Flammability is a niche construction trait: canopy architecture affects fire intensity.

By affecting local fire intensities or the probability of ignition, traits that influence plant flammability may indirectly control selection for fire-related life-history and physiological traits. The retention of dead branches in the canopy has been cited as contributing to plant flammability. No experiment, however, has demonstrated that differences in plant canopy architecture on the scale of observed variation in nature can affect local fire characteristics. I experimentally manipulated canopies of Adenostoma fasciculatum, a California shrub that naturally retains dead branches, to mimic degrees of self-pruning in four small-scale (4 m x 6 m) treatments: removal of all canopy dead wood, clipping of all dead wood with wood left as litter, an unmanipulated treatment, and a dead wood addition. Treatment plots were burned in large-scale prescribed fires. Fire temperatures and heat release were significantly higher in Unmanipulated and Addition treatments, demonstrating a significant local effect of dead branch retention. Removal and Clip and Leave treatments did not differ significantly; the observed effect is a result of canopy architecture rather than differences in total fuel load.