Data from: Nitrogen deposition suppresses ephemeral post-fire plant diversity
Data files
Dec 28, 2023 version files 179.52 KB
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cover_data.csv
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README.md
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richness_data.csv
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species_list.csv
Abstract
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.
README: Data from: Nitrogen deposition suppresses ephemeral post-fire plant diversity
https://doi.org/10.5061/dryad.zs7h44jg9
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-1 yr-1), M (medium = 15 kg N ha-1 yr-1), and H (high = 30 kg N ha-1 yr-1). 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-1 yr-1), M (medium = 15 kg N ha-1 yr-1), and H (high = 30 kg N ha-1 yr-1). 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
Methods
Materials and methods
Study site
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-1 yr-1. 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).
Nitrogen fertilization
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-1 yr-1) 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 (CaN2O6) and urea (CO(NH2)2), 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.
Vegetation monitoring
Each spring at the peak of the growing season (late April to early May), we sampled vegetation within all plots. We placed four 1m2 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.