Fire Effects on Lichen Biodiversity in Longleaf Pine Habitat

Abstract

Longleaf pine forests are economically important habitats that stabilize and enrich the soil and store carbon over long periods. When mixed with oaks, these forests provide an abundance of lichen habitats. The tree canopy lichens promote greater moisture capture and retention and encourage canopy insects. Ground lichens limit some vascular plant germination and growth, promoting a more open and healthy pine community. There is a longstanding mutualistic relationship between longleaf pine habitat and lichens. Longleaf pine habitat has a long history of natural summer burning, which promotes a diverse understory and limits tree densities. Lichen diversity exceeds vascular plant diversity in many mature longleaf pine habitats, yet information on the impacts of prescribed fire on lichen species in these habitats is limited. We assessed lichen diversity and abundance before and after a prescribed ground fire in a longleaf pine/wiregrass habitat near Ocala, Florida. Pre-burn, we found greater lichen abundance and diversity on hardwoods, primarily oak species, than on pines. Post-burn, lichen abundance on hardwoods dropped overall by 28%. Lichen abundance on conifers dropped overall by 94%. Ground lichen species were basically eliminated, with a 99.5% loss. Our study provides insights into retaining lichen diversity after a prescribed burn. Hardwood trees, whether alive or standing dead, help retain lichen biodiversity after burning, whereas conifer trees do not support as many species. Landscapes may need to be actively managed by raking pine needle litter away from ground lichen beds, moistening the ground, or removing some lichen material before the burn and returning it to the site post-fire. Based on these results, we suggest retaining some oaks and conducting burns in a mosaic pattern that retains unburned areas. This will allow for lichens to recover between burns, significantly enhancing biodiversity and the ecological health of these longleaf pine communities.

1. Introduction

Longleaf pine (Pinus palustris Mill.) forests are economically important habitats that stabilize, enrich the soil, and store carbon over long periods. Longleaf pines are called the “tree that built the South”, with good reason. This native of the southeastern U.S. was heavily harvested for decades to build homes, ships, and railroads. Less than 5% of longleaf pine forests remain [1]. These drought-tolerant and fire-resistant trees are known for efficiently capturing and storing carbon over long periods, making them important species as climate change intensifies [2]. With fire, these forests maintain an open canopy that lets sunlight spill down through a mostly vacant midstory to reach a forest floor tightly packed with grass and flowering plants. Though these forests can feel almost empty, the longleaf pine ecosystem is a trove of biodiversity. Some researchers estimate that its richness is surpassed only by tropical forests [3].

Lichens are often abundant in longleaf pine habitats [4,5]. However, little is known about the role lichens and their biodiversity play in these habitats. Longleaf pine habitats are highly variable [6], ranging from xeric sandhills to flatwoods to savannas. There are also transition stands, with various oak species present in the savanna habitat and xeric woodland series [7]. The lichens are similarly variable across these longleaf pine stands.

The oaks (Quercus spp.) are slow-growing hardwood trees that serve as a highly suitable substrate for lichens [8]. This is demonstrated by observations in the temperate zone of the northern hemisphere, where Quercus species are often dominant trees in natural forests. In Great Britain, for example, over 300 lichen species, or 22% of the total lichen flora, grow as epiphytes on oak [8]. The oak forests of the neotropical mountains frequently exhibit abundant lichen growth, seemingly no less than in the temperate zone. The montane environment, characterized by higher precipitation, frequent fog, and moderate temperatures, is conducive to lichen growth. Crown twigs may support loads of the yellowish, bushy beard lichen (Usnea spp.), while older branches are typically covered with whitish patches of leafy lichens belonging to the families Parmeliaceae and Physciaceae [8].

Oak trees worldwide provide abundant habitats for epiphytic lichens [8,9]. A total of 56 lichen species were found on a single oak tree in southern Florida [9]. Factors that seem to influence the presence of lichens on that particular oak, Quercus pumila Walter, include the tree’s location in the Neotropical realm, the tropical monsoonal climate, the tree’s size (DBH), and the relative open exposure where it grew [9].

Lichens serve as primary producers in the forest. Their biomass and nutrients contribute to forest litter and duff, enriching the soil and enhancing its moisture-holding capacity. Canopy lichens capture fog, retaining moisture within the forest. Birds and small mammals use lichens for nest building [10]. Lichen epiphytes play a critical role in the forest water cycle, increasing canopy interception and nutrient cycling [10]. Nitrogen-fixing cyanolichens are especially important for ecosystems. In the southern Appalachian Mountains, Becker [11] found that lichens annually contributed an estimated 0.8 kg N ha². Lichens alter the micro-environment to improve water capture and storage, while delaying water release [12]. Epiphytic lichens increase the abundance of invertebrates, providing habitat in the canopy for prey items of passerine birds [13]. Lichen diversity and abundance have consistently been significant predictors of invertebrate diversity [14,15]. In the tropics, lichens in the canopy layer have also shown high diversity and specific patterns [16]; large lateral limbs increase the bryophyte and decrease the lichen cover [16].

