Does overshoot in leaf development of ponderosa pine in wet years leads to bark beetle outbreaks on fine-textured soils in drier years？

Background:FrequentoutbreaksofinsectsanddiseaseshavebeenrecordedinthenativeforestsofwesternNorthAmericaduringthelastfewdecades,butthedistributionoftheseoutbreakshasbeenfarfromuniform.Insomecases,recentclimaticvariationsmayexplainsomeofthisspatialvariationalongwiththepresenceofexpansiveforestscomposedofdense,oldertrees.Forestmanagersandpolicymakerswouldbenefitifareasespeciallypronetodisturbancecouldberecognizedsothatmitigatingactionscouldbetaken.Methods:Weusetwoponderosapine-dominatedsitesinwesternMontana,U.S.A.toapplyamodelingapproachthatcouplesinformationacquiredviaremotesensing,soilsurveys,andlocalweatherstationstoassesswherebarkbeetleoutbreaksmightfirstoccurandwhy.Althoughtherewasageneraldownwardtrendinprecipitationforbothsitesovertheperiodbetween1998and2010(slope=-1.3,R2=0.08),interannualvariabilitywashigh.Someyearsshowedlargeincreasesfollowedbysharpdecreases.Bothsiteshadsimilartopographyandfirehistories,butbarkbeetleactivityoccurredearlier(circa2000to2001)andmoreseverelyononesitethanontheother.Theinitialcanopydensityofthetwositeswasalsosimilar,withleafareaindicesrangingbetween1.7-2.0m2?m-2.WewonderedifthedifferenceinbarkbeetleactivitywasrelatedtosoilsthatwerehigherinclaycontentatsiteIthanatsiteII.Toassessthispossibility,weappliedaprocess-basedstandgrowthmodel(3-PG)toanalyzethedataandevaluatethehypotheses.Results:Wefoundthatwhenwetyearswerefollowedbydrieryears,thesimulatedannualwoodproductionperunitofleafarea,ameasureoftreevigor,droppedbelowacriticalthresholdonsiteIbutnotonsiteII.Conclusion:Weconcludedthatthedifferenceinvulnerabilityofthetwostandstobeetleoutbreakscanbeexplainedlargelybydifferencesingrossphotosynthesisattributedtothefactthatanequivalentamountofstoredwaterintherootingzone(100m