Study design
Our three-year grassland experiment involved four treatments in a
partially factorial design: i) successional stage of recipient community
(early, mid, late); ii) disturbance (disturbed, undisturbed); iii)
invader type (fast invaders, slow invaders, both fast and slow
invaders); and iv) seed dose of invader species (none, low, high). The
invader type and seed dose treatments were not fully crossed for
logistical reasons (Fig. S1). This resulted in a total of 42 treatment
combinations (3 community levels x 2 disturbance levels x 7 invader
levels), replicated four times, giving a total of 168 plots, which were
surveyed over three growing seasons.
The experiment was carried out in three old field sites at Cedar Creek
Ecosystem Science Reserve (45.4°N, 93.2°W), Minnesota in 2013-2015.
Cedar Creek has nitrogen-limited sandy soils, annual precipitation of
~770 mm, mean summer temperatures of 27°C and winter
lows of -14°C. In 2013, the late succession site was approximately 80
years old, the mid succession site was 23 years old, and the early
succession site was 0 years old with no live standing vegetation when
the treatments were imposed. Community composition was distinct among
the three sites and soil fertility varied, reflecting the accumulation
of organic carbon and total nitrogen through succession (Appendix S1).
In each site, 56 1 m x 1 m plots were permanently marked, separated by 1
m wide walkways (Fig. S2). Plots were randomly assigned disturbance (0,
1) and seed sowing treatments, which were enacted in May 2013
(disturbance first). Disturbance involved mowing vegetation to 10 cm
followed by soil tilling, with the aim of killing approximately half of
the standing vegetation. Litter and dead plants were left in the plots.
Walkways were undisturbed.
We selected 23 grassland species for seed addition, all of which were
native or naturalized at Cedar Creek but were absent or rare in the
sites prior to seed addition. We separated the species into two groups
(fast and slow invaders) based on time of colonisation during succession
(Tilman 1988) and plant functional traits (Table S1, Figure S3). Within
the groups, we aimed to balance functional type, origin and lifespan. We
purchased seeds from local suppliers. We added 0.5 g and 4 g of viable
seed per species for the low and high seed dose treatments respectively.
Of 23 invader species sown, 15 were observed at least once (Table S1).Strophostyles leiosperma was only added to some of the intended
plots, resulting in an imbalanced study design. Where present, it had
mean cover of 0.3% (maximum 1%). It was hand-pulled to remove it from
plots when observed. Preliminary analysis showed that its inclusion or
exclusion did not affect model outputs, so it was excluded from further
analyses. This left 7 fast invaders and 7 slow invaders (Tables 1 & S1,
Appendix S1).