<sup>-1</sup> added as
 NH<sub>4</sub>NO<sub>3</sub>) and Other Nutrients
 (20 g m<sup>-2</sup> yr<sup>-1</sup>
 P<sub>2</sub>0<sub>5</sub>; 20 g
 m<sup>-2</sup> yr<sup>-1</sup>
 K<sub>2</sub>0; 40 g m<sup>-2</sup>
 yr<sup>-1</sup> CaCO<sub>3</sub>; 30.0 g
 m<sup>-2</sup> yr<sup>-1</sup>
 MgSO<sub>4</sub>; 18 ug m<sup>-2</sup>
 yr<sup>-1</sup> CuSO<sub>4</sub>; 37.7 ug
 m<sup>-2</sup> yr<sup>-1</sup>
 ZnSO<sub>4</sub>; 15.3 ug m<sup>-2</sup>
 yr<sup>-1</sup> CoCO<sub>2</sub>; 322 ug
 m<sup>-2</sup> yr<sup>-1</sup>
 MnCl<sub>2</sub>; and 15.1 ug m<sup>-2</sup>
 yr<sup>-1</sup> NaMoO<sub>4</sub>). Here we focus
 on two treatments: Control (No Nutrients added) and Fertilized (9.5 g N
 m<sup>-2</sup> yr<sup>-1</sup> added in
 combination with all other nutrients). Nutrients were applied twice per
 year in mid-May and mid-June. The complete list of treatments is presented
 in Table S1, and we present an analysis of all treatments in Figures
 S7-S10. The Disturbance treatment was replicated in the
 three old-fields (A, B, C) in a completely randomized block design (2
 treatments in each of 3 fields for a total of 6 35 x 55 m large plots).
 The nutrient treatments were replicated 6 times in a completely randomized
 design in each of the 35 x 55 m plots (54 4 x 4 m small plots). Thus, the
 complete experiment was composed of 3 fields, 6 35 x 55 m large plots, and
 324 4 x 4 m small plots (Figures S1 and S2). The
 establishment of the experiment proceeded as follows. Prior to the
 experiment, each of the fields was enclosed in a 1.8 m tall wire fence
 with 10 cm openings. In addition, woven wire fence with 6 mm openings was
 buried 84 cm in the ground and extended 60 cm above the ground. The fences
 reduced the density of mammalian herbivores, of which white-tailed deer
 (Odocoileus virginianus) and pocket gophers (Geomys bursarius) are of
 particular note at this site. Pocket gophers, in particular, can
 complicate interpretation of fertilization experiments, as they
 preferentially burrow in fertilized plots creating areas of bare soil and
 increasing spatial variability (Inouye et al. 1987; Inouye et al. 1997;
 Seabloom & Richards 2003). At this site, exclusion of deer and
 pocket gophers may have increased plant productivity, soil N, and
 colonization by woody plants (Inouye et al. 1987; Knops et al. 2000). It
 is unlikely that herbivore exclusion changed the relative effects of
 fertilization, based on experiments that manipulate both herbivore density
 and fertilization at our study site and other grasslands sites worldwide
 as effects tend to be additive (Inouye et al. 1987; Gruner et al. 2008;
 Borer et al. 2014b). In April 1982, two 35 x 55 m areas
 were designated in each of the three fields (A, B, and C). In each of the
 fields, one of these two 35 x 55 m areas was selected to be disturbed with
 a 45 cm diameter disk harrow pulled by a tractor 20 times in one
 direction, 20 times perpendicularly, and 5 times diagonally to the first
 passes. Following the disking, the soil was hand raked to smooth the soil
 and remove any remaining vegetation, so that subsequent colonization was
 solely from seeds or small rhizome fragments. Within each of the 6 large
 plots, the 54 small plots were arrayed in 6 x 9 grid with 1 m buffers
 between each plot. Aluminum flashing was buried to depth of 30 cm around
 each plot to prevent horizontal movement of nutrients and spreading of
 plants through vegetative growth. In our analyses, we
 focus on two nutrient treatments: 1. Control (no nutrients; Treatment I)
 and 2. Other Nutrients and 9.5 g of N (Treatment F)(Table S1). We chose
 the 9.5 g of N addition, as it is a rate that overcomes N limitation in
 our study systems and most grasslands systems without being toxic (Elser
 et al. 2007; Isbell et al. 2013a; Fay et al. 2015). We present analyses of
 the full N gradient in Figures S7 to S10 and Table S4. In brief, choosing
 a higher or lower N addition rate would change the strength of the N
 effect, but would not change the direction of the effects or change the
 strength of the interactions with the disking.
