Doubled density and increased resilience: Consequences of seven consecutive annual dry-season fires to the unburned Cerrado grass layer

Tropical grasslands and savannas are dominated by C4 grasses, which are crucial for the biodiversity, structure, and functioning of these ecosystems. Yet, grasses have been neglected in vegetation science, leading to a superficial understanding of their responses to fire and of the mechanisms/traits associated with it. This hinders the effective management and conservation of the tropical open ecosystems, especially because recent anthropogenic fire regimes have raised concerns about the long-term effects of repeated dry-season burns. Therefore, we investigated how prolonged frequent fires affect the Cerrado grass layer by exploring community properties, functional guilds, population responses, and persistence mechanisms. We sampled all grass individuals within 120 permanent 1-m² plots before the first fire in 2015 (single fire recorded in the last 30 years) and again after seven annual mid-dry-season fires in 2022. We also collected data on traits relevant to species’ regeneration and persistence. The data were analyzed using an effect size approach to assess changes in community properties and population sizes between sampling occasions. We also compared shifts in the proportions of functional guilds and partitioned the composition of populations to demonstrate persistence mechanisms. After seven annual burns, the grass layer maintained its composition, increased its homogeneity, increased its total richness (from 26 to 30 species), and doubled its density (from 8 to 17 individuals/m²), but did not change its richness/m² (4 species/m²). Functional composition was affected by fires, increasing the prevalence of traits related to fire tolerance and vegetative regeneration. Species responded differently to burns, with 25% benefited, 25% disadvantaged, and 50% unaffected. Resprouting, recruitment, and clonality contributed differently to species persistence, with clonal reproduction driving population growth. Synthesis and applications: the Cerrado grass layer benefits from frequent fires and is resilient even to an extreme fire regime, such as seven annual burns. Recurrent mid-dry-season fires are not harmful to the grass layer and should be less concerning than prolonged fire-suppression regimes, which diminish grasses’ biomass, cover, diversity, and functioning. Maintaining frequent fires or reintroducing them after prolonged fire exclusion (as in our study) are suitable management actions for the conservation of the grass layer.

热带草原与稀树草原以C4草本植物（C4 grasses）为优势类群，此类草本对于该类生态系统的生物多样性、结构与功能至关重要。然而，植被科学领域长期忽视草本植物，导致学界对其火响应机制及相关性状的认知较为浅显，这阻碍了热带开放生态系统的有效管理与保护——尤其是近年来人为火制度引发了学界对重复旱季火烧长期效应的担忧。
为此，本研究通过解析群落属性、功能群（functional guilds）、种群响应及种群持久性机制，探究了长期高频火烧对塞拉多（Cerrado）草本层的影响。
本研究于2015年首次火烧前（该区域过去30年仅记录过1次火烧），在120个永久性1平方米样方内对所有草本个体进行了取样，并于2022年经历7年连续旱季中期火烧后再次开展取样。
同时，本研究还采集了与物种更新及持久性相关的性状数据。
本研究采用效应量分析法，评估两次取样间群落属性与种群规模的变化；同时对比了功能群占比的变化，并通过拆分种群组成以阐释种群持久性机制。
历经7年连续火烧后，塞拉多草本层的物种组成得以维持，群落均匀度提升，总物种丰富度从26种增至30种，种群密度翻倍（从8株/平方米升至17株/平方米），但每平方米物种丰富度未发生变化（仍为4种/平方米）。
火烧显著改变了功能组成，使得与火耐受性及营养繁殖相关的性状占比升高。
不同物种对火烧的响应存在差异：25%的物种受益，25%的物种受抑制，剩余50%的物种未受明显影响。
萌芽更新、实生更新及克隆生长对物种持久性的贡献存在差异，其中克隆繁殖是种群增长的主要驱动因素。
综合与应用：塞拉多草本层受益于高频火烧，即便面对7年连续火烧这类极端火制度仍具有恢复力。周期性旱季中期火烧对草本层并无危害，相较于长期火抑制制度会降低草本生物量、盖度、多样性与生态系统功能的情况，前者无需过度担忧。维持高频火烧或在长期排除火烧（如本研究中的场景）后重新引入火烧，均是保护草本层的适宜管理措施。