Prescribed Burns Reduce Early-Stage Shrub Encroachment in Semi-arid Grassland

Abstract

Wildfire is a key factor in regulating ecological processes in grassland ecosystems; however, changes in land use/cover have modified the intensity and frequency of fires as they occurred naturally. Different factors have caused a rise in woody vegetation in these ecosystems, leading to changes in species composition, diversity, and biogeochemical cycles. Prescribed burns are a tool for controlling and eradicating shrubs; however, their effectiveness depends on vegetation composition, biomass availability, and the objectives of restoration. We evaluated the effectiveness of fire as a shrub controller in a semi-arid grassland ecosystem. We measured several shrub dasometric parameters and the percentage of damage in ten 2000 m² plots three months after a prescribed burning was performed. Both crown height and width and total height were the main variables that explained the percentage of shrub damage by fire. Individuals with a height greater than 1.6 m and wide crowns did not suffer damage. Moreover, even though 97% of the total shrubs presented some fire damage, 86% recovered after the rain period. Our results show that fire could be an effective strategy to control early-growing shrubs, but on overgrazed arid lands it would be difficult to have enough biomass to implement burning programs.

1. Introduction

Semi-arid natural grasslands are distributed across regions from central Mexico to the southern United States of America, in the Chihuahuan desert region [1]. The grasslands are mainly covered by various species of grasses, with a low density of shrubs [2]. However, grasslands have been increasingly threatened by human-induced changes, such as altered fire regimes, rising CO₂ levels, land conversion to agriculture, overgrazing by livestock, and the introduction of exotic grasses [3]. These disturbances have led to shifts in plant community composition, notably the encroachment of woody species, which have displaced native grasses, reduced forage availability for livestock, and altered nutrient cycling [4].

Woody species are particularly increasing in the Mexican central highlands, with genera Vachellia, Prosopis, Opuntia, Larrea, Brickellia and Isocoma being predominant in encroached grasslands. Research has shown that shrub expansion reduces soil water availability for the other plant species, exacerbating the competition for resources [5]. Furthermore, these species are highly adaptable to degraded environments, facilitating their dominance in disturbed areas [6]. Species such as Vachellia farnesiana and V. schaffneri (huisache) create microenvironments unsuitable for other plants. When these shrubs reach a certain height, their crowns widen, reducing the establishment of grasses under their canopy due to shading or the allelopathic effects of chemical compounds in their leaves and shoots [7]. This reduction in grass cover under the shrub canopy may result in lower fire intensity, as fine fuels such as grasses are less abundant.

The reduction in herbivores has been identified as a key driver of shrub expansion, as well as excessive grazing, which alters biomass (fuel) availability and limits the occurrence of natural fires [8,9]. The balance of herbaceous and woody species therefore depends on complex interactions involving grazing, climate, topography, fire frequency, and soil type [10].

The literature highlights that reversing shrub encroachment in grasslands is a challenging issue. Studies suggest that decreasing grazing intensity alone may not significantly reverse shrub colonization, and management efforts should include diverse techniques such as mechanical, chemical, pyric, and sometimes also biological treatments [11]. Fire, which once played a natural role in limiting shrub growth, has become less frequent due to reduced biomass, and in some regions, it has been eradicated altogether [12]. Historically, grasslands were fire-dependent ecosystems, with plant species developing fire-resistant adaptations such as meristems close to the ground, making them less susceptible to fire damage [13]. However, when natural fire cycles were disrupted, shrub species began to dominate, creating ecological and economic challenges by replacing key grassland species [14]. Recently, prescribed burning has been employed to mitigate shrub encroachment and restore grassland productivity, but its effectiveness is still debated. Some studies indicate that fire frequency every 3 to 4 years does not provide sufficient control over woody plants, as prescribed fires tend to be less intense than natural fires in non-fragmented landscapes [15]. Increasing fire intensity and frequency could enhance short- and long-term soil recovery, promoting the restoration of grassland ecosystems [16].