Epiphytic lichens’ life history and chemical composition may influence successional trends [10]. Epiphytic lichens vary by tree type, age, and physical features of the bark [10], and even on a single tree, lichen diversity can be high, such as in the tropics, where Aptroot [17] found 173 species on a single tree. Jarman and Kantvilas [18] found 76 lichen species on a single Huon pine in Tasmania. Lichens occurring on dead wood have been found to increase the diversity of a forest stand [19]. Alectorioid lichens (forage lichens, pendant Usnea, etc.) that hang from branches provide food and habitat for wildlife [20].

It is important to monitor lichen diversity and abundance. Many lichens are known to be dispersal-limited [21,22,23] and may be slow to recolonize a site, while those with asexual modes of reproduction can more quickly colonize the bark [23]. Unfortunately, burning occurs primarily to improve the structure of a forest and the understory vascular plant diversity and productivity, usually without regard to lichen diversity. Plant communities isolated within the landscape lack adjacent diaspores to recolonize [10]. Fragmented forest stands that have beenburned may not intercept lichen diaspores hindering recolonization. Prescribed burning can thin out small trees and canopy layers, allowing more light to enter a forest stand. A positive correlation often exists between greater lichen cover and light availability in the forest canopy [24,25]. Time is relative, and in the taiga, Lewis et al. [26] and Klein [27] concluded that fire can destroy lichens and other forage when observed over a short-term period of 50 years or less, potentially reducing the taiga’s ability to support caribou. However, over extended periods, typically a century or more, fire seems critical for maintaining ecological diversity and forage production for caribou. It is reported that both the frequency and intensity of fires determine the amount of ground lichens present in different ecosystems [26,27,28].

Lichens function as indicators and keystone species in many ecosystems, providing vital services for other community members [29]. Therefore, knowing more about how management practices impact this group of organisms is critical. We take a significant risk in not knowing how burning impacts a major component of this ecosystem. In other forest types, flying squirrels that are essential for mycorrhizal dispersal and forest food webs eat lichens [20]. Without lichens, the crucial mycorrhizal fungi that trees rely on would not be dispersed.

Longleaf pine habitats’ tolerance to burning has been found to correlate with historical and evolutionary adaptations across different forest types [30]. Fires have shaped forest communities and influenced plant specialization for millennia in eastern North America [31,32], and longleaf pine habitat is adapted to fire [6] (Figure 1). However, the impacts on lichens in longleaf pine communities have not been well studied [4,33,34]. Most studies on lichens in other habitats and disturbance regimes suggest that burning significantly affects lichens [35,36,37]. Most authors report reduced lichen cover, and some have noted that ground lichens were eliminated in response to burning [34,36]. Johansson and Reich [28] report that Cladonia-type ground lichen recovery is primarily a factor of fire intensity and frequency. Several authors have indicated that Cladonia-type ground lichens are slow to recover post-fire [35,36,38]. The presence and continuity of fuels can determine whether ground-dwelling lichens survive wildfires [34]. Like fire, common forestry practices such as site preparation, herbicide release, fertilization, and high-density stocking negatively affect lichens and overall biodiversity [3].

Our study aimed to assess lichen epiphyte diversity and abundance on longleaf pine and oak trees, as well as the diversity and abundance of understory ground lichens in this habitat, and to measure the effect of prescribed fire on lichen diversity. Because lichens serve as indicators and keystone species in many ecosystems, providing vital services for other community members, understanding how management practices impact this group of organisms is essential. To date, the effect of prescribed fire on lichens in longleaf pine communities has largely been overlooked.

2. Materials and Methods

2.1. Study Area

Topography and soils are well-drained and sandy with gentle slopes (1%–2%). Pre-burn surface fuels mainly comprised pine, some oak leaf litter, wiregrass, other natural grasses, and smilax (Smilax spp.) vines. Pine needle duff was 1–4 inches deep pre-burn. The study site is located 32 km (15 miles) west of Ocala, Florida, USA, in Marion County on private property (WGS 84; 29.19213, −82.38639; Elev: 30 m (100 ft) (Figure 1).

