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.