Furthermore, shrub colonization in grassland progresses in stages, initially at a slow pace, but eventually reaching a threshold where woody species spread rapidly and dominate the ecosystem [17]. Once shrub species reach significant size and density, the resilience of grasslands decreases, and the rapid expansion of woody plants exacerbates the issue [18]. Fire has been identified as a potential tool to reverse this process by reducing shrub populations and preventing new seedlings from being established. However, little research has been carried out to assess the effectiveness of fire management techniques at different shrub growth stages, and the specific variables that influence fire damage remain poorly understood. This gap in the literature highlights the need for studies that evaluate the impact of prescribed burning on shrub species at various growth stages. In a tropical woodland, small plants were 50% more susceptible to fire than large plants [19]; similarly, [20] found an inverse relationship between fire damage and trunk diameter in different shrub species of an arid shrubland.

On the other hand, fire treatment has shown limitations in combatting shrub encroachment, especially in those species that resprout after fire [21]. Topkill stimulates the regrowth of multiple stems that can become very competitive with grasses due to the increased density of stems and leaves [22]. In these cases, repeated burnings should be applied; however, the required frequency of burnings is not the same for all ecosystems [23,24]. Depending on the moisture regime, the full recovery of sprouting shrubs after fire treatments may take between 3 and 20 years, while in nonsprouting shrubs this may occur between 25 and 35 years [25,26]. Significant efforts to eradicate shrub/tree species of genera Juniper and Prosopis on grasslands have been carried out [24]; however, species of the genera Vachellia have received less attention. In some studies, these species have been treated by mechanical, prescribed fires and herbicides methods [27,28].

The objective of this study is to assess the effectiveness of prescribed burnings in controlling shrub species at different growth stages in the Llanos de Ojuelos, Jalisco, Mexico. We aim to identify the key dasometric variables, such as shrub height, crown width, and crown height, that influence the degree of damage caused by fire. We hypothesize that shrubs lower in size and stature, i.e., young shrubs, will be more susceptible to fire damage.

To evaluate the effects of prescribed burning on shrub species, we established experimental plots with varying shrub growth stages. Prescribed burns were conducted under controlled conditions, and post-fire damage was evaluated through the measurement of dasometric variables. Fire intensity and frequency were monitored throughout the study to assess their impact on shrub species.

Our study highlights the role of fire in shaping shrub dynamics in arid ecosystems. The ability of shrubs to resprout after fire underscores their resilience and the need for targeted management strategies. Prescribed burning during early growth stages emerges as a valuable tool to control shrub encroachment and maintain ecosystem balance.

2. Materials and Methods

2.1. Study Area

The study area is located in the arid and semi-arid region of the Central Plateau of Mexico, at the south of the Chihuahuan desert, in Santo Domingo ranch (Figure 1). This area has been dedicated to livestock production for 36 years. The production system is cattle, with grazing intensities ranging between 3 and 7 ha per animal unit and a forage consumption of less than 40% of the available aerial biomass. The study area is managed in a rotational grazing system with 8 paddocks of 60 ha.

2.2. Vegetation Cover

The predominant vegetation type in the Llanos de Ojuelos consists of scrub and grasslands. The grasslands are mainly composed of blue grama grass (Bouteloua gracilis), wolfstail (Lycurus phleoides), scorpion grama (Bouteloua scorpioides), three-bearded grass (Aristida divaricata), buffalo grass (Bouteloua dactyloides), silvery strawweed (Bothriochloa barbinodis), zacatón (Muhlenbergia rigida), and hairy grama (Bouteloua hirsuta). These grasslands are interspersed with shrubs such as huizache (Vachellia schaffneri), cat’s claw (Mimosa biuncifera), and mesquite (Prosopis leaviagata), which, while native to scrublands, are considered invasive. The herbaceous vegetation includes species like foxtail (Brickellia spinulosa), sawtooth candyleaf (Stevia serrata), toad grass (Eryngium carlinae F. Delaroche), and trompillo (Solanum elaeagnifolium), among others.

2.3. Climate

The climate of the study area is characterized as dry and semi-dry, with precipitation levels consistently lower than potential evapotranspiration. Rainfall is highly variable, ranging between 300 and 500 mm annually, with a mean annual precipitation of 424 mm. Precipitation typically occurs from June to September. The average annual temperature varies between 16 and 18 °C. The soil is shallow, with minimal organic matter and a cemented layer (caliche, tepetate) at 50 cm deep. Dominant soil types include durisols and phaeozem [29,30].

2.4. History and Grazing Management

The study site is a paddock planted with weeping lovegrass (Eragrostis curvula). Rainfed agriculture was practiced here until the early 1980s, after which the land was abandoned. Weeping lovegrass was sown 15 years prior to burning, and was primarily used for producing weeping lovegrass seed, with minimal cattle grazing.

