years" would have influenced the evolution of the tree species that they ate the seeds of — specifically, that masting trees that produced seeds during the spring nesting season (such as red oaks) evolved so that some portion of their seeds would be too large for passenger pigeons to swallow (thus allowing some of their seeds to escape predation and grow new trees), while white oaks, with its seeds sized consistently in the edible range, evolved an irregular masting pattern that took place in the fall, when fewer passenger pigeons would have been present. The study further concluded that this allowed white oaks to be the dominant tree species in regions where passenger pigeons were commonly present in the spring. With the large numbers in passenger pigeon flocks, the excrement they produced was enough to destroy surface-level vegetation at long-term roosting sites, while adding high quantities of nutrients to the ecosystem. Because of this — along with the breaking of tree limbs under their collective weight and the great amount of mast they consumed — passenger pigeons are thought to have influenced both the structure of eastern forests and the composition of the species present there. Due to these influences, some ecologists have considered the passenger pigeon a keystone species, with the disappearance of their vast flocks leaving a major gap in the ecosystem. Their role in creating forest disturbances has been linked to greater vertebrate diversity in forests by creating more niches for animals to fill, as well as contributing to a healthy forest fire cycle in the forests, as it has been found that forest fires have increased in prevalence since the extinction of the passenger pigeon, which seems to go against the idea that the tree limbs and branches they would bring down served as fuel for the fires. To help fill that ecological gap, it has been proposed that modern land managers attempt to replicate some of their effects on the ecosystem by creating openings in forest canopies to provide more understory light. The American chestnut trees that provided much of the mast on which the passenger pigeon fed was itself almost driven to extinction by an imported Asian fungus (chestnut blight) around 1905. As many as thirty billion trees are thought to have died as a result in the following decades, but this did not affect the passenger pigeon, which was already extinct in the