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National Park Service
U.S. Department of the Interior

Science Report NPS/SR—2026/424

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• Table of Contents
• List of Figures
• List of Tables

Status of Terrestrial Vegetation and Soils at Big Bend National Park, 2020–2024

Sarah E. Studd, J. Andrew Hubbard, Cheryl L. McIntyre, Jason Mateljak, Adam D. Pingatore

April 2026

Please cite this publication as:

Studd, S.E., J.A. Hubbard, C.L. McIntyre, J. Mateljak, and A.D. Pingatore. 2026. Status of Terrestrial Vegetation and Soils at Big Bend National Park, 2020–2024. Science Report NPS/SR—2026/424. National Park Service, Fort Collins, Colorado. https://doi.org/10.36967/2317772

Abstract

This report summarizes the status of vegetation and soils at 74 monitoring sites across Big Bend National Park in the context of management assessment points. These assessment points are designed to provide managers with a reference level for evaluating current conditions and determining if management action may be needed. Recent data indicate that exotic plants are common in low to middle elevations of the park but in low abundance. Overall, exotic plants were present in 21 of the 74 monitoring plots (28.3%), with Lehman lovegrass present in half of those plots (10) and buffelgrass only present in two plots. None of the exotic species observed had cover values (abundance) approaching our assessment point of 5%. Enhanced erosion potential appears moderate at most sites, based on mean soil stability values below 3 in half the strata and most plots having mature biocrust cover below the desired 2% cover stated in our management assessment point. This suggests that water erosion of upland soils may be a management concern. The Chisos High Elevations plots exceeded the assessment points for fine fuels cover and ladder fuels, with mean subcanopy (0.5–2.0 m height layer) woody cover approaching 30% and litter cover approaching 40–50%. Chisos Mid-Elevations sites also have high litter and duff cover (around 35%), but less woody subcanopy structure. Combined with other environmental stressors, such as prolonged drought, these conditions suggest severe wildfire risk. Finally, we evaluated the frequency of sensitive cactus species co-occurring with non-native grasses and found only four plots parkwide had both present during the most recent round of sampling (2020–2024). While this is encouraging, in future analyses we will evaluate the abundance of the non-native grasses at these sites with particular attention to any increases in cover that might warrant some focused management action.

Terrestrial vegetation and soils monitoring at site 302_011 within the Rocky Foothills strata.

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Table of Contents

• Abstract

• List of Figures

• List of Tables

• Introduction

• Methods

  • Study Area

  • Sampling Design

  • Field Methods

  • Analyses and Data Availability

• Results and Discussion

  • Erosion Potential

  • Exotic Plant Dispersal and Invasion

    • Any Exotic Species

    • Specific Exotic Species

  • Fire Hazard

  • Rare and Sensitive Cacti

• Conclusion and Management Recommendations

• Literature Cited

• Author Information

• About the National Park Service Science Report Series

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List of Figures

• Figure 1. Elevation and soils strata (n = 8) used in selection of uplands monitoring plot locations (74 plots actively monitored), Big Bend National Park.

• Figure 2. Soil aggregate stability by stratum in Big Bend National Park, 2020–2024.

• Figure 3. Cover of mature biocrusts in available habitat by stratum in Big Bend National Park, 2020–2024.

• Figure 4. Presence of Lehmann lovegrass (Eragrostis lehmanniana) in long-term monitoring plots at Big Bend National Park, 2020–2024.

• Figure 5. Cover of all exotic plants combined in any layer by stratum in Big Bend National Park, 2020–2024.

• Figure 6. Cover of dead plants in the subcanopy layer by stratum in Big Bend National Park, 2020–2024.

• Figure 7. Cover of live woody species in subcanopy layer by stratum in Big Bend National Park, 2020–2024.

• Figure 8. Cover of litter + duff by stratum in Big Bend National Park, 2020–2024.

• Figure 9. An example monitoring photo taken at plot V600_022 in the Chisos High Elevations stratum in 2024.

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List of Tables

• Table 1. Highest priority issues and corresponding management assessment points for Big Bend National Park terrestrial vegetation and soils monitoring.

• Table 2. Soil aggregate stability (mean ± standard error [se]) by stratum in Big Bend National Park, 2020–2024.

