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天然野火，常被认为是维持稀树草原（savanna）中不连续林木覆盖与草本主导地面层的生态机制。然而，此类野火未必能稳定维持湿润稀树草原的这一格局。不过，稀树草原树木的易燃凋落叶，或许可通过改变火行为特征，经由对地面层植物的作用来维持湿润稀树草原。我们依托一处高降雨、高频火烧的松稀树草原开展假说验证，该生境的优势树种为长叶松（Pinus palustris），其分泌树脂的针叶在脱落干燥后极具易燃性。我们提出假说：相较于其他丰富的细级可燃物（fine fuels）（脱落的栎叶与草本茎秆），易燃针叶对地面火行为特征，以及优势灌丛与草本的火烧后响应，具有更为显著的影响。我们进一步推测，此类影响会随针叶量的增加而增强。我们调控了影响可燃物的生境条件（火烧间隔时间、优势植被组成），对地面样方内的针叶量进行人工操控，对样方实施计划性火烧（prescribed burn），并测定了地面层的火行为特征。我们还分别在火烧前、火烧后2个月与8个月，测定了地面层栎类与草本的相关特征。我们针对易燃松木可燃物对火行为特征与植被恢复的影响验证了相关假说，并通过结构方程模型（structural equation model）探究了可燃物对火行为与植被的直接与间接效应。长叶松针叶确实对火行为特征产生了影响：相较于仅以其他稀树草原植物为可燃物的样方，针叶样方的最大升温幅度、60℃以上加热时长以及细级可燃物消耗量均有显著提升。针叶的存在抑制了火烧后栎类茎秆与草本茎秆的数量，尤其是在草本基株（genet）的内部区域，同时也抑制了草本的火烧后开花情况。结构方程模型结果显示，从针叶到栎类与草本的火烧后响应存在显著的直接与间接通路。经结构方程模型验证的野外假说实验结果表明，易燃细级可燃物会改变地面层计划性火烧的行为特征，对优势地面层栎类与草本产生负面影响。由易燃针叶作为主要可燃物的高频火烧，应当能够维持湿润稀树草原的格局，并产生空间火多样性（pyrodiversity），进而影响松稀树草原地面层植被的组成与动态。