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Article

Sequoia Groves of Yosemite: Visitor Use and Impact Monitoring

by
Sheri A. Shiflett
1,
Jeffrey S. Jenkins
2,*,
Rachel F. Mattos
1,
Peter C. Ibsen
3 and
Nicole D. Athearn
1
1
Division of Resources Management and Science, Yosemite National Park, 5083 Foresta Rd, El Portal, CA 95318, USA
2
Department of Management of Complex Systems, School of Engineering, University of California, Merced, 5200 Lake Rd, Merced, CA 95343, USA
3
United States Geological Survey, Geoscience and Environmental Change Science Center, Denver, CO 80232, USA
*
Author to whom correspondence should be addressed.
Forests 2024, 15(12), 2256; https://doi.org/10.3390/f15122256
Submission received: 8 November 2024 / Revised: 5 December 2024 / Accepted: 10 December 2024 / Published: 22 December 2024
(This article belongs to the Special Issue Forests and Nature Tourism: Navigating Conservation, Recreation, and Change in the Anthropocene)
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Abstract

:
Despite being long-lived and massive, giant sequoias (Sequoiadendron giganteum (Lindl.) J. Bucholz) are susceptible to erosion given their relatively shallow root structure. Human-caused soil compaction and vegetation loss through social trails are primary drivers of erosion in giant sequoia groves, particularly for trees that are near formal trails and access roads. We develop a method to observe and quantify the near-tree impacts from park visitors and to relate the overall amount of use with ground cover impact parameters to assess whether the desired conditions of each grove are being met for the park to maintain a spectrum of recreational opportunities. We collected data on visitation, ground cover, soil compaction, and social trailing using a combination of targeted surveys and observations at the three giant sequoia groves in Yosemite National Park. The Mariposa Grove receives the most visitation, and use levels among groves were consistent with relative size and facilities available. Selected parameters for ground cover data were analyzed by comparing values within undisturbed versus trampling-disturbed subplots at both 0–2 m and 2–8 m. Exposed soil cover and compaction were generally higher in anthropogenically disturbed subplots versus undisturbed subplots, and vegetation cover was reduced in some disturbed subplots. Each grove had one surveyed tree where average soil compaction was ≥2.2 kg/cm2, which may limit root growth and impact seedling regeneration. Each of the three groves had some trees with social trail presence, yet less than 7% of mature trees within any grove were impacted by social trails, and most social trails were rated as having low impairment. Coupling soil compaction measurements and estimates of trampling-disturbed areas with mapping of social trail conditions within groves provides a general assessment of visitor-associated impacts to sequoia groves and can facilitate a relatively rapid way to track hotspot (i.e., increasingly impacted) trees over time.

1. Introduction

1.1. Sequoia Groves of Yosemite

The natural distribution of the giant sequoia (Sequoiadendron giganteum (Lindl.) J. Bucholz, SEGI) is limited to approximately 65 isolated concentrations, traditionally known as groves, on the west slope of the Sierra Nevada of California [1]. The present disjunct distribution of these groves represents the remnants of what was once a relatively continuous SEGI forest that became dissected by conditions associated with glaciation thousands of years ago [2]. Giant sequoias are known for their massive size and longevity, with life spans reaching from 1500 to up to 3300 years [3]. The most mature and prominent of these trees are often referred to as monarchs. It is these long-lived and often conveniently located representatives of the species that draw much of the attention of visitors and with them associated biophysical and experiential impacts [4]. For instance, tourist trampling around the base of the Grizzly Giant in the Mariposa was at one time so pervasive it was described as “an artificial desert of more than an acre in extent” [5]. Sequoias are part of a broader forest ecosystem, at both the level of the grove and across the landscape, and are managed for a spectrum of biophysical and socially desired conditions that accommodate different levels of access and visitor facilities [6,7,8].
Giant sequoia distribution is influenced by several factors, including surface and ground water (often from snowmelt), topography, elevation, aspect, soil type, relative humidity, and temperature [9]. Giant sequoias have extensive, shallow, lateral root systems that radiate up to 200 feet from the base of the tree, with the majority of roots concentrated in the uppermost thirty to forty-five centimeters of soil [10]. Moreover, the roots have a symbiotic relationship with endomycorrhizal fungi that help transfer nutrients from the soil to the trees [11]. The condition and level of exposure of roots proximate to recreational trails can serve as an indicator of ecological sensitivity and be used to quantify environmental change due to human-caused trampling [12].
The Mariposa Grove of Giant Sequoias (MARI) supports approximately 500 mature giant sequoia trees and is the largest of three giant sequoia groves within Yosemite National Park (YNP). It contains 86 percent of the Park’s mapped adult giant sequoias and is estimated to receive more than one million visitors annually [13]. There are two additional giant sequoia groves at YNP: Tuolumne Grove (TUOL) and Merced Grove (MERC). They contain approximately 40 and 30 mature giant sequoia trees, respectively. These two groves receive far fewer visitors than Mariposa Grove.