• Table 3. Mature biocrust cover (mean ± standard error [se]) by stratum in Big Bend National Park, 2020–2024.

• Table 4. Total exotic plant cover (mean ± standard error [se]) by stratum in Big Bend National Park, 2020–2024.

• Table 5. Lehmann lovegrass cover (mean ± standard error [se]) by stratum in Big Bend National Park, 2020–2024.

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Introduction

Generating more than 99.9% of Earth’s biomass (Whittaker 1975), plants are the primary producers of life on our planet. Vegetation therefore represents much of the biological foundation of terrestrial ecosystems, and it comprises or interacts with all primary structural and functional components of these systems. Vegetation dynamics can indicate the integrity of ecological processes, productivity trends, and ecosystem interactions that can otherwise be difficult to monitor. To achieve park objectives, land management actions often focus on manipulating vegetation using strategies based on community structure or lifeform composition. Bailey’s ecoregion classification underscores the Chihuahuan Desert as a dynamic and ecologically complex region where abiotic factors such as precipitation, temperature, soil, and topography interact to shape the distribution and composition of its vegetation (Bailey 1998). Understanding these drivers is essential for effective conservation and management of the desert’s unique plant communities.

The Chihuahuan Desert Network, as part of the National Park Service (NPS) Inventory and Monitoring (I&M) Division, identified terrestrial vegetation and dynamic soil functional attributes as important ecosystem monitoring parameters, or “vital signs” (NPS 2010) that provide key insights into the integrity of terrestrial ecosystems. Since 2011, the Chihuahuan Desert Network has monitored terrestrial vegetation and soils at Big Bend National Park. The monitoring effort includes measurements of species and lifeform abundance, soil cover, biological soil crusts, and soil stability (Hubbard et al. 2012).

To achieve the NPS core mission of resource protection, resource management and monitoring must be explicitly linked (Bingham et al. 2007). Here, we use management assessment points to interpret monitoring results for resource management. Management assessment points are ” … pre-selected points along a continuum of resource-indicator values where scientists and managers have agreed to stop and assess the status or trend of a resource relative to program goals, natural variation, or potential concerns” (Bennetts et al. 2007). They do not define strict management or ecological thresholds, inevitably result in management actions, or reflect any legal or regulatory standard. They are only intended to serve as a potential early warning system, allowing scientists and managers to pause, review the available information in detail, and consider options. Professional opinion identified seven management issues and proposed 16 management assessment points for Big Bend National Park, which we evaluated and revised as needed (Chihuahuan Desert Network unpublished document). This report presents and interprets the results of recent monitoring in the context of the five highest priority issues and 14 related management assessment points at Big Bend National Park (Table 1).

Table 1. Highest priority issues and corresponding management assessment points for Big Bend National Park terrestrial vegetation and soils monitoring.

Priority Issues	Management Assessment Points
Erosion Potential	Mature biological soil crust cover is <2% of available habitat Soil stability <Class 3
Exotic Plant Dispersal and Invasion	Buffelgrass cover >5% Exotic plant cover >5% Exotic plants present on >25% of monitoring plots Lehmann lovegrass cover >5% Lehmann lovegrass present on >25% of monitoring plots
Fire Hazard	100-hour fuel cover >5% 1000-hour fuel cover >5% Dead plant cover in subcanopy (0.5–2 m height layer) >5% Litter + duff cover >30% Total woody plant cover in subcanopy (0.5–2 m height layer) >30%
Forest Health	Total woody cover in canopy (>2.0 m height layer) >30%
Rare and Sensitive Cacti	Invasive grass in plots with sensitive cacti

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Methods

Study Area

Big Bend National Park is the largest protected area of Chihuahuan Desert within the United States. It was established in 1944 and protects 324,219 hectares (801,163 acres) of land bordering Mexico. The Rio Grande River flows along the international border, defining the southern boundary of the park. In 1978, through an act of Congress a 315-kilometer (196-mile) section of the Rio Grande was designated as a wild and scenic river effectively creating the Rio Grande Wild and Scenic River park unit that starts in Big Bend National Park and extends eastward.

