DOI: 10.5061/dryad.7pvmcvf2h
title: High fire frequency in California chaparral reduces post-fire shrub regeneration and native plant diversity

description: Fire is crucial for maintaining species diversity and resilience in
 fire-adapted shrublands of the world’s Mediterranean climate zones (MCZs),
 which include the chaparral shrublands of the North American MCZ.
 Chaparral is adapted to high intensity burning, with long intervals
 between fires (30-100 years) typifying undegraded conditions. In much of
 the range of chaparral, modern fire frequencies are much higher, driven
 largely by high densities of human ignitions and coincidence between
 ignitions and severe weather conditions. This change in the fire regime
 has major implications for biodiversity, leading to exotic invasion,
 decreased ecosystem services, and potential type conversion of shrubland
 to grassland dominated by exotic species. We studied the impact of
 increased fire frequencies on the composition and abundance of herbaceous
 and woody species in the Interior Coast Range of northern California. Our
 study area is one of the most frequently burned areas in California, which
 afforded us the opportunity to investigate higher fire frequencies than
 heretofore reported in the scientific literature for California. We
 surveyed fifty-four 250-m2 plots to assess changes in plant community
 composition and postfire regeneration of chaparral shrubs across a wide
 range of fire frequencies, including plots that have burned up to six
 times in the past 30 years. Our findings reveal that short-interval fires
 significantly reduced post-fire native woody regeneration, with obligate
 seeding species experiencing a 99% reduction and facultative species
 showing an 83% reduction in regeneration in the most frequently burned
 plots. Moreover, the overall marginal effect of one additional short
 interval fire decreased the proportion of native species cover by 12% and
 both richness and Shannon diversity by 4%. Consequently, areas with higher
 fire recurrence supported a more structurally and botanically homogeneous
 landscape, dominated by a similar group of non-native species.
description: # High fire frequency in California chaparral reduces post-fire shrub
 regeneration and native plant diversity We surveyed fifty-four 250-m2
 plots to assess changes in plant community composition and postfire
 regeneration of chaparral shrubs across a wide range of fire frequencies,
 including plots that have burned up to six times in the past 30 years ```
 1) 'LNU_plot_description.csv' ``` Dataset of all potential
 covariates used in the models. Fields are as follows: * Site: Unique site
 name. *QuailRidge* for Quail Ridge UC Natural Reserve (38°30’ N, 122°08’
 W), *ColdCanyon* for Cold Canyon UC Natural Reserve (38°30’ N, 122°06’ W),
 *Bobcat* for Bobcat Ranch Audubon Reserve (38°31’ N, 122°04’ W), and
 *CacheCreek* for Berryessa Snow Mountain National Monument (38°54’ N,
 122°18’ W) and Valley Vista Regional Park (38°54’ N, 122°16’ W) * PlotID:
 A unique plot ID for each distinct transect. * num_burn: The number of
 prior fires, according to the California Fire Return Interval Departure
 (FRID) database, that occured after 1990. Since these fire perimeters
 generally ignore unburned patches within fires that are less than hundreds
 of acres in size, we used Google Earth historical imagery to examine the
 landscape for unburned patches after each fire. * preFireName: Name of
 fire prior to the LNU lightning complex * preFireYear: Year of fire prior
 to the LNU lightning complex * crrnFRI: The current fire return interval
 calculated by dividing the number of years in the fire record by the
 number of fires occuring between 1908 and the current year in a polygon. *
 mdnRFRI: (median Percent FRID) A measure of the extent to which
 contemporary fires (since 1908) are burning at frequencies similar to the
 frequencies that occurred prior to Euro-american settlement, with the
 median reference FRI as a basis for comparison. * mnPFRID: (mean Percent
 FRID) A measure of the extent to which contemporary fires (since 1908) are
 burning frequencies similar to the frequencies that occurred prior to
 Euro-american settlement, with the mean reference FRI as a basis for
 comparison. * mCC_FRI: A condition class categorization of the data in the
