Abstract
Eastern Joshua tree (Yucca jaegeriana) plays a central role in the ecology of the Mojave Desert ecosystem. However, the emergence of invasive grass fire-cycles in the last several decades brings into question Joshua trees’ tolerance and resilience to changing fire regimes. This study’s objective was to examine the effects of wildfires on the structure and regeneration potential of Joshua trees forests. We examined the density, size class structure, and regeneration response of Joshua tree populations on a network of one kilometer transects along the boundaries of four independent wildfires and the interior of the largest fire that occurred 15 years ago (2005) in the northeast Mojave Desert. Burned edge and burned interior transects had 23- and 4.1-fold lower Joshua tree stand densities than unburned transects. The more robust recovery of stand density along burned interior transects compared to burned edge transects appears to be primarily driven by more prolific vegetative sprouting. Our data show that Joshua trees can sprout vegetatively following fire, but it is not a strong or consistent post-fire resprouter. Limiting the spread of invasive annual grasses and novel fire regimes will be critical to maintaining healthy Joshua tree populations into the future, particularly on the edge of its ecological range.
Introduction
Wildfire activity is changing across earth’s ecosystems due to human activity resulting in novel fire regimes that can have lasting effects on vegetation (; ). Wildfires are increasing in deserts of North America, primarily driven by invasive grass fire cycles (; ). Red brome (Bromus rubens L.), an invasive annual grass, has played a significant role in increasing fire activity in the Mojave Desert by increasing plant fuels’ continuity and flammability upon drying at the end of the growing season (). Fire risk is especially high in years with early fall precipitation and warm winter temperatures that produces longer growing seasons that fuel red brome growth (; ). There is concern that increasing fire activity in North American deserts may favor the establishment and spread of exotic alien species to the exclusion of the native plant communities ().
Desert wildfires alter desert plant community composition and function and strongly influence plant community succession (). Plant community recovery following fire varies dramatically between sites depending on a variety of variables, including pre-fire vegetation composition and structure (), fire frequency and severity (), post-fire climate conditions (), and patterns of pollination, seed fate, and herbivory in burned desert landscapes (; ). Fire chronosequence studies show a high degree of variability in plant communities’ response in post-fire environments (; ). The biggest concern related to novel fire regimes in deserts is the potential for state and transition changes due to positive responses of invasive annual grasses to post-fire conditions shortening fire return intervals so that native vegetation has little time to recover (). Recent evidence suggests that both increasing fire frequency and higher burn severity can lead to pathways of dominance by invasive annual grasses and poor post-fire recovery of native vegetation ().
Successful post-fire re-establishment of native vegetation depends on species resilience to fire primarily driven by fire tolerance and regeneration characteristics (). summarized categories of major perennial species that tend to decrease or increase following fire based on these characteristics. At lower burn severity, desert succulents with high water content tend to be less vulnerable to burning, while two dominant Mojave species, Larrea tridentata and Yucca jaegeriana may escape because of their taller stature (). In contrast, annual and perennial grasses, forbs, and drought-deciduous shrubs still tend to be vulnerable to lower severity ground fire because of their short stature and low water content (). High burn severity driven by large woody fuel loads connected by continuous fine fuels can result in greater than 80% losses in plant cover (; ; ). A variety of perennial desert shrubs and grasses show the ability to sprout from surviving roots after fire (). Desert plants that survive or resprout following fire have shown significantly higher flower and seed production (; ), but environmental constraints in post-fire environments may limit plant recruitment from seed (; ). More research is needed on how seed banks influence post-fire plant community succession ().
Eastern Joshua tree (Yucca jaegeriana) () is a semi-succulent, arborescent that can exceed 5 m in height and 300 years in age (). Few studies have examined the post-fire ecology of Joshua trees. This is surprising considering Joshua trees’ central role in the Mojave Desert ecosystem and the dramatic increase in Mojave Desert wildfire in recent decades. Joshua trees can regenerate vegetatively following fire (; ) but are classified as having limited post-fire regeneration potential (; ). Furthermore, woody shrubs facilitate Joshua tree seedling establishment (), so reducing shrub cover following fire would impose additional limitations on Joshua tree regeneration. What is more poorly known are spatial patterns of Joshua tree regeneration as a function of distance from burn boundaries where there may be variation in burn effects or differences in the biological community’s characteristics. In other forest systems edge effects are known to strongly influence seedling establishment and patterns of recruitment (; ).