The vegetation at our study site, known as Gopher Tortoise Hill, is characterized as a xeric sandhill with natural longleaf pine, scattered turkey oak (Quercus laevis Walter), wiregrass (Aristida stricta Michx.), and other species such as saw palmetto (Serenoa repens (W. Bartram) Small), gopher apple (Geobalanus oblongifolius (Michx.) Small), blazing-star (Liatris sp.), Rosemary (Ceratiola ericoides Michx.), as well as an abundance of ground lichens (genus Cladonia). Longleaf pine is the dominant canopy tree species, with turkey oak the dominant understory tree, with scattered water oak (Q. nigra L.), bluejack oak (Q. incana W. Bartram), and sand live oak (Quercus geminata Small) also present in the understory. Multiple age classes of pine exist within the study area; the site index for longleaf pine on this soil type is 18 m (60 ft). Management objectives for Gopher Tortoise Hill are growing trees and wildlife conservation, including maintaining habitat for gopher tortoises, encouraging and promoting a variety of bird species, and maintaining the ecological function of this habitat type [39]. Based on a 1965 aerial photograph, the study area appeared to lack trees at that time (Jason Ballard 2022, personal communication). Hence, we estimate the oldest trees to be 58 years or less, suggesting the area had been logged (Jason Ballard 2022, personal communication). Gopher Tortoise Hill is a secondary forest formed through natural regeneration with a variety of age classes and different tree species. It was not a plantation, and there were no plantations in the nearby area.

2.2. Pre-Burn

Plots were randomly selected, with three plots within the areas to be burned and three in the unburned areas. Lichen communities were assessed by determining the presence and abundance of lichen species on longleaf pine and turkey oak trees in large circular plots of 0.378 ha (34.7 m radius). This is a standard US Forest Service Forest Health Monitoring (FHM) method used in their nationwide monitoring program [40]. Lichen species abundance was rated for the entire plot in the canopy and on the ground. A visual estimate rated the canopy lichen abundance as: rare ≤ 5%, infrequent = 5%–10%, common ≥ 10%, which is the rating the USFS uses in their FHM plots. We also recorded the total percentage cover of ground lichens (to the nearest 5%) in smaller compass-oriented (NE, SE, NW, SW) 1/4 pie-shaped plots within a radius of 5 m from the plot center. For our study, compared to the standard USFS Forest Health Monitoring (FHM) plots, we also sampled both the base and lower trunk of the tree and the forest ground layer [41]. These measurements add value to the forest components that comprise the longleaf pine–turkey oak–wiregrass plant community.

Voucher specimens of lichen species within the plots were collected, identified, and processed and are housed at the Snake River Plain (SRP) and University of Florida (FLAS) herbaria. Species were determined using field-oriented keys of the Florida lichens by Rosentreter et al. [42]. Lichen nomenclature follows Esslinger [43].

The prescribed fire was ignited on 1 February 2023, on the downwind side of the hill, starting at the top. Alachua Conservation Trust (https://www.alachuaconservationtrust.org/, accessed on 26 August 2025) conducted the burn, with Barry Coulliette as burn boss, with generous assistance and supervision provided by Florida Fish and Wildlife Commission (FFWCC) biologist Jason Ballard. The burn occurred mainly as a back-burn. The burn ignition time was 10:20 AM with winds of three mph out of the west and 62% relative humidity. Air temperature was 14 °C in the morning with an afternoon temperature of 27 °C. The day was partly cloudy. Relative humidity was 100% by evening. Before ignition, a fuel break was created on all sides of the 10-acre property, primarily by hand trimming and raking aside the organic duff. Six foil blankets were laid over some ground lichens and wiregrass in plot #1 to see how well they protected the ground lichens, as a potential form of burn management (Figure 2A).

2.3. Post-Burn

The three burned lichen plots were resampled following modified FHM protocols [41] to determine lichen diversity and abundance post-burn. When lichen cover was present but less than one percent, it was recorded as a trace and calculated as 0.5% cover. Rating cover as 0.5% was more consistent than rating at a finer % scale.

We randomly sampled ten live conifers and ten live hardwoods in each of the three burned and three unburned plots (total = 60 conifers and 60 oaks; combined total = 120 trees). We only selected the more mature trees with a DBH greater than 4 inches. This method excluded small, immature trees that might lack lichen epiphytes. For each tree, we recorded the maximum char height and the percent cover of lichens on the base (0–25 cm), trunk (25 cm–2.4 m or the lowest live branching height), and canopy (branching to the top of the tree). Canopy sampling was based on sight and the occasional fallen branches. Lichen cover was recorded by growth form—crustose, foliose, and fruticose—and was estimated to the nearest 5%. Most calculations of lichen cover were based on the combined cover of crustose, foliose, and fruticose lichens.