 mnPFRID field. * PFR: "Presettlement Fire Regime." This
 represents the dominant vegetation type linked to its probable historical
 fire regime (Van de Water and Safford 2011) * ppt: Point estimate of
 annual average precipitation (mm) using 4 km resolution from the PRISM
 dataset (PRISM Climate Group 2022). * tmean: Point estimate of annual
 average temperature (degrees F) using 4 km resolution from the PRISM
 dataset (PRISM Climate Group 2022). * elevation: Elevation of plot in
 meters * slope: Percent slope of plot in degrees * aspect: Aspect of plot
 in degrees * hli: Heat load index, which uses aspect and slope to account
 for the amount of solar radiation received (McCune and Keon 2002) *
 Distance_km: Distance from coast, measured in km * TSLF: 'Time Since
 Last Fire.' This designates the time between the LNU lightning
 complex and the most recent fire. * cool_warm_slope: Broad categorization
 of aspect, with cool slopes representing North and East aspects and warm
 slopes representing South and West aspects. ``` 2) LNU_severity.csv ``` *
 plotID: A unique plot ID for each distinct transect. * mean_diam_mm: The
 stem diameter (mm) one centimeter from the terminus of four stems from a
 chamise individual. One individual was chosen in or adjacent to each
 quadrat, and five additional individuals were measured at the entire 250m2
 transect scale. ``` 3) Subplot_species.csv ``` * PlotID: A unique plot ID
 for each distinct transect. * year: Year of survey. * spp: Species code,
 associated with SpeciesList.csv * seedling_resprout: Either *resprout*
 which designates a shrub that resprouted from burl, *seedling* which
 designates a shrub that germinated from a seed, or *na* for herbaceous
 species * cover_count: *Cover* designates a percent cover quantified in
 each quadrat. *Count* designates a count of each seedling or resprout in
 each quadrat. * Q1: Quadrat 1 of 5 * Q2: Quadrat 2 of 5 * Q3: Quadrat 3 of
 5 * Q4: Quadrat 4 of 5 * Q5: Quadrat 5 of 5 ``` 4) shrub_seedling_data.csv
 ``` * PlotID: A unique plot ID for each distinct transect. * spp: Species
 code, associated with SpeciesList.csv * year: Year of survey. *
 sum.count.5m2: Summation of seedling cover in each quadrat, across the
 entire transect * presence: Presence/absence of a seedling in each quadrat
 ``` 5) shrub_height.csv ``` * PlotID: A unique plot ID for each distinct
 transect. * spp: Species code, associated with SpeciesList.csv * status:
 *D* for dead individual or *L* for live individual * prefire_ht_m: Height
 (meters) before LNU lightning complex, measured by skeleton. Values
 containing *null* designate cases where the prefire ht was not recorded
 (survey year 2022). * postfire_ht_m: Postfire height (meters) of live
 vegetation after the LNU lightning complex. Values containing *na*
 designate instances without live regrowth (status = D). *
 diam_largest_stem: Diameter of largest stem (cm). Values containing *null*
 designate cases where the diameter was not recorded (survey year 2022). *
 year: Year of survey ``` 6) SpeciesList.csv ``` * spp: Species code,
 associated with all other files * Lifeform: Lifeform according to the
 University of California Jepson Herbarium eFlora
 ([https://ucjeps.berkeley.edu/eflora/](https://ucjeps.berkeley.edu/eflora/)) - TR (tree), SH (shrub), FB (forb), GR (graminoid), FERN (fern). * scientificNameAuthorship: Abbreviated name of person who named plant * Species: Species.  Values containing *na* designate instances when plant was only identified to genus * Genus: Genus * Native_nonnative: Origin of plant (native or nonnative), according to the University of California Jepson Herbarium eFlora ([https://ucjeps.berkeley.edu/eflora/](https://ucjeps.berkeley.edu/eflora/)).  Values containing *na* designate instances in which origin is unknown * fac.obl: Fire regeneration strategy according to USDA Forest Service Fire Effects Information System ([https://www.feis-crs.org/feis/](https://www.feis-crs.org/feis/)) - obligate seeder (OS), facultative seeder (FS), or obligate resprouter (OR).  Values containing *na* designate instances in which fire regeneration strategy is unknown. * subspecies: Subspecies of plant, if applicable. Values containing *na* designate instances in which subspecies is not applicable