2.5. Experimental Design

To evaluate the effect of fire on grasslands invaded by Vachellia schaffneri and Mimosa biuncifera, four different sites were selected as study areas. At each site, systematic plots measuring 20 × 100 m (2000 m²) each were established, with a 100 m separation between plots to cover the total area of the site. The plots were oriented from north to south. The number of plots at each site depended on the size of the area. At Sites 1 and 4, four plots were established, while at Sites 2 and 3, three plots of the same dimensions were delineated. Prescribed burns were conducted in these plots to generate the conditions needed to study fire’s impact on the shrubs (Figure 2).

To prevent the spread of fire to neighboring areas, the plots were delimited with mineral lines and black lines. All plots had approximately 20% shrub cover with varying heights. Measurements of all shrubs within the plots were taken before the burn and three months afterward. Data collected included diameter at 30 cm above ground (base diameter), total height, crown height, crown diameter, number of branches, and the shrub’s condition (alive or dead; Figure S1). Additionally, the presence of regrowth and the percentage of damage caused by the fire—such as black burn marks on dead stems or the absence of aerial biomass—were recorded. Each individual shrub was marked with a metal plate containing the plot number and the shrub number. It was assumed that individuals not located, or whose identification plates were missing, were consumed by the fire after the prescribed burn.

2.6. Vegetation Sampling

Aboveground biomass and species diversity were assessed using 1 m² plots, with a total of 20 plots sampled. To measure aboveground biomass, herbaceous vegetation was clipped, dried, and then weighed. The biomass was separated into two categories: living biomass, representing current-year growth, and dead biomass, consisting of residual plant material from the previous year. Additionally, the percentage of grasses and herbaceous plants present in the plots was identified and quantified, providing a detailed assessment of plant composition. This method was used for characterizing grass and herbaceous composition, whereas 20 × 100 m plots were used for measuring the density and cover of shrubs forms (V. shaffneri and M. biuncifera).

2.7. Application of Prescribed Burns

The prescribed burns were conducted on different dates to effectively control and manage the fire, minimizing risks and ensuring that the most favorable weather conditions were selected. In site 1, the prescribed burn was conducted on 29 March 2021, over an area of 6 hectares with a slope of less than 3%. A head-fire burn was applied with a fuel load of 11,160 t/ha, relative humidity exceeding 30%, and wind speeds below 10 km/h from the south-southeast.

In site 2, the prescribed burn was performed on 26 April 2021, in another 7-hectare area. This burn had a fuel load of 12,220 t/ha, with relative humidity again exceeding 30% and wind speeds under 7 km/h from the south. The higher fuel load resulted in flame heights reaching approximately 10 m.

In site 3, the prescribed burn was executed on 18 April 2022, over an area of 5 hectares. This site had grass cover and biomass accumulation similar to the previous sites. The recorded environmental conditions included a relative humidity of 69%, wind speeds of 3.7 km/h, and a temperature of 12 °C.

In site 4, the prescribed burn was conducted on 29 April 2022 over an area of 6 hectares with measurements showing a relative humidity of 47%, a temperature of 16 °C, and wind speeds of 9 km/h.

Biomass amount and environmental conditions were similar among sites and at the time of the prescribed burns, and this allowed us to consider each site as a replicate.

2.8. Environmental Variables

The air temperature (maximum and minimum) was obtained from the nearest weather station of the National Meteorological Service of Mexico (SMN, Rancho Las Papas), downloaded from the online service ESSENGER of the National Laboratory of Modeling and Remote Sensors (LNMYSR; [31]) for the two years of prescribed burnings. Precipitation measurements were made with a rain gauge located near to the study sites, whereas soil moisture was extracted from TerraClimate [32].

Air and soil temperatures were measured during the prescribed burning at site 3. This site had grass cover and biomass accumulation similar to the other sites. Soil temperature was measured every second using four temperature sensors (TMC20-HD, Onset, Bourne, MA) connected to a data logger (U12-006, Onset, Bourne, MA, USA). The sensors were placed at depths of 1, 3, 5, and 10 cm in the soil. Air temperature was monitored using three type-K thermocouples positioned at heights of 50, 100, and 150 cm above the ground, while three thermocouples were placed directly over tussocks to monitor air temperature at the crown level.

