Adaptive flammability syndromes in thermo-Mediterranean vegetation, captured by alternative resource-use strategies

Fire affects and is affected by leaf functional traits indicative of resource allocation trade-offs. Global change drivers constrain both the resource-use strategies and flammability of coexisting species. However, small attention has been given in identifying links among flammability and plant economics. Ambiguity comes from the fact that flammability is a multidimensional trait. Different flammability attributes (i.e. ignitibility, sustainability, combustibility and consumability) have been used to classify species, but no widely-accepted relationships exist between attributes. We hypothesised that flammability is a spectrum (defined by its four attributes) and the alternative flammability syndromes of coexisting species can be captured by their resource-use strategies. Furthermore, we argue that flammability syndromes are adaptive strategies that ensure persistence in the post-fire community. We conducted a large-scale study to estimate all flammability attributes on leaves from nine, dominant, thermo-Mediterranean species with alternative resource-use and fire-response strategies across a wide environmental and geographic gradient. We assessed the interdependence among attributes, and their variation across ecological scales (genus, species, individual, site and region). Furthermore, we collected 10 leaf functional traits, conducted a soil study and extracted long-term climatological data to quantify their effect on flammability attributes. We found that leaf flammability in thermo-Mediterranean vegetation is a continuous two-dimensional spectrum. The first dimension, driven by leaf shape and size, represents heat release rate (combustibility vs. sustainability), while the second, controlled by leaf economics, presents ignition delay and total heat release (i.e. consumability). Alternative flammability syndromes can increase fitness in fire-prone communities by offering qualitative differences in survival or reproduction. Trade-offs and constraints that control the distribution of resource-use strategies across environmental gradients appeared to drive leaf flammability syndromes as well. Tying the flammability spectrum with resource allocation trade-offs on a global scale can help us predict future ecosystem properties and fire regimes and illustrate evolutionary constraints on flammability.