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Frontiers in Forests and Global Change ·

Volume 9 - 2026 | https://doi.org/10.3389/ffgc.2026.1950264

Understory vegetation development six years after a wildfire in the temperate forests of Patagonia, Argentina

  • M. M. Rago1,2*corresponding author
  • M. F. Urretavizcaya1,2,3
  1. 1Terrestrial Ecosystem Ecology Area, Centro de Investigación y Extensión Forestal Andino Patagónico (CIEFAP), Esquel, Argentina
  2. 2Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Buenos Aires, Argentina
  3. 3Facultad de Ingeniería, Carrera Ingeniería Forestal, Universidad Nacional de la Patagonia San Juan Bosco, Esquel, Argentina
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Abstract

Fire is a fundamental driver in many forest ecosystems, yet increasing fire severity, frequency and extent threaten recovery processes. In the Andean-Patagonian forests, early post-fire succession remains poorly understood despite its importance in shaping long-term ecosystem structure and function. We evaluated the effects of three fire severity levels (unburned, moderate, and high) and five forest types (Nothofagus pumilio, N. antarctica, N. dombeyi, mixed N. dombeyi-Austrocedrus chilensis, and A. chilensis) on the plant community developed 6 years after a large wildfire in the Cholila Lake basin (Argentina). We assessed understory species composition and traits, vertical structure, and regeneration of dominant species. Both fire severity and forest type significantly influenced understory composition. High-severity sites showed pronounced compositional differences, including higher richness and cover of exotic species, greater dominance of annual herbs, and a more developed upper stratum (1.5–3 m height) dominated by shrubs. In contrast, moderate-severity sites showed assemblages more similar to unburned forests, higher total richness, greater cover of nitrogen-fixing species, and a more developed middle stratum (0.5–1.5 m height). Differences among forest types were also detected, with the highest total and native species richness in A. chilensis forests and the highest exotic species cover in N. antarctica and N. dombeyi forests. Regeneration of foundational tree species and the bamboo Chusquea culeou was present in moderate-severity sites but strongly limited in high-severity sites. Our results highlight that fire severity acts as a filter on early post-fire dynamics, while forest type further shapes community responses, consistent with potentially divergent successional pathways that require long-term monitoring. As fire regimes intensify and early post-fire landscapes expand, understanding the structure and dynamics of understory communities will assist restoration efforts aimed at achieving desirable states while maintaining the ecological processes supported by early successional communities.

Keywords

1 Introduction

Fire has been part of many ecosystems since the origin of terrestrial plants, and many species have evolved mechanisms that allow them to resist or recover from fire in response to the historical fire regime within their range (Keeley et al., 2011; Pausas and Keeley, 2009). However, the ongoing intensification of fire regimes in forest ecosystems worldwide that are characterized by more severe, frequent, and larger fires has important implications for plant communities (Grau-Andrés et al., 2024; Nolan et al., 2021; Pausas and Keeley, 2021; Rogers et al., 2020). One consequence is the increase in the extent of early successional stages (McDowell et al., 2020). These stages perform specific ecological functions and can serve as a template for future forest development (Swanson et al., 2011). In addition, fire intensification may hinder tree establishment, particularly in forests dominated by fire-sensitive species, where recovery depends on seed dispersal from nearby unburned patches (Enright et al., 2015; Landesmann and Morales, 2018; Nolan et al., 2021; Urretavizcaya et al., 2022). Therefore, a comprehensive understanding of early successional stages is necessary to guide restoration and management actions in assisting the transition toward mature forests or other desirable alternative stages (Gann et al., 2026; Swanson et al., 2011).

Early post-fire succession is characterized by rapid changes driven by both surviving organisms and the establishment of species arriving from outside the affected area (Falk et al., 2022; Swanson et al., 2011). Changes in the microenvironment, such as increased sunlight, temperature, and soil moisture (Urretavizcaya et al., 2006; Wolf et al., 2021), may favor shifts in plant communities by creating suitable conditions for early successional species that may not persist in later stages (Swanson et al., 2011). This stage may also play an important role in nutrient replenishment, as species with different functional attributes can establish, including nitrogen-fixing or fast-growing species (Fernández-García et al., 2020). Ecological legacies further support ecological resilience (Johnstone et al., 2016). For example, standing dead trees can serve as perches for frugivorous birds that contribute to post-fire seed dispersal and vegetation recovery (Cavallero et al., 2013). These legacies can also ameliorate microenvironmental conditions and facilitate forest recovery when restoration actions are implemented (Urretavizcaya and Defossé, 2019).

The Andean-Patagonian forests are located along the Andes mountain range between latitudes 37° and 54° S. These temperate forests comprise different forest types generally dominated by one foundational tree species, which are replaced along elevation gradients and across a west-east precipitation gradient (Donoso, 1993). Evidence of fire in these forests dates back approximately 13,000 years (Whitlock et al., 2006), with periods of higher and lower fire activity associated with both lightning and human activity. As a result, most present-day forests have developed after fire disturbances (Veblen et al., 2008). Post-fire recovery of many foundational tree species depends on seed dispersal from nearby unburned individuals, since their wind-dispersed seeds typically travel less than a few hundred meters (Landesmann and Morales, 2018; Urretavizcaya et al., 2022). Furthermore, most of these species lack the capacity to resprout and have transient seed banks (Cuevas and Arroyo, 1999; Urretavizcaya and Defossé, 2004; Veblen et al., 2003). Consequently, recovery of forest structure is more likely after low-severity fires (Cavallero et al., 2015). Following high-severity fires that affect large areas, events that have become more frequent in recent decades and are projected to increase in the future (Kitzberger et al., 2022; Kitzberger and Bürgesser, 2025; Tiribelli et al., 2024), forest recovery may fail. In such cases, forests may shift toward more flammable non-forest vegetation communities such as shrublands or grasslands (Landesmann et al., 2020; Rusch et al., 2016, 2017; Tiribelli et al., 2018). Additionally, land-use history, including livestock grazing and the introduction of exotic species, as well as broader environmental changes, can influence post-fire successional trajectories (Blackhall et al., 2008, 2017; De Paz and Raffaele, 2013; Rago et al., 2024).

