Gaps critical for the survival of exposed seeds during Cerrado fires

Abstract

The fine-scale effects of fire and the consequences for seed survival are poorly understood, especially in the Cerrado (Brazilian savannas). Thus, we investigated whether vegetation gaps (bare soil patches) influence the survival of exposed seeds during fire events in the Cerrado by serving as safe sites. We performed field fire experiments in Central Brazil to examine how gap size (% of bare soil) influences fire heat (fire temperatures and residence time) and seed survival (Experiment 1) and to determine how seed survival is affected by fixed conditions: gaps vs grass tussocks during fires (Experiment 2). We used seeds of two common Cerrado legumes, Mimosa leiocephala Benth. and Harpalyce brasiliana Benth. Seed survival was analysed using GLMMs with a binomial distribution. In Experiment 1, seeds survived (38 and 35% for M. leiocephala and H. brasiliana respectively) only when the gaps had >40% of bare soil. In Experiment 2, all seeds under grass tussocks died when exposed to fire, whereas up to 40% of seeds survived in vegetation gaps, relative to their respective controls. Because vegetation gaps influence fire heat, they are important as safe sites for seed survival in the Cerrado, allowing a significant proportion of seeds to survive when exposed at the soil surface.

Post‐burning germination responses of woody invaders in a fire‐prone ecosystem

“…As very high heat exposures are expected to occur under fire on the soil surface (Daibes et al . ) and beneath trees (i.e. equal to or higher than those of the most intense heat treatments of this study), post‐dispersed seeds in these areas are not expected to survive most fire events.…”

“…) or large vegetation gaps with bare soil (Pausas & Keeley ; Daibes et al . ). Such small‐scale variation in fuel loads that cause heterogeneous patterns of surface fire severity (Rocca ) may be found in the secondary grasslands and degraded shrublands – where these invaders regularly occur – where fuel loads are reduced by intense livestock grazing (Giorgis et al .…”

“…; Daibes et al . ). Most post‐dispersed seeds of the study species might remain on the soil surface (García et al .…”

“…This is in line with results reported by Daibes et al . (), who found that in Cerrado open savannas most seeds of legumes died when exposed directly to fire. Still, our findings indicate that G. triacanthos generally has the highest germination rate of the studied species regardless of heat index, except at the high heat index level.…”

“…; Daibes et al . , ). For example, the heat intensity to which seeds are exposed during a wildfire depends on seed distribution (i.e.…”