Data from: Pyrogenic fuels produced by savanna trees can engineer humid savannas

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. Pyrogenic shed leaves of savanna trees, however, might engineer fire characteristics in ways that maintain humid savannas through effects on ground layer plants. 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.

干旱后由雷击引发的自然野火，常被认为是维持稀树草原中非连续树木覆盖与草本主导地面层的生态机制。然而，此类野火或无法稳定维持湿润稀树草原。不过，稀树草原树木的致火性凋落叶，或可通过影响地面层植物，调控火行为特征（fire characteristics），以此维持湿润稀树草原。我们在一处高降水、高频火烧的松稀树草原中验证该假说：该区域的优势树种为长叶松（longleaf pine, Pinus palustris），其分泌树脂的松针脱落干燥后具有极强可燃性。我们提出假说：相较于其他丰富的细可燃物（fine fuels，即脱落的栎叶与禾草茎秆），致火性松针对地面火行为特征以及优势灌丛与禾草的火烧后响应的影响应更为显著。我们进一步推测，此类影响会随松针存量的增加而增强。我们调控了影响可燃物的样地条件（火烧间隔时间、优势植被组成），对地面层样地内的松针存量进行人工干预，随后对样地实施计划性火烧（prescribed burn），并测量了地面层的火行为特征。我们还在火烧前、火烧后2个月与8个月，分别测量了地面层栎类与禾草的相关特征。我们通过结构方程模型（structural equation model），验证了关于致火性松可燃物对火行为特征与植被恢复的影响的假说，并探究了可燃物对火行为特征与植被的直接及间接效应。研究结果显示，松针会显著影响火行为特征：相较于仅以其他稀树草原植物为可燃物的情况，松针可大幅提升最高温升、60℃以上加热时长以及细可燃物消耗量。松针的存在会抑制火烧后栎类茎秆与禾草茎秆的数量，尤其是在禾草基株的内部区域，同时也会抑制禾草的火烧后开花。结构方程模型显示，从松针到栎类与禾草的火烧后响应，存在显著的直接与间接作用路径。经结构方程模型佐证的野外假说实验结果表明，致火性细可燃物会改变地面层计划性火烧的特征，对优势地面层栎类与禾草产生负面影响。由致火性松针驱动的高频火烧，或可维持湿润稀树草原，并产生空间火多样性，进而影响松稀树草原地面层植被的组成与动态。