Patchy Fires Promote Regeneration of Longleaf Pine (Pinus palustris Mill.) in Pine Savannas

Abstract

Research Highlights: Spatial patterns of fire spread and severity influence survival of juvenile pines in longleaf pine savannas. Small areas that do not burn during frequent fires facilitate formation of patches of even-aged longleaf pine juveniles. These regeneration patches are especially associated with inner portions of openings (gaps) and where canopy trees have died in recent decades. Patterns of prescribed fire can thus have an important influence on stand dynamics of the dominant tree in pine savannas. Background and Objectives: Savannas are characterized by bottlenecks to tree regeneration. In pine savannas, longleaf pine is noted for recruitment in discrete clusters located within gaps away from canopy trees. Various mechanisms promoting this pattern have been hypothesized: light limitations, soil moisture, soil nutrients, pine needle mulching, competition with canopy tree roots, and fire severity associated with pine needle litter. We tested the hypothesis that regeneration patches are associated with areas that remain unburned during some prescribed fires, as mediated by gaps in the canopy, especially inner portions of gaps, and areas re-opened by death of canopy trees. Materials and Methods: We mapped areas that were unburned during prescribed fires applied at 1–2 year intervals from 2005–2018 in an old-growth pine savanna in Georgia, USA. We compared the maps to locations of longleaf pine juveniles (<1.5 m height) measured in 2018 and canopy cover and canopy tree deaths using a long-term (40 year) tree census. Results: Logistic regression analysis showed juveniles to be associated with unburned areas, gaps, inner gaps, and areas where canopy trees died. Conclusions: Patterns of fire spread and severity limit survival of longleaf pine juveniles to patches away from canopy trees, especially where canopy trees have died in recent decades. These processes contribute to a buffering mechanism that maintains the savanna structure and prevents transition to closed canopy forest or open grassland communities.

Fire Survival of Longleaf Pine (Pinus palustris) Grass Stage Seedlings: The Role of Seedling Size, Root Collar Position, and Resprouting

“…Previous studies have demonstrated that fire behavior (e.g., season, intensity, patchiness) significantly affected the mortality of longleaf pine grass stage seedlings (e.g., [7,25]). Consequently, we acknowledged that the absolute fire-induced mortality rate of longleaf pine seedlings was necessarily varying with fire behavior that is largely determined by the weather conditions and fuel properties during the burn.…”

“…Furthermore, the studied forest was planted and had a relatively uniform distribution of longleaf pine trees. With needle litter, the main fuel component, cast evenly on the ground, fire may have burned uniformly and impacted all seedlings [36], making survival due to fire escape (i.e., on the unburned spots) less likely [25]. Therefore, the mortality estimation from this study might represent the upper threshold of fire-induced mortality for seedlings of the same size.…”

“…Currently, a relatively fixed burning schedule with a short return interval has been widely used in the management of longleaf pine forests. Fire-induced mortality of longleaf pine seedlings under such burning regime would be high, and unburned patches would become important to seedling survival [25]. Given the uniformed topography in sandhills, the presence of any unburned patches is likely dependent on within-stand variation in the fuel complex [22,23].…”

“…Given the uniformed topography in sandhills, the presence of any unburned patches is likely dependent on within-stand variation in the fuel complex [22,23]. In longleaf pine plantations, unburned patches become less likely because planted trees are regularly spaced and provide continuous coverage of dead needles [25,36]. Therefore, we recommended a more flexible burning schedule with a variable return interval in longleaf pine plantations in sandhills to ensure the success of natural longleaf pine regeneration.…”

“…As a result, some longleaf pine seedlings are killed by fire even during the grass stage. Previous studies suggested that seedling size [14,[16][17][18][19][20][21] and fire behavior [16,[22][23][24][25] could significantly affect fire-induced mortality of longleaf pine seedlings.…”