DOI: 10.5061/dryad.zs7h44jg9
title: Data from: Nitrogen deposition suppresses ephemeral post-fire plant diversity

description: Fire is a dominant force shaping patterns of plant diversity in
 Mediterranean-type ecosystems. In these biodiversity hotspots, including
 California’s endangered coastal scrub, many species remain hidden
 belowground as seeds and bulbs, only to emerge and flower when sufficient
 rainfall occurs after wildfire. The unique adaptations possessed by these
 species enable survival during prolonged periods of unfavorable
 conditions, but their continued persistence could be threatened by
 nonnative plant invasion and environmental change. Furthermore, their
 fleeting presence aboveground makes evaluating these threats in situ a
 challenge. For example, nitrogen (N) deposition resulting from air
 pollution is a well-recognized threat to plant diversity worldwide, but
 impacts on fire-following species are not well understood. We
 experimentally evaluated the impact of N deposition on post-fire
 vegetation cover and richness for three years in stands of coastal sage
 scrub that had recently burned in a large wildfire in southern California.
 We installed plots receiving four levels of N addition that corresponded
 to the range of N deposition rates in the region. We assessed the impact
 of pre-fire invasion status on vegetation dynamics by including plots in
 areas that had previously been invaded by nonnative grasses, as well as
 adjacent uninvaded areas. We found that N addition reduced native forb
 cover in the second year post-fire while increasing the abundance of
 nonnative forbs. As is typical in fire-prone ecosystems, species richness
 declined over the three years of the study. However, N addition hastened
 this process, and native forb richness was severely reduced under high N
 availability, especially in previously invaded shrublands. An indicator
 species analysis also revealed that six functionally and taxonomically
 diverse forb species were especially sensitive to N addition. Our results
 highlight a new potential mechanism for the depletion of native species
 through the suppression of ephemeral post-fire biodiversity.
description: <strong>Materials and methods</strong>
 <strong><em>Study
 site</em></strong> We conducted field
 experiments at a site in the Santa Monica Mountains National Recreation
 Area (34.151125, -118.964891) in stands of coastal sage scrub shrublands
 which burned in the May 2013 Springs Fire. The site last burned in 1993.
 The Santa Monica Mountains are adjacent to the highly urbanized Los
 Angeles metropolitan area and experience a steep gradient of anthropogenic
 N deposition. The study site where field plots were installed is located
 in the western end of the mountain range which receives moderate levels of
 exogenous N inputs, approximately 8 kg N ha<sup>-1</sup>
 yr<sup>-1</sup>.<sup> </sup>Using aerial imagery
 and initial site surveys, we selected study areas of coastal sage scrub
 that were previously uninvaded by nonnative annual grasses before the
 fire, as well as adjacent areas at the ecotone of native shrublands and
 annual grasslands where both native shrub species and nonnative grasses
 were present before the fire (hereafter referred to as uninvaded and
 invaded, respectively).
 <strong><em>Nitrogen
 fertilization</em></strong> We installed
 plots (5m x 5m) in September 2013 (Fig. 1A) and monitored vegetation
 dynamics for three growing seasons. In a randomized block design, we
 treated plots  with one of four levels of N addition (0, 5, 15, and 30 kg
 N ha<sup>-1</sup> yr<sup>-1</sup>) annually in the
 fall before the first rains of the growing season to simulate the
 accumulation of dry deposition to the soil surface throughout the dry
 season. Plots within each block were separated by a buffer of at least two
 meters. Plots were fertilized by hand-broadcasting a mix of granular
 calcium nitrate (CaN<sub>2</sub>O<sub>6</sub>) and
 urea (CO(NH<sub>2</sub>)<sub>2</sub>), an approach
 previously used to examine impacts of N addition in this ecosystem. We
 installed blocks that included replicate plots of each N treatment (n =
 10) in stands of coastal sage scrub that were previously invaded by
 nonnative annual grasses (prior to the fire), as well as in directly
 adjacent areas of shrublands that were not. Invasion status (invaded or
 uninvaded) was determined based on aerial imagery and site surveys (both
 before and after the fire). Blocks in invaded and uninvaded shrublands
 were matched in terms of aspect and slope.