1.2. Impacts of Visitation

Formally designed trails rarely provide access to all the locations of interest to visitors, causing them to sometimes create social trails, which are unmaintained and unplanned, visibly discernible pathways [14]. The impact of informal trails (i.e., social trails) on protected resources is much greater than that of sanctioned trails due to the former’s lack of professional design, construction, and maintenance [15]. Informal trails also cause habitat fragmentation, which alters soil moisture regimes, increases movement barriers for soil invertebrates, and reduces habitat quality [16,17,18]. Fragmentation may be further increased by visitors creating duplicative routes that are close in proximity to one another [19]. Higher levels of soil disturbance are associated with greater intensity of use, and slower travel speeds of hikers, generally associated with more crowded conditions, lead to faster soil compaction; thus, higher levels of use accelerate the process of fragmentation [20].
Once established, informal trails can be slow to recover and challenging to conceal due to vegetation disturbance (i.e., trampling and loss). Trampling alters the appearance and composition of trailside vegetation by reducing vegetation height and cover and promoting trampling-resistant species [21,22,23]. Visitors can also unintentionally introduce exotic plant species along trail corridors, which may replace native vegetation [16,24]. In addition, trampling may alter soils by causing soil compaction. In some ecosystems, the relationship between frequency of use and intensity of impact to vegetation and soil occurs rapidly with initial use, whereas in others, more impact can be tolerated before a noticeable degradation of the vegetation and/or soils occurs [25,26,27,28,29].
Soil compaction, which is associated with visitation and trampling, has been shown to limit plant growth by reducing soil macroporosity and restricting gas, water, and root movement, as well as lead to soil erosion [30,31,32,33]. Soil compaction also reduces understory cover and regeneration and may harm endomycorrhizal fungi directly by physical stress or indirectly by hindering vegetation growth. However, not all studies show that root growth is hindered by soil compaction.
Hartesveldt [10] studied visitor impacts on giant sequoia at Yosemite and did not observe a significant correlation between annual growth increments and loss of vegetation cover or soil compaction. Yet, he observed a slight decline in growth that he suggested might lead to significant decline if compaction were not relieved. Busse et al. [34] found that conifer root production in Sierra Nevada forests, which included giant sequoia, was similar in compacted and uncompacted plots 20 years after planting occurred, such that lateral and fine root numbers were comparable throughout the rooting profile for both groups. In contrast, studies on coastal redwood trees (Sequoia sempervirens (D. Don) Endl.), closely related to giant sequoia, have shown that soil compaction results in increased soil density, reduced microporosity and water infiltration, reduced feeder root density, and ultimately reduced ability of redwoods to absorb nutrients and moisture from the soil [5,35,36,37]. Voigt [38] found that tree size (in diameter at breast height, DBH) was significantly positively related to trampling disturbance in multiple coastal redwood groves. Conflicting results of direct impacts of trampling disturbance or soil compaction on long-lived mature trees may be due in part to the short time span in which field studies have been conducted relative to the lifespan of redwood species. Differences may also be due to interspecific responses of S. giganteum compared to S. sempervirens.
The potential for visitor impacts to be injurious to the scenery with higher levels of visitation was first anticipated by Olmsted in his Yosemite Report of 1865 in reference to the Mariposa Grove [39]. In the decade prior, from 1855 to 1863, hotelier James Hutchings recorded 653 visitors to the park, and from 1864 to 1870, use increased to 4936 visitors [40]. During this time at the beginning of the Anthropocene, Yosemite became popular with those wanting to escape the urban blight associated with the industrial revolution. Tourists sought out the sublime; a feeling of natural grandeur that obfuscated the role of people from inhabiting or actively maintaining the landscape [41]. As the effects of industrial society on people have increased, so too has demand for access to naturalized areas perceived to be beyond the impact of human influence. The visitation growth of today is in part the product of broader anthropogenic discourses and human-nature binaries reinforced through marketing the recreational sublime [42]. YNP visitation growth is reflective of a similar trajectory of growth to that of protected areas worldwide, who must accommodate the increased use that comes with population growth, economic mobility, and development pressures. This increase in use presents additional pressure to forest systems already facing anthropogenic climate impacts and potential hazards, like with more common drought conditions, tree mortality, and increased wildland fire risk, in what has been termed the Pyrocene [43]. Old-growth tree species tend to owe their success to previously stable climatic conditions that have contributed to their recruitment and longevity but are otherwise sensitive to intense and prolonged changes in environmental conditions in addition to the reoccurring impacts of human use [44].
In 1906, when annual park use was first officially counted, it reached 5414 visitors. By 1922 annual use reached 100,506, and by 1940 use first exceeded half a million with 506,781 visitors [45]. In 1954, annual use passed one million, and by 1956, the National Park Service approved the first ecological study of MARI as part of a multi-park initiative to collect scientific data, assess impacts, and inform management actions [10]. By 1967, annual park use surpassed 2 million visitors, and growing concerns about traffic congestion and visitation impacts led to the decision in 1970 to prohibit private vehicles in the upper MARI area and at Wawona Point [13]. By 1980, the National Park Service recognized the need to reduce infrastructure and traffic within the Mariposa Grove, control the intensity of visitor use, and reestablish a more natural fire regime in this rare ecosystem, addressing these issues in the General Management Plan [46]. Annual use surpassed 3 million total park visitors in 1987 and 4 million visitors in 1996. In 2013, Yosemite NP developed an Environmental Impact Statement for restoration of the MARI to further address these issues for the purpose of increasing the resiliency of the grove and improving visitor experience [13]. By 2016, with the centennial of the National Park Service, parkwide use surpassed 5 million visitors, with approximately one million visitors to MARI. Currently, formulation of a Visitor Access Management Plan (VAMP) is underway at YNP, which will address targets for visitor use within each of the giant sequoia groves [47]. Broader goals of this plan include reducing overcrowding and avoiding resource impairment thresholds. Protecting the trees within the groves for their natural significance is one of the key management objectives. The VAMP entered public review under a National Environmental Policy Act Environmental Assessment in August 2024 [47].

1.3. Desired Conditions

Desired conditions are being developed for the three groves such that there is a spectrum of accommodating the most visitors at MARI and the least at MERC. The groves provide visitors with a range of experiences, from more accessible services at MARI to undeveloped and relatively remote experiences at MERC [47]. Draft desired conditions were modified from previous planning documents [13,46] to highlight commonalities and also distinctions among the groves applicable to how they should be managed and are expected to be finalized with completion of the Yosemite VAMP. Grove-specific desired conditions are as follows:

1.3.1. Mariposa Grove (MARI)

  • The primary location for visitor enjoyment and interpretation of the giant sequoia, providing the most accessible experience and accommodating the highest levels of use among the three groves. Visitor facilities are consistent with the preservation of the unique ecosystem, and any facilities not necessary for visitor enjoyment of the resource are removed.

