Burning for grassland pollination: recently burned patches promote plant flowering and insect pollinators

Abstract

Grasslands are historically and evolutionarily associated with disturbances, such as fire, that drive biodiversity assembly patterns and biotic interactions. Disturbance suppression in fire- prone ecosystems usually leads to a decline in forb diversity and flowering due to biomass accumulation, which could jeopardize pollinator diversity. In this study, we investigated patterns and drivers of plant flowering and flower insect visitor communities in a chronosequence of patches from different time-since-fire categories in Southern Brazilian grasslands. Old-burnt patches (more than 2 years since fire) had taller vegetation, more biomass and grass cover than intermediate (about 1 year after fire) and freshly-burnt patches (less than 6 months since fire), which had increased bare soil cover. Forb flower abundance was much higher in freshly-burnt patches, directly predicted by the degree of habitat openness. Pollinator insects were then benefited by floral resource aggregation in freshly-burnt patches, increasing in abundance (bees and butterflies) and species richness (bees). Beetle communities were positively influenced by vegetation height. Furthermore, plant species flowering and bee species composition varied between freshly and old-burnt grasslands, with indicator species found for all recovery stages but mainly freshly-burnt patches. Altogether, these results indicate the importance of maintaining freshly-burnt patches in the grassland landscape: it helps to sustain flower diversity, pollination services, and flowering plant reproduction. Our findings support the idea that a mosaic of grasslands from different times-since- fire should be considered for grassland conservation.