Fire effects on a fire-adapted species: response of grass stage longleaf pine seedlings to experimental burning

During the past few decades, forest managers have increasingly attempted to incorporate patterns of natural disturbance regimes into forest management practice (Attiwill 1994, Franklin et al. 2002, Palik et al. 2002), with particular interest in understanding the effects of fire regimes on forested ecosystems in both ecological and management contexts. In North America, longleaf pine (Pinus palustris Mill.) ecosystems of the southeastern United States exemplify the tight coupling of ecosystem function and fire regime (Van Lear et al. 2005, Mitchell et al. 2006). Much of the southeastern Coastal Plain region was historically dominated by longleaf pine ecosystems that were maintained with fire return intervals generally < 5 yr (Frost 2006, Huffman 2006, Stambaugh et al. 2011). In these ecosystems, frequent fire generates a positive feedback among vegetation, fuels, and fire (O’Brien et al. 2008, Mitchell et al. 2009). The combination of long, resinous pine needles shed from the canopy trees and herbaceous ground-layer vegetation creates a fuelbed that ignites easily and burns quickly along the forest floor. Consumption of the forest floor creates space for regenerating individuals of the characteristically species-rich ground flora (Hiers et al. 2007, Veldman et al. 2014) and germination sites for longleaf pine seed (Croker and Boyer 1975, Brockway et al. 2006). Longleaf pines are shade intolerant and poor competitors compared to faster-growing tree species (Boyer 1990). Frequent fires inhibit the development of other woody vegetation (Glitzenstein et al. 1995, Brockway and Lewis 1997, Addington et al. 2015) that would displace longleaf pine regeneration and the herbaceous ground flora. Without fire, longleaf pine ecosystems can quickly transition to alternative states and may be difficult to re-establish without intensive treatments (Martin and Kirkman 2009).

Longleaf pines have a seedling morphology described as a “grass stage,” which is also expressed by other pine species that occur within frequent-fire ecosystems across the world (e.g., Pinus merkusii Jungh. & Vriese ex Vriese, P. montezumae Lamb, P. michoacana Lindl.; Koskela 2000, Keeley 2012). Following germination, grass stage seedlings allocate growth to root and needle production rather to than stem elongation (Brockway et al. 2006, O’Brien et al. 2008). During this stage, the terminal bud is located near the ground surface and is surrounded by a dense tuft of needles that may be consumed by fire but protects the terminal bud from damage (Wahlenberg 1946, O’Brien et al. 2008, Keeley 2012). Although generally considered to be resistant to fire, grass stage longleaf pine seedlings have also been documented to sprout following top-kill, providing an additional mechanism for survival with frequent fire (Farrar 1975). For longleaf pine, seedlings emerge from the grass stage and begin stem elongation when the root collar diameter is approximately 25 mm (Boyer 1990, Ramsey et al. 2003, Knapp et al. 2006), and the thick bark of saplings provides protection from fire during tree recruitment (Schafer et al. 2015).

The grass stage is widely accepted as an adaptation to frequent fire (Brockway et al. 2006, Keeley 2012), yet several studies have reported high mortality rates of small grass stage seedlings following fire (Croker and Boyer 1975, Grace and Platt 1995a, Moule 2013). Brockway et al. (2006) suggested a threshold root collar diameter of 13 mm for increasing the probability of survival from surface fire, although seedling mortality has also been shown to increase with fire intensity (Jack et al. 2010). Observations of seedling aggregation within canopy openings in naturally regenerated longleaf pine stands may be related to fire-induced mortality of longleaf pine seedlings beneath canopy pines (Grace and Platt 1995a, Avery et al. 2004). However, effects of seedling size and fire intensity on mortality are difficult to separate in observational studies because greater canopy densities often correspond to greater competition (i.e., smaller longleaf pine seedlings) and greater fuel loads from needlefall (i.e., greater fire intensity).

Fire adaptation of longleaf pine seedlings is often associated with traits that allow for survival, whereas the impacts of fire on seedling growth have not been well documented. The consumption of foliage by fire would necessitate the allocation of stored carbohydrate to replace needles and restore pre-burn photosynthetic capacity. Long-term experimental studies have reported growth reductions of longleaf pine saplings following repeated prescribed burning (Boyer 1987). However, the release of nutrients following burning and the consumption of competing vegetation may also provide opportunities for increased growth of survivors (Brockway et al. 2006). Grelen (1978) reported that prescribed burning increased the proportion of seedlings that had emerged from the grass stage in a four-year study in Louisiana, USA. It is not clear to what degree burning alone, in the absence of indirect effects such as changes in resource availability, affects longleaf pine seedling growth responses.

