DOI: 10.5061/dryad.7pvmcvf2c
title: Germination phenology alters species coexistence outcomes

description: Species-specific phenological responses to changing climate are
 reshuffling the timing of species interactions, however, we don’t fully
 understand the consequences of these changes for species’ population
 dynamics and community composition. In this study, we
 experimentally manipulated the timing of germination for five annual plant
 species from southern California and used pairwise competition experiments
 and coexistence theory to quantify how phenological shifts may impact
 species interactions and coexistence. We found that phenological
 shifts may help promote coexistence when they confer an advantage for
 competitively inferior species, but in other cases promote dominance by
 competitively superior species. Earlier germination generally increased
 species’ performance relative to competitors, but the relative changes in
 intra-and inter-specific interactions caused more complex effects on niche
 and fitness differences. Phenological differences tended to reduce
 stabilizing niche differences for many species pairs and reduced overall
 coexistence probabilities. While phenological differences among species
 have typically been considered a form of niche partitioning, it seems
 increasingly likely that phenological offsets could destabilize species
 coexistence. The net effects of changing phenology on species coexistence
 will depend on the complex combinations of effects on intra- and
 inter-specific interactions, which remain challenging to predict.
description: We studied five study species: two non-native grasses,
 <em>Bromus diandrus </em>(BRDI) and <em>Bromus rubens
 </em>(BRRU), and three native forbs, <em>Lasthenia
 gracilis</em> (LAGR), <em>Nemophila menziesii
 </em>(NEME), and <em>Plantago erecta </em>(PLER). The
 native species are common members of the herbaceous community, and the two
 non-native grasses are both species that are well-known to become abundant
 at the expense of local native species diversity and abundance.
 These data come from a common garden competition experiment at
 the University of California, Riverside’s Agricultural Operations
 facilities, in Riverside, CA. The experiment was designed to calculate
 pairwise interaction coefficients (including intra-specific interactions)
 for all combinations of the five species, yielding 10 unique species
 pairs. Each study species was sown into three plots
 (1m<sup>2</sup>) at low, medium and high densities, to form a
 competitive density gradient for each species. Into each plot we sowed
 focal individuals of each species, establishing all pairwise combinations
 of competitive interactions. The relative timing of germination between
 the focal species and competitors was manipulated by offsetting the
 introduction of seeds by three weeks in both directions. These species of
 annual plants all have fast germination rates when they are exposed to
 enough water (within 3-4 days), as they are adapted to germinate with the
 beginning of seasonal rains during the cool wet winters of southern
 California. Thus, we watered them after sowing seeds, causing 3-week
 differences in germination dates.  In "focal first" plots, the
 focal seeds were sown and then three weeks later the competitor species
 were sown (Figure S1). Similarly, "background first" plots were
 established by waiting three weeks after sowing competitor species to
 introduce the focal species. Finally, a "same time" treatment
 was established by sowing focal plants and competitors at the same time,
 mid-way between the first and last dates.  Each species was also sown into
 plots without competitors to help estimate the performance of each species
 in the absence of competition. This design resulted in a total of 2850
 focal individuals in 114 plots (5 species x 3 background densities x 2
 replicates x 3 phenology treatments + 24 “no competition” plots). The
 garden was irrigated and weeded regularly to maintain suitable growing
 conditions and as “weed-free” pairwise competition plots as
 possible. The reproductive output of focal individuals
 was measured at the end of the growing season, along with the number and
 identity of competitors within a 20cm diameter neighborhood. As seed
 counts were logistically impossible, we counted flowers of all focals and
 obtained estimates of number of seeds per flower for each species. Prior
 to counting a total of flowers, 10 flowers per species had seeds extracted
 and counted to determine an average seed count per flower. These
 quantities were used to convert flower counts to seed estimates for each
 focal plant. Seed germination rates were estimated from a combination of
 buried seeds in nylon mesh bags, and germination trials in a greenhouse.
 We tried a variety of seed survival rates to mirror the typical pattern
 that the native species in this ecosystem have longer-lived seed banks
 than the non-native grasses, however, all results were quantitatively and
 qualitatively robust to very different seed survival values.
 Thus, the core dataset contains individual plant seed production
 as a function of the composition and density of neighbors.
description: # Germination phenology alters species coexistence outcomes
 [https://doi.org/10.5061/dryad.7pvmcvf2c](https://doi.org/10.5061/dryad.7pvmcvf2c) #### Description of the data and file structure. The core dataset contains individual plant seed production as a function of the composition and density of neighbors.  **Variable definitions (in dataset):** Phenology: Experimental treatment:  In "focal first" plots, the focal seeds were sown, and then three weeks later the competitor species were sown (Figure S1). Similarly, "background first" plots were established by waiting three weeks after sowing competitor species to introduce the focal species. Finally, a "same time" treatment was established by sowing focal plants and competitors at the same time, mid-way between the first and last dates.  background.spp:  Identity of the competitor species:  *Bromus diandrus* (BRDI), *Bromus rubens* (BRRU), *Lasthenia gracilis* (LAGR), *Nemophila* *menziesii* (NEME), and *Plantago erecta* (PLER) focal.spp: Identity of the focal species for whose fitness is listed fitness: Estimated number of seeds produced by the focal plant log.fitness: : Log of the estimated number of seeds produced by the focal plant N.background.species: Number of competitor plants in the neighbor of the focal plant