 <strong><em>Vegetation
 monitoring</em></strong> Each spring at the
 peak of the growing season (late April to early May), we sampled
 vegetation within all plots. We placed four 1m<sup>2</sup>
 subplots within each plot and visually estimated percent cover in a
 gridded plot frame of all plant species present. From these data, we
 calculated plant cover by native and nonnative plant functional groups,
 including for native shrubs, native forbs, native grasses, nonnative
 annual grasses, and nonnative forbs. We also surveyed entire plots for
 plant species richness to ensure we captured species not present within
 the smaller subplots sampled for plant cover.
description: # Data from: Nitrogen deposition suppresses ephemeral post-fire plant
 diversity Data includes
 plant cover ("cover_data.csv") and richness
 ("richness_data.csv") from nitrogen fertilization plots over
 three years following wildfire, including in both previously invaded and
 uninvaded shrublands. Also included is a species list of all plant species
 recorded during the study ("species_list.csv"). ## Description
 of the data and file structure **species_list.csv**: This file contains
 full species names (genus and species) for all plant species recorded
 during the study. Also included are species codes used in the plant cover
 and richness data file, as well each specie's growth form (i.e.,
 perennial or annual, forb, grass or shrub) and origin (either native to
 the site or nonnative), and plant family. **cover_data.csv**: Data file
 includes percent cover (%) of each plant species within field plots, as
 well as percent bare ground ("Bare") and litter
 ("Litter"). Nitrogen fertilization plots were installed in a
 randomized block design, and identifying numbers for each block are shown
 in the "Block" column. We included four nitrogen addition
 treatments in each block, indicated in the "Nitrogen" column.
 These treatments are abbreviated by C (control = no added nitrogen), L
 (low = 5 kg N ha<sup>-1</sup> yr<sup>-1</sup>), M
 (medium = 15 kg N ha<sup>-1</sup>
 yr<sup>-1</sup>), and H (high = 30 kg N
 ha<sup>-1</sup> yr<sup>-1</sup>). Year refers to
 the sampling year (2014, 2015, and 2016). Status refers to whether or not
 plots were previously invaded by annual grasses ("Invaded") or
 not ("Uninvaded") prior to the wildfire. Species are abbreviated
 by six letter codes (first three letters of genus and species). Full
 species names and other information are included in the
 "species_list.csv" file. Also included is percent cover totals
 for different native and nonnative plant function groups, including native
 shrub cover, native forb cover, nonnative grass cover, and nonnative forb
 cover. ***Column headers*** **Block** = block identifier for all field
 plots **Nitrogen** = nitrogen fertilization treatment **Status** =
 invasion status prior to fire **Bare** = percent bare ground **Litter** =
 percent litter **Columns G through CQ** = six letter species codes (full
 names and codes are included in the "species_list.csv" file);
 data in these columns is percent cover (%) of each species **shrub cover**
 = percent cover of all native shrubs (%) **native forb cover** = percent
 cover of all native forb species (%) **nonnative grass cover** = percent
 cover of all nonnative annual grasses (%) **nonnative forb cover** =
 percent cover of all nonnative forb species (%) **richness_data.csv**:
 File includes a matrix of presence ("1") and absence
 ("0") data for all plant species observed within field plots
 over three years (from 2014-2016). Nitrogen fertilization plots were
 installed in a randomized block design, and identifying numbers for each
 block are shown in the "Block" column. We included four nitrogen
 addition treatments in each block, indicated in the "Nitrogen"
 column. These treatments are abbreviated by C (control = no added
 nitrogen), L (low = 5 kg N ha<sup>-1</sup>
 yr<sup>-1</sup>), M (medium = 15 kg N
 ha<sup>-1</sup> yr<sup>-1</sup>), and H (high = 30
 kg N ha<sup>-1</sup> yr<sup>-1</sup>). Year refers
 to the sampling year (2014, 2015, and 2016). Status refers to whether or
 not plots were previously invaded by annual grasses ("Invaded")
 or not ("Uninvaded") prior to the wildfire. Species are
 abbreviated by six letter codes (first three letters of genus and
 species). Full species names and other information are included in the
 "species_list.csv" file. Also included are the total number of
 native species ("native richness") and the total number of
 native forb species ("native richness"). ***Column headers***
 **Block** = block identifier for all field plots **Nitrogen** = nitrogen
 fertilization treatment **Status** = invasion status prior to fire
 **Columns E through CO** = six letter species codes (full names and codes
 are included in the "species_list.csv" file); numbers in these
 columns indicate presence ("1") or absence ("0") of a
 given species in field plots **native richness** = total number of native
 species **native forb richness** = total number of native forb species