In contrast to the overstory, which is composed of relatively few, mostly fire-sensitive tree species, the understory of these forests is highly diverse, including species with varied growth forms, traits, and recovery strategies following fires (Joelson et al., 2025). In humid western sites, a dominant understory species is the bamboo Chusquea culeou E. Desv., which becomes less abundant toward the drier eastern sites, where it is gradually replaced by xeric shrubs and small trees. Chusquea culeou has the capacity to resprout after fire (Fuentes-Ramirez et al., 2020). However, this semelparous bamboo reproduces by seeds only during synchronous regional flowering events that occur approximately every 60 years (Guerreiro, 2014) and may or may not coincide with fire events.

In this study, we aimed to characterize the initial plant assemblages following a wildfire that burned at different severities across several forest types in the Andean-Patagonian region. Our goal was to contribute to the limited knowledge of early post-fire succession in these forests under increasingly intense fire regimes. Specifically, we evaluated whether 6 years after the fire: (1) understory plant assemblages differed among fire severity levels and forest types, and which species contributed most to these differences; (2) traits of understory plants varied among fire severity levels and forest types; (3) the vertical structure of the understory and overstory differed among fire severity levels and forest types; and (4) regeneration of C. culeou and the foundational tree species was occurring. We expected to find an association among fire severity and understory vegetation assemblages, with greater differences between high and unburned severity sites, regardless of forest type. We also expected regeneration of C. culeou and the foundational tree species to be more frequent in sites with moderate- fire severity.

2 Materials and methods

2.1 Study area

During the middle of summer and the beginning of autumn of 2015 (February–April), a large wildfire, named “Las Horquetas,” burned across the Cholila Lake basin in northwestern Patagonia, Argentina (Figure 1). The fire affected native forests and shrublands in a mountainous landscape ranging from 300 to 1,600 masl, located in a west-east oriented valley with a pronounced precipitation gradient, from 2,400 mm in the west to 800 mm in the east (Bianchi and Cravero, 2010). The region has a temperate Mediterranean-type climate, characterized by cold, wet winters and warm, dry summers (Köppen and Geiger, 1936). Soils are derived from volcanic ash and are classified as Andisols in the wetter areas, gradually transitioning to Mollisols in the drier portions of the basin (La Manna et al., 2020; Pereyra and Bouza, 2019).

Study area in the Cholila Lake basin, Argentina (point black), and location of the sampled plots at each forest type and fire severity, with the classification based on normalized burn ratio (NBR) thresholds.
Figure 1. Study area in the Cholila Lake basin, Argentina (point black), and location of the sampled plots at each forest type and fire severity, with the classification based on normalized burn ratio (NBR) thresholds.

The fire burned a total of 28,960 ha, making it the largest wildfire recorded in the region up to that time. Prior to the fire, the main land use in the area was livestock grazing, with an average stocking rate of 0.155 livestock units per hectare, primarily cattle (Subsecretaría de Bosques (SSB) et al., 2015). The fire coincided with the post-flowering death of the bamboo Chusquea culeou, which had flowered the previous year. The large amount of dry biomass produced by this species acts as highly flammable fuel (Kitzberger et al., 2016). Historical records indicate that part of the area had burned previously, including fires more than a century ago and during the widespread fire year of 1944, which was also associated with C. culeou post-flowering (Rothkugel, 1916; Tortorelli, 1947; Veblen et al., 2003). The main types of vegetation burned were pure forests dominated by Nothofagus pumilio (Poepp. & Endl.) Krasser; Nothofagus antarctica (G. Forst.) Oerst.; Nothofagus dombeyi (Mirb.) Oerst.; Austrocedrus chilensis (D.Don) Pic-Serm. & Bizzarri; mixed forest of N. dombeyi—A. chilensis, and shrublands dominated by species such as Diostea juncea (Gillies & Hook. ex Hook.) Miers., Schinus patagonica (Phil.) I.M. Johnst. ex Cabrera, Ochetophyla trinervis (Gillies ex Hook.) Poepp. Ex Endl., and Aristotelia chilensis Molina (Stuntz), among others, that reach a height of 1 to 3 m.

2.2 Plot establishment

This study is part of a long-term research on post-fire recovery and the establishment of the permanent plots was carried out during the first growing season after the fire (November 2015–February 2016) (Urretavizcaya et al., 2025).