2.9. Analysis of Data

Species composition was evaluated by measuring the proportion of dry biomass of each species within the cutting plots. For shrub composition, we quantified the number of individual shrubs per species in the fire plots to assess changes in species abundance due to the burns.

To determine the variables that most significantly affected shrub species and to identify the thresholds for vegetation response to fire, we employed boosted regression trees (BRTs). This analytical method helps uncover complex relationships between environmental variables and vegetation responses. The “GBM” (Generalized Boosted Regression Models) library in R software (Ver. 4.2.3) [33] was used for this analysis. This analysis provided us with insights into how different factors, such as fire dasometric variables, influence fire damage and shrub recovery and survival.

By using BRTs, we aimed to capture the most influential factors affecting shrub species and to identify critical thresholds for vegetation resilience to fire. This approach allows for a more nuanced understanding of fire’s impacts on different shrub species. The response variables used in the analysis were the percentage of damage and the number of resprouts. Because most Mimosa biuncifera individuals experienced 100% damage, this variable (percentage of damage) was not usable. Therefore, only the number of resprouts is reported in the BRTs.

Data failed to accomplish assumptions of normality and homoscedasticity, and thus the nonparametric Mann–Whitney U test (α = 0.05) was used to determine differences in size between resprouting and non-resprouting shrubs that experienced 100% fire damage. Base diameter, plant height, and crown diameter of resprouted individuals were compared with those of shrubs that did not show any resprouting after the growing season (dead plants). By comparing these metrics, we aimed to infer the effect of shrub growth stage on fire resistance.

3. Results

3.1. Species Composition

The herbaceous stratum at the study site consisted of 13 herbaceous species, including 8 native grasses and 1 introduced species, Eragrostis curvula (

Table 1. Plant species composition and biomass contribution (%) in the study site.

Species	Biomass Contribution (%)
Living grass standing biomass	63.305
Dalea Bicolor	14.961
Bidens sp.	6.936
Dead grass standing biomass	6.545
No ID	3.837
Plantago lanceolata	2.695
Stevia serrata	1.075
Euphorbia	0.243
Paspalum sp.	0.190
Zornia reticulata	0.079
Phaseolus sp.	0.057
Dichondra sp.	0.041
Tagetes sp.	0.013
Cyperus sp.	0.012
Sida sp.	0.010
Vachellia schaffneri	0.002

). Grasses contributed most to biomass productivity, comprising 75% of the total (1.9 t ha⁻¹). The shrub layer was predominantly composed of Vachellia schaffneri and Mimosa biuncifera, with 97% of individuals identified as V. schaffneri. Shrub heights ranged from 30 cm to 3.5 m, with an average crown width of 3 m. Mimosa biuncifera was absent from biomass samples because this species forms isolated, dense groups that the small quadrants used for vegetation sampling missed.

3.2. Prevailing Weather Conditions

Daily maximum and minimum air temperatures before prescribed burnings in 2021 were lower than in 2022 (Figure 3). Dry conditions prevailed for more than two months previous to burnings, with the lowest soil water content levels in both years (Figure 3). Even though annual precipitation greatly differed between the two years (548 and 243 mm, for 2021 and 2022, respectively), when measurements were taken, precipitation amounted to 250 mm in 2021 (July) and 223 mm in 2022 (November).

3.3. Temperatures in Prescribed Burnings

Soil temperature did not show significant changes due to the fire effect (Figure 4a); however, larger variations were observed at a depth of 1 cm due to a darkened soil surface from the deposition of charcoal residues that remained on soil several days after the burning (Figure 3 and Figure 4b). Air temperatures decreased with height, from 180 °C at 50 cm to 100 °C at 150 cm aboveground (Figure 4c). In contrast, tussocks experienced crown temperatures as high as 500 °C during the burn, although this peak temperature lasted for less than a minute (Figure 4d).

3.4. Fire Effects on Shrubs

Ninety-seven percent of the total shrubs exhibited some level of damage, while the remaining shrubs were unaffected by fire. Of the damaged shrubs, 86% showed regrowth after the rainy season, even when the damage was complete (100%).

