Tropical riparian forests in danger from large savanna wildfires

In their latest research, Flores and colleagues combine satellite image analysis with detailed field assessments, to quantify the impact caused by large wildfires on riparian forests in the Chapada dos Veadeiros National Park (CVNP).

Large savanna wildfires are increasing across the tropics because of a synergism between climate change and unsustainable management practices. In Brazil, for instance, savannas of the Cerrado and Pantanal have recently experienced the most severe wildfires on record. Whilst most trees and grasses in the savanna have evolved under frequent fires, the wet riparian zones rarely burn, meaning they can be extremely fire-sensitive.

In October 2017, the Chapada dos Veadeiros National Park (CVNP), a UNESCO World Heritage Site in Central Brazil, experienced a large wildfire that burnt more than 86,000 hectares of native and well-preserved vegetation, including the fire-sensitive riparian forests.

The park landscape is dominated by fire-prone savannas, yet riparian forests are common along streams. These habitats are critical for large animals, such as jaguars, because they provide water and shelter. Their trees usually act as barriers for wildfire spread, and reduce soil erosion, maintaining water security

Riparian forests surrounded by grasslands and savannas at the Chapada dos Veadeiros National Park. Photo: Bernardo M. Flores.

In our study, we examined the impacts of the large 2017 wildfire on riparian forest ecosystems, as requested by managers of the CVNP. First, we used Google Earth images to assess how forest cover changed between 2003 and 2019 across 90 hectares of riparian forest. Since 2003, forest cover remained above 90 %, but after 2017, it dropped, ranging from 20 to 100 % cover.

In the field, we looked at fire impacts in detail using data from 36 forest plots, including information about all trees, herbaceous plants and soil properties. Surprisingly, forests that flood during the rainy season were the most severely damaged, suffering on average 52 % tree mortality and 39 % topsoil loss. In contrast, non-flooded forests suffered only mild impacts and appear to be more fire-resistant.

Only six months after the wildfire, invasive grasses and other opportunistic plants, such as vines and ferns, were already dominating many burnt forests, probably facilitated by the nutrients released in the soil (e.g. 18-fold increase in exchangeable phosphorus).

Aerial view of the 2017 wildfire at the Chapada dos Veadeiros National Park. Photo: Fernando Tatagiba.

In the coming years, we intend to monitor burnt forests to assess whether they will recover or persist in a state of open vegetation, degraded by invasive and opportunistic plants. However, if Mauritia flexuosa palms (buriti), which also occur along streams, recruit massively in burnt sites, forests could be replaced by palm swamp savannas, locally known as veredas.

The loss of riparian forests by large savanna wildfires could have catastrophic consequences in the future. These forests are critical elements of the system, and they contribute to maintain ecological networks and ecosystem processes stable.

Our study demonstrates that it is necessary to manage landscape flammability. In many tropical savannas, prescribed controlled fires are already being used to help restore landscape heterogeneity, and reduce wildfire spread. The control of non-native grasses is another important measure to reduce their invasion risk.

Managing landscape flammability can be extremely challenging, particularly in Brazil, where environmental governance is now weakened, which has allowed fire outbreaks to boost across the country. However, it will become increasingly necessary to preserve tropical riparian forests in the future.

The full article, Tropical riparian forests in danger from large savanna wildfires, is free to read for a limited time in Journal of Applied Ecology

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