Sampling sites were selected in two stages. In the first, potential sites were selected at random based on the fire severity classification map elaborated for the Las Horquetas fire (Subsecretaría de Bosques (SSB) et al., 2015) and the forest-type cover map of the area (Centro de Investigación y Extensión Forestal Andino Patagónico [CIEFAP] and Ministerio de Ambiente y Desarrollo Sostenible [MAyDS], 2016). The fire severity map was based on a severity index with original values obtained by satellite data and later calibrated with field surveys in the affected area. To estimate the index, the authors used Landsat 8 OLI (USGS) satellite data with a capture date of April 11, 2015, which were previously atmospherically corrected using the ATCOR3 algorithm, and the post-fire Normalized Burn Ratio (NBR). TheNBR is a Normalized ratio between near infra-red (NIR) band (~785–899 nm, Landsat-8 band # 5) and short wave infra-red 2 (SWIR2) band (~2,100–2,280 nm, Landsat-8 band # 7). These estimates were calibrated with field surveys conducted at the plot level, where fire effects on vegetation were recorded across different strata (Subsecretaría de Bosques (SSB) et al., 2015). The authors obtained a map of fire severity classified into three classes, using NBR thresholds: high severity (NBR −1.00 to −0.35), low and moderate severity (NBR −0.35 to 0.35) (from this point on “moderate severity”), and unburned (NBR + 0.35 to + 1.00) (Subsecretaría de Bosques (SSB) et al., 2015). The forest types selected for the study were: (1) Nothofagus pumilio; (2) Nothofagus antarctica; (3) Nothofagus dombeyi; (4) mixed N. Dombeyi - Austrocedrus chilensis, and (5) A. chilensis. Altogether, they accounted for 72% of the burned area (Subsecretaría de Bosques (SSB) et al., 2015).

In the second stage, given that the affected area is located in a mountainous region with difficult access, 45 of the potential sites were selected based on the presence and condition of 4 x 4 vehicle tracks, the feasibility of river crossings, travel times on foot to the sampling sites, and landowner permissions. The resulting 45 plots were distributed ensuring 3 replications per combination of fire severity (3 levels) and forest type (5 levels) while capturing the environmental variability within the burned area. Distances among plots varied from >70 m to <30 km. In addition, the presence of C. culeou, either alive or identified through burned culms, was recorded in half of the plots, ensuring at least one occurrence within each combination of fire severity level and forest type.

At each sampling site, a permanent plot with a radius of 15 m (~700 m2) was established to assess fire damage and characterize forest structure, ensuring the homogeneity of the forest type (Urretavizcaya et al., 2025). Fire damage was assessed following a protocol based on the modified US Forest Service fire severity classification (Lutes et al., 2006). Across five strata: litter and duff, herbs, shrub and tree regeneration (0–1 m height), shrub and low trees (1–7 m), and dominant trees (more than 7 m), the authors visually estimated the percentage of vegetation or fuel consumed and the percentage of charred material (Urretavizcaya et al., 2025). The resulting estimates were then used to assign each plot to one of three fire-severity categories: (1) unburned, (2) low and moderate-severity (from this point of “moderate severity”), and (3) high-severity, based on a scale adapted from Mutch and Swetnam (1993) and Ryan (2002). Unburned plots showed green and unaltered plants, without any direct effect from heat; moderate-severity plots include trees with part of the canopy dead, but leaves not entirely consumed, all understory plants charred or consumed, fine twigs on soil surface consumed and logs charred, and pre-fire soil organic layer largely consumed; high-severity plots showed dead tree canopy and consumed leaves, surface litter of all sizes and soil organic layer largely consumed, white ash deposition, and organic matter charred to several centimeter depth.

2.3 Sampling procedure

Six years after the fire (January–March 2021), we revisited the sampling sites to sample the plant community. We used a methodology with characteristics of rapid ecological assessment (Sayre et al., 1999). At a distance of 4 m from the plot center, we established four 1 m2 quadrats, one in each of the four cardinal directions, and a circular subplot with a radius of 4 m (~ 50 m2) in one cardinal direction (Supplementary Figure S1). Plant community was classified by height into the understory (<3 m height) and the overstory (>3 m height). Within the understory vegetation, we recorded all vascular plant species and visually estimated the percentage of aerial cover of each species, further classified into three vertical strata: (1) lower stratum (0.0–0.5 m), sampled within the four quadrats; (2) middle stratum (0.5–1.0 m), and (3) upper stratum (1.0–3.0 m), both sampled within the circular subplot. Also, for the lower stratum, species detected in the circular subplot but not recorded in any quadrat were recorded as present.

At each sampling site, species richness was defined as the total species composition recorded at the four quadrats plus the circular subplot, and a single cover value was assigned to each recorded species. For the lower stratum, cover was calculated as the mean across the four 1 m2 quadrats, whereas species detected only in the circular subplot were assigned a minimal cover value of 0.1% to capture the complete understory composition. For the middle and upper strata, the recorded cover value was used directly. Plant species were identified following Anton and Zuloaga (2026).

The overstory cover, that included the foundational trees either alive or dead, was visually estimated in the subplot. At the plot, we recorded the presence of C. culeou regeneration, identifying whether individuals originated from resprouts (we could observe the burned culms from which they had resprouted) or from seed (Seedlings appeared in clusters and were smaller than the individuals originated from resprouts) (Figure 2). We also recorded the presence of seedlings and saplings of the affected foundational tree species that regenerate exclusively from seeds (i.e., A. chilensis, N. dombeyi, and N. pumilio). For tree species occurring in the 1 m2 quadrats, we counted individuals and classified them into four height classes: (1) <10 cm, (2) 10 to 50 cm, (3) 50 to 130 cm, and (4) >130 cm with DBH <5 cm (Supplementary Figure S1).

*Chusquea culeou* postfire regeneration from seeds **(A)** and resprouts **(B)** in moderate-severity; from resprouts **(C)** in high-severity; and evidence of *C. culeou* but with no regeneration in high-severity **(D)**.
Figure 2. Chusquea culeou postfire regeneration from seeds (A) and resprouts (B) in moderate-severity; from resprouts (C) in high-severity; and evidence of C. culeou but with no regeneration in high-severity (D).