1.3.2. Tuolumne Grove (TUOL)

  • Visitors can experience relative quiet and levels of low-to-moderate visitation while having access to basic amenities such as restrooms, paved parking, and a walking trail to and through the grove. Visitors can learn about the grove through interpretive signage and basic orientation panels located at the parking area and throughout the walk into the grove.

1.3.3. Merced Grove (MERC)

  • The least-developed grove provides enhanced opportunities for quiet where natural sounds dominate, and visitors can hear the breeze rustling through the foliage or the sound of a woodpecker calling or tapping against a tree. Minimal facilities are available.
Sequoia Grove desired conditions provided a basis for this pilot study to assess if visitor use and impacts from visitation are consistent with these desired conditions and to also provide information for the purposes of informing the VAMP process. Setting desired conditions is important to maintain levels of use and mitigate impacts to avoid impairment to physical settings and visitor experience. Indicators such as those analyzed here related to soil compaction, ground cover, and the number of users allow management to track changes over time and act if impacts fall beyond a certain threshold. The use of a rapid method to track visitor impacts is especially relevant for forest systems in the Anthropocene, where change can unfold rapidly, and if not mitigated, small but cumulative impacts can impair resources and be long-lasting.

2. Materials and Methods

In each of the three giant sequoia groves (MARI, TUOL, and MERC) at Yosemite NP (Figure 1), data were collected in 2023 regarding visitor use (e.g., trail counter estimates) and visitor impacts (e.g., ground cover effects, social trailing). Soundscape data were not collected for this study.

2.1. Trail Counter Estimates

To document visitor use, TRAFx (Canmore, AB, Canada) infrared trail counters were deployed within each grove. Counters were set to collect data of hourly totals (period = 001) with a 1.0 or 1.25 s delay between counts (delay = 020 or 025). Trail counters were placed June–July and collected data from 6/14 to 9/30 in MARI, 6/21–7/5 in TUOL, and 7/5–7/10 in MERC. The trail counter in TUOL experienced a malfunction after 7/5, while MERC closed to the public on 7/10, accounting for the shortened data collection periods in these groves relative to MARI. Trail counter data were analyzed using an ANOVA to compare all data for hourly trail usage at each grove. R software (ver 4.4.0) was used for statistical analysis.

2.2. Ground Cover and Compaction

Data for ground cover types and compaction were collected by modifying methods of Voigt [38]. Five trees were selected per grove to primarily include mature sequoia trees, which likely had some amount of visitor-related ground impacts based on cursory visual inspection. Trees were selected that were adjacent to the main trail or road in each grove. Thirteen of fifteen trees were within 16 m of an official trail or road, while two trees were located within 28 m of the trail or road. For each selected tree, a circular plot was laid out following the plot design of Voigt (2016), who evaluated the impacts of social trails on old-growth redwood groves. Plot lines extended 2 m and 8 m from the base of the tree in each cardinal direction (N, E, S, and W) (Figure 2). Measurements for ground cover were collected within each of the 8 subplots: 0–2 NW, NE, SE, and SW and 0–8 NW, NE, SE, and SW. Percent cover was estimated for organic cover elements (e.g., exposed roots, organic litter, vegetation cover), other elements (e.g., established trail/road, rocks, streams), and social trail/trampling cover (e.g., disturbed area attributed to human foot traffic). Organic cover elements and other elements included mutually exclusive categories that added up to 100%, and social trail cover also included additional mutually exclusive categories that added up to 100%. Descriptions of each percent cover category are documented in Table 1.
An AMS pocket soil penetrometer (American Falls, ID, USA) was used to measure soil compaction. This device is a simple, lightweight, handheld device that provides instant estimates of unconfined compressive strength of cohesive soils. The hand penetrometer provides readings ranging from 0 to 4.5 kg/cm2. Three measurements were taken randomly per subplot, and for each sample, the litter/duff layer was removed.
One of two researchers was always present for percent cover and soil compaction measurements to ensure consistency of estimation, and both of these researchers worked together for 2/3 of the plots.
Selected parameters for ground cover data (e.g., exposed soil cover, vegetation cover) and soil compaction were analyzed by comparing values within trampling undisturbed versus trampling disturbed subplots at both 0–2 m and 2–8 m. For MARI and MERC, plots were considered disturbed if there was >5% cover of trampling disturbed area within the subplot, whereas for TUOL, subplots were considered disturbed if there was >2.5% trampling disturbed area within the subplot. There were too few disturbed plots for statistical comparison in TUOL when using the threshold of greater than 5% anthropogenically disturbed area. A minimum of 5% disturbance was preferred for comparison to reduce the likelihood of disturbance being erroneously attributed to anthropogenic causes. T-tests were used to compare data between subplots for each parameter. Soil compaction data were also averaged using all soil compaction data collected around each tree.