Situated in the Chihuahuan Desert, Big Bend National Park has a hot and dry climate—summer temperatures routinely reach over 100°F (38°C) and fall and winter are mild. The average annual rainfall ranges from 48 cm (19 in) in the Chisos Basin to about 25 cm (10 in) along the Rio Grande (Cogan and Lea 2021), with most rainfall occurring in the summer months and the remainder falling as winter rains.

Vegetation communities within the park primarily change along elevation (548–2,387 m [1,798--7,831 ft]) and rainfall gradients. The low elevation and drier areas host mixed cacti scrublands and semi-desert grasslands, while the higher elevation and more mesic Chisos Mountains support montane pine-oak woodlands. The park has a rugged landscape with a diversity of landforms and geologic formations that have contributed to the development of varied soils. A 2011 soil survey (USDA NRCS 2011) describes the soils in the context of supporting five broad vegetative zones: Hot Desert Shrub, Desert Grassland, Southern Edwards Plateau, Mixed Prairie, and Mountain Savannah. The rugged landscape, abundance of geological formations, wide range of elevations, and diverse ecological influences from the surrounding ecoregions all contribute to the high biodiversity and variety of habitats found in Big Bend National Park (Cogan and Lea 2021).

Sampling Design

Hubbard et al. (2012) describes the temporal and spatial sampling design for vegetation monitoring at Big Bend National Park. Briefly, we allocated monitoring plots in a stratified, random, spatially balanced design. Strata were based on elevation zones and surface soil rock fragment content. We excluded all areas around roads, buildings, and parking areas (including a 100 m buffer), trails, washes, and streams (including a 50 m buffer), steep slopes, and areas with cultural resources. The park archeological staff provided site review prior to establishment of plots in areas that had not been previously surveyed for cultural resources. Plots were allocated to eight strata that together span 77% of the park.

• Non-Rocky Valley Bottom (V101): ≤2,500 ft elevation and <35% surface soil rock fragments

• Non-Rocky Bajada (V201): 2,501–3,700 ft elevation and <35% surface soil rock fragments

• Rocky Bajada (V202): 2,501–3,700 ft elevation and 35–90% surface soil rock fragments

• Bajada Rock Outcrops (V203): 2,501–3,700 ft elevation and bedrock or rock outcrops

• Non-Rocky Foothills (V301): 3,701–4,500 ft elevation and <35% surface soil rock fragments

• Rocky Foothills (V302): 3,701–4,500 ft elevation and 35–90% surface soil rock fragments

• Chisos Mid-Elevations (V500): 5,000–6,000 ft elevation

• Chisos High Elevations (V600): >6,000 ft elevation

Ninety-eight monitoring plots were established between 2011 and 2015. Subsequently, we reduced our sampling effort to 76 plots based on estimates of statistical power. In this report, we summarize data from 74 plots sampled from October 29, 2020, to November 11, 2024: 15 plots in the Non-Rocky Valley Bottom stratum; 20 plots in the Non-Rocky Bajada stratum; 12 plots in the Rocky Bajada stratum; six plots in the Bajada Rock Outcrops stratum; five plots in the Non-Rocky Foothills stratum; six plots in the Rocky Foothills stratum; five plots in the Chisos Mid-Elevations stratum; and five plots in the Chisos High Elevations stratum (Figure 1).

Figure 1. Elevation and soils strata (n = 8) used in selection of uplands monitoring plot locations (74 plots actively monitored), Big Bend National Park. Any strata that comprises less than 5% of total park area (i.e., low relative area) and buffers surrounding waterways, trails, and roads were excluded from plot selection.

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Field Methods

Field data collection followed Hubbard et al. (2012). Briefly, data were collected within 20 × 50 m plots with six parallel 20 m transects. Vegetation was recorded within three height categories using the line-point intercept method (Herrick et al. 2005a), with points spaced every 0.5 m. The three height categories were field (0.025–0.5 m), subcanopy (>0.5–2.0 m), and canopy (>2.0 m). Only the first species encountered were recorded at each line intercept per height category, even if multiple species were present. Perennial vegetation was recorded to species. Annual vegetation was recorded to lifeform, except for non-native plants that were recorded to the species level. Soil cover was recorded by substrate class (e.g., rock, gravel, bare ground), and litter and woody debris were recorded to fuel type (e.g., fine fuels, 1000-hour fuels; Deeming et al. 1977). In the space between the transects (10 × 20 m subplots), perennial vegetation was recorded by species and columnar cacti and ocotillo were counted by height class. Surface soil aggregate stability was measured using a modified wet aggregate stability method (Herrick et al. 2005a), with 18 samples per plot. Biological soil crust cover was measured using 0.25 m² quadrats. Three quadrats per transect were measured using the point-quadrat method, with 16 intercept measurements per quadrat. At each intercept, biological soil crusts were recorded as light cyanobacteria, dark cyanobacteria, moss, liverwort, or lichen by growth form.