2.4 Data analyses

2.4.1 Understory composition by fire severity and forest type

To compare understory plant assemblages among fire severity levels and forest types, we performed a permutational multivariate analysis of variance (PERMANOVA) using the function adonis2 from the vegan package (Oksanen et al., 2020). The analysis was based on Bray-Curtis distances calculated from species cover at each sampling site and 999 unrestricted permutations. To assess the spatial structure of assemblages among plots, we estimated distance-based Moran’s eigenvector maps (dbMEM) through the function dbmem from the adespatial package (Dray et al., 2006, 2026). Spatial variables significantly associated with understory assemblages were identified using forward selection (function forward.sel) and subsequently incorporated as spatial covariates in PERMANOVA. To assess whether the inclusion of present species and spatial structure modified the results, we performed sensitivity analyses. To test multivariate homogeneity of dispersion, we used the function betadisper from the vegan package. When differences were significant (p < 0.05), pairwise comparisons were conducted using the pairwise.adonis function from the pairwiseAdonis package (Martinez Arbizu, 2017). To visualize patterns in understory plant assemblages, we performed a non-metric multidimensional scaling (NMDS) ordination based on species cover with Wisconsin standardization and square-root transformation using Bray-Curtis distances with the metaMDS function from the vegan package.

To identify indicator species associated with each fire severity level and forest type, we conducted an indicator species analysis using species cover with the multipatt function from the indicspecies package (De Cáceres and Legendre, 2009). This method evaluated both the specificity and fidelity of each species to particular habitats (Dufrêne and Legendre, 1997). Indicator values reach a maximum of 1 when a species occurs exclusively in a single habitat (specificity) and is present in all sites of that habitat (fidelity). All analyses were conducted in the R software (R Core Team, 2026).

2.4.2 Understory traits by fire severity and forest type

To assess which plant traits may drive differences in understory assemblages across fire severity levels and forest types, we grouped the species and their cover at each sampling site according to variables related to composition, structure, and functional attributes in (1) total species richness, richness by origin in (2) native and (3) exotic species, (4) total cover, cover by origin in (5) native, and (6) exotic species, cover by growth form in (7) trees, (8) shrubs, (9) perennial herbs, and (10) annual herbs, (11) cover of nitrogen-fixing species, and (12) cover of species with resprouting capacity, when there was information available. Information on species origin and growth form was obtained from the “Flora Argentina” database (Anton and Zuloaga, 2026). Classification of species as nitrogen fixers and resprouters was based on information available in the TRY database (Kattge et al., 2020), published literature, and field observations (Supplementary Table S1).

We carried out factorial models with a factorial design (3 x 5), including fire severity (unburned, moderate-severity, and high-severity) and forest type (N. pumilio (Np), N. antarctica (Na), N. dombeyi (Nd), mixed N. dombeyi—A. chilensis (Nd-Ac), and A. chilensis (Ac)) as fixed factors. The design included three replicate plots for each combination of fire severity and forest type. The probability distribution and link function were selected according to the nature of each response variable and model diagnostics. Continuous response variables were analysed using Gaussian linear models (lm function, stats package) or Gamma generalized linear models (glm function, stats package), whereas count variables were analysed using Poisson (glm function, stats package) or negative binomial (glm.nb function, MASS package) generalized linear models (Venables and Ripley, 2002). For linear models, normality and homogeneity of residual variance were assessed using graphical diagnostics and Shapiro–Wilk (shapiro.test function, stats package) and Levene’s tests (leveneTest function, car package), respectively. When necessary, response variables were log or square-root transformed to meet the model assumptions. For generalized linear models, residual uniformity, dispersion and zero inflation were assessed using simulated residuals (functions testUniformity, testDispersion, and testZeroInflation, DHARMa package) (Hartig, 2026). Spatial autocorrelation was assessed using Moran’s I on model residuals (function moran.test, spdep package). When significant spatial autocorrelation was detected, dbMEM associated with the response variables were identified, and the selected spatial variables were subsequently included in the corresponding models. For all models, the effect of fire severity, forest type, and their interaction were initially evaluated. Significance of model terms (p < 0.05) was assessed using Type II tests (Anova function, car package). When the interaction was not significant, an additive model containing the two main effects was fitted. Post-hoc comparisons were performed using estimated marginal means (emmeans function, emmeans package), with Tukey adjustment for multiple comparisons (Lenth and Piaskowski, 2026). When the interaction was significant, simple effects were compared within levels of the interacting factor, and when the interaction was not significant, pairwise comparisons were conducted among levels of the main effects. Pairwise contrasts are reported on the response scale as differences or ratios, as appropriate, with 95% confidence intervals (IC). All analyses were conducted in the R software (R Core Team, 2026)

2.4.3 Understory and overstory vertical structure

To compare the vertical structure of plant assemblages, we performed analyses of variance using the same factorial design and procedure described above. Response variables included the cover of (1) lower stratum, (2) middle stratum, (3) upper stratum within the understory, and (4) overstory cover.

2.4.4 Bamboo and tree regeneration

For plots that showed evidence of C. culeou, we calculated the percentage of plots with regeneration originating from resprout or from seed. We also calculated the percentage of plots with seedling regeneration of the foundational tree species (N. pumilio, N. dombeyi and A. chilensis) within the corresponding forest types and across fire severity levels.

3 Results

3.1 Understory composition by fire severity and forest type

We recorded a total of 124 taxa belonging to 48 families. Of these, 116 taxa were identified at the species level, five at the genus level, and three at the family level. Most species were native, with a high proportion of South American endemics. The predominant growth form was native perennial herbs, followed by native shrubs (Supplementary Table S1).

