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

Abstract

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 massattack (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.

Forest structure and climate mediate drought‐induced tree mortality in forests of the Sierra Nevada,USA

“…, Egan et al. ), the proximate drivers of most mortality in this event (Fettig et al. ), likely because the beetles can more readily colonize and reproduce within trees that are weakened by drought.…”

“…The influence of different stand variables also varies through the course of an infestation, which can complicate comparisons among studies (Egan et al. ). Some bark beetle species tend to target larger pines (generally >10 or 15 cm diameter), presumably because of their larger carbohydrate stores (Furniss and Carolin ).…”

“…However, in some cases, the largest trees may be less prone to attack due to their ability to access deep water stores, though even these trees become susceptible in later outbreak stages (Egan et al. ).…”

Detecting Drought-Induced Tree Mortality in Sierra Nevada Forests with Time Series of Satellite Data

“…The majority of these affected areas were subject to infestations of fir engraver beetles (Scolytus ventralis), western pine beetles (Dendroctonus brevicomis), and Jeffrey pine beetles (D. jeffryi) ( Figure S4). Outbreaks of these beetles typically occur in conjunction with weakened trees, and the severe drought likely contributed to the elevated mortality from infestations in 2015 and 2016 [15,18,58,59]. …”

“…However, if temperatures continue to rise throughout the region, these forests may also experience greater mortality as warmer temperatures allow insects and pathogens to thrive, as bark beetles in large numbers can attack and kill previously healthy trees [14,58,84]. Firs engraver populations, which devastated red fir in the Eastern Sierra in 2014-2015, have recently started to attack white fir trees ( Figures S3 and S4), and Jeffrey pine trees are beginning to succumb to infestations of Ips beetles.…”