DOI: 10.5061/dryad.2z34tmpr2
title: Seed size, seed dispersal traits, and plant dispersion patterns for native and introduced grassland plants

description: Most terrestrial plants disperse by seeds, yet the relationship between
 seed mass, seed dispersal traits, and plant dispersion is poorly
 understood.  We quantified seed traits for 48 species of native
 and introduced plants from grasslands of western Montana, USA, to
 investigate the relationships between seed traits and plant dispersion
 patterns.  Additionally, because the linkage between dispersal
 traits and dispersion patterns might be stronger for actively dispersing
 species, we compared these patterns between native and introduced plants.
 Finally, we evaluated the efficacy of a global trait database, the TRY
 plant traits database, versus locally collected data for examining these
 questions. This archive contains species-level data used in analyses,
 including species metadata (origin, growth form), mean values of measured
 seed traits (size metrics and type of dispersal structures), two metrics
 of dispersion (local and broad scales, respectively) derived from
 grassland surveys in the study region, and information on the seed mass
 accessed from the TRY traits database. Note that the latter seed mass data
 could not be included in the archive, but can be acquired directly from
 the TRY plant traits database (https://www.try-db.org/TryWeb/Home.php).
description: Our study took place in semi-arid grasslands of the Intermountain
 Region in western Montana, U.S.A. The native system is dominated primarily
 by bluebunch wheatgrass (<em>Pseudoroegneria spicata</em>)
 with other grasses and a great variety of forbs diversifying the system,
 but it is heavily invaded by exotics. We identified our study species,
 comprised of 23 native and 25 exotic species, to reflect a range of
 dispersion patterns by using data from 620 1-m<sup>2</sup>
 vegetation plots from 31 grassland sites spread over 20,000
 km<sup>2</sup> of western Montana. Plant dispersion patterns
 were defined at a local-scale by the proportion of plots occupied within a
 site and at a broad-scale by the proportion of sites occupied per
 species. For each species, we collected at least 50
 seeds from each of 10 plants at each of 3 locations in Missoula and Lake
 County, Montana in either 2020 or 2021. Collection locations were chosen
 opportunistically based on species presence and hence differed by species.
 Although these locations did not align with sites surveyed for species
 dispersion per se, they were generally drawn from the central portion of
 the study area. Seeds were stored in a laboratory under ambient conditions
 until measurements were taken, at which point they were cleaned by hand
 and sorted based primarily on visual characteristics to remove potentially
 non-viable seeds. To determine the mean seed mass per species, we weighed
 a fixed number of samples (three or four) from each of the three
 locations. The number of seeds weighed per sample was set per species to
 ensure a total mass &gt;1.5 mg, the minimum reading needed for an
 accuracy of 2% per the specifications of the balance. For 32 of our 48
 species, only 10 seeds were needed to reach this minimum. For remaining
 species, we increased the number of seeds included per sample in
 increments of 10 (range 20–150 seeds/sample) until the minimum mass was
 reached. Seed mass included the entire diaspore (e.g., endosperm, seed
 coat, awns, and dispersal appendages) to ensure that all species could be
 treated in the same way (e.g., dispersal appendages such as wings would
 have been very difficult to remove from small-seeded species). Though the
 inclusion of dispersal appendages potentially biases seed mass estimates
 for this subset of species, we note that this bias should be small
 relative to the large variation in seed mass across species. Indeed,
 estimates for three exotic species (<em>Lactuca
 serriola</em>,<em> Taraxacum officinale</em>, and
 <em>Tragopogon dubius</em>) with pappuses showed that these
 structures increased seed mass measures by &lt;12%. For the remaining
 measurements, we used a ProgRes C10 camera (Jenoptik, CCD/CMOS) to create
 images of 20 seeds per species drawn from the 3 sampling locations (n=6
 from two locations and n=8 from the third, chosen randomly). We used the
 images to obtain the following measurements for each seed via ImageJ
 software (Rasband 1997-2018): seed length (maximum), seed width (maximum),
 and seed surface area. These seed measurements excluded dispersal
 structures. Mean values per species for all seed size measurements are
 included in the species-level dataset archived here.  Finally, we
 inspected seeds to determine whether seeds of each species possessed
 dispersal structures including pappuses, awns, wings, or plumes. For
 smaller-seeded species, we accomplished this using the seed images and
 also checked the literature to assure that dispersal structures were not
 missed. To enable comparison of empirical seed measures
 to those available in online trait databases, we used the TRY plant trait
 database (accessed 22 September – 7 October 2022), a global database
 integrating ~700 datasets including other major collective databases. This
 database included seed mass data for 44 of our 48 species but contained
 insufficient data to evaluate the other seed traits (i.e., length, width,
 and surface area) we measured (i.e., for only 2-40% of our study species).
 Importantly, 63% of n=831 seed mass records obtained from the TRY database
 could not be used in analyses. This is because these contained duplicate
 data that resulted from the consolidation of many datasets with common
 sources. See the publication for a full description of our process for
 identifying duplicate values. Remaining seed mass values from the TRY
 database were averaged to generate the mean estimate used in analyses. See
 the archive for sample size information per species.
description: See the README tab within (or the README.md file included in this
 archive) for detailed information about each data field
 included.