In the PERMANOVA, understory plant assemblages varied significantly among fire severity (p = 0.001) and forest type (p = 0.001) levels, with no significant interaction between fire severity and forest type (p = 0.165). Both the significance and the amount of variation explained by each effect remained after including present species and controlling for the spatial structure. Further, no evidence of multivariate dispersion was detected among fire severity (p = 0.659) or forest type levels (p = 0.446). Fire severity accounted for 7% of the variation in understory assemblages, with significant differences between unburned and high-severity sites (p = 0.006). Forest type explained 14% of the variation, with significant differences between Np and Na (p = 0.01), Np and Nd-Ac (p = 0.01), Np and Ac (p = 0.01), and Na and Nd-Ac (p = 0.04) (Supplementary Figure S2).

Indicator species analysis identified species associated with all levels of fire severity and forest type (Table 1). In unburned sites, only native species were detected as indicators, with high specificity and variable fidelity. In moderate-severity sites, indicator species were also exclusively native but showed moderate specificity and high fidelity. In high-severity sites, both native and exotic species were detected, with moderate to high specificity and fidelity. For forest types, many species showed maximum specificity. Nothofagus pumilio forests had three exclusive native indicator species. Exotic indicator species were detected only in N. antarctica and N. dombeyi forests, both with moderate fidelity.

Table 1. Understory species associated with each level of fire severity and forest type, obtained with indicator species analysis.

Indicator speciesIVSFp-value
Fire severity
Unburned
Osmorhiza berteroi0.760.780.730.007
*Maytenus spp.0.660.720.600.041
Rumohra adiantiformis0.581.000.330.008
Bromus coloratus0.540.870.330.043
Moderate-severity
Vicia nigricans0.700.610.800.037
Viola magellanica0.690.510.860.048
High-severity
Crepis capilaris<sup>e</sup>0.720.650.800.005
Epilobium brachycarpum<sup>e</sup>0.680.980.470.003
Oxalis valdiviensis0.670.740.530.013
Phacelia secunda0.590.580.600.034
Diostea juncea0.590.810.400.039
Fabiana imbricata0.570.880.330.044
Forest type
Nothofagus pumilio
Leucheria glacialis0.751.000.560.002
Berberis serratodentata0.671.000.440.005
Ribes magellanicus0.650.540.780.03
Nothofagus pumilio0.581.000.330.046
Nothofagus antarctica
Trifolium repens<sup>e</sup>0.800.970.670.001
Nothofagus antarctica0.730.800.670.002
Ovidia andina0.671.000.440.003
Plantago lanceolata<sup>e</sup>0.630.900.440.023
Potentilla chiloensis0.570.960.330.038
Nothofagus dombeyi
Cerastium fontanum<sup>e</sup>0.581.000.330.037
N. dombeyi-A.Chilensis
Nothofagus dombeyi0.640.730.560.014
Austrocedrus chilensis
Lomatia hirsuta0.680.690.670.003
Quinchamalium chilense0.630.900.440.012
Fabiana imbricata0.600.650.560.022
Acaena splendens0.581.000.330.031
Orchidaceae0.581.000.330.034

*Maytenus spp. includes M. disticha and M. chubutensis.

IV for indicator value, S for specificity, F for fidelity, *e*indicates exotic species.

3.2 Understory traits by fire severity and forest type

Understory traits showed different responses to fire severity and forest type (Tables 2, 3). Regarding origin, total richness as well as the richness of native species differed significantly with fire severity (p < 0.004 and p = 0.005, respectively) and forest type (p < 0.001, and p < 0.001, respectively). Total richness was predominantly native species, with the highest values in moderate-severity fire and A. chilensis forests. Exotic richness showed a significant interaction (p = 0.005) with the highest values in burned N. dombeyi and N. dombeyi-A. chilensis forests. Total cover was also predominantly of native species, for which there were no significant differences among either fire severity or forest type, whereas exotic cover differed significantly with fire severity (p < 0.001) and forest type (p = 0.001) with the highest values in burned sites and in N. antarctica and N. dombeyi forests. Among growth forms, only tree cover differed significantly among forest types (p = 0.014), with the highest values in A. chilensis forests, whereas only annual herbs differed significantly among fire severity levels (p = 0.009), showing the highest values in high-severity sites. Perennial herbs showed significant interaction (p = 0.006) with the highest values in moderate-severity N. dombeyi and A. chilensis forests and in unburned N. antarctica forests, whereas shrubs did not show any significant effect. The cover of N-fixing and resprouting species differed significantly with fire severity (p = 0.004 and p = 0.031, respectively), with the highest values in moderate-severity sites (Tables 2, 3).

Table 2. Mean and standard error (in parenthesis) for understory traits by each fire severity (unburned, moderate, and high) and forest type (Np: N. pumilio, Na: N. antarctica, Nd: N. dombeyi, Nd-Ac: N. dombeyi-A. Chilensis, Ac: A. chilensis) level or by each fire severity within forest types, when significant differences were detected for the main effects or the interaction, respectively (p ≤ 0.05).

