Death and Taxus: the contribution of mechanical damage to the multiple factors associated with Taxus brevifolia mortality

Abstract Background Recent increases in tree mortality are often attributed to climate, but climate extremes may just be the last of many stressors that have unfolded over many years resulting in tree death. Potentially it is only those trees weakened by mechanical damage or attacks by insects or fungi that are susceptible to climate-mediated mortality, whereas vigorous trees resist periods of unfavorable climate. Although previous studies have explored immediate and catastrophic effects of mechanical damage via blowdowns and stem breakage, few have investigated the delayed effects of mechanical damage on mortality. Taxus is a shade-tolerant genus of subcanopy trees or shrubs distributed throughout the northern hemisphere. Populations of Taxus are in decline worldwide but owing to the tenacity of Taxus in the face of stressors, this decline is poorly understood. Here, we provide spatial evidence that cumulative stress interactions, particularly mechanical damage, contribute to Taxus mortality. Methods We examined 14 years of annual demographic data from the 27.2 ha Wind River Forest Dynamics Plot (WFDP), where woody stems, snags, and deadwood have been tagged, mapped and identified to species. We analyzed the multiple factors associated with tree death, including mechanical damage, pathogens, suppression, beetles, animal damage, and their combinations. We performed spatial pattern analyses with the pair correlation function to investigate the prevalence of density-dependent mortality, effects of neighboring large-diameter trees, and effects of nearby snag fall and deadwood. Results In 2011, there were 29,827 trees within the WFDP of which 2119 were Taxus . Between 2011 and 2024, Taxus declined to 1523 trees (mean annual mortality of 2.79% yr −1 ). Taxus mortality was not influenced by intraspecific density dependence or the presence of snags but was mainly driven by mechanical damage, pathogens, and suppression. Dead Taxus were strongly associated with deadwood within 2 m of the bole. Structural equation modeling showed that mechanical damage likely increased the vulnerability of Taxus to pathogen infection and exposure to drier gap areas. Conclusions These results suggest that Taxus mortality is rarely due to a single event, but a cumulative process of interacting stressors initiated by falling wood and often culminating in death by other stressors—potentially many years later. The association of newly dead trees with deadwood suggests that falling snags likely contributed to past crown damage, initiating or accelerating a decline spiral. The results emphasize that previous mechanical damage—evidenced by mapped and persistent deadwood—can disproportionately affect tree species, influencing successional dynamics.