Functional traits influence woody stem productivity of Cerrado trees across altered fire regimes

Across ecosystems worldwide, fire regimes are shifting – some landscapes experience more frequent and intense burns, while others see fire exclusion altogether. These changes have profound implications for ecosystem functioning, particularly for net primary productivity (NPP). Francisco Navarro-Rosales, a DPhil student supervised by Dr Imma Oliveras Menor, presented his second DPhil paper from the CERFogo project, which investigates how fire alters the carbon balance of tropical savannas. His earlier work showed that fire reduces tree density and associated carbon fluxes. In this latest study, he asked a deeper question: what mechanisms underlie changes in woody stem productivity across altered fire regimes?

The central hypothesis was that tree productivity after fire reintroduction would be maintained by survival traits such as resprouting ability, bud protection, and bark thickness; while fire exclusion would shift productivity towards a balance between acquisitive and conservative strategies. To test this, Francisco examined how Cerrado tree communities respond to different fire intervals, and whether these responses can be explained by plant functional traits.

An experimental burn at the CERFogo project in Brazil.

At the community level, unburnt plots showed productivity patterns reflecting species abundance, but heavily skewed towards generalist species. Fire reduced overall productivity and shifted dominance towards savanna-adapted species, with stronger effects under higher fire intensity. At the species level, he explored whether there is a trade-off between NPP and fire tolerance. Indeed, after prescribed burns of different frequencies, species with higher NPP tended to be more fire-vulnerable. However, this relationship was obscured by the dominance of Tachigali vulgaris, a short-lived generalist species whose productivity increased disproportionately under low to moderate fire intensities—an ecological outlier that complicates the broader pattern.

Leaf ecophysiology measurements being taken at the CERFogo project field site.

Exploring trait relationships via principal component analyses revealed two main axes of variation: structural investment strategies and photosynthetic strategies. By modelling productivity as an interaction between traits, fire regime, and interval, Francisco was able to test if the influence of traits on NPP had change due to the long-term effect of fire regimes. In fire exclusion and annually burnt plots, productivity was highly variable and not strongly linked to traits. Under biennial burns, NPP was associated with specific leaf area (SLA) and wood density, largely driven by T. vulgaris. Triennial burns supported more intense fires and favoured species with bark protection and rapid carbon assimilation, highlighting how resource acquisition strategies are constrained by fire in terms of structure but not photosynthesis.

The take-home message is clear: Cerrado trees exhibit a productivity–fire tolerance trade-off. Bark protection and light adaptation help maintain NPP under high fire intensities, as both structural and acquisitive strategies mediate growth responses across fire regimes. These findings underscore how functional traits shape ecosystem productivity under shifting fire dynamics, offering insights into the resilience and vulnerability of tropical savanna systems.

The CERFogo project team during a field campaign in Brazil.