2.4. Post-Burn—Year 2

Two years after the burn, we surveyed the hardwood trees in the 10-acre study area for live standing versus fallen trees. We only surveyed the larger trees with a diameter at breast height (DBH) of ≥4 in (10.16 cm). Hardwood trees became rare after the fire, so a full count was conducted to assess the loss. Many small pine trees burned, but since pines are the dominant species in the area, their overall numbers stayed high, and pines remained common. Therefore, we did not find it necessary to count pines.

2.5. Data Analysis

Trace percent cover was calculated as 0.5%. Percent lichen cover on trees was analyzed by adding the cumulative cover of the crustose, foliose, and fruticose growth forms, capped at 100%. Cumulative cover could exceed 100%, given that lichens can be physically layered on top of each other.

We conducted independent t-tests between burned and unburned lichen cover for both hardwood and conifer trees and for ground lichen cover. We also conducted t-tests between tree species for both burned and unburned plots. We performed two-way ANOVAs to assess interactions between burn state (burned vs. unburned) and lichen location (base, trunk, canopy) on lichen cover for each tree type. We then ran Tukey’s HSD post hoc analyses to identify significant ANOVA interactions. Finally, we ran Pearson correlation analyses to assess the relationship between char height and lichen cover for each tree type (conifer/hardwood).

3. Results

3.1. Pre-Burn Lichen Inventory

Thirty-seven macrolichen species were found in the six FHM plots at our study site (

Table 1. Lichen species (on-plot, off-plot, downed) found within the study site and their abundance rating, based on the number of individuals or colonies, denoted as: rare (R) < 5, infrequent (I) = 5–10; common (C) > 10. Preferred substrate for this site is indicated as: P = pine, H = hardwood, B = both, G = ground, L = log or tree base. Reproductive mode is indicated as: S = sexual (spores), A = asexual (isidia, soredia = specialized fragmentation structures). ** Denotes that the species is a local indicator for this substrate. ^^ Represents species eliminated from the site post-burning. Nomenclature follows Esslinger [43].

