DOI: 10.25338/b8dw6f
title: Rancho Seco vernal pool community data

description: Disturbance often increases local-scale (α) diversity by suppressing
 dominant competitors. However, widespread disturbances may also reduce
 biotic heterogeneity (β diversity) by making the identities and abundances
 of species more similar among patches. Landscape-scale (γ) diversity may
 also decline if disturbance-sensitive species are lost. California’s
 vernal pool plant communities are species-rich due in part to two scales
 of β diversity: (1) within pools, as species composition changes with
 depth (referred to here as vertical β diversity), (2) between pools, in
 response to dispersal limitation and variation in pool attributes
 (referred to here as horizontal β diversity). We asked how grazing by
 livestock, a common management practice, affects vernal pool plant
 diversity at multiple hierarchical spatial scales. In terms of
 abundance-weighted diversity, grazing increased α both within local pool
 habitat zones and at the whole-pool scale,  as well as
 γ  at the pasture scale without influencing horizontal or
 vertical β diversity. In terms of species richness, increases in α
 diversity within habitat zones and within whole pools led to small
 decreases in horizontal β diversity as species occupancy increased. This
 had a dampened effect on species richness at the γ  (pasture)
 scale without any loss of disturbance-sensitive species. We conclude that
 grazing increases species richness and evenness (α) by reducing
 competitive dominance, without large disruptions to the critical spatial
 heterogeneity (β) that generates high landscape-level diversity
 (γ).  
description: <i>Site selection:</i> Our study
 took place at Rancho Seco (38.34˚ N, -121.11˚ W), a 458.10-ha conservation
 site in Northern California. Rancho Seco is located on a high-terrace
 alluvial formation that hosts Northern Hardpan Vernal Pools on Redding
 Gravelly Loam and Corning Complex soils (USGS SoilWeb) (Figure 1). The
 climate is Mediterranean with an average annual precipitation of 526.2 mm
 per water year (1 Oct – 30 Sep, CIMIS Weather Station, 21-year avg.
 1997-2018, Fair Oaks, CA). Annual plants germinate with the first
 significant fall rains (generally Oct.-Nov.) and flower as the rainy
 season ends (Apr.-May), and seeds are dormant through the dry summers. 
 Our study included the last 2 years of a multi-year drought:  2014-15
 (39.06 cm, 75.27% of 21-year avg.), the slightly wetter year of 2015-2016
 (43.60 cm, 82.83% of 21-year avg.), and the extremely wet year of
 2016-2017 (93.06 cm, 176.84% of 21-year avg.) (based on the Oct 1-Sep
 30<sup>th</sup>  water year, CIMIS Weather
 Station, 1997-2018, Fair Oaks, CA). Pool standing water depths vary
 greatly both between pools and within pools between years. The pools at
 our site ranged from
 maximum water depths of  0.00 (no water) to 38.00 cm  across all pools
 across all three wet seasons (Oct-May).  The site includes a
 20.9 ha pasture, where grazing has been in place for 150 years, and the
 current regime is 1 Animal Unit (AU) per 2.4 ha, where AU is defined as
 the forage demand of a 450 kg-cow. While typical stocking rate varies
 greatly by region (Herrero-Jáuregui & Oesterheld 2017),this regime
 is moderate for conservation grazing  and typical for vernal pool
 landscapes in this area (Marty 2015, George et al. 2016). In montane
 vernal pool landscapes, this stocking rate may be higher (1 AU/1.68 ha)
 (Merriam 2017). This site also includes an adjacent ungrazed pasture of
 24.35 ha from which cattle were removed 40 years ago when a fence was
 built to delineate property management boundaries.
   In winter 2014, we selected 14 pools each
 from the grazed and ungrazed areas that spanned two soil types, Corning
 Complex and Redding Gravelly Loam, (USGS SoilWeb) and a range of pool
 characteristics affecting plant communities, including, size, shape and
 slope around the pool perimeter (Gerhardt and Collinge 2003). We matched
 each grazed pool with an ungrazed pool with as many similar key
 characteristics as possible (Appendix S-1: Table S1).<i>
 </i>    We were interested in
 the effects of grazing at the pasture scale in addition to the pool
 (4-6800 m<sup>2</sup>) and local (&lt;1
 m<sup>2</sup>) scale. To achieve this, we chose a site in
 which grazing was applied at the pasture level rather than in a spatially
 random pattern, typical of many grazing experiments. We therefore expected
 to see some spatial autocorrelation across the whole site driven by
 vegetation differences between the grazed and ungrazed pastures.  Within
 grazing treatments, however, we also wanted to ensure that the similarity
 between any set of pools (horizontal β diversity) that we observed were
 not simply due to their spatial proximity. To determine whether spatial
 autocorrelation needed to be accounted for in our analyses, we conducted a
 partial Mantel test using spatial coordinates of each pool centroid. After
 accounting for grazing treatment, we found no significant spatial pattern
 in community composition (Mantel statistic based on Pearson’s
 product-moment correlation = 0.09, P = 0.10; Appendix S1: Figure
 S1<b>)</b>. Thus, we can rely on our multivariate analyses to
 assess differences in plant composition that are not confounded by spatial
 proximity.   <i>Vegetation
 Sampling:</i> We followed established sampling
 methods for vernal pools that stratify based on vertical habitat zones and
 randomly sample within each zone (Marty 2005, Gerhardt & Collinge,
 2007). In early spring 2015, after the pools dried down and before forb
 taxa were identifiable, we delineated three vertical habitat zones
 (inundated, transition, and upland) by recording slope and visual differences
 (e.g., soil color and texture, algal mats, water marks, and matted
 litter/vegetation) that indicated differences in inundation time. Two
 water lines were visible in each pool—one low-elevation distinct line
 indicating inundation throughout the season, and another, fainter
 high-elevation line suggesting  peak wet-season inundation level . We
 delineated the lowest point in the pool up to the inner line as the
 ‘inundated’ zone and the area between the two lines as the ‘transition’
 zone.  We delineated the ‘upland zone’ as the area outside the basin
 within 5-m of the upper edge of the transition zone, beyond which we
 expect little interaction with the vernal pool ecosystem (Marty 2005,
 Harpel 2008). Biweekly from March-May, we visited each pool and tracked
 the phenology of forb species. When we determined that a pool had reached
 ‘peak flowering’ in which the majority of forbs were blooming and
 identifiable, we placed quadrats in randomly chosen locations within each
 zone. Each quadrat was 50x50 cm, divided into 100 5 x 5 cm squares. We
 recorded the number of cells in which each species occurred. Each year,
 new locations were randomly chosen for the quadrats within each habitat
 zone in each pool. Due to the short phenological sampling window, we were
 limited to three quadrats per zone in each pool (9 quadrats per pool, 216
 quadrats/year total).