VariableFire severityForest type
UnburnedModerateHighNpNaNdNd-AcAc
Richness (n.53 m−2)
Total17.4 (2.0)b23.6 (1.7)a19.9 (1.4)ab15.0 (1.6)c19.8 (2.5)abc18.1 (1.2)bc21.9 (2.5)ab26.7 (1.9)a
Natives13.9 (1.4)b17.9 (1.3)a13.8 (1.3)b12.9 (1.5)c13.3 (2.0)abc13.1 (1.2)bc16.7 (1.8)ab19.9 (1.4)a
Exotic
Unburned0.0 (0.0)4.3 (2.2)1.3 (1.3)b1.0 (1.0)b4.7 (2.3)
Moderate0.3 (0.3)5.0 (1.0)5.7 (0.9)a5.3 (0.7)a5.7 (0.3)
High3.0 (1.2)6.3 (1.9)6.0 (1.0)a4.0 (1.2)ab5.3 (1.2)
Cover (%)
Total36.0 (6.9)46.2 (4.5)44.2 (4.1)36.1 (3.1)49.3 (10.5)45.6 (7.4)35.6 (4.8)44.3 (6.5)
Natives27.6 (2.4)36.2 (3.5)34.9 (4.7)33.5 (2.7)28.2 (4.6)34.1 (5.1)30.6 (4.8)38.1 (6.5)
Exotic2.7 (1.2)b8.9 (3.4)a8.6 (1.7)a1.0 (0.7)b12.0 (3.8)a11.0 (4.5)a3.6 (1.3)ab5.5 (1.4)ab
Trees1.9 (0.6)2.4 (0.7)2.3 (1.2)0.6 (0.4)b4.0 (2.0)ab1.0 (0.7)b1.6 (0.5)ab3.8 (0.5)a
Shrubs16.6 (2.2)18.1 (2.9)24.6 (4.4)21.4 (3.5)16.6 (4.4)18.4 (5.3)17.2 (3.9)25.1 (4.7)
Perennial herbs
Unburned16.2 (5.2)37.9 (24.3)a7.9 (1.9)b12.4 (5.1)4.8 (0.8)b
Moderate12.6 (4.5)19.3 (4.5)ab29.2 (9.0)a14.0 (6.1)18.8 (6.9)a
High6.2 (0.3)7.8 (4.0)b12.7 (2.2)ab14.7 (4.1)10.7 (3.3)ab
Annual herbs1.0 (0.5)b6.0 (2.8)ab6.0 (1.6)a0.8 (0.6)6.3 (2.4)9.1 (4.5)1.8 (0.7)3.6 (1.4)
N-fixing3.8 (1.5)b9.2 (2.7)a2.5 (0.5)b3.8 (2.0)5.3 (2.3)8.8 (4.3)4.2 (1.8)3.7 (1.3)
Resprouting19.4 (2.2)b31.2 (3.9)a24.6 (3.2)ab25.1 (3.5)24.8 (3.7)32.4 (5.9)19.2 (3.4)25.9 (3.8)

Different letters indicate significant differences (p ≤ 0.05). No letters indicate no significant differences (p > 0.05).

Table 3. Significant (p ≤ 0.05) pairwise comparison of estimated marginal means from the adjusted models for each variable of richness and cover, for fire severity (Unb: unburned, Mod: moderate, and high), and forest type (Np: N. pumilio, Na: N. antarctica, Nd: N. dombeyi, Nd-Ac: N. dombeyi-A. Chilensis, Ac: A. chilensis) main effects, or their interaction.

VariableFire severityForest typeEstimatesLowerUpperp-value
RichnessRatio
TotalMod vs. unb1.361.101.690.003
Ac vs. Nd1.481.072.030.008
Ac vs. Np1.781.282.49<0.001
Nd-Ac vs. Np1.461.032.050.024
NativesMod vs. unb1.291.101.600.017
Mod vs high1.301.101.600.015
Ac vs. Nd1.481.072.030.008
Ac vs. Np1.781.282.49<0.001
Nd-Ac vs. Np1.461.032.050.024
ExoticsNd: high vs. Unb4.001.2316.000.018
Nd: mod vs. unb4.001.1616.000.025
Nd-Ac: mod vs. Unb5.001.2223.000.021
CoverDifference
ExoticsHigh vs. unb5.171.468.870.002
Mod vs. unb4.971.308.650.003
Nd vs. Np7.571.4813.660.001
Na vs. Np6.420.7012.150.004
TreesAc vs. Nd3.22−0.296.740.040
Ac vs. Np3.37−0.086.810.020
Perennial herbsAc: mod vs. unb4.900.649.160.024
Nd: mod vs. unb7.381.5813.180.034
Na: high vs. Unb7.011.5212.500.009
Annual herbsHigh vs. unb3.990.427.560.009
N-fixingHigh vs. mod5.280.999.580.008
Mod vs. unb5.010.669.360.015
ResproutingMod vs. unb9.730.2219.240.037
OverstoryUnb vs. high60.1045.6374.60<0.001
Unb vs. mod49.1034.5763.60<0.001
Upper understoryHigh vs. mod12.462.8222.110.009
High vs. unb12.022.4121.640.011
Middle understoryMod vs. high6.921.6412.200.005
Mod vs. unb6.320.9011.740.014

Estimates are expressed on the response scale, in ratio for richness variables (n.53 m−2) and in difference for cover variables (%). The first level indicated for each contrast has the higher value. Lower and Upper refer to the lower and upper limits of the adjusted 95% confidence intervals. P-values were adjusted by the Tukey method for multiple comparisons.

3.3 Understory and overstory vertical structure

The overstory, as well as the upper and middle strata of the understory, differed significantly in cover only with fire severity (p < 0.001, p = 0.004, p = 0.003, respectively). Overstory cover, which corresponded to the foundational tree species at each forest type, showed the highest values in unburned sites, where individuals were alive. Upper stratum cover showed the highest values in high-severity sites, whereas middle stratum cover showed the highest values in moderate-severity sites. The lower stratum showed no significant differences for either main factor or their interaction (Figures 3, 4; Table 3).