2.3. Social Trail Extent and Condition Assessment

The extent of social trails was mapped for each plot by using a blank copy of the plot design (showing cardinal direction and 2 m and 8 m plots) and hand-drawing social trails in the field after plots were flagged and visually inspected. Social trail widths were measured initially and at points of obvious change per trail. The social trail was assigned a condition according to condition class descriptions presented in Table 2. If multiple conditions were present, this was marked on the drawing. For instance, in some cases, adjacent to a tree in a 0–2 m subplot, a trail might present a condition of 1, whereas in a 2–8 m plot, the condition changed to 2. A condition class of 0.5 was included to detect areas that may not currently be social trails but are areas that appear to be disturbed by human activities and might be targeted for management actions if conditions worsen.
Drawings were digitized around point locations of selected sequoias using ArcGIS Pro (ver. 3.2.2, ESRI). The total area associated with each trail condition class within each grove was computed using the ArcGIS summary function. Data were extracted within the boundaries of each 8 m radius plot around each of the five trees per grove. These values were compared to the total area sampled (5 plots × 200 m2 each). Average social trail widths were determined for social trails within each grove and limited to condition classes 1–3, given that condition classes of 0.5 and 5 have no boundaries to determine trail tread, and a condition class 4 was not observed in any grove. Social trail widths were compared using a Kruskal–Wallis test due to unequal sample sizes. R software was used for statistical analysis.
As slope and grade of trail are known to affect conditions of trails [51], topographic and spatial metrics of each sampled social trail in relation to trail condition were analyzed. Topographic metrics were determined by first deriving the elevation using a 1 m USGS Digital Elevation Model [52], then deriving the maximum, minimum, and mean slopes of the trail using the Slope Tool in ArcGIS Pro. Length and area of each social trail were calculated from the polygons. Collected data demonstrated unequal variance in each category of trail conditions; therefore, analyzed variable influences on trail conditions were analyzed with Kruskal–Wallis tests.
In addition to digitized drawings of social trail cover for select sequoia trees, data about the presence or absence of social trails were also collected throughout each grove during annual sequoia grove inventory monitoring. Every year, SEGI inventory work is performed by trained and typically repeat volunteers, where data are collected about life history traits and the condition of the trees in each of the three groves. In 2023, volunteers were asked to add the questions below to their geospatial data collection protocol for Yosemite’s sequoia groves:
  • Is there evidence of social trailing w/in ~8 m of the tree base (i.e., does there appear to be a footpath leading to the tree and/or a bare area due to visitor activity)?—Y/N;
  • Does it have clear/unclear boundaries (i.e., is it faint or well-defined)?—Faint/well-defined;
  • Is any portion of the social trail greater than 0.5 m wide?—Y/N.
These data were collected and compiled in ArcGIS Pro. Data were compared to the data collected for five selected trees per grove to estimate how many trees have been impacted by visitor use per grove.

3. Results

3.1. Visitor Use, Volume, and Timing

Trail counter data showed the highest usage in MARI, followed by TUOL, and lastly MERC (Figure 3A). Around 12:30 PM, usage in the MARI peaked at approximately 200 visitors per hour. Total daily trail counter passes were typically around 2000, with some days approaching 3000, whereas in TUOL, daily counter passes were <1200 maximum, with several days at 900 or more, and in MERC, daily visitation was <150 maximum trail counter passes, with a couple of days <50 passes (Figure 3B). Moreover, weekend and weekday visitation were similar in each grove, with the exception that on weekends from the late morning through the afternoon there is slightly higher visitation in TUOL and slightly lower visitation in MARI as compared with each during weekdays (Figure 3C). Fridays were included in the analysis of weekdays, which may have lowered weekend averages, however. It should also be noted that the trail counter in MARI is likely to represent ~2× or 1.5 times the number of visitors because it was placed on a loop trail with multiple entrances/exits, whereas both the TUOL and MERC counters were placed on out-and-back trails, where the counts should theoretically represent ~2× the number of visitors (Figure 3). We repeated the analysis with Fridays being considered as a weekday as well, and the only statistical variation was an additional significant difference between weekday and weekend hourly trail usage at 10 AM in MERC.

3.2. Ground Cover and Compaction

Some differences between undisturbed and human-disturbed plots were observed for percent cover of exposed soil and vegetation (Figure 4A,B). It generally seemed as though exposed soil cover was greater in disturbed plots compared to undisturbed plots, but the only significant difference was observed in the 8 m subplots in MERC. Vegetation cover appeared to decrease for four out of six comparisons between trampling undisturbed versus trampling disturbed plots, yet the only significant comparisons were in 2 m plots in TUOL and MERC.
Soil compaction tended to be higher in anthropogenically disturbed relative to undisturbed plots and was significantly higher for all subplot categories and groves except for 8 m subplots in TUOL (Figure 4C). Each grove also had one tree where average soil compaction was ≥2.2 kg/cm2 (Figure 5).

3.3. Social Trail Extent and Condition Assessment

For MARI and MERC, the majority of social trail-impacted areas was associated with trail condition five (i.e., widespread compaction, with no discernible trail margins), followed by two (i.e., social trail obvious, but likely not recently used), and one (i.e., social trail just discernible), respectively (Table 3). For TUOL, no social trails were observed with a class greater than 1, and the majority of impacted area was classified as a 1 (Table 3, Figure 6). For all three groves, <10% of the total area mapped was associated with social trail impacts. Average social trail width across trail condition classes 1–3 in MARI was 0.6 ± 0.2 m compared to 0.8 ± 0.5 in TUOL and 0.5 ± 0.2 m in MERC. There was no difference in trail width across condition classes (p = 0.3). Slope, trail length, or trail area did not significantly affect trail conditions (p > 0.05). Upon full ground survey, it was also discovered there were two selected trees that did not have anthropogenic impacts (MARI 199 and TUOL 176).
MARI had the most impacted trees (25), followed by MERC and TUOL (Figure 7). Analysis of SEGI volunteer–inventory data revealed MARI also had the most trees with social trails that were deemed as well-defined and with a portion exceeding 0.5 m width. However, proportionally, more mature trees were impacted in MERC (6%) relative to TUOL (3%) or MARI (4%). When considering all sequoia trees within a grove, less than 7% of the trees within a grove were impacted by social trails, regardless of grove.

4. Discussion

Trail counter data provided a relatively low-effort way to assess visitor use within groves. Trail counter data confirmed aspects of the desired conditions, such that MARI accommodates the highest levels of visitor use and MERC, the least. MARI was the most visited of the three groves, and it also appeared to have the greatest number of trees impacted by visitation, with more trees with well-defined and wider social trails. However, when considering the total number of mature trees within MARI, this grove has the fewest impacts on mature trees proportionally, relative to the other two groves. Overall, few trees were impacted by social trailing in each grove, and in TUOL, impacts were limited to the least impacted categories of condition class 1 or less, as indicated by a combination of field assessment by researchers and SEGI-trained volunteers.
Although trail counters data were only collected for 5 days in MERC compared to 109 days in MARI, the amount of visitation was consistent with observations by Yosemite NP rangers in this grove. Moreover, the parking lot only has space for 10 passenger vehicles at a time, with 6 unmarked overflow spots, compared to 58 spaces (20 of which accommodate buses) at TUOL and 300 parking spaces at the MARI welcome plaza. While it is unlikely that this MERC receives more than 100 visitors per day during the busiest times of the year, the park has set the capacity of this grove at 200 visitors per day [47]. The closing of MERC early in the season (July 10) may have influenced some visitors to go to TUOL instead, as these two groves are close in proximity (~7 km apart), so it is unlikely that visitors who could not access MERC would have been rerouted to MARI, as this grove is ~71 km from MERC. In either case, the number of visitors rerouted would have been minimal and not likely to substantially alter the visitation at either of the other two groves. If MARI closed, however, especially during peak season in the summer, this would likely place a strain on the facilities at the other two groves. Continued data collection with trail counters is suggested to further document these dynamics, determine if desired conditions are being met, and understand the relationship between the number of visitors and the extent of recreation impacts within each grove.
Monitoring ground cover provided information related to anthropogenic impacts. For instance, exposed soil cover tended to be higher in anthropogenically disturbed (i.e., trampling) subplots versus undisturbed subplots, while vegetation cover was reduced in some disturbed subplots relative to undisturbed subplots. Of these metrics, exposed soil cover seems to be a more responsive indicator of anthropogenic impacts. One consideration for ground cover types is that sequoia groves occur on west and southwest aspects [53], which have been documented as 21 to 33% less productive, respectively, than north and east aspects in forest studies from other ecosystems [54]. If similar growth phenomena with aspect occur in giant sequoia groves, then the background amount of vegetation cover in giant sequoia groves may be less than if they occurred on north- and east-facing slopes, potentially creating fewer obstacles and more openings in the understory, which may be inviting to visitors. This may also explain the limited coupling of vegetation cover with disturbed areas. It should also be noted that data were only collected at one timepoint during one season for a small number of trees, which limits conclusions that may be drawn about the utility of ground cover types. These data were the most time-intensive to collect of that collected during this study. If repeated, ground cover monitoring consisting of fewer variables measured (e.g., exposed roots, exposed soil, vegetation cover, and trampling disturbed area) at multiple timepoints in a season could alleviate some of the time burden while providing managers with useful information for anthropogenic impact assessment.
Soil compaction differed between almost all trampling undisturbed versus disturbed plots for all three groves, indicating that this may be a better rapid indicator of visitor impacts to trees than ground cover [55]. Moreover, assessing average soil compaction around trees selected for monitoring may give a rapid snapshot of not only which trees exhibit higher impacts but also changes over time, if repeatedly measured [48]. Vaquero [56] reported that soil compaction values lower than 2.0 kg/cm2 were suitable for crop root growth, values higher than 2.0 kg/cm2 limited root growth, and bulk root density was drastically reduced in areas with high compaction (>2.5 kg/cm2). A USDA research paper from 2004 quantified soil compaction on 16 compacted trails (i.e., trails with absence of vegetation) in Cache National Forest, UT, and adjacent undisturbed soils and found that compression strength of compacted soils was 4.4 ± 0.4 vs. 1.0 ± 0.2 kg/cm2 for undisturbed soils [57]. In a soil compaction study on rangeland plots, Gifford et al. [58] created four compaction treatments, each receiving a differing number of passes with a gasoline-driven soil tamper. Their control, light, medium, and heavy treatments were 0.83, 1.87, 2.59, and 3.11 kg/cm2, respectively. Most trees monitored during this pilot study had low soil compaction, yet a few trees showed light to medium soil compaction per the soil compaction classification scheme of Gifford et al. [58]. There was also one tree per grove where average soil compaction exceeded the 2.0 kg/cm2 threshold.
Increased soil compaction around sequoia trees poses a management concern given the suite of associated ecosystem impairments (e.g., decreased water filtration, increased runoff and erosion, reduced nutrient absorption, inhibition of plant germination and growth). Although the scientific literature provides conflicting, though limited, evidence of direct impacts of trampling disturbance or soil compaction on long-lived trees, such as coastal redwoods and giant sequoias, indirect impacts may also be important to consider for giant sequoia grove management. For instance, seed germination and thus, regeneration of giant sequoia may be negatively impacted by soil compaction, especially during times of drought when water availability is critical for seedling establishment amid reduced water infiltration capacity [59]. Unlike coastal redwoods, giant sequoias do not sprout from roots or stumps, and seed germination and seedling survival are critical to the regeneration of the species. Giant sequoia seeds germinate most readily in moist soil approximately 1 cm below the surface [60].
Giant sequoias are also known to form associations with arbuscular mycorrhizal fungi [61], which may be impaired by increased soil compaction [62], though these associations may also help to alleviate other negative impacts of soil compaction, such as reduced nutrient uptake [63]. Recent work has shown that bacterial/archaeal richness was greater under giant sequoia than sugar pine, with a core microbial community nearly double the size and likely related to increased soil moisture under giant sequoia [11]. Recovery of organic litter and soil functions, including plant-mycorrhizal associations and communities, to pre-disturbance levels can take years, and in many cases decades [62,64]. Future work could include measuring soil microbial activity at anthropogenically disturbed and undisturbed subplots around giant sequoias to determine if soil compaction limits microbial processes.
Social trail mapping data are valuable for determining the extent of social trails and the condition of impacts and can be updated to track changes over time [38]. During this pilot study, drawings were created in the field and digitized later. Future work should incorporate social trail mapping updates and/or add additional trees for monitoring via tablets using a program such as ArcGIS Field Maps to increase data collection efficiency and for assessment of trail extent and condition changes over time. Additionally, pairing social trail condition maps with average soil compaction provides a general assessment of visitor-associated impacts to a given tree and could provide a relatively rapid way to track hotspot trees over time, as well as to compare to unimpacted tracked trees.
SEGI inventory volunteer observations represent a large value added for gathering quick information about an entire grove that not only informs about the potential condition of all trees within a grove but can aid resource managers in targeting trees for future data collection, especially in cases where volunteers identify trees with new impacts or heavily impacted trees that were missed during initial sampling. It was a low time investment to leverage the efforts of motivated volunteers who had already been trained to add a few qualitative and semi-quantitative questions to their planned sampling [61]. Including trained volunteers in forestry research has been known to expand the spatial and temporal reach of research questions, allowing results to have broader management impacts [65]. Continuing to collect these data can inform managers of changing impact conditions within the groves and where to take management actions in response. These data should be interpreted with a recognition that observationally collected data are a best available tally and that the full extent of visitor impacts is difficult to assess [66]. The total impacts presented in this study are likely undercounted relative to the actual extent of human disturbance.