Analyses and Data Availability

Data used in this report are available to park staff on the NPS DataStore (McIntyre et al. 2025) and can be provided upon request. All data have undergone certification processes to ensure they have been verified and validated for accuracy, are complete, and are fully documented. Our focus in this report is on status, based on plots sampled during the most recent visits in 2020–2024. For most of our parameters, we evaluated plot-specific presence/absence or calculated basic summary statistics (mean and standard error [se]) on untransformed data (summary statistics are rounded to nearest tenth except for values <0.05, which are shown to four decimal places; true zeros appear as 0.0). The R scripts to summarize the data and initiate the report are also available to NPS staff on the NPS DataStore (McIntyre and Thomas 2025) and can be provided upon request. Not all management assessment points apply to all areas of the park, so assessment point results are presented for applicable strata.

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Results and Discussion

Erosion Potential

There are two management assessment points associated with erosion potential:

• Soil stability <Class 3 (used to gauge the ability of the soil surface to resist water erosion in all strata)

• Mature biological soil crust cover is <2% of available habitat (used to assess the extent of biocrust development and distribution in Non-Rocky Valley Bottom, Non-Rocky Bajada, Rocky Bajada, Bajada Rock Outcrops, Non-Rocky Foothills, and Rocky Foothills strata)

Soil surface aggregate stability (mean ± se) for each stratum is shown in Table 2. Scores between 2 and 3 indicate moderately stable soils, which are vulnerable under intense rain or disturbance. Low stability ratings are common in semi-degraded or recovering soil conditions, such as areas impacted by grazing, past cultivation, or with low vegetation cover. Values between 4 and 6 indicate high stability from good soil structure and therefore better resistance to erosion (Herrick et al. 2005b).

Table 2. Soil aggregate stability (mean ± standard error [se]) by stratum in Big Bend National Park, 2020–2024.

Stratum	Mean ± se
Non-Rocky Valley Bottom	2.5 ± 0.2
Non-Rocky Bajada	2.8 ± 0.3
Rocky Bajada	3.9 ± 0.3
Bajada Rock Outcrops	3.0 ± 0.7
Non-Rocky Foothills	2.9 ± 0.3
Rocky Foothills	3.6 ± 0.2
Chisos Mid-Elevations	4.2 ± 0.5
Chisos High Elevations	3.4 ± 0.9

Mean stability values were generally higher in higher elevation strata and in strata with rockier soils (Figure 2). Soil aggregate stability values are correlated with vegetation cover (Herrick et al. 2005b), and the higher elevation strata tend to have more vegetation cover and grasses, providing fibrous roots and higher organic inputs from leaf litter.

Figure 2. Soil aggregate stability by stratum in Big Bend National Park, 2020–2024. The dashed orange line is the management assessment point value (class 3). Black dots are individual plot means, and open diamonds are the stratum means. Thick black lines are the median, and the lower and upper horizontal lines are the 25^th and 75^th percentiles, respectively.

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Open spaces on the soils in arid and semi-arid regions are often covered by biocrusts (biological soil crusts), which are communities of cyanobacteria, algae, lichens, and bryophytes that provide key ecosystem functions, such as increasing water and wind erosion resistance, contributing to soil organic matter, and fixing atmospheric nitrogen (Belnap 2003). “Mature” biocrusts, or later successional biocrusts (Belnap and Eldridge 2003), provide more resistance to erosion than biocrusts dominated by light cyanobacteria, or early successional biocrusts (Belnap et al. 2008; Warren 2003). The abundance and pattern of mature biocrusts within our monitoring sites likely depends on plant canopy cover, micro-aspect, and soil nutrients (Bowker et al. 2006), whereas park-scale biocrust abundance is driven by soil chemistry and disturbance history (Belnap et al. 2001).