Scientific Name	Common Name	Growth Form	Abundance Rating	Preferred Substrate	Reproductive Mode
Buellia sp.	Button lichen	crustose	R	H	S
Buellia stillingiana J. Steiner	Button lichen	crustose	R	P	S
Bulbothrix confoederata (W.L. Culb.) Hale	Smooth eyelash lichen	foliose	C	P **	S
Bulbothrix laevigatula (Nyl.) Hale	Matted eyelash lichen	foliose	I	P **	A
Canoparmelia caroliniana (Nyl.) Elix and Hale	Carolina shield lichen	foliose	C	B	A
Canoparmelia cryptochlorophaea (Hale) Elix and Hale	Powdery-headed shield lichen	foliose	C	B	A
Chrysothrix xanthina (Vain.) Kalb	Sulfur dust lichen	crustose	C	B	A
Cladonia evansii (Abbayes) Hale and W.L. Culb.	Powder-puff lichen	fruticose	C ^^	G	S
Cladonia leporina Fr.	Jester lichen	fruticose	C	G and L	S
Cladonia parasitica (Hoffm.) Hoffm.	Fence-rail cladonia	fruticose	I ^^	G and L	A
Cladonia peziziformis (With.) J.R. Laundon	Turban lichen	fruticose	Off-plot (R) ^^	G	S
Cladonia rappii A. Evans	Slender ladder lichen	fruticose	Off-plot (I) ^^	G	S
Cladonia ravenelii Tuck.	Ravenel’s cup lichen	fruticose	R ^^	L	A
Cladonia subcariosa Nyl.	Peg lichen	fruticose	I ^^	G and L	S
Cladonia subradiata (Vain.) Sandst.	Powdery peg lichen	fruticose	C ^^	B	A
Cladonia subtenuis (Abbayes) Mattick	Dixie reindeer lichen	fruticose	C ^^	G	S
Dirinaria aegialita (Afz.) B. Moore	Grainy medallion lichen	foliose	I	B	A
Dirinaria picta (Sw.) Clem. and Shear	Powdery medallion lichen	foliose	C	B	A
Haematomma accolens (Stirt.) Hillm.	Bloodstain lichen	crustose	C	H	S
Hypotrachyna livida (Taylor) Hale	Wrinkled loop lichen	foliose	C	H	S
Hypotrachyna minarum (Vain.) Elix	Hairless-spined shield lichen	foliose	R	B	A
Hypotrachyna osseoalba (Vain.) Y.S. Park and Hale	Grainy loop lichen	foliose	R	H	A
Hypotrachyna pustulifera (Hale) Skorepa	Pustulate loop lichen	foliose	R	H	A
Hypotracyna spumosa (Asahina) Elix and Hale	Pustuled shield lichen		I	B	A
Lecanora sp.	Rim lichen	foliose	I	B	S
Lepraria sp.	Dust lichen	crustose	I	B	A
Parmotrema gardneri (C.W. Dodge) Hale	Ruffle lichen (K-, P+ red)	foliose	C	P	A
Parmotrema hypoleucinum (J. Steiner) Hale	P+ orange powdered ruffle lichen	foliose	C	P	A
Parmotrema perforatum (Jacq.) A. Massal.	UV- perforated ruffle lichen	foliose	C	B	S
Parmotrema rampoddense (Nyl.) Hale	Long-whiskered lichen	foliose	C	B	A
Parmotrema subisidiosum (Mull. Arg.) Hale	Cracked and salted ruffled lichen	foliose	C	H	A
Parmotrema submarginale (Michx.) DePriest and B. Hale	Unperforated ruffle lichen	foliose	C	H	S
Parmotrema sulphuratum (Nees and Flotow) Hale	Sulphur ruffle lichen	foliose	R	H **	A
Parmotrema tinctorum (Despr. ex Nyl.) Hale	Palm ruffle lichen	foliose	C	B	A
Pertusaria amara (Ach.) Nyl.	Wart lichen	crustose	C	H	A
Pertusaria texana Mull. Arg.	Texas wart lichen	crustose	I	H	S
Phaeophyscia pusilloides (Zahlbr.) Essl.	Wreath lichen	foliose	I	H	A
Physcia sorediosa (Vain.) Lynge	Black-bottomed rosette lichen	foliose	I	H	A
Pyxine albovirens (G. Mey.) Aptroot	White rosette lichen	foliose	R	H	A
Pyxine caesiopruinosa (Nyl.) Imshaug	Buttoned rosette lichen	foliose	I	H	A
Pyxine eschweileri (Tuck.) Vainio	Thin rosette lichen	foliose	I	H	A
Trapeliopsis granulosa (Hoffm.) Lumbsch.	Granular mottled-disk lichen	crustose	Off-plot, I ^^	G and L	A
Usnea dimorpha (Mull. Arg.) Mot.	Powder-tipped beard lichen	fruticose	C	H	A
Usnea mutabilis Stirt.	Bloody beard lichen	fruticose	I	B	A
Usnea rubicunda Stirt.	Red beard lichen	fruticose	C	B	A
Usnea strigosa (Ach.) A. Eaton	Bushy beard lichen	fruticose	C	B	A
Usnea subscabrosa Nyl. ex Motyka	Horny beard lichen	fruticose	C	B	A

). Five common crustose lichen species were also identified within the same six plots. Three additional lichen species were off-plot, all growing on relatively rare decomposed wood (logs or stumps; two species of Cladonia and Trapeliopsis granulosa (Hoffm.) Lumbsch.). Down-decomposed wood is rare on this site due to historic logging and rapid decomposition rates. Based on our six FHM plots and additional species found on stumps, 45 lichen species in total were present pre-burn. Twenty-four of these taxa had an abundance rating as common (>10 individuals), fourteen were infrequent (5–10 individuals), and eight of the forty-five taxa were rated as rare (<5 individuals). We also found one leafy liverwort, Cheilolejeunea conchifolia (A. Evans) W. Ye and R.L. Zhu, growing epiphytically on several longleaf pine tree trunks.

We identified 18 lichen genera at the study site, with the highest diversity concentrated in 3 genera: nine Cladonia, eight Parmotrema, and five species in the genus Usnea. The highest species richness and percent cover were on oak trees. Oak and pine tree bases were typically colonized by soil-dwelling Cladonia species.

3.2. Post-Burn Ground Lichens

Ground lichens (Cladonia spp.) were nearly eliminated, with a 99.5% loss in abundance observed across plots following fire. Ground lichens were either completely blackened or had disappeared. While some ground lichens maintained their three-dimensional structure, they were no longer green, even at the base. Overall, ground lichens appeared blackened and felt fragile to the touch. The fire completely consumed the six foil blankets, leaving only the metal stakes at the four corners (Figure 2B). The heat from the fire was intense enough to vaporize the foil blankets, rendering them ineffective in protecting the underlying ground lichens. Foil blankets proved an ineffective method of protecting ground lichens from fire.