 <strong>Sampling and Statistical Analyses
 </strong> At peak biomass (mid-July to late
 August), all aboveground biomass was clipped in a 3 m by 10 cm strip (0.3
 m<sup>2</sup>) in each plot. Note that there were 4 years when
 the disturbed plots were not sampled or only sampled in a single field.
 Inclusion or exclusion of years with missing data does not change the
 qualitative results. The biomass was sorted into dead, previous-year’s
 growth (litter) and live, current-year’s growth (live biomass). Live
 biomass was sorted to species, dried to constant mass at
 40<sup>o</sup> C, and weighed to the nearest 0.01 g. We
 estimated total aboveground biomass as the summed biomass of all non-woody
 species in each 0.3 m<sup>2</sup> sample, converted to g
 m<sup>-2</sup>. We excluded woody biomass, because our goal
 was to estimate annual productivity and most of the woody biomass is from
 previous year’s growth. Woody plant biomass composed less than 1% of total
 biomass across the data set. Species richness is the number of species in
 each 0.3 m<sup>2</sup> sample.  We
 quantified plant diversity as the Effective Number of Species based on the
 Probability of Interspecific Encounter
 (<em>ENS<sub>PIE</sub></em>), a measure of
 diversity that is more robust to the effects of sampling scale and less
 sensitive the presence of rare species than species richness (Jost 2006,
 2007; Chase & Knight 2013).
 <em>ENS<sub>PIE</sub> </em>is equivalent to the
 Inverse Simpson’s index of diversity which is calculated as where
 <em>S </em>is the total number of species (i.e., species
 richness) and <em>p<sub>i</sub></em> is the
 proportion of the community biomass represented by species
 <em>i</em> (Jost 2006, 2007; Chase & Knight 2013).
 Simpson’s evenness (<em>E</em>) satisfies the main
 requirements of an evenness index (Smith & Wilson 1996). In
 addition, it is directly related to
 <em>ENS<sub>PIE</sub> </em>through the
 relationship <em>E = ENS<sub>PIE</sub> /S
 </em>(Smith & Wilson 1996), thus we can factor diversity
 directly into its richness and evenness components through the
 relationship: <em>ENS<sub>PIE</sub> =
 S * E</em> Across all data,
 <em>ENS<sub>PIE</sub> </em>was positively
 correlated with richness (r=0.63) but uncorrelated with evenness (r=0.03).
 Richness and evenness were negatively correlated (r= -0.60).
 All analyses were conducted in R (v. 3.5.3; R Foundation for
 Statistical Computing, Vienna, Austria). Mixed effects models were fit
 using the lmer function in the lme4 R library. We
 log<sub>10</sub> transformed all response variables prior to
 analysis to stabilize the variance of the residuals. In addition, the
 transformed data tested for proportional changes in the variables, which
 was biologically the most relevant given the among-field and among-year
 variability. In all models, among field and among year variability were
 treated as random effects. In the analyses of individual groups of species
 (e.g., annual or perennial plants), we analyzed
 log<sub>10</sub>(biomass + 1), because there were some plots
 with zero mass of certain groups. All model specifications are included
 with each table of results. (Tables S5-S7).
description: Please refer to cdr-e001-e002-output-data-variable-descriptions
 for file descriptions.