Cover of mature biocrusts (mean ± se) in available habitat is shown in Table 3. Mature biocrusts are more developed biocrusts, which include dark cyanobacteria, lichen, moss, and other bryophytes (after Bowker and Belnap 2008). Available habitat are areas that were not occupied by duff, rock, bedrock, embedded litter, or vegetation. Mature biocrust cover within our plots was highly variable, with some plots having no biocrust cover and some having about 16% of available habitat covered by biocrusts (Figure 3). The Rocky Bajada stratum had the highest average cover. Across our strata, most plots had mature biocrust cover below the desired 2% cover stated in our management assessment point, suggesting that water erosion may be a current and future management concern (Figure 3). The desired cover value was based on expert opinion and will be reevaluated in the future as needed.

Table 3. Mature biocrust cover (mean ± standard error [se]) by stratum in Big Bend National Park, 2020–2024.

Stratum	Mean ± se
Non-Rocky Valley Bottom	0.2% ± 0.2
Non-Rocky Bajada	0.9% ± 0.8
Rocky Bajada	2.9% ± 1.1
Bajada Rock Outcrops	0.8% ± 0.7
Non-Rocky Foothills	0.0% ± 0.0
Rocky Foothills	0.8% ± 0.6

Figure 3. Cover of mature biocrusts in available habitat by stratum in Big Bend National Park, 2020–2024. Mature biocrusts include dark cyanobacteria, lichen, moss, and other bryophytes. Available habitat are areas that were not occupied by duff, rock, bedrock, embedded litter, or vegetation. The dashed orange line is the management assessment point value (2%). Black dots are values for individual plots, and open diamonds are the stratum means. Thick black lines are the median, and the lower and upper horizontal lines are the 25^th and 75^th percentiles, respectively.

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Exotic Plant Dispersal and Invasion

During 2020–2024, we observed a total of four exotic (non-native) plant species across 74 plots: Lehmann lovegrass (Eragrostis lehmanniana; perennial grass), buffelgrass (Cenchrus ciliaris; perennial grass), spreading fanpetals (Sida abutilifolia; perennial forb), and prickly Russian thistle (Salsola tragus; annual forb). There are five management assessment points associated with exotic plant dispersal and invasion:

• Exotic plant cover >5% (all strata)

• Exotic plants present on >25% of monitoring plots (all strata)

• Buffelgrass cover >5% (Non-Rocky Valley Bottom, Non-Rocky Bajada, Rocky Bajada, Bajada Rock Outcrops, Non-Rocky Foothills, and Rocky Foothills strata)

• Lehmann lovegrass cover >5% (all strata)

• Lehmann lovegrass present on >25% of monitoring plots (all strata)

Any Exotic Species

Exotic plant cover values across the height layers (absolute cover; mean ± se) were low across all elevation strata and well below our management assessment point value of 5% (Table 4).

Table 4. Total exotic plant cover (mean ± standard error [se]) by stratum in Big Bend National Park, 2020–2024.

Stratum	Mean ± se
Non-Rocky Valley Bottom	0.0% ± 0.0
Non-Rocky Bajada	0.1% ± 0.1
Rocky Bajada	0.0% ± 0.0
Bajada Rock Outcrops	0.0% ± 0.0
Non-Rocky Foothills	0.9% ± 0.9
Rocky Foothills	0.2% ± 0.2
Chisos Mid-Elevations	0.0% ± 0.0
Chisos High Elevations	0.0% ± 0.0

Overall, exotic plants were present in 21 of the 74 monitoring plots (28.3%). Exotic plants were present in

• 2 of 15 plots (13.3%) in the Non-Rocky Valley Bottom stratum,

• 4 of 20 plots (20.0%) in the Non-Rocky Bajada stratum,

• 4 of 12 plots (33.3%) in the Rocky Bajada stratum,

• 1 of 6 plots (16.7%) in the Bajada Rock Outcrops stratum,

• 4 of 5 plots (80.0%) in the Non-Rocky Foothills stratum,

• 4 of 6 plots (66.7%) in the Rocky Foothills stratum,

• 2 of 5 plots (40.0%) in the Chisos Mid-Elevations stratum, and

• 0 of 5 plots (0.0%) in the Chisos High Elevations stratum.