A paired-samples t-test showed a significant difference in ground lichen cover between pre-burn (M = 12.0, SD = 11.2, n = 12) and post-burn (M = 0.08, SD = 0.19, n = 12) samples (t(11) = 3.73, p < 0.01; Figure 3). Following the fire, ground lichen species experienced nearly complete elimination, with a 99.5% loss in abundance observed across the plots. The severity of the ground fire was rated as level 3, indicating that the fire consumed 50%–99% of the litter and some of the duff.

Ground lichen species that were lost from burning included Cladonia evansii, C. parasitica, C. peziziformis, C. rappii, C. ravenelii, C. subcariosa, C. subradiata, C. subtenuis, and Trapeliopsis granulosa.

Figure 3. Mean ground lichen cover (%) before and after prescribed fire across repeated plots. Error bars represent the standard error (SE).

Figure 3. Mean ground lichen cover (%) before and after prescribed fire across repeated plots. Error bars represent the standard error (SE).

3.3. Post-Burn Tree Canopy Lichens

Post-burn, lichen abundance on hardwoods dropped overall by 28%, with base of tree lichen abundance reduced by 90%, trunk abundance reduced by 56%, and canopy abundance reduced by 24%. Lichen abundance on conifers dropped by 94%, with base of tree lichen abundance reduced by 99%, trunk abundance reduced by 93%, and canopy abundance reduced by 29%.

Total lichen cover was significantly higher on unburned hardwood trees (M = 97.7, SD = 12.5, n = 30) compared to burned hardwood trees (M = 70.6, SD = 29.7, n = 30; t(38.97) = −4.61, p < 0.001). Similarly, unburned conifer trees (M = 33.9, SD = 24.1, n = 30) had significantly greater lichen cover than burned conifers (M = 2.1, SD = 0.95, n = 30; t(29.09) = −7.21, p < 0.001).

When comparing hardwood and conifer trees within burn categories, burned hardwoods (M = 70.6, SD = 29.7, n = 30) had significantly higher lichen cover than burned conifers (M = 2.1, SD = 0.95, n = 30; t(29.06) = −12.62, p < 0.001). Unburned hardwoods (M = 97.7, SD = 12.5, n = 30) also had significantly greater lichen cover compared to unburned conifers (M = 33.9, SD = 24.1, n = 30; t(43.56) = −12.88, p < 0.001).

A two-way ANOVA revealed a significant interaction between burn state and lichen location on lichen cover for hardwood trees, F (2, 534) = 10.11, p < 0.001. Tukey’s HSD post hoc tests showed that unburned hardwood trees had significantly greater lichen cover than burned hardwoods at both the base (p < 0.001) and trunk (p < 0.001) locations, but no significant difference was detected in the canopy (p = 0.531; Figure 4). Similarly, a two-way ANOVA for conifer trees revealed a significant burn state × lichen location interaction, F (2, 534) = 31.69, p < 0.001. Tukey’s HSD post hoc tests indicated that unburned conifers had significantly greater lichen cover than burned conifers at the base (p < 0.001) and trunk (p < 0.001) locations, with no significant difference observed in the canopy (p = 0.999).

Pearson correlations between hardwood char height and lichen cover were weak across locations (base: R = −0.15, trunk: R = −0.14, canopy: R = −0.07; Figure 5). Pearson correlations between conifer char height and lichen cover were weak across locations (base: R = −0.22, trunk: R = −0.16, canopy: R = 0.14). We found no tree canopy lichens that were completely absent post-burn.

There were fewer lichens on the base of the trees post-fire, but the char at the base of the trees was highly variable (Figure 5). Vines, especially Smilax spp., which commonly grew on some trees, appeared to decrease fire damage to lichens and lowered the char level on the tree trunks. Char height on the conifers (mean: 62 inches, SE 5.35) was greater than twice that of the hardwoods (mean: 29.4 inches, SE 1.56).

3.4. Post-Burn Vascular Vegetation

Many of the small trees and shrubs were consumed by the burn. Longleaf pine does not resprout after fire, but many oak species are capable of resprouting. Understory perennial bunchgrasses and forbs were released and responded by flowering or producing leaves (Figure 6). Bunchgrasses such as wiregrass, little bluestem (Schizachyrium scoparium (Michx.) Nash) and bluestem (Andropogon spp.) and various forbs including gopher apple (Figure 6), wavyleaf noseburn (Tragia urens L.), showy dawnflower (Stylisma abdita Myint), sandhill milkweed (Asclepias humistriata Walter), honeycomb-head (Balduina sp.), Florida paintbrush (Carphephorus corymbosus (Nutt.) Torr. and A. Gray), silver croton (Croton agryranthemus Michx.), blackroot (Pterocaulon pycnostachyum (Michx.) Elliott), and trailing ratany (Krameria lanceolata Torr.) exhibited extensive flowering and abundant regrowth post-fire. Many of these plants had not been observed above ground at the study site in recent years. Florida rosemary, which requires fire for germination, did not respond as hoped. Two years post-burn, there was no sign of rosemary seedling recruitment or survival.