Mean and standard error (error bars) for the cover of the overstory and of each understory stratum at the unburned, moderate-severity, and high-severity. Different letters indicate significant differences (*p* ≤ 0.05) for each variable for the fire severity main effect. No letters indicate no significant differences (*p* > 0.05).
Figure 3. Mean and standard error (error bars) for the cover of the overstory and of each understory stratum at the unburned, moderate-severity, and high-severity. Different letters indicate significant differences (p ≤ 0.05) for each variable for the fire severity main effect. No letters indicate no significant differences (p > 0.05).
Representative images of the unburned **(A)**, moderate- **(B)** and high- **(C)** severity sites, 6 years after the wildfire in *Nothofagus dombeyi* and *Austrocedrus chilensis* forests.
Figure 4. Representative images of the unburned (A), moderate- (B) and high- (C) severity sites, 6 years after the wildfire in Nothofagus dombeyi and Austrocedrus chilensis forests.

3.4 Bamboo and tree regeneration

Among plots with evidence of pre-fire presence of Chusquea culeou, regeneration occurred in 78% of moderate-severity plots and in 20% of high-severity plots. Under moderate-severity fire, regeneration from seeds and rhizome resprouts occurred across all forest types (Figure 2). In high-severity sites, the few cases of regeneration were detected only in N. pumilio forest and occurred exclusively through rhizome resprouting (Figure 2).

Seed regeneration of the foundational tree species was observed in 53% of unburned plots and in 47% of moderate-severity plots, whereas it was detected in only 7% of high-severity plots, corresponding to a single plot in N. dombeyi forests (Figure 5). Seedling and sampling density, estimated from the 1m2 quadrats, varied among fire severity and forest type combinations and was generally dominated by the smallest size class (Figure 5).

Mean and standard error of seed regeneration of the main tree species by fire severity: unburned (Unb), moderate-severity (Mod) and high-severity (High), in the corresponding forest types: *Nothofagus pumilio* (Np), *N. dombeyi* (Nd), mixed *N. dombeyi-Austrocedrus chilensis* (Nd-Ac), and *A. chilensis* (Ac), classified by size. Numbers indicate the plots with regeneration occurrence (the maximum number for each Fire severity-Forest type combination is three).
Figure 5. Mean and standard error of seed regeneration of the main tree species by fire severity: unburned (Unb), moderate-severity (Mod) and high-severity (High), in the corresponding forest types: Nothofagus pumilio (Np), N. dombeyi (Nd), mixed N. dombeyi-Austrocedrus chilensis (Nd-Ac), and A. chilensis (Ac), classified by size. Numbers indicate the plots with regeneration occurrence (the maximum number for each Fire severity-Forest type combination is three).

4 Discussion

Understanding how vegetation recovers after wildfire is essential for predicting the dynamics of Andean-Patagonian forests under changing fire regimes. Our results show that 6 years after wildfire, vegetation varied with both fire severity and forest type. High-severity fire emerged as the main driver of vegetation differences across all forest types, influencing plant assemblages with an increase in exotic species and an alteration of vegetation structure. In contrast, moderate-severity sites showed greater similarity to unburned forests, although with differences in vegetation structure. Together, these patterns suggest distinct successional trajectories following large wildfires in northern Patagonia (Kitzberger et al., 2012; Paritsis et al., 2015; Davis et al., 2019).

Understory plant assemblages in Andean-Patagonian forests vary among forest types and are influenced by multiple environmental drivers, including elevation, precipitation, forest structure, land use, and soil characteristics (Joelson et al., 2025). Although we did not directly evaluate the effect of environmental variables, we detected differences in understory plant assemblages among forest types, with the most pronounced differences involving N. pumilio forests. Within this environmental context, fire severity also significantly influenced understory assemblages. Fire severity explained a relatively small proportion of the variation in understory assemblages (7%), which likely reflects the strong environmental heterogeneity among sites, a factor known to influence understory plant assemblages in these forests (Joelson et al., 2025) as well as possible pre-fire site differences. Nevertheless, differences between high-severity and unburned sites were significant. These results are consistent with those reported in Andean-Patagonian forests during the first year after wildfire (Fuentes-Ramirez et al., 2020; Rago et al., 2024) and 5 years later (Arroyo-Vargas et al., 2022). Understory assemblages in moderate-severity sites showed an intermediate position between high-severity and unburned sites, with no significant differences. This pattern may reflect the coexistence of fire-sensitive species that survived the disturbance and new species that established after the fire, which may also explain the higher total richness observed in moderate-severity sites.

The predominance of native species, many of which are endemic to South America, across all forest types and fire severity levels suggests a generally favorable post-fire recovery. However, fire promoted the richness and cover of exotic species, especially in high-severity sites. This pattern is consistent with the release of resources and reduced competition following severe fires (Richardson et al., 2000). Among the species detected, the annual herb Epilobium brachycarpum was a highly specific indicator of high-severity sites, which is consistent with its role as an indicator of recently disturbed environments within its native range (Smith Dicarlo et al., 2019). Although we did not quantify it, signs of livestock grazing (dung and browsing) were recorded in several plots during sampling and this may have facilitated the spread of exotic species, as previously reported in post-fire sites in Andean-Patagonian forests (Arroyo-Vargas et al., 2019; Martínez et al., 2023; Raffaele et al., 2011). Differences in exotic species richness and cover among forest types may also be associated with variation in grazing intensity, as well as other drivers such as forest structure and human activities (Chillo et al., 2020; Joelson et al., 2025).

Fire had a major immediate impact on the vegetation, as observed during the field assessment of fire severity, in which a large consumption of vegetation was detected (Urretavizcaya et al., 2025). Thus, although in this study we only measured vegetation at one post-fire time, we can infer that the total understory cover values greater than 30% in all fire severities indicates it had largely recovered 6 years after the fire, regardless of forest types. This rapid recovery of vegetal cover, was particularly evident in the lower stratum, whereas significant differences among fire severities were detected in the upper and middle vertical strata, consistent with potentially divergent successional pathways that require long-term monitoring. Shrubs and perennial herbs were the dominant growth forms in all cases. The upper stratum (1.5 to 3 m) was more developed in the high-severity sites, where shrub species, including Fabiana imbricata, Schinus patagonica, Aristotelia chilensis, and Ribes magellanicus were dominant. In contrast, the middle stratum (0.5 to 1.5 m) was more developed in moderate-severity sites and was dominated by perennial herbs and vines, including Vicia nigricans, Mutisia decurrens, and M. spinosum, as well as the shrub Senecio filaginoides and the bamboo Chusquea culeou. The increased radiation resulting from reduced upper stratum cover (Urretavizcaya and Defossé, 2019), as observed in high-severity sites where trees were completely burned, may be favoring the shrub species. Therefore, these sites may continue to develop into shrub-dominated communities (Kitzberger et al., 2012, 2016), whereas moderate-severity sites seem more likely to recover into a forest structure, particularly in areas where tree regeneration was detected, supporting previous findings (Cavallero et al., 2015; Landesmann et al., 2020).

Many of the understory species recorded in this study have the capacity to resprout (Supplementary Table S1), representing more than 50% of vegetation cover across all fire severity levels. This proportion may even be higher, as information on post-fire regeneration mechanisms remains incomplete for many Andean-Patagonian understory species, particularly herbs. Similarly, because information about obligate seeders and facultative species sensu Pausas and Keeley (2014) is still scarce for these forests, these categories were not included in our analyses. Nevertheless, we recorded some obligate seeders, such as F. imbricata (Ghermandi et al., 2013), as well as facultative species, such as Aristotelia chilensis, indicating that species with these regeneration strategies also contribute to post-fire recovery. Re-establishment from nearby unburned patches may also contribute to post-fire recovery, with bird-mediated dispersal representing an important mechanism in these forests (Bravo et al., 2015; Cavallero et al., 2013).

Among the understory species detected, the bamboo C. culeou deserves particular attention. As described above, the wildfire occurred shortly after a regional flowering event of this semelparous species, which had flowered and produced seeds the previous year and subsequently largely died off. In this context, the post-fire regeneration observed in our study provides insight into the early recovery of this species following a flowering-related die-off. Regeneration occurred mainly in moderate-severity sites and was associated with small unburned patches within these areas, whereas in high-severity sites, detected regeneration was rare and restricted to rhizome resprouting. These patterns suggest that fire severity may influence the post-fire recovery and distribution of C. culeou. Given the structural and ecological roles of this bamboo in Andean-Patagonian forests, changes in its post-fire distribution may have important consequences for understory structure and ecosystem functioning (Rivas et al., 2016; Urretavizcaya et al., 2025). Additionally, nitrogen-fixing species are known to influence soil properties following fire (Certini, 2005; Johnson and Curtis, 2001). Therefore, the recovery and increased cover of these species detected in moderate-severity sites may contribute to the recovery of soil nitrogen pools, one of the nutrients most strongly affected by fire in Andean-Patagonian forests (Urretavizcaya et al., 2025).

Forests in the temperate Mediterranean-type climate of northwestern Patagonia are typically dominated by one or two foundational tree species, yet they support a diverse understory. This study contributes to a better understanding of the understory plant assemblages that develop during the early stages following wildfires. Our results highlight both the capacity of many species to develop after a large wildfire and the compositional differences that may occur in these assemblages. In particular, these differences included greater exotic species at burned sites and limited short-term recovery of some native species. Early successional species perform key ecological functions, including soil protection, nutrient cycling, and habitat provision for fauna. As fire regimes intensify and early post-fire landscapes expand, understanding the structure and dynamics of understory communities becomes increasingly important. Such knowledge is essential for informing restoration and management strategies that guide post-fire ecosystems toward desirable states while maintaining the ecological processes supported by early successional communities and sustaining the resilience of Patagonian temperate forests under changing fire regimes.

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Supplementary material

The Supplementary material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/ffgc.2026.1950264/full#supplementary-material

Supplementary material Supplementary material at the source

Footnotes

Edited by: Hugh Safford, University of California, Davis, United States

Reviewed by: Daniel Cadena, CONACYT—Colegio de Postgraduados, Mexico

José Francisco Díaz-Nájera, Colegio Superior Agropecuario del Estado de Guerrero, Mexico

Statements

Conflict of interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Funding

The author(s) declared that financial support was received for this work and/or its publication. This investigation was funded by Project PE7A11703 Centro de Investigación y Extensión Forestal Andino Patagónico (CIEFAP) and Project PUE 22920160100128 Consejo Nacional de Investigaciones Científicas y Técnicas de Argentina (CONICET). This research was carried out within the frame of a Postdoctoral fellowship from CONICET to MM Rago.

Data availability

The original contributions presented in the study are included in the article/Supplementary material, further inquiries can be directed to the corresponding author.

Author contributions

MR: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Visualization, Writing – original draft, Writing – review & editing. MU: Conceptualization, Funding acquisition, Investigation, Methodology, Project administration, Writing – review & editing.

Acknowledgements

We are grateful to the landowners for their assistance and permission to do the sampling. We thank Stefano Gianolini and Secretaría de Bosques for their support in the sampling.

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Citation

Rago, M. M. & Urretavizcaya, M. F. (2026). Understory vegetation development six years after a wildfire in the temperate forests of Patagonia, Argentina. Frontiers in Forests and Global Change, Volume 9. https://doi.org/
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9 - 2026

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Copyright © 2026 Rago and Urretavizcaya.

License: https://creativecommons.org/licenses/by/4.0/

*Correspondence: M. M. Rago

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