Specific Exotic Species

Buffelgrass was not encountered on the line-point transects during our most recent monitoring, so there are no percent cover data to report. However, this species was recorded as “present” in at least 1 of 5 subplots in two of the Non-Rocky Valley Bottom stratum plots. One plot, V101_005, had buffelgrass present in all 5 of the subplots, indicating it is widespread at this location. This may also mean there is broader infestation in this part of the park. Buffelgrass was not observed in any plots in the Non-Rocky Bajada, Rocky Bajada, Bajada Rock Outcrops, Non-Rocky Foothills, and Rocky Foothills strata.

The absolute cover (mean ± se) of Lehmann lovegrass in any layer was low across all strata and well below our management assessment point value of 5% (Table 5).

Table 5. Lehmann lovegrass cover (mean ± standard error [se]) by stratum in Big Bend National Park, 2020–2024.

Stratum	Mean ± se
Non-Rocky Valley Bottom	0.0% ± 0.0
Non-Rocky Bajada	0.1% ± 0.1
Rocky Bajada	0.0% ± 0.0
Bajada Rock Outcrops	0.0% ± 0.0
Non-Rocky Foothills	0.9% ± 0.9
Rocky Foothills	0.2% ± 0.2
Chisos Mid-Elevations	0.0% ± 0.0
Chisos High Elevations	0.0% ± 0.0

Lehmann lovegrass was present (Figure 4) in a total of 10 of the 74 plots (13.5%). This species was found in

• 0 of 15 plots (0.0%) in the Non-Rocky Valley Bottom stratum,

• 2 of 20 plots (10.0%) in the Non-Rocky Bajada stratum,

• 1 of 12 plots (8.3%) in the Rocky Bajada stratum,

• 1 of 6 plots (16.7%) in the Bajada Rock Outcrops stratum,

• 2 of 5 plots (40.0%) in the Non-Rocky Foothills stratum,

• 2 of 6 plots (33.3%) in the Rocky Foothills stratum,

• 2 of 5 plots (40.0%) in the Chisos Mid-Elevations stratum, and

• 0 of 5 plots (0.0%) in the Chisos High Elevations stratum.

Figure 4. Presence of Lehmann lovegrass (Eragrostis lehmanniana) in long-term monitoring plots at Big Bend National Park, 2020–2024. This species was present in 10 of the 74 plots during recent surveys.

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Collectively, these results suggest that while current exotic species occurrence is moderately widespread and exceeds our assessment point (present in >25% of all plots within a stratum) in 4 of the 8 strata, exotic plant cover is relatively low (Figure 5). While non-native plants have colonized many sites, low cover indicates they are not currently abundant across plots and are often only present in localized patches. Patterns of invasion can frequently be tied to vectors of spread such as roads, trails, and waterways and to disturbance factors such as fire, fire suppression activities, flooding, and earthworks. The presence of non-native species in the park interior away from common vectors may indicate a more widespread distribution and make treatment efforts much more difficult.

Figure 5. Cover of all exotic plants combined in any layer by stratum in Big Bend National Park, 2020–2024. The dashed orange line is the management assessment point value (5% cover). Black dots are values for individual plots, and open diamonds are the stratum means. Thick black lines are the median, and the lower and upper horizontal lines that represent the 25^th and 75^th percentiles are not visible here because they are equal to the median in every stratum. This graph shows that the majority of sites had low cover, far below the assessment value, but one site in the Non-Rocky Foothills stratum has individual plot cover values between 4 and 5%.

