Tropical riparian forests in danger from large savanna wildfires

1. Tropical savannas are known for the fire-prone ecosystems, yet,
riparian evergreen forests are another important landscape feature. These
forests usually remain safe from wildfires in the wet riparian zones. With
global changes, large wildfires are now more frequent in savanna
landscapes, exposing riparian forests to unprecedented impact. 2. In 2017,
a large wildfire spread across the Chapada dos Veadeiros National Park, an
iconic UNESCO site in central Brazil, raising concerns about its impact on
the fire-sensitive ecosystems. By combining remote sensing analysis of
Google Earth images (2003-2019) with detailed field information from 36
sites, we assessed wildfire impacts on riparian forests. For this, we
measured the structure of trees, saplings and herbaceous plants, as well
as topsoil variables. 3. Since 2003, all riparian forests had canopy cover
above 90 %, but after 2017, canopy cover dropped to 20 % in some forests,
indicating large variation in wildfire damage. A closer look in the field
revealed that, on average, the wildfire killed 52 % of adult trees and 87
% of tree saplings in flooded forests. In non-flooded forests, impacts on
adult trees were negligible, but fire killed 75 % of tree saplings.
Opportunistic vines and the invasive grass Melinis minutiflora were
already present in severely disturbed flooded forests. In all forests,
impacts on many ecosystem variables were related to canopy damage, a
variable measurable from satellite. Overall, seasonally flooded riparian
forests were the most severely impacted, possibly due to the relatively
thinner barks of their trees. 4. Synthesis and applications. Our findings
reveal how riparian forests embedded in tropical savanna landscapes are in
danger from large wildfires. The destruction of some forests has opened
space for new plant species that may propel a shift to an alternative
ecosystem state. Riparian forests are habitat of large savanna animals and
their loss could affect entire trophic networks. Managing wildfires and
invasive grasses locally is probably the best strategy to maintain
riparian forests resilient. As wildfire regimes intensify in tropical
savanna landscapes, our findings stress the need for an integrated
management that considers riparian forests as a vulnerable element of the
system.