DOI: 10.5061/dryad.n5tb2rbwg
title: Survey of spatial associations of California buckwheat (Eriogonum fasciculatum) and California sagebrush (Artemisia californica) in coastal sage scrub

description: While models of species coexistence largely focus on how competition
 defines biological communities, over recent decades, a number of studies
 show positive plant-plant species interactions (facilitation) can also
 promote stable coexistence. The long-lived, co-dominant shrubs California
 buckwheat (E. fasciculatum) and California sagebrush (A. californica)
 share a well-documented positive association at the habitat level in their
 native California coastal sage scrub ecosystem, but mechanisms underlying
 their interactions remain unclear at finer spatial scales. Here, a
 hypothesis that E. fasciculatum acidifies CSS’s alkaline soils and
 facilitates A. californica through amelioration of alkalinity stress is
 tested in a greenhouse experiment and association tests in the field.
 Greenhouse results demonstrate facilitation at early growth stages. In
 late growth stages, water competition is known to determine the shrubs’
 interactions with each other, but here, field observations of the shrubs
 in late growth stages show positive associations between A. californica
 and E. fasciculatum that have a positive linear relationship to increasing
 soil pH. These results highlight the importance of understanding
 lifecycle-long interactions among species in evaluating facilitation’s
 impacts on community structure.
description: The presence of <i>E. fasciculatum </i>and
 <i>A. californica </i>in late growth forms<i>
 </i>(canopies &gt; 1 m diameter) as well as all other plant
 species grouped together were recorded in 560 1 m<sup>2
 </sup>quadrats. Sampling sites were located in five CSS preserves
 and nature parks<b> </b>(see Table S1) along a N-S coastal
 gradient from San Onofre State Beach (33.376068,
 -117.567275) to Zuma Canyon State Park in Malibu (34.044757,
 -118.809499). At each site, 10 m<sup>2
 </sup>quadrats were chosen to filter microsites that supported
 growth of both target species. Within each 10 m<sup>2</sup>
 quadrat, three parallel 1 x 10 m belt transects were randomly chosen,
 except in Zuma Canyon and San Onofre Beach. In these two latter sites,
 1x10 m belt transects were sampled in the only areas in which <i>E.
 fasciculatum </i>and <i>A. californica</i> were observed
 growing: along washes with thin soils in Zuma Canyon and along the edges
 of vernal pools atop seaside bluffs in San Onofre Beach. The presence of
 <i>E. fasciculatum </i>and <i>A. californica
 </i>as well as any other plants was recorded in the 10 1
 m<sup>2 </sup>quadrats within the 1 x 10 m belt transects.
 Plant species other than the target spcies were analyzed as one group.
 Associations were analyzed at 1 m<sup>2</sup>, because it was
 assumed <i>E. fasciculatum,</i> <i>A. californica
 </i>and other species would interact at a species-to-species level
 for limited water, nutrients, and light. (See Table1 for site-specific
 count numbers.) The 560 1 m<sup>2</sup>
 quadrats were aggregated with species data in five 50 m point-intercept
 transects from five additional CSS sites that were independently surveyed
 the same year by UC Irvine’s Center for Environmental Biology (CEB) along
 a coast-to-inland gradient (see Table S1 for coordinates of all
 locations). The presence of <i>E. fasciculatum, A. californica
 </i>and all other plant species grouped together<i>
 </i>in CEB transects was counted in adjacent 1 m blocks to conform
 surveying methods as much as possible with belt transects. For example, if
 <i>E. fasciculatum </i>was present at the 4 m point-intercept
 and <i>A. californica </i>was present at the 5 m
 point-intercept, then they were counted as co-occurring at a scale of 1
 m.<i> </i>Since both shrub species typically have canopies
 &gt; 1 m in late growth stages, these counting methods mean some
 shrubs were likely counted twice, but these methods better reflect the
 relatively large area of vegetative cover and effects on the distribution
 of other species that <i>E. fasciculatum </i>and <i>A.
 californica </i>in adult growth forms exhibit.
description: For analysis of spatial associations using chi-squared
 contingency tables, percent covers were converted to presence/absence
 of <em>A. californica, E. fasciculatum, </em>and all other
 plant species.