Outbreak and population history of a native wood borer in the Ozark and Ouachita National Forests, Arkansas, in relation to climatic factors, host physical resistance and individual tree and stand characteristics

Native wood boring beetle population dynamics are poorly understood. I utilized tree-ring techniques to examine historical dynamics of Enaphalodes rufulus (Haldeman), the red oak borer, and its primary host, Quercus rubra L., within the Ozark and Ouachita Mountains of Arkansas throughout the past century. This time period included an unexplained and unprecedented outbreak of this historically endemic, non-outbreak species. I first developed a sampling system that optimized the number of host Q. rubra sampled and minimized the amount of bole sampled per tree to confidently obtain historical information on borer activity within hosts. I then utilized a combination of borer densities dated from larval gallery scars preserved within tree-rings and population growth, i.e. intrinsic rate of increase, estimated from density per generation to describe endemic, incipient (i.e. growing), outbreak and declining borer population phases at the site level. An exactly dated history of borer populations in 107 Q. rubra from 1938--2008 indicated that timing of these population phases varied geographically, although peak borer densities at most sites occurred in 2001, consistent with timing of observed regional host mortality. Borer numbers were growing across the region since the mid-1970s, at least eleven generations prior to observed host tree mortality. Borer population growth was positively correlated with drought severity, which suggested that drought, an exogenous factor, may have been an important causal factor in the recent outbreak. Endogenous factors such as Q. rubra maturity acting at the stand level were also likely important due to geographic variation in timing of borer population phases.Tree and stand data collected from an additional 869 recently dead, declining or healthy Q. rubra revealed that trees which died during the borer outbreak were likely suppressed individuals and poor competitors for resources, which allowed neighboring healthy individuals to experience a growth release during the outbreak. A time-dependent regression technique revealed changes in the relationship between Q. rubra growth and climate from 1920--2007. Decline in tree growth relationship to an index of July soil moisture availability occurred between 1950 and 1980 for most individuals. Timing of growth decline and deteriorating tree growth-climate relationships were linked to the combination of a severe drought event in the early 1950s followed by an unusually wet period, which likely resulted in increased leaf area that could not be supported by existing root systems and sapwood area during subsequent droughts. These back-to-back unusually extreme weather events weakened those Q. rubra, in terms of radial growth and relationship to climate, which later became suitable hosts for E. rufulus.To examine important factors affecting Q. rubra physical resistance to E. rufulus attack and subsequent pathogen invasion, I measured an indicator of vascular tissue compartmentalization and an indicator of vascular tissue repair, vertical lesion size and healing time of borer scars, respectively. Host Q. rubra decline status proved a valuable predictor of resistance, wherein healthy trees exhibited better compartmentalization capacity, as indicated by shorter lesions and faster tissue repair, than declining trees and those that died. I utilized recursive partitioning to determine whether variables associated with hosts, borer feeding density or the environment were more important predictors of physical resistance. Variables related to borer density consistently explained more variation in lesion length than host or environmental variables. No variables of the 16 examined were useful for describing healing time variability in surviving hosts. Only healthy Q. rubra exhibited a significant declining lesion length trend from 1988--2006 and were likely able to re-allocate resources in favor of maintaining resistance throughout time as feeding pressure from borers increased and then relaxed post-outbreak. These trees were able to effectively tolerate feeding pressure from borers and survive the outbreak.