Joshua tree forests are common in the northeast Mojave Desert, where fires related to red brome invasion are becoming larger and more frequent (). Several large and independent fires that occurred in the summer of 2005 in the northeast Mojave provided an opportunity to assess Joshua trees’ regeneration in post-fire environments. This study’s objective was to characterize the post-fire structure and regeneration of Joshua tree stands 15 years after the 2005 wildfires. We addressed the following questions: (1) What are the characteristics of Joshua tree stands 15 years after wildfire occurrence? (2) Is there evidence of post-fire Joshua tree regeneration, and does it vary spatially between burned edges along fire boundaries to locations deeper into the fire’s interior? We hypothesized greater Joshua tree regeneration along burned edges due to higher seed dispersal from adjacent unburned vegetation and lower burn severity.
Materials and Methods
Study Area
This study was conducted in the Beaver Dam Wash of southwestern Utah in May 2020. The study area is characterized by native shrubs of which Coleogyne ramosissima Torr., Larrea tridentata (DC.) Coville, and Ambrosia dumosa (A. Gray) Payne are the most common. Two yucca species are common in the study area Yucca baccata Torr. and Yucca jaegeriana. Common native herbaceous plants included Sphaeralcea ambigua A.Gray spp., Baileya multiradiata Harv. & A. Gray, Astragalus nuttaliianus DC., Plantago spp. (P. patagonica and P. ovata), and Descurainia pinnata (Walter) Britton. Soil in the area is classified as a gravelly sandy loam (). Mean annual precipitation is 272 mm and mean annual temperature is 16°C (). In recent years the landscape has been altered by plant invasions resulting in changing fire regimes characterized by larger and more frequent fires in the Mojave Desert (; ; ). Invasive annual grasses, Schismus arabicus Nees., and Bromus rubens L. responded positively to early fall precipitation in 2004 resulted in the build-up of fine fuels that triggered multiple, large-scale fires in the study area (Beaver Dam Wash) in the summer of 2005 (). Burn transects were placed within four independent fires that occurred during the summer of 2005: Westside (June, 27,059 ha), Duzak (July, 7,065 ha–within Utah), Burgess 1 (July, 60 ha), and Burgess 2 (July, 712 ha) (Utah Bureau of Land Management).
Study Design
We established a network of 1 km long paired transect lines. Six transect pairs were positioned along adjacent burned/unburned boundaries within the four independent fires in 2005 (Figure 1). Paired transects were placed in areas where they would be adjacent to one another within 200 m on either side of a burn boundary. For the three transects pairs in the Westside fire we added an additional criterion of spreading them as far apart as possible to get broader coverage of the burned area. A single transect pair was placed in the Burgess 1, Burgess 2, and Duzak fires to get statistical replication in independent fires. The remaining three transects were spread across the Westside fire which was fourfold larger than the Duzak fire and several orders of magnitude larger than the Burgess 1 and 2 fires making up the majority of the area burned during the 2005 fires. Five additional transects were established in the interior of the Westside fire to compare Joshua tree regeneration responses in the interior of burned landscapes compared to adjacent burned edges. All transect pairs were selected on these criteria using GIS mapping software to prevent any bias from choosing sites while on location. We estimated pre-fire Joshua tree stand densities along burned-edge and burned-interior transects by counting the number of Joshua trees in a 100 m–2 square area in the center of each transect. Density counts were generated from 2004 image layers of the study area using Google Earth Pro. There were no differences in pre-fire stand density between burned-edge and burned-interior transects (P = 0.71) or burn severity ().
FIGURE 1
Measurements
At each 100-increment point along the transect, we measured the distances to the two nearest living Joshua trees on each side of the transect tape to estimate population density. This included surviving trees that had burned partially in the 2005 fires but still had living branches. On each of the trees (20 trees/transect), we measured tree height and trunk diameter. For Joshua trees under 1 m in height that would likely have regenerated after the fire based on sprout growth estimates (), we measured the distance to the nearest dead Joshua tree. Tree height was determined using a telescoping meter stick. Trunk diameter was measured 20 cm above the soil surface using tree calipers.