For Vachelia shrubs, 84% of individuals suffered 100% of damage, and 13% of them did not show resprouts; we assumed that these shrubs were dead. The number of resprouts increased with the base diameter and with the crown area (Figure 5d,e). Larger (in size and crown area) shrubs showed a lower number of resprouts because they were less affected by fire. The most influential variable on fire damage was crown area, contributing 53% to the relative influence, followed by diameter and total height (24% and 22%, respectively;

Table 2. Relative importance of variables for explaining fire damage and number of resprouts after fire treatment in two shrub species.

Species	Explanatory Variable	Relative Importance (%)
		Fire damage	Number of resprouts
Vachellia schafneri
	Crown diameter	53.1	28.7
	Base diameter	24.7	65.2
	Height	22.2	6.1
Mimosa biuncifera
	Crown diameter		60.0
	Base diameter		30.2
	Height		9.8

; Figure 5g–i). Site, crown and stem shape did not significantly affect damage levels and were therefore excluded from the final regression trees.

Shrubs with the highest percentage of damage were those with crown areas lower than 5 square meters, lower than 5 cm base diameter, and heights between 0 and 2 m (Figure 5). This suggests that the fire caused total damage to younger Vachelia shrubs with smaller crowns. As shrub height and crown width increased, the fire inflicted little or no damage.

For Mimosa shrubs, the crown diameter was the variable that most explained the number of resprouts after fire treatment (60%); it was followed by base diameter (30%) and plant height (10%). Since all individuals of Mimosa shrubs were damaged at 100%, this variable was not used for the boosted regression tree analysis (Table 2). The recovery of shrubs increased with the crown diameter, i.e., shrubs with smaller crowns produced a lower number of sprouts than the bigger ones. This same pattern was observed with the height of shrubs; taller plants produced more resprouts after the fire. The fire completely consumed 99% of Mimosa shrubs, leaving only 18% without any resprouts.

For both Vachellia and Mimosa, plants lower in stature and in crown area were more susceptible to fire (Figure 5). These variables were related to the number of resprouts after fire, which could be interpreted as resistance or the capacity of shrubs to recover from disturbances. Base diameter is related to the age of shrubs or vigor and is correlated with plant height. This suggests that smaller and younger shrubs were more susceptible to fire damage, while larger, more established individuals demonstrated higher resilience.

With respect to individuals totally damaged by fire (100% of damage), when the base diameter, height, and crown area of shrubs was compared between those plants that resprouted after the rain season and those that were considered dead since they did not show any resprouting, shrubs of M. biuncifera that were smaller in base diameter were killed by fire (

Table 3. Median values (± confidence intervals at 95%) of base diameter, height, and crown area of shrubs of Mimosa biuncifera and Vachellia schaffneri that were 100% affected by fire. Dead shrubs correspond to those that did not resprout after the growing season, while alive shrubs showed at least one resprout.

	M. biuncifera		V. schaffneri
	Dead Shrubs	Alive Shrubs	Dead Shrubs	Alive Shrubs
Base diameter (cm)	${0.6}_{1.2}^{0.47}$	${1.1}_{1.1}^{1.0}$ *	${0.25}_{1.0}^{0.25}$	${1.9}_{2.0}^{1.8}$ *
Height (m)	${0.9}_{1.15}^{0.7}$	${1.0}_{1.1}^{0.9}$	${1.2}_{1.3}^{1.0}$	${1.25}_{1.3}^{1.2}$
Crown area (m2)	${0.69}_{1.23}^{0.23}$	${0.69}_{0.95}^{0.54}$	${0.74}_{1.14}^{0.57}$	${1.23}_{1.38}^{1.09}$ *

The * stands for significant differences between medians of dead and alive shrubs (Mann–Whitney U, α = 0.05).

), and there was no difference in height and crown area. In contrast, dead individuals of V. schaffneri were smaller in base diameter and in crown area than individuals that resprouted after the rainy season (Table 3).

