10.25338/B8DW6F
Michaels, Julia
0000-0003-4548-0469
University of California, Davis
Rancho Seco vernal pool community data
Dryad
dataset
2020
FOS: Natural sciences
2021-11-20T00:00:00Z
2021-11-20T00:00:00Z
en
191353 bytes
4
CC0 1.0 Universal (CC0 1.0) Public Domain Dedication
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).
Site selection: 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
30th 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). We were interested in the effects of grazing at the pasture
scale in addition to the pool (4-6800 m2) and local (<1 m2) 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). Thus, we can rely
on our multivariate analyses to assess differences in plant composition
that are not confounded by spatial proximity. Vegetation Sampling: 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).