Statistical Analysis
Mixed-effects linear models were used to examine the effects of burn condition and location (unburned, burned edge, burned interior) on Joshua tree stand density, tree height, trunk diameter, and percent of sprouts next to dead Joshua trees. In our mixed models, transect type (unburned, burned-edge, burned-interior) and height class were specified as a fixed effect, and site (transect number) was specified as a random effect. Tukey’s LSD was used to test mean differences across burn conditions within each height class. Assumptions of normality and homoscedasticity were visually inspected using residual plots (e.g., histogram, scatterplot, and qqnorm of residuals). To meet equal variance assumptions, data were log-transformed when appropriate. All calculations, models, assumption analyses, and statistical significance determinations were performed using JMP statistical software (version 15).
Results
The average stand density, height, and stem diameter varied significantly between Joshua trees growing along burned interior transects compared to unburned and burned edge transects. Joshua tree stand density was dramatically lower along burned edge (23-fold) and burned interior transects (4.1-fold) compared to unburned transects (P < 0.0001) (Figure 2). Stand density was sixfold greater along burned interior transects compared to burned edge transects. The average height and trunk diameter of Joshua trees along burned interior transects were approximately 3.2- and 11-fold lower than the height and diameter averages of Joshua tree stands along burned edge and unburned transects (P < 0.0001) (Figure 2).
FIGURE 2
Height class distribution varied dramatically between Joshua trees growing along burned interior transects compared to unburned and burned edge transects. Nearly 80% of Joshua trees growing along burned interior transects were less than a meter tall, compared to less than 15% in the same height class for unburned and burned edge transects (P < 0.0001) (Figure 3). There was no statistical difference in the proportion of Joshua trees in the 1–2 m height class among the three transect types (P = 0.13). Unburned and burned edge transects had approximately 6-, 21-, and 5-fold higher proportions of Joshua trees in the 2–3, 3–4, and 4–5 m height classes compared to burned interior transects (P < 0.0001) (Figure 3). The majority of Joshua trees growing along burned edge and unburned transects were in the 1–4 m height range, with the largest proportion (near 40%) in the 3–4 m height class (Figure 3).
FIGURE 3
There was evidence of vegetative regeneration through root sprouting of Joshua trees, particularly along burned interior transects. Nearly 75% of Joshua trees (<1 m tall) growing along burned interior transects were growing from the base of dead Joshua trees compared to 33% of trees (<1 m tall) next to the base of dead Joshua trees along burned edge transects (P = 0.0005) (Figure 4). We did not find any young Joshua trees growing in proximity to dead Joshua trees along unburned transects.
FIGURE 4
Discussion
Our results provide evidence that Joshua tree forests exhibit significant changes in stand structure in post-fire environments depending on fire effects and regeneration responses. Reductions in stand density along burn transects were expected due to fire effects, but contrary to our hypothesis we found four-fold greater Joshua tree density along burned interior transects compared to burned edge transects (Figure 2). suggested that Joshua tree’s higher vegetative regeneration could be related to lower burn severity. However, vegetation surveys of post-fire plant community cover and density showed similar burn severity in the burned, interior locations compared to burned edges in an earlier study along these same study transects (). Instead, it appears that increases in stands density in burned interiors is driven by significantly higher post-fire sprouting based on our size class analysis (Figure 3). This is further supported by the significantly shorter and smaller diameter stand structure of Joshua trees growing along burned interior transects (Figure 2).
Joshua trees have been shown to sprout from the root crown of burned Joshua trees the year following a fire (). Joshua trees’ average height along burned interior transects was just under 100 cm (Figure 2) due to the high proportion of sprouts less than 1 m tall (Figure 3). Growth rates of 3.1–3.8 cm/year have been estimated for regenerating Eastern Joshua tree (; ). Over the 15 years since the fire, this would have resulted in approximately 47–57 cm of vertical growth. This suggests that Joshua trees under 1 meter in height common in the burned interior transects were established soon after the 2005 fires.