5. Conclusions

Rapid condition assessments are useful tools for land managers who typically lack time, financial, and/or staff resources for monitoring and assessment [67]. Mapping social trail impacts around selected trees on an annual or semi-annual basis is a rapid way to assess visitor impacts to the groves. Rapid condition assessments are a useful and essential approach and provide relative ease of tracking these types of data over time for land managers who are often strapped for both time and financial resources. Trained community scientist observations can amplify the spatial and temporal reach of social trail impact data and inform managers of changing conditions. Pairing social trail impact data with soil compaction data and estimates of percent trampling disturbed area within subplots enhances the utility of the data and yields a quick assessment of the condition around a tree. Denoting which trees and subplots include soil compaction values greater than 2.0 kg/cm2 aids in determining the severity of impacts and locations to target for corrective management actions. Intervention could include the installation of temporary signage, indicating that restoration is in progress, and/or fencing, symbolic or permanent.
Data were not collected about signage or fencing within the groves, though future efforts could document which trees have signage nearby and/or fencing and the social trail extent and condition at nearby trees. TUOL had several signs asking visitors not to trample the soil and mentioning the impact on roots. This grove also had the least observed impacts (e.g., fewest trees impacted, fewest social trails > 0.5 m width, no social trails in condition class > 1, least total area impacted, etc.). An informational sign was observed in MARI within the first ~0.5 mi of the trailhead, but there may have been fewer signs at the base of trees compared to TUOL. Fencing, symbolic or obtrusive, also appeared to keep people on the trail, but there were areas where fencing was broken, which is likely inviting for visitors.
Further, analysis of soundscape data within each grove is another important facet of assessing if desired conditions are being met. Two of the three grove-specific desired conditions reference sound levels within the grove. Assessing soundscape should include how acceptability of noise conditions may vary between groves based on different trip motivations and soundscape expectations associated with each [68]. It is suggested that soundscape data are coupled with trail counter data to not only assess if soundscape-based grove desired conditions are met but also to determine if there are certain times of the season, week, or day that visitors have increased impacts on natural grove soundscapes.
We envision the methodological framework presented here and the citizen science approach being adopted by other parks and agencies that manage giant sequoia groves, or similar systems, from those that are experiencing persistently high use and impacts to those that receive less use and where baseline conditions can be assessed as indicators for future environmental change. This is especially important in the Anthropocene with persistently high levels of visitation to parks and other public and privately conserved land that pose impacts to SEGI groves along with climatic vulnerabilities such as drought and fire that threaten long-term species survival. Monitoring and management actions for long-lived tree species and the constituencies that carry out these activities must necessarily be intergenerational efforts for the species that humans value to survive through the Anthropocene and in perpetuity [69].

Author Contributions

Conceptualization, S.A.S., R.F.M. and N.D.A.; data curation, S.A.S. and R.F.M.; methodology, S.A.S.; formal analysis, S.A.S., J.S.J. and P.C.I.; writing—original draft preparation, S.A.S. and J.S.J.; writing—review and editing, S.A.S., J.S.J., R.F.M., P.C.I. and N.D.A. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Data were collected on behalf of the National Park Service for park planning purposes and are available upon public request.

Acknowledgments

Trevor Denson, Alexis Klopack, and Kyle Ricio assisted with fieldwork. The authors thank Kane Russell and SEGI inventory volunteers for providing additional field data on social trails during their annual sequoia inventory monitoring. The authors also thank Garrett Dickman for discussions about impacts to giant sequoia and previous studies and Trevor Denson for providing GIS support and insight about similar methodology in coastal redwoods. We give special thanks to the Physical Science and Landscape Ecology Branch at Yosemite National Park for logistical support. Thank you to the UCNRS Yosemite Field Station for their support with scholarly coordination and park resources (doi:10.21973/N3V36C).

Conflicts of Interest

The authors declare no conflicts of interest. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