4. Discussion

4.1. Fire Effects on Shrubs

Our results showed that the burning affected 97% (Vachellia) and 100% (Mimosa) of the shrubs; however, on average, 86% of the affected shrubs recovered after the first rains. The taller individuals with larger base diameters and with wide crowns did not suffer any effects from the burning (Figure 5). This indicates that as height and crown width increase, fire causes little or no damage to the shrubs. We confirmed our hypothesis that shrubs lower in both size and crown area would be more susceptible to fire. This low fire damage of large shrubs is likely explained by thicker bark at the base of the shrubs, and taller growth structures that lessen the effect of fire on adult plants. It also suggests that shrubs become less vulnerable to fire damage as they grow [34] A larger crown area reduces grass cover due to shading or allelopathic effects just below the shrub crown, which lowers fire intensity by decreasing the availability of fuel [7]. On the other hand, a greater height allows the growth structures to escape from the fire [35,36]. This indicates that fire causes total damage to low-growing shrubs with small crowns, which coincides with other studies where the size of shrubs was an important characteristic of fire resistance [19,20,37]. Interestingly, of those shrubs that were totally affected by fire (100% damage), plants with thicker base diameters and with larger crown diameters resprouted after precipitation. However, there were some differences between species.

Available biomass is a factor to consider during burning. In our plots, there was an average biomass of 12 tons per hectare, and although the fire reached flame heights of around 10 m, its duration in certain areas was only of a few seconds due to the speed of the fire’s movement (Figure 4). Our results showed that a single burn reduces shrub density by 15%, primarily affecting small (and likely younger) plants while having minimal impact on other herbaceous species that are senescent or are found in the soil seed bank. One year after the burning, net primary productivity recovered at 1.9 t ha⁻¹, which is over the average of natural semi-arid grasslands in the Chihuahuan Desert region. Some studies in semi-arid grasslands suggest that full biomass recovery after burning takes about 3–5 years [38,39]. Our study site is not the typical grassland in Mexico; overgrazing by cattle results in very low standing biomass levels (<1 t ha⁻¹), which prevents severe wildfires, but that might not represent a threat for shrub survival. To replicate our results, previous land management to increase biomass must be in place.

In the early stage of shrub colonization, grasses can suppress shrub dominance through competition for near-surface soil resources, slowing shrub growth. However, when grasses are inactive, shrubs use these resources to accelerate growth [40]. In the later stages of the transition from grassland to shrubland, shrub–shrub competition does not slow the expansion rate of shrubs [41]. An adult-stage shrub can reach heights of 3 to 4 m and develop better roots, allowing for greater nutrient reserves [42], which could translate into greater fire resistance and the ability to regrow even after 100% damage. The capacity of shrubs for resprouting is achieved early in development; for instance, V. farnesiana can resprout from 6 months of age [43]. This indicates that even in early shrub invasions, a single fire treatment might not be enough to eradicate shrubs. However, even when woody plants benefit from disturbance regimes as they are capable of resprouting, these sprouts tend to produce fewer seeds than older branches, limiting reproduction by seeds [42,44]. We obtained a shrub reduction of 15%, which involved individuals that did not present resprouts after fire. Plants of M. biuncifera with low base diameters and plants of V. schaffneri with both small base diameter and crown areas did not recover after fire (Table 3). It is likely that very small plants did not have the reserves to resprout or to stay alive until the rains, or that fire affected all their meristems. These results have implications for management and conservation programs that use fire to control shrubs.

The small stature of M. biuncifera made it more susceptible to fire, with 99% of the observed individuals being completely damaged, but those individuals with a median of 1.1 cm of base diameter resprouted after the rainy season. In agreement with Kittams [26], this species could recover within 5 years after fire. Senescence in this species could be beneficial. Although low-moisture tissues are entirely consumed by fire during the dry season, plant reserves remain assured and can be used for resprouting when the rains return.

Our results reveal that a diameter of 30 cm in V. schaffneri is an important factor for lowering fire damage and for favoring resprouting (Table 2 and Table 3; Figure 5). This finding can be explained by several ecological and morphological factors. First, a diameter at 30 cm from the ground serves as an indicator of the shrub’s age and vigor. Shrubs with a larger diameter, which are typically older and more vigorous, have a greater capacity to generate biomass and support a more extensive branching structure [44]. This is because a larger diameter provides a more robust base capable of supporting a greater number of branches. On the other hand, the crown area is closely related to the shrub’s photosynthetic capacity, which influences branch formation. A shrub with a larger crown can capture more sunlight and produce more energy, facilitating the development of a greater number of branches [45]. In contrast, total height is not necessarily linked to branching, as some species can grow in height without proportionally increasing the number of branches [46]. Vachellia and Mimosa showed similar responses; even though Mimosa are lower in stature than Vachellia, burned plants resprouted, and only a minimum percentage of plants died (17%). Fire and grazing have shaped plant communities for millennia, and environmental conditions have selected species with morphological characteristics for deep water exploration and root reserves. This suggests that shrub morphology and branching capacity have been determined by environmental conditions and disturbances, with a high capacity for resistance and survival [47]. The post-fire resilience of shrubs may be associated with their branching capacity; shrubs with a larger diameter and bigger crown have more resources to recover after a disturbance such as fire [48]. Base diameter and crown area are indicative of recovery capacity, as more robust shrubs with a larger crown seem better equipped to withstand damage and regenerate branches more easily [49].