There is evidence in the literature of up to 30% of burned Joshua trees resprouting after fire (). Other studies tend to show significantly less post-fire Joshua tree regeneration (; ), which may represent differences in post-fire sprouting responses of the Eastern Joshua tree (Yucca jaegeriana) compared to the Western Joshua tree (Yucca brevifolia). Our study similarly shows a wide range of regeneration responses within a burned desert landscape. In our study, the high proportion of young Joshua trees regenerating in burned interior locations (Figure 4) appears to be driven by vegetative sprouting from the base of Joshua trees that burned in the 2005 fire (Figure 5). It is unclear why Joshua tree sprouting would have been higher along interior transects than those along burned edges (Figure 2). Burned interior transects were higher in elevation but only slightly on average (960 m vs. 1,050 m) and elevation showed no correlation with regeneration density (R2 = 0.05, P = 0.37). The literature hints at one possible hypothesis. Both Jackrabbits and gophers are known to eat regenerating Joshua trees, resulting in high mortality rates (; ). Jackrabbits prefer shrubland habitat typical of the unburned transect lines and utilize burned habitats less (). This could result in lower rates of Joshua tree herbivory along burned interior transects than burned edge transects adjacent to unburned habitat where rabbits may be more active. Additional studies would need to be conducted to test his hypothesis.
FIGURE 5
Our data are consistent with previous studies showing the Joshua tree can sprout vegetatively following fire (), but it is not a strong or consistent post-fire resprouter (). The literature indicates that post-fire resprouting constraints on Joshua tree may be compounded by drought events () that are expected to become more frequent in the future () and animal herbivory (). Hotter and drier future climates are expected to physiologically contract Joshua tree’s current range (; ). In addition, extended and more severe droughts are likely to promote more fire in the Mojave Desert, causing direct mortality of Joshua trees and limiting seedling establishment by burning native nursing shrubs (). Limiting the spread of invasive annual grasses and novel fire regimes will be critical to maintaining healthy Joshua tree populations into the future, particularly on the edge of its ecological range ().
Conclusion
Our study shows a high degree of variability in post-fire regeneration patterns of Joshua tree based on distance from burn boundaries. Joshua tree, which is thought of as a poor-post fire sprouter, especially under high burn severity (; ) did show evidence of post-fire sprouting success in our study (Figure 4). This seems especially important since we found little evidence of post-fire regeneration of Joshua tree from seed. Surprisingly, regeneration density was higher along interior transects than along burn edges although additional surveys in larger burned areas are needed to validate our findings and conclude that it is a broader geographic phenomenon in the Mojave Desert. However, additional experiments are needed to better identify the factors driving the patterns we observed. For example, monitoring of herbivory and environmental correlates such as soil resource availability and topographical variation that may contribute to variability in Joshua tree resprouting success in post-fire environments.
While we observed Joshua tree vegetative regeneration following high severity fire, regeneration densities were still dramatically lower than typical Joshua tree stands in unburned areas even 15 years after the fire (Figure 2). This suggests that post-fire vegetative sprouting alone isn’t sufficient to re-establish typical stand densities in the years following fire. Since climate change is likely to increasingly constrain Joshua tree regeneration from seed () management approaches that limit the establishment and spread invasive grasses and human ignitions that spread wildfire into Joshua tree stands is of high priority. Where fire does occur and Joshua tree stand re-establishment is desired, outplantings may be a restoration approach to re-establish Joshua tree stands. However, research is needed to explore the viability of this option and even then it would probably only be economically feasible at small spatial scales ().
Publisher’s Note
All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.
Statements
Data availability statement
The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.
Author contributions
SeS and ES conducted the field surveys. SeS curated the data. SeS and SaS conceive the study, analyzed the data, and wrote the manuscript. ES reviewed the manuscript and provided feedback. All authors contributed to the article and approved the submitted version.
Funding
Brigham Young University provided funding support for this research.
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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Summary
Keywords
invasive grass fire cycle, Mojave Desert, vegetative regeneration, wildfire, Yucca brefifolia
Citation
St. Clair SB, St. Clair EA and St. Clair SB (2022) Spatio-Temporal Patterns of Joshua Tree Stand Structure and Regeneration Following Mojave Desert Wildfires. Front. Ecol. Evol. 9:667635. doi: 10.3389/fevo.2021.667635
Received
14 February 2021
Accepted
09 December 2021
Published
27 January 2022
Volume
9 - 2021
Edited by
Eddie John Van Etten, Edith Cowan University, Australia
Reviewed by
Lynn Sweet, University of California, Riverside, United States; Daniel Moya, University of Castilla-La Mancha, Spain
Updates
Copyright
© 2022 St. Clair, St. Clair and St. Clair.
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Samuel B. St. Clair, stclair@byu.edu; orcid.org/000-0001-6612-0329
This article was submitted to Biogeography and Macroecology, a section of the journal Frontiers in Ecology and Evolution
Disclaimer
All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher.