Root traits in response to frequent fires: Implications for belowground carbon dynamics in fire-prone savannas

Abstract

Predicting how belowground carbon storage reflects changes in aboveground vegetation biomass is an unresolved challenge in most ecosystems. This is especially true for fire-prone savannas, where frequent fires shape the fraction of carbon allocated to root traits for post-fire vegetation recovery. Here I review evidence on how root traits may respond to frequent fires and propose to leverage root traits to infer belowground carbon dynamics in fire-prone savannas. Evidently, we still lack an understanding of trade-offs in root acquisitive vs. conservative traits in response to frequent fires, nor have we determined which root traits are functionally important to mediate belowground carbon dynamics in a frequently burned environment. Focusing research efforts along these topics should improve our understanding of savanna carbon cycling under future changes in fire regimes.

Keywords: belowground carbon allocation; frequent fire; root functional trait; savannas; soil carbon storage; trait variation.

Figure 1

Linking root functional trait to understand belowground carbon dynamics in fire-prone savannas. Panel (A) shows fire frequency (fires/year) across tropical and subtropical savannas. Dataset were from the Moderate Resolution Imaging Spectroradiometer (MODIS) burned area product (Giglio et al., 2018). Panel (B) shows potential response of root traits to frequent fires in savannas based on root economics space (Bergmann et al., 2020). Frequent fires may shape root traits toward resource conservation and longer-lived, for example, higher root tissue density, higher root mass fraction, higher nonstructural carbohydrate, and lower root nitrogen content. Panel (C) shows an example of root mass fraction response to frequent fires at the individual species and ecosystem level (Zhou et al., 2022). Root mass fraction of Terminalia sericea resprouters that experienced annual fires can reach up to 95%, and savannas experienced more frequent fires have higher root mass fraction at the ecosystem level than less and unburned ones in Kruger National Park, South Africa. Panel (D) shows a conceptual framework linking root traits to understanding belowground carbon storage in fire-prone savannas. Savanna species experiencing more frequent fires are expected to allocate more carbon for root traits (H1), which is hypothesized to lead to a higher belowground biomass carbon storage (H2) as well as soil carbon storage (H3).