Obligate resprouting, obligate seeding, and facultative seeding shrub species in California’s Mediterranean-type climate region

Underwood, Emma1 ; Sorenson, Quinn1 ; Schrader-Patton, Charlie2

Published Jun 30, 2023 on Dryad. https://doi.org/10.25338/B8FS9V

Data files

Jun 30, 2023 version files 151.46 MB

Abstract

Mediterranean-climate region (MCR) shrublands have evolved a set of regeneration strategies in response to periodic, high-intensity wildfires: obligate seeding (OS), obligate resprouting (OR), and facultative seeding (FS) species. Spatial variation is seen in different regeneration strategies. In California, previous studies have found a higher abundance of OR species in mesic environments and OS species in xeric environments (Meentenmeyer et al. 2001). To date, however, data on their spatial distribution at a regional scale in California is limited and presents a significant information gap for resource managers of shrub-dominated landscapes. We developed a multinomial model using temporally dynamic and static variables to predict the distribution of the three shrub post-fire regeneration strategies, plus trees and herbs, in southern California. Cross-validation showed 50% of the predicted values for each of the five plant groups were within 8–24 percent of the actual value (Underwood et al. 2023). Spatial data for OS, OR, and FS provide an important contribution to resource management to help quantify carbon storage of shrublands and prioritize areas for post-fire restoration.

Methods

Our study area consists of shrublands within a 31,069 km² (7,677,317 acres) footprint that encompasses all 36 Level IV ecoregions (Omernik and Griffith, 2014) that overlap with the Angeles, Cleveland, Los Padres, and San Bernardino National Forests in southern California, USA (Figure 1). Vegetation types in the study region are dominated by shrubland (62%), grassland (16%), broadleaf woodland (8%), and conifer and mixed conifer-broadleaf forests (8%). The predominant shrublands communities (following Wildlife Habitat Relations classification, Barbour et al., 2007; https://wildlife.ca.gov/Data/CWHR/Wildlife-Habitats [CWHR]) are: mixed chaparral (29%; dominated by scrub oak [Quercus berberidifolia], various species of Ceanothus and manzanita [Arctostaphylos], and other mostly resprouting shrub species); sage scrub (12%; dominated by California sagebrush [Artemisia californica], purple sage [Salvia leucophylla], black sage [Salvia mellifera], and California buckwheat [Eriogonum fasiculatum]); and chamise/redshank chaparral (6%; dominated by chamise [Adenostoma fasciculatum] and/or redshank [Adenostoma sparsifolium]).

We analyzed 222 plots from the USDA Forest Service Forest Inventory and Analysis (FIA) program (Burkman, 2005) for which we estimated aboveground biomass using shrub species-specific allometric equations (McGinnis et al., 2010; Wakimoto, 1978) or a generalized shrub-herb biomass equation (Lutes et al., 2006, see Schrader-Patton and Underwood, 2021 for details). We assigned each species in the FIA plots to one of three lifeform categories: shrub, tree or herb (forbs and grasses). Shrub species were further categorized into one of three post-fire regeneration strategies: obligate seeder (OS), facultative seeder (FS), or obligate resprouter (OR) using descriptions of regeneration strategy and life history reported in primary literature and public databases (Gordon and White, 1994; Borchert et al., 2004; CNPS, 2021; FEIS, 2021). For each plot, we calculated the proportion of aboveground live biomass for each of these five plant groups: OS, FS, OR, tree, and herb, by dividing the estimated biomass of each by the summed total biomass across all groups.

To predict the distribution of OS, FS, OR, trees and herbs, we used a multinomial regression model (mnet package and function multinom in R software, R Core Team, 2016; Ripley and Venables, 2021) using: average annual solar radiation, actual evapotranspiration (AET), climate water deficit (CWD), average annual precipitation, the normalized difference vegetation index (NDVI, using the maximum composite value from July to August each year), modeled aboveground biomass (a proxy for productivity), eastness (a measure of continentality and dryness), slope, flow accumulation, soil bulk density, soil clay content, and soil percent carbon (see Table S1). Finally, we included vegetation type from the CWHR classification system from the FVEG vegetation data (FRAP, 2015) and time since last fire from the Fire Return Interval Departure geodatabase (USDA, 2015). We omitted any variables that were strongly correlated (r > 0.55).

To select the best model, we started with a full model and removed predictors sequentially using Akaike Information Criteria (AIC) to evaluate model fit. Predictor variables remained in the final model if they improved model fit by a minimum ΔAIC of –2 (Anderson and Burnham, 2004). For the final model, p-values were generated with a Wald-z test. Obligate seeder shrubs were selected as the baseline variable for the multinomial model (as we expected OS to differ most from FS and OR) against which the other four plant groups were calculated. To evaluate model performance, we performed leave-k-out cross-validation with k = 8 (Hastie et al., 2009) and examined the distribution of cross-validation errors to evaluate predictive accuracy: mean, standard deviation, kurtosis, skew, and interquartile range. We then used the raster surfaces corresponding to each predictor as model inputs into the ‘predict’ function in R software. We created a raster spatial layer (30 m resolution) with the proportion of biomass for each of the five plant groups by applying the model predictions of the proportion of each plant group to each pixel in our study area (see Underwood et al. 2023 for details).

Figure 1. The boundary of the study area in southern California within which we identify the distribution of obligate resprouter, obligate seeder, and facultative seeder species.

CAVEATS

In using these spatial data, users should note that the USFS FIA program that provided the input plots for the multinomial model is designed to measure forest conditions across the US, so FIA sampling is biased to upland interior, moister sites which contain trees. Consequently, OS species in coastal and desert scrub communities may not be well represented. In addition, studies (e.g., Keeley 2023) have shown some shrub species, such as Ceanothus leucodermis, vary temporarily and spatially in their post-fire regeneration strategy, changing from OR to OS with longer fire-fire intervals.

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