Figure 6. Gopher apple (Geobalanus oblongifolius) three months after the burn.

Figure 6. Gopher apple (Geobalanus oblongifolius) three months after the burn.

3.5. Post-Burn—Year 2 Results

By year 2 within the ten-acre burn, more than 90% of the above-ground turkey oak had either fallen over or blown over from strong hurricane winds during the second year (

Table 2. Status of oak trees (≥4-inch DBH) two years post-burn.

Oak Species	Live	Dead—Above Ground	Total	Percent Mortality Above Ground
Turkey	4	42	46	91.3
Water	2	5	7	71.4
Bluejack	0	2	2	100
Sand live	Many shrub-like (12)	0	12	0
Total	18	49	67	73.1

). The mean DBH of the fallen oaks was between 4 and 24 inches. The turkey oaks ≥4-inch DBH had 80%–90% lichen cover on their branches (Figure 7). Sand live and bluejack oak also died back or blew over during the second year, significantly impacting lichen cover within the burn treatment area.

4. Discussion

Our study sought to assess lichen epiphyte diversity and abundance on longleaf pine and oak trees, the diversity and abundance of understory ground lichens, and to quantify the effect of prescribed fire on lichen diversity in this longleaf pine/turkey oak/wiregrass habitat…

By 2 February 2023, 24 h after burn ignition, many of the small (<4″ DBH) to medium-sized (<6″ DBH) oak trees had burned at the base and fallen over; some were still smoking. Several American Robins and Red-Bellied Woodpeckers were already feeding in the area. On 30 April 2023 (three months after ignition), the property was revisited the day after an inch of rain was recorded. The study site had previously received little rainfall in 2023, which is common in North Florida in winter and early spring. Despite being mostly dry, the burn released many bunchgrass and forb species.

The hardwoods supported higher lichen cover and abundance pre-burn (Figure 7), with the greatest cover on the canopy twigs of some apparently slow-growing or repressed oaks. Tree species determine the structure and diversity of the macrolichen community because they regulate substrate suitability by modifying the temperature, humidity, and light conditions on which the epiphytes depend [10]. Thus, species richness of epiphytic macrolichens varies significantly between host tree species. The physical bark properties also influence lichen species composition and abundance [9,44]. Structural stability of bark matters, with the more stable bark supporting greater lichen diversity and abundance. Burning reduced overall lichen abundance and diversity in an Oregon prairie [37], but given our broader abundance classes, we found little change in canopy lichen species abundances. At our study site, canopy lichen decline is attributed to increased wind damage post-fire rather than direct damage by fire.

Oak trees are known to support an abundance and diversity of lichen species (Figure 7). The genus Quercus is also recognized for the vital habitat it provides for wildlife. Native oak trees are a keystone species that provide essential resources and habitats for various insects, including more than 550 species of caterpillars in some regions [45]. One of the reasons oaks are such biodiversity powerhouses is that they host more insect species than any other tree in North America. Caterpillars serve as the mainstay of most bird diets in North America, particularly when birds are rearing their young [45]. Holes and crevices of various sizes provide breeding and shelter sites for wildlife, including cavity-nesting birds, flying squirrels, bats, and other mammals.

Post-fire, the longleaf pine forest at our study site was more open, with sunlight now reaching the forest floor. The prescribed fire killed more hardwoods than conifers, although it was difficult to quantify the number of small trees consumed. Hardwoods charred at the base also slowly fell over, and some were still falling two years post-burn.

The lichen diversity and abundance of lichens on the base and trunks of burned trees were lower than in unburned forest stands in southern pine forests of the mid-Atlantic coastal plain [30]. However, the lichen cover within the canopy was unaltered by burning [30]. The canopy layer represents a refugium from fire damage for this group of lichens. Lichen taxa that are limited to growing at the base of trees, on downed coarse woody material, or on the ground lack such a refugium [46]. We found a similar pattern at our study site, with lichen cover significantly reduced at the base of trees and on the trunks of trees in the burn treatment (Figure 4).

Ground lichens (Cladonia spp.) were blackened and fragile to the touch or completely gone at our study site post-fire. Some still showed their three-dimensional structure, but they were not green at the base. Mechanical damage post-fire is an issue at sites where ground lichens were previously abundant, and it is possible that avoidance post-fire could facilitate ground lichen recovery [47].