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Fire Hazard

There are five management assessment points associated with fire hazard, and these are evaluated only across the 10 plots in the two Chisos Mountains strata:

• Dead plant cover in subcanopy layer (0.5–2.0 m height layer) >5% (Chisos Mid-Elevations and Chisos High Elevations strata)

• Total woody plant cover in subcanopy layer (0.5–2.0 m height layer) >30% (Chisos Mid-Elevations and Chisos High Elevations strata)

• 100-hour fuel cover >5% (Chisos Mid-Elevations and Chisos High Elevations strata)

• 1,000-hour fuel cover >5% (Chisos Mid-Elevations and Chisos High Elevations strata)

• Litter + duff cover >30% (Chisos Mid-Elevations and Chisos High Elevations strata)

Dead plant cover in the subcanopy layer (0.5–2.0 m height layer) was 0.2% ± 0.2 in the Chisos Mid-Elevations stratum and 3.1% ± 0.9 in the Chisos High Elevations stratum (Figure 6). Here, “dead cover” refers collectively to standing dead trees and shrubs and suspended woody debris, such as a fallen tree limb caught in the canopy. These values do not exceed our management assessment points for dead woody cover. Total live woody plant cover (mean ± se) in the subcanopy layer (Figure 7) was 19.2% ± 5.4 in the Chisos Mid-Elevations stratum and 27.7% ± 8.9 in the Chisos High Elevations stratum.

Figure 6. Cover of dead plants in the subcanopy layer by stratum in Big Bend National Park, 2020–2024. The dashed orange line is the management assessment point value (5%). Black dots are values for individual plots, and open diamonds are the stratum means. Thick black lines are the median, and the lower and upper horizontal lines are the 25^th and 75^th percentiles, respectively (for the Chisos Mid-Elevations stratum, the 25^th and 75^th percentiles equal the median).

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Figure 7. Cover of live woody species in subcanopy layer by stratum in Big Bend National Park, 2020–2024. The dashed orange line is the management assessment point value (30%). Black dots are values for individual plots, and open diamonds are the stratum means. Thick black lines are the median, and the lower and upper horizontal lines are the 25^th and 75^th percentiles, respectively.

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We measured fuel loading on the ground in two ways: measuring fine and coarse woody debris by size class and looking at litter and duff depths at each sampling point across the site. The cover of 100-hour fuels (woody material 2.54–7.62 cm [1--3 in] in diameter) was 0.2% ± 0.2 in the Chisos Mid-Elevations stratum and 1.4% ± 0.7 in the Chisos High Elevations stratum. The cover of 1,000-hour fuels (woody material >7.62 cm [>3 in] in diameter) was 0.0% ± 0.0 in the Chisos Mid-Elevations stratum and 0.1% ± 0.1 in the Chisos High Elevations stratum. These values do not exceed our management assessment points for 100-hour or 1000-hour fuels. The cover of litter + duff was 39.4% ± 3.8 in the Chisos Mid-Elevations stratum and 49.4% ± 6.9 in the Chisos High Elevations stratum (Figure 8). Both values substantially exceed the fuel loading threshold set out in our management assessment points for litter cover.

Figure 8. Cover of litter + duff by stratum in Big Bend National Park, 2020–2024. The dashed orange line is the management assessment point value (30%). Black dots are values for individual plots, and open diamonds are the stratum means. Thick black lines are the median, and the lower and upper horizontal lines are the 25^th and 75^th percentiles, respectively. Data do not represent the total fuel loading since litter and duff depth are not part of this cover measurement, which only represents surface cover (%).

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The plots in the Chisos High Elevations stratum are exceeding the desired amount of fine fuel cover and ladder fuels, with mean subcanopy woody cover approaching 30% and litter cover approaching 40–50%. Chisos Mid-Elevations stratum sites also have high litter and duff cover (around 35%), but less woody subcanopy structure. Plant litter can support fire spread and a dense woody understory can provide ladder fuels to carry fire into the tree canopies (Figure 9), often leading to more severe fire effects. It should be noted that litter + duff cover in this report is not equivalent to a fuel load calculation but instead indicates the percentage of the ground that had some (albeit potentially shallow) litter or duff cover. Fuel loading calculations will be included in future reports. Nevertheless, the combination of fine fuels and subcanopy ladders fuels—particularly at higher elevations of the Chisos Mountains—suggests the potential for extreme wildfire in these locations.

Figure 9. An example monitoring photo taken at plot V600_022 in the Chisos High Elevations stratum in 2024. Litter cover appears continuous, with low-growing, intertwined tree and shrub branches in the foreground.

