Multi-temporal ecological analysis of Jeffrey pine beetle outbreak dynamics within the Lake Tahoe Basin

From 1991 to 1996, Jeffrey pine beetles (Dendroctonus jeffreyi Hopkins) (JPB) caused tree mortality throughout the Lake Tahoe Basin during a severe drought. Census data were collected annually on 10,721 trees to assess patterns of JPB-caused mortality. This represents the most extensive tree-level, spatiotemporal dataset collected to-date documenting bark beetle activity. Our study was an exploratory assessment of characteristics associated with the probability of successful JPB mass-attack (P_JPB) and group aggregation behavior that occurred throughout various outbreak phases. Numerous characteristics associated with P_JPB varied by outbreak phase although population pressure and forest density had positive associations during all phases. During the incipient phase, JPBs caused mortality in individual trees and small groups within toeslope topographic positions and P_JPB had a negative relationship with stem diameter. In the epidemic phase, JPB activity occurred in all topographic positions and caused mortality in spatially expanding clusters. P_JPB had a curvilinear relationship with tree diameter and a negative relationship with proximity to nearest brood tree. Majority (92–96 %) of mass-attacked trees were within 30 m of a brood tree during the peak epidemic years. During the post-epidemic phase, mortality clusters progressively decreased while dispersal distances between mass-attacked and brood trees increased. Post-epidemic P_JPB had a negative relationship with stem diameter and mortality was concentrated in the mid and upper-slope topographic positions. Results indicate mortality predictions are reasonable for the epidemic phase but not for incipient and post-epidemic phases. Ecological factors influencing JPB-caused tree mortality, clustered mortality patterns, and transitions from environmental to dynamic determinism are discussed.