Pyrogenic fuels produced by savanna trees can engineer humid savannas

Abstract

Natural fires ignited by lightning strikes following droughts frequently are posited as the ecological mechanism maintaining discontinuous tree cover and grass‐dominated ground layers in savannas. Such fires, however, may not reliably maintain humid savannas. We propose that savanna trees producing pyrogenic shed leaves might engineer fire characteristics, affecting ground‐layer plants in ways that maintain humid savannas. We explored our hypothesis in a high‐rainfall, frequently burned pine savanna in which the dominant tree, longleaf pine (Pinus palustris), produces resinous needles that become highly flammable when shed and dried. We postulated that pyrogenic needles should have much greater influence on fire characteristics at ground level, and hence post‐fire responses of dominant shrubs and grasses, than other abundant fine fuels (shed oak leaves and grass culms). We further reasoned that these effects should increase with amounts of needles. We managed site conditions that affect fuels (time since fire, dominant vegetation), manipulated amounts of needles in ground‐layer plots, prescribed burned the plots, and measured fire characteristics at ground level. We also measured characteristics of ground‐layer oaks and grasses before, then 2 and 8 months after fires. We tested our hypotheses regarding effects of pyrogenic pine fuels on fire characteristics and vegetation regrowth and explored direct and indirect effects of fuels on fire characteristics and vegetation using a structural equation model. Pine needles influenced fire characteristics, elevating maximum temperature increases, durations of heating above 60°C, and fine fuel consumption considerably above measurements when fuels only included other savanna plants. Presence of pine needles depressed post‐fire numbers of oak stems and grass culms, especially in the interior of grass genets, as well as post‐fire flowering of grasses. The structural equation model indicated strong direct and indirect pathways from pine needles to post‐fire responses of oaks and grasses. The experimental field tests of hypotheses, bolstered by structural equation modeling, indicate pyrogenic fine fuels modify characteristics of prescribed fires at ground level, negatively affecting dominant ground‐layer oaks and grasses. Frequent fires fueled by pyrogenic needles should maintain humid savannas and generate spatial pyrodiversity that affects composition and dynamics of pine savanna ground‐layer vegetation.

Frequent fire reorganizes fungal communities and slows decomposition across a heterogeneous pine savanna landscape

“…Locations near or away from pines and in litter or soil substrates may result in compositionally different fungal communities that, in turn, are differentially reorganized by fire. For example, fire intensity increases with greater density of pine needles (Ellair & Platt, ; Platt et al ., ), so fungal communities in the vicinity of pines may be altered by fire more than communities away from pines are. Postfire shifts in fungal communities should be tied to fire‐induced shifts in soil properties and vegetation, as suggested by prior studies in similar systems (Hart et al ., ; Ponder et al ., ; Oliver et al ., ). For example, fungal taxa associated with flammable plants favoured by fire ( sensu Gagnon et al ., ) might respond positively to fire, whereas fungi associated with less‐resistant plant hosts may decline ( sensu Platt et al ., ).…”

“…Pine savannas of the North American Coastal Plain are a key example characterized by discontinuous tree cover within a continuous ground‐layer vegetation. This pine–grassland ‘matrix’ supports a diverse, highly endemic flora and fauna (Platt, ; Noss et al ., ; Peet et al ., ) and structures the spatial distribution of fuels, soil properties, and plants that, in turn, influence fire characteristics (Ellair & Platt, ; Platt et al ., , ; Varner et al ., ; Veldman et al ., ). For example, fuel loads from overstory pines ( Pinus palustris ) directly influence fire intensity (Platt et al ., ), and dominance of pines defines the chemical composition of litter, soil pH and nutrient concentrations, loads of photosynthetic carbon (C), and vegetation demand for water and nutrients (Priha et al ., ; Osono et al ., ; Deng et al ., ; Stoppe et al ., ).…”

“…Fungal taxa adapted to alkaline or high‐nutrient conditions might also be favoured by postfire shifts in soil properties, such as increased soil pH, N, phosphorus (P), and C associated with low‐intensity fires (Raison & McGarity, ; Certini, ; Ponder et al ., ). Fine‐fuel decomposition should be governed by both fire and proximity to pine. Pine needles should decompose slower than grass litter, leading to reduced decomposition below pines (Platt et al ., ). Fire effects on decomposition, however, might be tied to fungal taxa that fire enhances or suppresses.…”

“…For example, fire intensity increases with greater density of pine needles (Ellair & Platt, ; Platt et al ., ), so fungal communities in the vicinity of pines may be altered by fire more than communities away from pines are. Postfire shifts in fungal communities should be tied to fire‐induced shifts in soil properties and vegetation, as suggested by prior studies in similar systems (Hart et al ., ; Ponder et al ., ; Oliver et al ., ). For example, fungal taxa associated with flammable plants favoured by fire ( sensu Gagnon et al ., ) might respond positively to fire, whereas fungi associated with less‐resistant plant hosts may decline ( sensu Platt et al ., ). Fungal taxa adapted to alkaline or high‐nutrient conditions might also be favoured by postfire shifts in soil properties, such as increased soil pH, N, phosphorus (P), and C associated with low‐intensity fires (Raison & McGarity, ; Certini, ; Ponder et al ., ). Fine‐fuel decomposition should be governed by both fire and proximity to pine.…”

“…Frequent fires that alter fungal communities may modify ecosystem functions that, in turn, influence future fires. Fire intensity in pine savannas is directly related to accumulation of fine fuels produced by treesshed pine needles, cones, and bark (Platt et al, 2016;Peet et al, 2018). These fine fuels are decomposed primarily by microbial saprotrophs, so fire-induced changes to functioning of these organisms should influence decomposition of new litter.…”