Squirrels reduce post-fire regeneration potential in serotinous pines

Serotiny is a key trait for population persistence in fire-prone ecosystems, allowing species to accumulate an aerial seed bank until fire triggers seed release. However, serotinous cones also provide a food source for specialized pre-dispersal seed predators, potentially counteracting the benefits of serotiny. These predators may also exert selection pressures on cone traits that can further reduce the size of the canopy seed bank. Here we evaluate the mechanisms by which seed predation reduces the size of the canopy seed bank and quantify the magnitude of these effects.

We sampled six sites dominated by mature Pinus halepensis forests, three with European red squirrels, Sciurus vulgaris, and three without. For each tree, we quantified cone production, determined the degree of serotiny, assessed seed predation (in squirrel-present populations) and characterized cones and seeds. This allowed us to estimate the size of the seed bank and the effects of seed predation on it. We also conducted a literature review for further validation.

Our findings indicate that, in populations with squirrels, pre-dispersal seed predation reduces the canopy seed bank by ∼66 %, not only through the ecological effect of cone removal, but also by exerting negative selection on serotiny and promoting increased seed defences.

Pre-dispersal seed predation may compromise the post-fire regeneration of serotinous species by reducing the canopy seed bank, which is particularly concerning in the context of increasing fire activity in the Mediterranean basin.