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Figure 1. Location of giant sequoia (Sequoiadendron giganteum (Lindl.) J. Bucholz) groves within Yosemite National Park (top left), with inset maps of MARI (bottom left), TUOL (top right), and MERC (bottom right), which each include names and locations of trees sampled and formal trails within the groves.
Figure 1. Location of giant sequoia (Sequoiadendron giganteum (Lindl.) J. Bucholz) groves within Yosemite National Park (top left), with inset maps of MARI (bottom left), TUOL (top right), and MERC (bottom right), which each include names and locations of trees sampled and formal trails within the groves.
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Figure 2. Plot design for 2 m and 8 m radius tree plots with four subplots at each radius.
Figure 2. Plot design for 2 m and 8 m radius tree plots with four subplots at each radius.
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Figure 3. (A) Trail counter results showing hourly trail use within each giant sequoia (Sequoiadendron giganteum (Lindl.) J. Bucholz) grove at Yosemite National Park. All three groves (MARI, TUOL, MERC) had significantly different trail usage (p < 0.5). (B) Trail counter results showing daily total trail usage within each grove for weekdays (blue) and weekends (red, Fri–Sun). (C) Trail counter results showing hourly average trail use within each grove for weekends compared to weekdays. Points are means ± 1 SD. Significant differences between weekday and weekend use are denoted by an *.
Figure 3. (A) Trail counter results showing hourly trail use within each giant sequoia (Sequoiadendron giganteum (Lindl.) J. Bucholz) grove at Yosemite National Park. All three groves (MARI, TUOL, MERC) had significantly different trail usage (p < 0.5). (B) Trail counter results showing daily total trail usage within each grove for weekdays (blue) and weekends (red, Fri–Sun). (C) Trail counter results showing hourly average trail use within each grove for weekends compared to weekdays. Points are means ± 1 SD. Significant differences between weekday and weekend use are denoted by an *.
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Figure 4. Exposed soil percent cover data (A), vegetation percent cover (B), and soil compaction (C) for trampling undisturbed and trampling disturbed plots at 2 m and 8 m distances ± 1SD within giant sequoia (Sequoiadendron giganteum (Lindl.) J. Bucholz) groves (MARI, TUOL, MERC). For MARI and MERC, plots were considered disturbed if anthropogenic disturbance percent cover was estimated at 5% or greater, whereas in TUOL, plots were considered disturbed if anthropogenic disturbance percent cover was estimated at 2.5% or greater. There were too few disturbed plots in TUOL when using the threshold of greater than 5% anthropogenically disturbed area. Significant differences between pairs (i.e., 2 m or 8 m) are represented by * such that * < 0.05, ** ≤ 0.01, *** ≤ 0.001.
Figure 4. Exposed soil percent cover data (A), vegetation percent cover (B), and soil compaction (C) for trampling undisturbed and trampling disturbed plots at 2 m and 8 m distances ± 1SD within giant sequoia (Sequoiadendron giganteum (Lindl.) J. Bucholz) groves (MARI, TUOL, MERC). For MARI and MERC, plots were considered disturbed if anthropogenic disturbance percent cover was estimated at 5% or greater, whereas in TUOL, plots were considered disturbed if anthropogenic disturbance percent cover was estimated at 2.5% or greater. There were too few disturbed plots in TUOL when using the threshold of greater than 5% anthropogenically disturbed area. Significant differences between pairs (i.e., 2 m or 8 m) are represented by * such that * < 0.05, ** ≤ 0.01, *** ≤ 0.001.
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Figure 5. Average soil compaction ± 1 SD (kg/cm2) across all 8 subplots around a given tree within each giant sequoia (Sequoiadendron giganteum (Lindl.) J. Bucholz) grove (MARI, TUOL, MERC, n = 24).
Figure 5. Average soil compaction ± 1 SD (kg/cm2) across all 8 subplots around a given tree within each giant sequoia (Sequoiadendron giganteum (Lindl.) J. Bucholz) grove (MARI, TUOL, MERC, n = 24).
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Figure 6. Mapped extent of social trails around five selected trees within giant sequoia (Sequoiadendron giganteum (Lindl.) J. Bucholz) groves (A) MARI, (B) TUOL, and (C) MERC. Digitized drawings include the name of the tree, social trail condition (Table 2), average soil compaction, and also show formal trail locations mapped by researchers (black outline) and as an existing National Park Service base layer (brown lines). N.B. The inner ring around each tree occurs at 2 m distance, whereas the outer ring occurs at 8 m distance. Spatial arrangement of individual trees in this figure has no bearing on physical distances among them, with the exception of MERC 30 and MERC 34 (Figure 1).
Figure 6. Mapped extent of social trails around five selected trees within giant sequoia (Sequoiadendron giganteum (Lindl.) J. Bucholz) groves (A) MARI, (B) TUOL, and (C) MERC. Digitized drawings include the name of the tree, social trail condition (Table 2), average soil compaction, and also show formal trail locations mapped by researchers (black outline) and as an existing National Park Service base layer (brown lines). N.B. The inner ring around each tree occurs at 2 m distance, whereas the outer ring occurs at 8 m distance. Spatial arrangement of individual trees in this figure has no bearing on physical distances among them, with the exception of MERC 30 and MERC 34 (Figure 1).
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Figure 7. Number of trees impacted by social trails within each giant sequoia (Sequoiadendron giganteum (Lindl.) J. Bucholz, SEGI) grove, including the total number impacted, those with a well-defined social trail (ST), and those where the width of the ST was greater than 0.5 m at any point. These data are an aggregate of sampling conducted by National Park Service staff of five trees per grove and data collected by SEGI inventory volunteers. The percentage of mature trees impacted by social trails within each grove is displayed to the right of the dashed line.
Figure 7. Number of trees impacted by social trails within each giant sequoia (Sequoiadendron giganteum (Lindl.) J. Bucholz, SEGI) grove, including the total number impacted, those with a well-defined social trail (ST), and those where the width of the ST was greater than 0.5 m at any point. These data are an aggregate of sampling conducted by National Park Service staff of five trees per grove and data collected by SEGI inventory volunteers. The percentage of mature trees impacted by social trails within each grove is displayed to the right of the dashed line.