Previous studies have shown season of burn to be an important ecological driver within frequent-fire ecosystems, although effects vary across taxa and by response variable (Platt et al. 1988a, Streng et al. 1993, Hiers et al. 2000). Generally, burning within the growing season has been reported as more effective at killing small hardwood trees than burning in the dormant season (Waldrop et al. 1992, Brose et al. 1999). Seasonal effects of burning on trees and shrubs have been attributed to differences in fire intensity due to differences in weather conditions during burns, as well as to differences in plant physiology at the time of burning (Glitzenstein et al. 1995, Drewa et al. 2002). Although previous studies have reported no effects of season of burn on longleaf pine survival (Glitzenstein et al. 1995, Jack et al. 2010), these studies either considered trees that were out of the grass stage, or grouped grass stage seedlings as a single size class.

This study was designed to examine the direct effects of fire on grass stage longleaf pine seedlings in a controlled field experiment. Whereas previous experimental studies have manipulated fire intensities in longleaf pine ecosystems by removing or adding fuels during burns (e.g., Thaxton and Platt 2006, Jack et al. 2010), we used a prescribed burn simulation tool (PBST) to deliver controlled fire treatments to individual longleaf pine seedlings. We used thermocouple (TC) temperatures as a surrogate of fire intensity. Our specific objectives were to determine: 1) effects of seedling size, fire intensity (measured as TC temperature), and season of treatment application on longleaf pine seedling survival and growth; 2) the contribution of sprouting to longleaf pine seedling survival following burning; and 3) impacts of fire on the initiation of height growth. We hypothesized that: 1) seedling size is positively related to post-fire survival while fire intensity is negatively related to post-fire survival; 2) direct effects of fire on longleaf pine seedlings result in short-term growth reductions; and 3) season of burn would not affect longleaf pine growth or survival, similar to results reported from previous studies.

With historical fire return intervals reported to have been as frequent as 1 to 3 years (Frost 2006, Huffman 2006, Stambaugh et al. 2011), longleaf pine must be able to survive fire during the seedling stage for successful regeneration to occur. Similar to previous studies, our results provide evidence that longleaf pine seedlings are fire resistant but not fireproof, and that seedling size and fire intensity each affect seedling survival (Hypothesis 1). In a prescribed fire study of in situ longleaf pines that ranged in size from grass stage seedlings to saplings, Jack et al. (2010) found that grass stage seedlings (defined as < 0.2 m tall) had greater mortality than larger seedlings and saplings. Results from our study demonstrate that the size of seedlings within the grass stage also affects the probability of survival. Previous studies have reported a range of root collar diameter thresholds for fire resistance. Bruce (1951) reported that all grass stage seedlings with root collar diameters greater than 5.1 mm survived a winter burn in Mississippi, USA; Croker and Boyer (1975) cite 7.6 mm as the threshold for fire resistance; and Brockway et al. (2006) cited a threshold of 13.0 mm. Generally, our results suggest that the probability of seedling survival one year after fire exceeds 50% for seedlings with root collar diameters greater than 15 mm for the high-temperature burn treatment. Our results indicate that the low-temperature burn treatment had little effect on seedling mortality. However, reduced survival of small seedlings after the second growing season, even in the low-temperature burn treatment, demonstrates the potential importance of the interaction between seedling size and fire intensity for grass stage seedling mortality.

The best models for seedling survival (of only burned seedlings) differed for survival near the end of the first (August 2013) and second (August 2014) growing seasons, although initial seedling size was included in both models. In addition to initial seedling size, the area under the time−temperature curve above 60 °C was an important predictor of seedling survival after the first growing season. Several publications have discussed the relevance of Area60, as a measure of heat dosage, to biotic response (Bova and Dickinson 2005, Kennard et al. 2005, Wally et al. 2006). The measure of maximum temperature, used to standardize treatment delivery in our study, may not as strongly indicate conditions experienced by plant tissues exposed to fire because both temperature and duration affect tissue damage (Bova and Dickinson 2008, Stephan et al. 2010). Strong et al. (2013) found that the area under the time−temperature curve was a better predictor of bunchgrass (Aristida purpurea Nutt.) mortality than maximum temperature or the duration above 60 °C. In contrast, the best model for seedling survival after the second growing season in our study included maximum temperature rather than the area under the curve. Given the highly controlled treatment delivery in this study, correlations among fire intensity variables (Max, Area60, Dur60) were likely higher than what would be expected from a natural fire environment (Bova and Dickinson 2008). In addition, the thermocouples, having been placed immediately adjacent to the terminal bud during treatment delivery, provided an estimate of the fire environment of each seedling but did not measure actual tissue temperature.

