Small-Scale Variation in Fuel Loads Differentially Affects Two Co-Dominant Bunchgrasses in a Species-Rich Pine Savanna

Ecological disturbances frequently control the occurrence and patterning of dominant plants in high-diversity communities like C4 grasslands and savannas. In such ecosystems disturbance-related processes can have important implications for species, and for whole communities when those species are dominant, yet mechanistic understanding of such processes remains fragmentary. Multiple bunchgrass species commonly co-dominate disturbance-dependent and species-rich pine savannas, where small-scale fuel heterogeneity may influence bunchgrass survival and growth following fires. We quantified how fire in locally varying fuel loads influenced dynamics of dominant C4 bunchgrasses in a species-rich pine savanna in southeastern Louisiana, USA. We focused on two congeneric, co-dominant species (Schizachyrium scoparium and S. tenerum) with similar growth forms, functional traits and reproductive strategies to highlight effects of fuel heterogeneity during fires. In experimental plots with either reduced or increased fuels versus controls with unmanipulated fuels, we compared: 1) bunchgrass damage and 2) mortality from fires; 3) subsequent growth and 4) flowering. Compared to controls, fire with increased fuels caused greater damage, mortality and subsequent flowering, but did not affect post-fire growth. Fire with reduced fuels had no effect on any of the four measures. The two species responded differently to fire with increased fuels – S. scoparium incurred measurably more damage and mortality than S. tenerum. Logistic regression indicated that the larger average size of S. tenerum tussocks made them resistant to more severe burning where fuels were increased. We speculate that locally increased fuel loading may be important in pine savannas for creating colonization sites because where fuels are light or moderate, dominant bunchgrasses persist through fires. Small-scale heterogeneity in fires, and differences in how species tolerate fire may together promote shared local dominance by different bunchgrasses.

生态干扰（ecological disturbances）常调控C4草原（C4 grasslands）、稀树草原（savannas）等高多样性群落中优势植物的出现与分布格局。在这类生态系统中，与干扰相关的过程对物种以及作为优势种的整个群落均具有重要意义，但目前对这类过程的机制性理解仍较为零散。多种丛生禾草（bunchgrass）通常共同占据依赖干扰且物种丰富的松树稀树草原，其中小型尺度的燃料异质性可能会影响火灾后丛生禾草的存活与生长。

我们以美国路易斯安那州东南部一处物种丰富的松树稀树草原为研究对象，量化了局域燃料载荷变化下的火烧如何影响优势C4丛生禾草的种群动态。我们聚焦于两个同属的共优种（Schizachyrium scoparium与S. tenerum），二者拥有相似的生长形态、功能性状与繁殖策略，旨在阐明火烧过程中燃料异质性所产生的效应。我们在燃料削减、燃料增加以及未干预的对照实验样地中，对比了四项指标：1）丛生禾草的火烧损伤程度；2）火烧死亡率；3）灾后生长情况；4）开花情况。

相较于对照组，燃料增加处理下的火烧会造成更严重的植株损伤、更高的死亡率以及更多的后续开花，但并未影响灾后生长。燃料削减处理下的火烧对上述四项指标均无影响。两个物种对燃料增加组火烧的响应存在差异：Schizachyrium scoparium的损伤程度与死亡率均显著高于S. tenerum。逻辑回归（logistic regression）分析显示，S. tenerum的丛生体平均尺寸更大，使其在燃料载荷增加的环境中能够耐受更剧烈的火烧。

我们推测，在松树稀树草原中，局域燃料载荷的增加可能对创建定植位点至关重要，因为在燃料量较轻或适中的区域，优势丛生禾草可在火烧后存活下来。火烧的小型尺度异质性，以及不同物种对火烧的耐受差异，可能共同促进了不同丛生禾草在局域范围内的共优共存。