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Rare and Sensitive Cacti

There is one management assessment point associated with rare and sensitive cacti:

• Presence of invasive non-native grass in plots with sensitive cacti (Non-Rocky Valley Bottom, Non-Rocky Bajada, Rocky Bajada, Bajada Rock Outcrops, Non-Rocky Foothills, Rocky Foothills strata).

For this assessment point, we looked for any non-native grass present in the same plots as sensitive cacti—with “sensitive” defined as having any park, state, or federal listings ranging from “species of concern” to “endangered.”

• In the Non-Rocky Valley Bottom stratum, 3 of 15 plots had sensitive cacti present, but none of those 3 also had non-native grasses present.

• In the Non-Rocky Bajada stratum, 6 of 20 plots had sensitive cacti present, and only 1 of those 6 plots also had non-native grasses present.

• In the Rocky Bajada stratum, 8 of 12 plots had sensitive cacti present, and only 1 of those 8 plots also had non-native grasses present.

• In the Bajada Rock Outcrops stratum, 4 of 6 plots had sensitive cacti present, and only 1 of those 4 plots also had non-native grasses present.

• In the Non-Rocky Foothills stratum, 3 of 5 plots had sensitive cacti present, and only 1 of those 3 plots also had non-native grasses present.

• In the Rocky Foothills stratum, 1 of 6 plots had sensitive cacti present, and that plot did not have non-native grasses present.

Only 4 plots parkwide had both sensitive cacti and non-native grasses present during this most recent round of sampling. While this is encouraging, in future data analyses, we will include abundance of non-native grasses at these sites with particular attention to any increases in cover that might warrant some focused management action to remove the grasses and restore and protect the cactus habitat.

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Conclusion and Management Recommendations

Terrestrial vegetation monitoring data at Big Bend National Park was assessed in the context of the five highest priority issues: erosion potential, exotic plant dispersal and invasion, fire hazard, forest health, and rare and sensitive cacti. For each issue, data were evaluated against 14 related management assessment points. With a focus on current status rather than trend, this report shows there are no current conditions that warrant immediate management actions. However, there are several issues that should continue to be closely monitored and some that should be given increased attention during subsequent in-depth trend reporting.

Exotic plant presence and cover values (abundance) at sites within the park interior away from major vectors of spread should continue to be monitored and evaluated for any sharp increases in abundance or new occurrences at any single plot. Rapid response to these early warning signs can substantially increase management successes. Figure 4 shows Lehman lovegrass observations are clustered in the northwest-central part of the park, which may be strongly tied to park use patterns, historical management actions, or some combination of these. Park staff are aware of Lehman lovegrass (and buffelgrass) occurrences within the front country and roadway edges, and we will continue to provide insights into dispersal within the backcountry areas.

Erosion potential, as measured by soil stability values, showed half the strata had values at or below 3, indicating moderate to low stability, and half the strata had values above 3, indicating better resistance to erosion. While lower values can be concerning, these values are common in many arid landscapes with low plant cover and low organic inputs. Therefore, it is our intent to further explore this issue during trend reporting by including other indicators like estimated extent of erosion features (such as rills or wind erosion) across each plot or any correlated decreases in plant or biocrust cover.

Continuous fine fuels (leaf litter and small woody material) in the high elevation Chisos Mountains sites may be a cause for concern, especially during particularly dry years with high fire potential. Our data indicated that the Chisos High Elevations stratum plots have a mean subcanopy woody cover approaching 30% and litter cover approaching 40–50%. Collectively, these results indicate the potential for extreme wildfire and severe fire effects in the Chisos Mountains, particularly at high elevations.

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Literature Cited

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Author Information

Sarah E. Studd  ¹ https://orcid.org/0009-0007-1330-2058

J. Andrew Hubbard ¹   https://orcid.org/0000-0002-4223-7730

Cheryl L. McIntyre  ²   https://orcid.org/0000-0001-7554-7586

Jason Mateljak   ²   https://orcid.org/0009-0006-3604-375X

Adam D. Pingatore ¹   https://orcid.org/0009-0000-1117-7475

¹ National Park Service, Sonoran Desert Network
12661 E. Broadway Blvd.
Tucson, AZ 85748

² National Park Service, Chihuahuan Desert Network
3655 Research Dr., Genesis Building D
Las Cruces, NM 88003

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About the National Park Service Science Report Series

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