Post‐fire trajectories reveal a legacy effect and decoupled structural and floristic responses in a seasonally dry subtropical mountain region

Fire is a key driver maintaining non‐forest vegetation in tropical and subtropical seasonal climates that could otherwise support forest. However, a deeper understanding of long‐term post‐fire dynamics requires tracking both structure and floristic composition in the same locations over time while accounting for pre‐vegetation conditions—an approach seldom achieved. Here, we leverage a 12‐year dataset from resurveyed sites with pre‐fire vegetation data to assess how pre‐existing site variation influences post‐fire trajectories (i.e. legacy effects).

In the seasonally dry subtropical mountains of central Argentina, we surveyed floristic composition and vegetation structure in 33 permanent sites in 2008. Following a wildfire in 2011 that burned 18 of these sites, we resurveyed all sites in 2012 (first post‐fire growing season, representing short‐term change) and biennially until 2020. This design allowed us to analyse 12‐year change without fire (2008–2020), short‐term pre‐ to post‐fire change (2008–2012) and 8‐year post‐fire change (2012–2020).

Twelve years without fire promoted shifts from non‐forest to forest, a trend more pronounced in floristic composition than in structure. In the first post‐fire growing season, fire shifted vegetation towards a less woody structure, with greater changes in sites with higher pre‐fire woodiness. Floristically, burned sites converged towards a community dominated by species typical of shrublands during the same period. Over 8 years, vegetation structure in shrubland recovered to its pre‐fire condition, while in forest it remained stable. Floristic composition showed a recovery to pre‐fire conditions only in grasslands, while remaining stable in shrublands and forests.

Synthesis . Our study demonstrates that fire mediates the trade‐off between forest expansion and the persistence of non‐forest communities. Fire‐free periods drive gradual, asynchronous shifts in floristic composition and vegetation structure towards forest, whereas fire disrupts these trajectories, maintaining non‐forest communities. Our decadal‐scale data capture the initial stages of these non‐forest to forest transitions, suggesting that a full grassland to forest transition may require fire‐free intervals extending over a century. Our findings underscore the importance of long‐term monitoring and suggest that adaptive fire management strategies are essential either to conserve non‐forest communities or to facilitate woody expansion.