Masting facilitates seed predator escape in white pine blister rust-infected landscapes

Background/Question/Methods

Satiation of predispersal seed predators by mast years has been demonstrated in many intact ecosystems. Most high elevation five-needled pine ecosystems in North America have declined considerably from the introduced white pine blister rust (WPBR; Cronartium ribicola), jeopardizing seed availability for mutualistic seed dispersers. We used 17 populations of the endangered limber pine (Pinus flexilis) in Alberta, Canada, to examine whether the benefits of interannual variation in cone production vary depending on the severity of WPBR infections, and the abundance of seed predator populations. We compared cone production, cone predation, and abundance of red squirrels (Tamiasciurus hudsonicus) in low versus high WPBR landscapes (1% and 38% live tree infection, respectively), over two mast years (2007, and 2010), and two nonmast years (2008, and 2009).

Results/Conclusions

Significant variability in interannual cone production (CV = 193.4), and proportionately greater cone escape in the 2010 mast year (0.54 vs. 0.21 in low cone years) led to 10 times greater cone escape overall in the mast year than in nonmast years. High severity WPBR landscapes produced 69.8% more cones per tree , and supported half as many squirrels, but the proportion of cones predated, and mean cone escape, did not differ between landscapes. Our study suggests there is temporal resiliency in seed escape in declining limber pine ecosystems, even under different disease and seed predator threats at both population and landscape scales. These findings also suggest that evolved seed escape strategies may be resilient to a variety of ecological conditions, including:  disease infestations, natural variation in seed production between stands, lower seed production at the northern limits of a species range, and different forest types.