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Table 1. Description of elements for percent cover used in this study. Modified from Voigt [38].
Table 1. Description of elements for percent cover used in this study. Modified from Voigt [38].
Mutually Exclusive Set of Categories, Which Total 100%Description
Organic Cover ElementsExposed roots, burl, or woody debrisRoots or burl with damaged or removed bark. Woody debris damaged by trampling.
Exposed soilLitter layer is completely removed. Depending on the depth of the O-horizon, either black, well-decomposed soil or mineral soil is visible.
Organic litterDead plant material, e.g., twigs, bark, needles, and leaves, that have fallen to the ground and have not yet been incorporated into the decomposed top humus layer. N.B. Litter that has been pulverized by trampling is not included and is classified as bare soil. Additionally, twigs > 5 cm diameter, bark > 5 cm length or width, and all pinecones are included in woody debris estimates.
Woody and organic debrisWoody material, slash and debris, fallen dead trees, and the remains of branches on the ground (>5 cm diameter). See the above category for more details.
VegetationIncluding trees, shrubs, ferns, forbs, and graminoids. For canopy vegetation (i.e., trees), cover was limited to the ground footprint of the mainstem(s), whereas for understory and forest floor vegetation, cover estimates included foliage.
Other Cover ElementsEstablished trail/roadIncludes dirt and paved surfaces, marked and used as official trails and/or roads.
RocksIncludes masses of mineral matter > 5 cm length or width.
StreamChannel with water present.
StructureFootprint of physical structure.
Additional set of categories, which total 100%
Social Trail
Cover		Disturbed area (social trail(s)/trampling disturbance present)Area affected by trampling and/or covered with social trails.
Undisturbed area (social trails absent)Area without visible trampling disturbance and without discernible social trails.
Table 2. Comparison of trail condition class descriptions for Marion and Leung [48], Leung et al. [49], and Cole et al. [50], and this study of giant sequoia (Sequoiadendron giganteum (Lindl.) J. Bucholz) groves in Yosemite National Park. Table modified from Voigt [38].
Table 2. Comparison of trail condition class descriptions for Marion and Leung [48], Leung et al. [49], and Cole et al. [50], and this study of giant sequoia (Sequoiadendron giganteum (Lindl.) J. Bucholz) groves in Yosemite National Park. Table modified from Voigt [38].
Condition ClassMarion and Leung [48], Assessment of Informal TrailsLeung et al. [49], Social Trails, and Cole et al. [50], High-Use Wilderness AreasSequoia Groves of Yosemite NP
0.5 Trail not obvious, though the area appears as if disturbed by human foot traffic, slight loss of vegetation cover and/or minimal disturbance of organic litter.
1Trail distinguishable; slight loss of vegetation cover and/or minimal disturbance of organic litter.Trails are disturbed but not well established. They retain at least 20% of vegetation cover on the treads. The boundaries between trail treads and off-trail areas are often unclear.Social trail(s) (just) distinguishable, the boundaries between trail treads and off-trail areas are often unclear; slight loss of vegetation cover and minimal disturbance of organic litter
2Trail obvious; vegetation cover lost and/or organic litter pulverized in primary use areas.Trails are well established. They retain less than 20% of vegetation cover on the treads. These trails are less than 0.3 m wide. The boundaries between trail treads and off-trail areas are often discernible.Social trail obvious, but maybe not used recently; on-trail vegetation cover lost and/or litter diminished in primary use areas.
3Vegetation cover lost and/or organic litter pulverized within the center of the tread, some bare soil exposed.Trails are well established. They retain less than 20% of vegetation cover on the treads and are between 0.3 and 0.6 m wide. The boundaries between trail treads and off-trail areas are usually discernible.Social trail obvious and well used, vegetation cover lost and/or organic litter pulverized within the center of the tread, some bare soil exposed.
4Nearly complete or total loss of vegetation cover and organic litter within the tread, with bare soil widespread.Trails are well established. They retain less than 20% of vegetation cover on the treads and are more than 0.6 m wide. The boundaries between trail treads and off-trail areas are usually discernible.Social trail(s) are hard to distinguish from formal trails (i.e., have a similar appearance). Nearly complete or total loss of vegetation cover and organic litter pulverized within the tread, with bare soil widespread.
5Soil erosion is obvious, as indicated by exposed roots and rocks and/or gullying. Disturbance spread over a large area, with no boundaries to identify trail tread, nearly complete or total loss of vegetation cover, and organic litter pulverized in the whole area, bare soil widespread
Table 3. Area (m2) within each social trail condition class (see Table 2) in each of the three giant sequoia (Sequoiadendron giganteum (Lindl.) J. Bucholz) groves. The total area is presented, and the percentage of the total area sampled within each grove (i.e., out of ~1000 m2 per grove).
Table 3. Area (m2) within each social trail condition class (see Table 2) in each of the three giant sequoia (Sequoiadendron giganteum (Lindl.) J. Bucholz) groves. The total area is presented, and the percentage of the total area sampled within each grove (i.e., out of ~1000 m2 per grove).
Trail Condition Class
Grove0.512345Total Area (m2)% of Area Sampled
MARI01193041646
TUOL6170000232
MERC02263050818
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Shiflett, S.A.; Jenkins, J.S.; Mattos, R.F.; Ibsen, P.C.; Athearn, N.D. Sequoia Groves of Yosemite: Visitor Use and Impact Monitoring. Forests 2024, 15, 2256. https://doi.org/10.3390/f15122256

AMA Style

Shiflett SA, Jenkins JS, Mattos RF, Ibsen PC, Athearn ND. Sequoia Groves of Yosemite: Visitor Use and Impact Monitoring. Forests. 2024; 15(12):2256. https://doi.org/10.3390/f15122256

Chicago/Turabian Style

Shiflett, Sheri A., Jeffrey S. Jenkins, Rachel F. Mattos, Peter C. Ibsen, and Nicole D. Athearn. 2024. "Sequoia Groves of Yosemite: Visitor Use and Impact Monitoring" Forests 15, no. 12: 2256. https://doi.org/10.3390/f15122256

APA Style

Shiflett, S. A., Jenkins, J. S., Mattos, R. F., Ibsen, P. C., & Athearn, N. D. (2024). Sequoia Groves of Yosemite: Visitor Use and Impact Monitoring. Forests, 15(12), 2256. https://doi.org/10.3390/f15122256

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