The unburned base of the conifers occasionally supported one ground lichen species, Jester lichen, C. leporina. Cladonia leporina is one of the most abundant and widespread Cladonia species. There was no correlation between surviving fire and abundance rating by reproductive mode, i.e., Cladonia species with asexual reproductive modes (C. subradiata) were eliminated, as were Cladonia with only sexual reproductive modes (C. peziziformis).

Deciduous oaks generally allow more sunlight and moisture through the canopy. Turkey oaks, a deciduous species, provided the best lichen substrate at our study site. Canopy lichen diversity was also highest on the turkey oaks, the tallest Quercus at our site (to 10 m). Sand live oak, an evergreen, did not offer suitable lichen substrate, while the bluejack oak were young and generally unoccupied by lichens. The larger water oaks generally do not support many macrolichens, but crustose lichens are often present on their trunks. Crustose lichen diversity was similar in our burned and unburned plots; however, smooth-barked oak trees abundant with crustose lichens declined post-fire.

Crustose lichens are the least susceptible to fire unless the tree they grow on dies. They are more common on smooth-barked trees, but because smooth-barked trees often have thin bark, they are vulnerable to fire damage. In contrast, foliose and fruticose lichens grow best on rough, thick-barked trees [9,10] and are both susceptible to fire; fire can directly damage them, and it often kills oak trees despite their thick bark.

Johansson et al. [48] correlated lichen survival after wildfire with factors such as life histories, growth forms, soredia presence, habitat preference, population size, and reproductive mode. Our study did not find these factors to be linked to lichen survival. Ground lichens may restrict some vascular plant germination and growth, promoting a more open and healthy thinned pine community [49]. This growth of ground lichens functions similarly to the open forest structure that fire aims to create. Since ground lichens have been shown to limit pine seedling germination and growth, they help establish a natural open forest structure similar to the effects of fire. Due to the water-holding capacity, texture, and secondary compounds in their thalli, ground lichens act as an allelopathic filter for plant seedling establishment. Their role in stabilizing open woodlands by hindering tree regeneration and growth [49] is often overlooked. Ground and canopy lichens serve as hydrobuffers, absorbing water and gradually releasing humidity into the forest. Insects find shelter within the lichen thalli on the ground and in the trees.

Epiphytic fruticose lichens may better recolonize bark and canopy twigs due to their asexual reproductive strategies. Most ground lichens lack asexual reproductive propagules and are slow to recolonize a site. Habitat quality and a lack of disturbance generally increase lichen diversity [50]. This forest is not very tall (generally less than 12 m), and lichen differences within the canopy layers were not observed. This may be partly due to the frequency of fog in this area, which provides moisture for lichens regardless of their vertical position.

Based on our results, further investigations of inventory methods and monitoring ground and tree-base lichens relative to fire effects would be beneficial. Recent studies on forest diversity in the eastern US found that species diversity was the best indicator of overall forest health and productivity [51]. Natural wildfire and prescribed burning create a crazy quilt of habitats across the landscape. Fire, both wild and prescribed, could be better used to manage and understand our rich forest biodiversity in the future, including the lichen component.

5. Conclusions and Conservation Recommendations for Management

Prescribed fire in longleaf pine communities may reduce lichen cover on the boles and trunks of trees, although most epiphytic lichen species can survive a low—to moderate-intensity fire. Ground lichens may need special consideration and management. However, the outcome should be interpreted considering the overall restoration goals and the altered disturbance regime brought about by fire exclusion. Some studies have documented the detrimental effects of forest densification on lichens, which can occur without fires [52].

We suggest the following management practices for maintaining lichen diversity and abundance in longleaf pine communities:

• Maintain some hardwoods within the longleaf pine stand to increase lichen diversity and to serve as refugia for those hardwood-associated lichen species.
• Maintain some tree age and size diversity to encourage greater lichen diversity.
• Create a landscape mosaic that includes some unburned areas within the treatment area.
• Use low-intensity burning rather than high-intensity methods.
• Weather conditions and the resulting fire intensity matter to lichens responding to prescribed fire [28].
• Lichens will do better when the fire frequency is low since they are slow-growing organisms [37,53].
• Mechanical clearing around the base of a tree or around a ground lichen colony is the most efficient preparation method to avoid being consumed by fire [54]. This suggests that raking litter away from ground lichen colonies (Cladonia) could be the most practical way to protect them from prescribed fire.
• Retain some standing and dead wood in the forest for lichen diversity.
• Basic lichen floristic surveys and practical applied research into maintaining lichens in the SE US managed forests are needed. Additional work will need to be carried out to elucidate effective strategies for maintaining and restoring lichens in burned portions of longleaf pine habitats.