Top-down vs bottom-up models for boreal forest fire regimes
What are the ultimate factors controlling fire regimes? North American boreal forests primarily experience crown-fire regimes, whereas Eurasian boreal forests are characterized by surface-fire regimes. What are the ultimate drivers of these two contrasted fire regimes? One hypothesis suggests that the variability in environmental factors (e.g. productivity in different continents) shapes fire regimes, which assemble different communities with divergent plant traits (top-down mechanism; Fig. 1 left). Another hypothesis suggests that the different biogeographical history (in the same environment, i.e., boreal climate) assembles different communities with different plant traits that subsequently shape different fire regimes (bottom-up mechanism; Fig. 1, right).
Given the similar climate across the boreal forests, it is often assumed that bottom-up forces drive the two fire regimes. However, by analyzing congeneric Pinus and Picea species from both continents, we tested the alternative top-down hypothesis—that the environment primarily governs fire regimes [1]. We compared environmental conditions where the species occur using remote sensing data and found that Eurasian tree species occupy warmer and more productive environments than their North American congeners. These findings support a top-down model where environmental factors drive the continental dichotomy between surface- and crown-fire regimes in boreal systems.
References
[1] Pausas JG, Keeley JE, Syphard AD 2025. Are fire regimes the result of top-down or bottom-up drivers? Philosophical Transactions of the Royal Society of London, Series B 380: 20230447 [doi | web | pdf]