As shrub encroachment increases, grasses and other flammable fuels decrease, so reintroducing fire after prolonged suppression may not necessarily be beneficial [50]. It is critical to determine the frequency of fire needed to prevent shrubs from reaching a crown height or width at which fire is no longer an effective control tool. Other factors, such as the effects of fire frequency and intensity on biodiversity, also need consideration [51].

Interestingly, the relationship between the percentage of damage and crown diameter and height is not linear (Figure 5), which agrees with the observations in Juniperus deppeana in dry shrubland [20]. This could indicate that prolonged periods without fire pressure or more favorable climatic events for shrub growth (e.g., winter rains: [52]) could lead to “no return” colonization stages that are not controllable with fire. The woody species that resprout are the most difficult to control with fire alone and require additional management strategies to reduce their abundance and eventually remove them from the ecosystem. In agreement with some studies, combining prescribed burns with the browsing of ruminant species can reduce shrub density by up to 90% and promote grass cover by improving the light environment for herbaceous species [53,54,55]. Although controlled grazing can be a valuable supplementary technique, it cannot replace fire for controlling shrubs in these systems [36,55]. Properly timed prescribed burns can be a key ally in controlling shrubs before they become a serious problem.

4.2. Research Limitations

Our study has several limitations. First, the high biomass availability in our study area exceeds that of typical grasslands in Central Mexico, potentially limiting the generalizability of our findings to other sites with lower fuel loads. Second, even though the study was carried out in two contrasting years of precipitation (548 and 243 mm, for 2021 and 2022, respectively), the observed recovery in 86% of the shrubs after the first rains suggests that the post-fire climatic conditions, such as the amount of rainfall, were favorable for regeneration. Interannual and seasonal variations in precipitation were not considered in this study. The amount of winter precipitation and the previous year’s precipitation are key factors for productivity in the grasslands and shrublands of the north of Mexico and the south of the United States of America [52,56,57]. High resprouting levels observed in 2022 could be influenced by the high amount of precipitation in 2021. Third, although we observed substantial post-fire recovery within a year (86% of shrubs resprouted after the first rains), this timeframe may not capture longer-term effects such as shifts in species composition or ecosystem dynamics. These effects could have more significant implications for long-term management.

4.3. Future Research

Extensive research has focused on solving the problem of shrub encroachment, a phenomenon observed globally in mainly grassland ecosystems. Despite these efforts, a single technique or process to eradicate shrubs remains absent, and worryingly, many current procedures rely on the use of herbicides [27,28]. Modeling and experimental simulations to investigate burns at small scales, control of fire intensity, the age of shrubs, and the season of prescribed fires will give valuable information for developing better and more effective management programs to reduce shrub cover. Moreover, further investigations about triggers and key variables of climate, land use, or grazing regimes that promote encroachment will be crucial to prevent future invasions and halt advanced stages of shrub establishment. The legacy effect of precipitation [57] is a line of research that should be addressed to clarify trends of shrub invasion, and this information should be used to develop more effective prescribed burning programs.

5. Conclusions

Prescribed burning caused total damaged in 99% of M. biuncifera, and in 84% of V. schaffneri shrubs; however, it only caused death in 18% of Mimosa and 13% of Vachellia shrubs. Both species showed a high capacity for resprouting, even after being almost completely burned. Fire primarily affected shrubs that were small in both base diameter and crown area. When shrubs had a large crown area, base diameter, or height, the fire caused minimal or no damage. Based on our results, we can infer that these species are more vulnerable during their early growth stages. However, it is necessary to continue this research, testing recurrent burnings in different climatic conditions and applying different methods for determining the best conditions in which to apply prescribed burns. Continuous prescribed burning could potentially improve shrub control [58]. However, in overgrazed arid lands, it may be challenging to maintain sufficient biomass for a continuous two-year burning program.