The best model for seedling survival at the end of the second growing season, which encompassed both the direct, immediate mortality as well as delayed effects, included the month of treatment application. There was greater mortality within the second growing season across all size classes for both burn treatments in the May application, whereas mortality patterns for the January application were similar after the first and second growing season. We speculate that interactions among the fire treatment and environmental conditions may have contributed to our results. Although we achieved similar maximum temperatures with each burn application month, the area under the time−temperature curve and the duration above 60 °C were both greater in the May application than in the January application, perhaps due to higher air temperature during burn application in May. Moreover, the 2014 growing season was drier than average, and it is not known how interactions among environmental stress and fire affect mortality patterns in longleaf pine seedlings. Additional research is warranted to explore the mechanistic drivers of seasonal fire effects on longleaf pine seedling responses.

Sprouting is widely accepted as a persistence mechanism for hardwood trees (Bond and Midgley 2001, Del Tredici 2001) and is commonly discussed as an important fire adaptation for several pine species (e.g., shortleaf pine (Pinus echinata Mill.), pitch pine (P. rigida Mill.); Mattoon 1915, Welch et al. 2000). However, sprouting of longleaf pine seedlings has received relatively little attention. Farrar (1975) used mechanical clipping to test for sprouting responses and reported that up to 60% of small grass stage seedlings sprouted, with lower sprouting rates as tree size increased. We observed sprouting across all study treatments, including the control, but the rate of sprouting did not exceed 30% of the original population for any seedling size and treatment combination. However, sprouting was more common for small seedlings and seedlings in the high-temperature burn treatment, suggesting that the sprouting response was associated with severity of seedling damage. Although sprouting appears to contribute to the short-term fire resistance of longleaf pine seedlings, additional research is needed to determine if sprouting affects the vigor, development, or survival of longleaf pine seedlings through time.

Our results support the hypothesis that prescribed burning causes short-term reductions in seedling growth (Hypothesis 2), although responses depended on fire treatment and varied between the two application months. We did not quantify foliar consumption following the burns but observed that the high-temperature burn treatment completely consumed seedling foliage (Fig. 2c), while low-temperature burn treatment had more variable needle consumption. Regardless, all burned seedlings experienced either nearly complete needle consumption or scorch. These results suggest that short-term growth reductions of longleaf pine seedlings may be due to allocation of carbohydrates for needle production rather than to direct effects of heat damage. This is further supported by the lack of significant differences in needle biomass among treatments, as seedlings prioritized needle growth following consumption or removal.

Regardless of treatment, root collar area increment was low for seedlings treated with the May application, and the majority of root collar diameter growth of seedlings in the Control treatment of the January application occurred between January and May 2013 (data not shown). Low growth rates from May 2013 through the end of the study for seedlings in all treatments (including the Controls) in both treatment application months suggest that additional factors (external to study treatments) were limiting growth. For example, abnormally high rainfall in the 2013 growing season (e.g., Knapp et al. 2008) and abnormally low rainfall in the 2014 growing season (e.g., Rodrı́guez-Trejo et al. 2003) may have reduced the growth potential for all seedlings and contributed to the growth difference observed between the Control seedlings from the January and May application months. Assuming that external factors were limiting growth of all seedlings in this study, it is likely that growth reductions due to treatment effects may have been more pronounced in years with more favorable growing conditions.

Emergence from the grass stage is a critical event for longleaf pine seedling development. Previous studies reported that burning in May can stimulate height growth of longleaf pine seedlings, possibly due to control of competition or reduced infection of brown-spot needle blight (Maple 1977, Grelen 1978). We did not address effects of our experimental burning on the incidence of brown-spot needle blight by excluding infected seedlings from the sampling population. We removed understory competition adjacent to each seedling across all treatments in this study and found no evidence that burning further improved grass stage emergence. The size of the root system (often expressed by the surrogate measure of root collar diameter) tends to provide consistent indication of height growth emergence (Knapp et al. 2006). Although other studies have suggested that competition control may affect the relationship between seedling root collar diameter and height (Ramsey et al. 2003), our results support the rule of thumb that height growth begins when the root collar reaches approximately 25 mm diameter.

In contrast to our expectation, several of our results indicate that season of burn affected longleaf pine seedling response (Hypothesis 3). Although previous experimental studies have generally reported no effects of burn month or season on longleaf pine mortality across tree size classes (Glitzenstein et al. 1995, Jack et al. 2010), our study, which considers a smaller range of seedling sizes, suggests that season of burn may interact with seedling size and fire intensity to affect survival response. For example, mortality of Small seedlings was greater for the growing season application than the dormant season application for the low-temperature burn treatment, but the high-temperature burn treatment resulted in high mortality regardless of season. In contrast, Medium seedlings were large enough for high survival regardless of burn season for the low-temperature burn treatment but had lower survival with growing season burns than dormant season burns for the high-temperature burn treatment (Fig. 3). For Large seedlings, burn season had no effect regardless of fire intensity. Given that ambient temperatures were higher during burns in May than in January, and the fire intensity measures of Area60 and Dur60 were greater in May than in January, we cannot determine if the burn season effect would be attributed to seedling physiology or to differences in fire treatment intensity. However, the mortality models suggest that the effect of burn season on seedling mortality was primarily attributed to differences in mortality that took place during the second growing season.

