Seed Release in Lodgepole Pine Forests After Mountain Pine Beetle Outbreak

Serotinous lodgepole pine (Pinus contorta var. latifolia) usually regenerates after fire or harvesting provided conditions that are warm enough to open the cones. There are concerns that large-scale stand mortality due to mountain pine beetle (MPB) outbreak could greatly reduce natural regeneration of lodgepole pine because the closed cones are held in place in the tree canopy without any seed release. We selected 15 stands (five gray-attacked, five red-attacked, and five green) in the Sub-Boreal Spruce biogeoclimatic zone of British Columbia to determine loss of canopy seed via breakage of twig-bearing cones and cone opening (i.e., loss of serotiny) throughout the 2008 growing season. We also quantified seed loss of fallen cones via predation and cone opening. Red-attacked stands lost an estimated 175 000 seed-bearing canopy cones ha-1 yr -1 due to crown friction resulting in twig breakage, representing an over three-fold increase compared to green stands. This result was considered ecologically important since it equated to over 25 % of canopy cones lost to the forest floor. Red- and gray-attacked stands also had 15 % of canopy seed lost due to cone weathering resulting in cone opening. Additional seed losses occurred in the gray-attacked stands due to additional cone opening (58 % yr-1) on the forest floor and predation (12 000 fallen seed-bearing cones ha-1 yr-1). MPB-killed stands released some canopy seed through breakage of twig-bearing cones, partial loss of serotiny, and forest floor cone opening. The implications are: i) seed supply is gradually lost in the first years after attack; ii) if adequate levels of regeneration are to occur, either anthropogenic or fire disturbances must happen shortly after tree mortality. We conclude that lodgepole pine is poorly-adapted to disturbances such as MPB because seed is slowly released onto an unfavorable seed bed.