DOI: 10.5061/dryad.qv9s4mwqj
title: Environmental resistance strongly deters, but does not eliminate, propagule pressure

description: This study investigates the dynamics between propagule pressure and
 environmental resistance in plant invasion using Oxalis stricta L. (common
 yellow woodsorrel) as a model species. Oxalis stricta is a North American
 forb that can become invasive in native and novel habitats. A simulated
 invasion was conducted in a controlled glasshouse environment with
 propagule pressure from proximate reproducing plants and environmental
 resistance from planted turfgrass communities (biotic) and environmental
 heterogeneity in moisture and fertility (abiotic). Whereas O. stricta
 successfully established and proliferated in bare soil (essentially
 disturbed) environments with high propagule pressure, its invasion into
 established plant communities – and subsequent reproduction – was largely
 inhibited. Additionally, O. stricta reproduction was highest at
 intermediate levels of O. stricta cover, suggesting that negative density
 dependence somewhat limited reproductive output. Still, O. stricta was
 able to establish and persist in some trays with minimal propagule input,
 which may be enough for self-pollinating plants that may thrive in small
 populations. Such findings might challenge the propagule pressure versus
 environmental resistance dichotomy as environmental resistance may
 outweigh propagule pressure in determining invasion success, particularly
 in environments with well-established plant communities, but the ability
 to self-pollinate with high germination rates might allow even low-density
 invaders to establish and persist. The research underscores the importance
 of considering both propagule pressure and environmental resistance, as
 well as the reproductive strategies of the invader, in understanding
 invasion dynamics.
description: The experiment was conducted in a climate-controlled glasshouse
 bay (3.5 x 6.5 m) November 2020-March 2021. The glasshouse bay walls and
 floor were thoroughly cleaned before initiating the experiment, and all
 outside ventilation was kept closed. Eight mature <em>O.
 stricta</em> plants were collected from plants that naturally
 colonized plant containers in the glasshouse and placed (2 each) in 4,
 27.8 x 54.5 x 6.2 cm trays filled with commercial potting soil. The four
 <em>O. stricta</em> source trays were placed on growing tables
 at one end of the glasshouse bay (Fig. 1, S1). The plants were producing
 ballistic dispersing seeds at the beginning of the experiment. Thirty-two
 experimental trays were filled with commercial planting soil and half (16)
 were planted with an equal mix (30 cm<sup>3</sup> seeds per
 species) of the field/turf graminoids and herbaceous plants (hereafter,
 “plant resistance trays”). Eight experimental trays were each placed on
 tables at 1, 2, 3 and 4 m distance from the <em>O.
 stricta</em> source trays. Each plant resistance and bare soil tray
 was then randomly assigned a treatment so that each group got the same
 four treatments: control, supplemental watering, supplemental fertilizer
 and supplement watering + supplemental fertilizer (<em>n</em>
 = 8 replicates each treatment). All trays were watered thoroughly (until
 water drained through tray bottom) once per week; the high moisture trays
 were watered three times per week. Each tray was lightly fertilized (6-5-4
 NPK) every 4 weeks (2 g · l<sup>-1</sup>); the high fertility
 (“fertilized”) trays were fertilized weekly. All plants were grown in
 ambient light augmented with full spectrum LED lights (~1200-1500 max.
 PAR) to create 12 hours of photosynthetic light per day. Glasshouse
 temperatures ranged from 13-30° C. To track <em>O.
 stricta</em> establishment in the experimental trays, <em>O.
 stricta</em> cover and seedpod production were measured weekly
 (visually assessed). Four additional trays were filled with the same
 commercial soil as used in the experiment and maintained in similar
 conditions in a separate glasshouse bay to account for potential
 contamination seeds – none appeared. To track
 <em>O. stricta</em> seed input that was ballistically
 dispersed from the source trays, two 25 x 30 cm white Styrofoam seed traps
 (S1) were placed at each row (<em>n</em> = 8 total).
 <em>Oxalis stricta</em> seeds are sticky, and they were easy
 to observe and count against the white background (hereafter, “seed
 input”). The seeds were rinsed off the traps with each weekly
 count.
description: # Environmental resistance strongly deters, but does not eliminate,
 propagule pressure The experiment was conducted in a climate-controlled
 glasshouse bay (3.5 × 6.5 m) from November 2020 to March 2021. Eight
 mature *O. stricta* plants producing ballistic seeds were placed in four
 trays at one end of the bay. Thirty-two experimental trays, half planted
 with a mix of graminoids and herbaceous plants, were positioned at 1–4 m
 from the source trays and assigned one of four treatments (control,
 watering, fertilization, or both). Trays were watered and fertilized
 according to treatment, and *O. stricta* cover, seedpod production, and
 seed input (via Styrofoam traps) were monitored weekly. **Description of
 the data and file structure** Cover.data.csv * Ident = line identifier -
 Week = study week * Date = date of measurement - Tray = experimental tray
 identification (number = row; letter = tray) * Row = experimental row; -
 number = meters from source population * Cover = percent cover of O.
 stricta - Flowers = number of O. stricta flowers * Pods \= number of O.
 stricta seedpods - Planted = Plant resistance treatment (0 \= bare soil; 1
 = turfgrass community) * Water = supplemental watering (0 = control; 1 =
 supplemental watering) - Fertilizer = supplemental fertilizing (0 =
 control; 1 = supplemental fertilizing) * Seed.dist.n = seed input *
 Seed.cat = seed input categories (high = 20+ seeds, medium = 11-20 seeds,
 low = 1-9, none = 0)