The manipulative, experimental approach taken in this study provides unique insight into fire’s effects on naturally regenerated, grass stage longleaf pine seedlings, yet comes with important trade-offs and limitations. For example, the thermocouples used in the study were positioned adjacent to the terminal bud of the target seedling but did not reflect the true temperature of the plant tissue. Thermocouples are commonly used to describe characteristics of fire behavior, and, while having been found to provide useful information, were limited to recording the temperature of the thermocouple device itself, as influenced by its materials and construction, rather than the energy output of the fire (Kennard et al. 2005, Bova and Dickinson 2008, Kremens et al. 2010). Although the treatments we applied were developed based on comparable data collected from actual prescribed burns (Fig. 1), the use of a PBST does not recreate all aspects of prescribed burning (Kral et al. 2015); for example, the delivery of heat from above with the PBST differs from fire movement with an operational prescribed burn, and we purposefully removed most surrounding fuels to better control the time−temperature curves. Thus, the study design includes a trade-off between experimental control and direct representation of operational conditions.

In naturally regenerated longleaf pine forests and woodlands, seedlings and saplings are commonly observed in aggregations within canopy openings (Platt et al. 1988b, Pecot et al. 2007), which previous studies have attributed to competitive exclusion by canopy trees (Brockway and Outcalt 1998, Gagnon et al. 2004) and seedling mortality beneath the canopy due to greater fire intensities caused by needle accumulation (Grace and Platt 1995a, Avery et al. 2004). Our results suggest that these two factors likely work in combination to affect the spatial distribution of natural regeneration. Growth suppression of longleaf pine seedlings by canopy trees has been well documented (Boyer 1963, Croker and Boyer 1975, Grace and Platt 1995b), with little seedling growth occurring when basal area exceeds 9 m² ha^− 1 (Mitchell et al. 2006). Likewise, fire intensity has been reported to increase with proximity to longleaf pine canopy trees (Williamson and Black 1981). Our results demonstrate that seedlings across the size range observed in this study had relatively good survival with low-temperature burns, but survival was stratified by seedling size with greater fire intensity. Fire behavior is variable at fine scales within longleaf pine ecosystems (Hiers et al. 2009), suggesting that seedling survival may also be spatially variable. As indicated by our findings, the probability of survival would decrease beneath canopy trees, where seedling size is reduced by competition and fire intensity is likely greater due to needle accumulation.

Longleaf pine plantations are established as an initial step in ecosystem restoration where canopy longleaf pines no longer exist (Brockway et al. 2005) or to meet objectives related to wildlife, timber, or pine needle production (South 2006). In a restoration context, reintroducing fire is considered to be critical for maintaining ecosystem function over long timescales (Freeman and Jose 2009, Martin and Kirkman 2009, Addington et al. 2012). Results from this study indicate that the risk of mortality in longleaf pine plantations is relatively high for seedlings with root collars less than 15 mm diameter. High-quality container-grown longleaf pine seedlings are recommended to have root collar diameters of 7 to 9 mm before planting (Barnett and McGilvray 2000), although studies have reported root collar diameters of container-grown seedlings to range from 2.2 to 11.0 mm at planting (South et al. 2005). Growth rates of longleaf pine seedlings are variable but have been reported to range from 2 to over 10 mm per year without canopy competition (South et al. 2005, Hu et al. 2012a). Depending on size at planting and subsequent growth rates, longleaf pine seedlings may take several years to develop root collars greater than 15 mm diameter. Recently, underplanting has been suggested as a viable option for converting existing pine stands to longleaf pine (Kirkman et al. 2007, Hu et al. 2012b, Knapp et al. 2013). The retention of canopy trees has been reported to reduce growth rates of planted seedlings, which potentially extends the vulnerability of longleaf pine seedlings to fire mortality.

Although fire simulation methods are imperfect in mimicking prescribed burn conditions and effects, the controlled and repeatable application of burn treatments, as used in this study, cannot be achieved with prescribed burning. Our results demonstrate fire resistance of longleaf pine seedlings, but at the same time reveal that greater fire intensity selects against small longleaf pine seedlings. Moreover, fire season may impact survival of grass stage longleaf pine seedlings but further interacts with seedling size and fire intensity. Other factors that may affect seedling response to fire, such as seedling age and vigor, environmental conditions, or interactions with nearby plants, were not tested in this study; thus, additional research is needed to determine mechanisms for seedling responses. Understanding the effects of fire on grass stage longleaf pine seedlings can inform burn prescriptions to ensure that natural or artificial regeneration needs are met while maintaining a frequent-fire regime for ecosystem management.